Hard carbon negative electrode material and preparation method thereof, and sodium ion battery

Abstract

The application discloses a hard carbon negative electrode material and a preparation method thereof and a sodium ion battery, and belongs to the technical field of batteries. The preparation method of the hard carbon negative electrode material comprises the following steps: nitration treatment is performed on pitch by using nitric acid to obtain nitration pitch; oxidation heat treatment is performed on the nitration pitch in an oxygen-containing atmosphere to obtain oxidation pitch; and carbonization heat treatment is performed on the oxidation pitch in an inert atmosphere to obtain the hard carbon negative electrode material. By means of the treatment mode of the synergistic modification of nitration and oxidation, nitrogen elements are introduced into the pitch and the pitch is oxidized, and pore formation is promoted, the crystallinity of the pitch is reduced, and the pitch is microporous, so that the prepared hard carbon negative electrode material has high reversible capacity and excellent rate performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a hard carbon negative electrode material, a preparation method thereof and a sodium ion battery. BACKGROUND

[0002] With the non-renewable of traditional fossil energy and the increasingly serious environmental problems, new energy technologies represented by lithium ion batteries are more and more applied in electronic devices, electric vehicles and other fields due to their advantages in environmental protection, renewable, energy and power density. Sodium ion batteries, which have similar working principles to lithium ion batteries, are expected to replace lithium batteries in large-scale energy storage equipment, small low-speed electric vehicles and other fields due to their rich reserves in the earth's crust, low cost and mature production process. In recent years, the research progress related to sodium ion batteries has attracted close attention from the academic and industrial circles.

[0003] In combination with the working principle of sodium ion batteries, hard carbon is considered to be the most potential industrialized sodium battery negative electrode material due to its short-range order, long-range disorder, graphite-like domain structure, rich closed pores and defects, high reversible capacity and low working potential. The types of hard carbon precursors generally include biomass, organic polymers and pitch. Among them, biomass has abundant raw material sources, complex natural components, and more defects after carbonization, which are beneficial to provide sodium storage sites. However, the pre-treatment process is complex, the raw material sources are unstable, and the carbonization has a low yield and a high ash content. Organic polymers such as resins can effectively inhibit the planar ordered rearrangement of carbon layers during carbonization due to their stable cross-linked structure, thereby obtaining hard carbon with excellent sodium storage performance. However, this type of material needs to be synthesized and processed in a chemical plant, which is costly. Pitch, as a common petroleum chemical extract, has a wide source and low cost. The aliphatic chain segments, unsaturated aromatic rings, condensed ring structures and heteroatoms in its components can produce more defects during carbonization, expand the carbon layer spacing, and are beneficial to the storage and deintercalation of sodium ions. It is a common precursor of hard carbon material.

[0004] Compared with biomass and organic polymer precursors, hard carbon prepared by using pitch as a precursor has advantages in cost and yield. However, pitch is prone to ordered rearrangement of carbon layers during high-temperature carbonization, forming a large-area planar stacked graphite-like domain structure. The reduction of amorphous structure and the narrowing of carbon layer spacing are not conducive to the stable deintercalation of sodium ions in the structure. Therefore, the specific capacity of hard carbon negative electrode material prepared by using pitch as a precursor is usually low (<260 mAh / g), the capacity in the low-voltage platform area is less, and the rate performance is poor.

[0005] CONTENT

[0006] Therefore, the present application provides a hard carbon negative electrode material, a preparation method thereof and a sodium ion battery, aiming to obtain a sodium ion battery hard carbon negative electrode with high reversible capacity and excellent rate performance.

[0007] The embodiment of the present application is implemented in this way, and provides a preparation method of a hard carbon negative electrode material, comprising: nitration treatment of asphalt by using nitric acid to obtain nitration asphalt; oxidative heat treatment of the nitration asphalt in an oxygen-containing atmosphere to obtain oxidized asphalt; carbonization heat treatment of the oxidized asphalt in an inert atmosphere to obtain the hard carbon negative electrode material.

[0008] In some embodiments, the nitration treatment of asphalt by using nitric acid comprises: mixing an aqueous solution of nitric acid with asphalt, and then performing a mixing reaction in an inert atmosphere.

[0009] In some embodiments, the mass percentage of nitric acid in the aqueous solution of nitric acid is 30% to 65%; and / or, the ratio of the mass of the asphalt to the volume of the aqueous solution of nitric acid is 1:1 to 1:15; and / or, the aqueous solution of nitric acid comprises sulfuric acid; and / or, the inert atmosphere comprises an atmosphere formed by one or more of nitrogen, helium, neon, argon, and xenon; and / or, the temperature of the mixing reaction is 0 to 20℃; and / or, the time of the mixing reaction is 12 to 24 hours.

[0010] In some embodiments, after the mixing reaction, the method further comprises: solid-liquid separation and drying to obtain a powder of the nitration asphalt, wherein the temperature of the drying is 80 to 120℃.

[0011] In some embodiments, the asphalt comprises one or more of petroleum asphalt and coal tar pitch; and / or, the softening point of the asphalt is 100℃ to 350℃; and / or, the asphalt is asphalt particles or asphalt powder; and / or, the average particle size of the asphalt is 10 to 100μm.

[0012] In some embodiments, the oxygen-containing atmosphere is a pure oxygen atmosphere, an air atmosphere, or a mixed atmosphere formed by oxygen and an inert gas; and / or, the holding temperature of the oxidative heat treatment is 150 to 400℃; and / or, the holding time of the oxidative heat treatment is 2 to 8 hours; and / or, the heating rate of the oxidative heat treatment is 3 to 5℃ / min.

[0013] In some embodiments, the inert atmosphere in the carbonization heat treatment comprises an atmosphere formed by one or more of nitrogen, helium, neon, argon, and xenon; and / or, the temperature of the carbonization heat treatment is 1100 to 1700℃; and / or, the time of the carbonization heat treatment is 2 to 8 hours; and / or, the carbonization heat treatment further comprises a pre-carbonization treatment before the carbonization heat treatment, and the temperature of the pre-carbonization treatment is 600 to 900℃.

[0014] Correspondingly, the embodiment of the present application also provides a hard carbon negative electrode material, which is prepared by the above preparation method.

[0015] In some embodiments, the median particle size of the hard carbon negative electrode material is 7-20 μm.

[0016] Correspondingly, the application also provides a sodium ion battery comprising the hard carbon negative electrode material described above, or comprising the hard carbon negative electrode material prepared by the preparation method described above.

[0017] The preparation method of the hard carbon negative electrode material provided by the application comprises: nitration treatment of pitch by using nitric acid to obtain nitration pitch; oxidative heat treatment of the nitration pitch in an oxygen-containing atmosphere to obtain oxidation pitch; and carbonization heat treatment of the oxidation pitch in an inert atmosphere to obtain the hard carbon negative electrode material. By the treatment mode of cooperative modification of nitration and oxidation, nitrogen element is introduced into the pitch and the pitch is oxidized, and pore formation is promoted, the crystallinity is reduced and the pitch is microporous, so that the prepared hard carbon negative electrode material has high reversible capacity and excellent rate performance. BRIEF DESCRIPTION OF DRAWINGS

[0018] In order to more clearly illustrate the technical solutions in the embodiments of the application, the drawings needed in the embodiment description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the application, and other drawings can be obtained by those skilled in the art without creative labor.

[0019] Figure 1 is a flowchart of an embodiment of the preparation method of the hard carbon negative electrode material provided by the application;

[0020] Figure 2 is the charge-discharge curve of Example 1, Comparative Example 1;

[0021] Figure 3 is the charge-discharge curve of Example 1, Comparative Example 2, Comparative Example 4;

[0022] Figure 4 is the charge-discharge curve of Example 2, Comparative Example 3, Comparative Example 5;

[0023] Figure 5 is the rate graph of Example 1 and Comparative Example 1;

[0024] Figure 6 is the rate graph of Example 1 and Comparative Example 2;

[0025] Figure 7 is the rate graph of Example 1 and Comparative Example 4. DETAILED DESCRIPTION

[0026] Clearly, the embodiments described are only a part of all the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by a person of ordinary skill in the art without creative work shall fall within the scope of the present application. In addition, it should be understood that the specific implementation described herein is only used to illustrate and explain the present application, and is not used to limit the present application. In the present application, the orientation words such as "upper" and "lower" are the direction of the drawing surface. In addition, in the description of the present application, the term "comprising" means "including but not limited to".

[0027] Various embodiments of the present application can exist in the form of a range; it should be understood that the description in the form of a range is only for the convenience and brevity, and should not be understood as a hard limit to the scope of the present application; therefore, it should be considered that the described range has been specifically disclosed all possible sub-ranges and single values within the range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. In addition, whenever a numerical range is indicated in the present application, it refers to any cited number (fraction or integer) within the indicated range.

[0028] In the present application, the association relationship of the associated objects described by "and / or" indicates that there can be three kinds of relationships, for example, A and / or B can represent the cases of A alone, A and B together, and B alone. Wherein A and B can be singular or plural.

[0029] In the present application, "at least one" means one or more, and "multiple" means two or more. "At least one", "at least one of the following" or the like means any combination of these items, including any combination of single item or multiple items. For example, "at least one of a, b, or c", or "at least one of a, b, and c", can represent a, b, c, a-b (i.e. a and b), a-c, b-c, or a-b-c, wherein a, b, and c can be single or multiple.

[0030] The present application provides a preparation method of a hard carbon negative electrode material, referring to Figure 1 , Figure 1 is a flowchart of an embodiment of the preparation method of the hard carbon negative electrode material provided by the present application, which specifically comprises the following steps:

[0031] Step S1: nitration treatment of asphalt using nitric acid to obtain nitration asphalt;

[0032] Step S2: oxidative heat treatment of the nitration asphalt under an oxygen-containing atmosphere to obtain oxidation asphalt;

[0033] Step S3: carbonization heat treatment of the oxidation asphalt in an inert atmosphere to obtain the hard carbon negative electrode material.

[0034] In the embodiment, asphalt is used as a precursor and raw material, nitric acid is selected as an electrophilic reagent, nitronium ions generated by decomposition of nitric acid are used to undergo electrophilic aromatic substitution reaction with aromatic rings in asphalt, and oxygen-containing functional groups are further introduced by combining with air pre-oxidation to jointly prevent ordered rearrangement of asphalt molecules in the carbonization process, so as to reduce crystallinity and microporous. The method has simple process, strong controllability, and low raw material cost. The asphalt modified by the above method has high sodium storage capacity, rich low-voltage platform capacity and excellent rate performance when used for sodium ion storage after carbonization.

[0035] Further, in the embodiment, the asphalt is treated by the cooperative modification of nitration by nitric acid and pre-oxidation. Before the asphalt raw material is subjected to oxidation treatment such as air oxidation, there are fewer oxygen-containing functional groups and more aromatic hydrogens on the surface of the asphalt. In the nitration process, the active hydrogen protons of part of the sites are removed as nitronium ions, which promotes electrophilic aromatic substitution reaction with asphalt, so that nitrogen-containing functional groups are introduced into the asphalt molecules. When the oxidation treatment is carried out in an oxygen-containing atmosphere, part of the nitrogen-containing functional groups will react with oxygen to generate N2O, NO2, N2O4, N2O5 and other small molecules, so as to realize gas phase pore formation, increase the generation of micropores and defects of asphalt in the carbonization process; another part of the nitrogen-containing functional groups are doped into the bulk structure of the asphalt as a nitrogen source, so as to reduce the ordered rearrangement of the asphalt in the carbonization process, expand the carbon layer spacing and increase the disorder degree. The two parts of nitrogen jointly play a role in adjusting the structure of the asphalt-based hard carbon, preventing the ordered rearrangement of the asphalt molecules in the carbonization process, and promoting the reduction of crystallinity and microporous.

[0036] The step S1:

[0037] The asphalt can include one or more of petroleum asphalt, coal pitch. In an embodiment, the softening point of the asphalt can be 100-350℃, and specifically can be 100℃, 150℃, 200℃, 250℃, 300℃, 350℃, etc. The asphalt itself is a complex mixture of hydrocarbons of different molecular weights and their non-metallic derivatives. Asphalts of different softening points have different degrees of aromatization. The asphalt with a lower softening point corresponds to fewer aromatic nuclei and more aliphatic chain segments. In the thermal decomposition process, gases are produced, which distort the carbon lattice and produce a large number of intrinsic defects and active sites. The asphalt with a higher softening point corresponds to fewer short aliphatic chains and more substantial aromatic nuclei. In the thermal decomposition process, due to its higher condensation degree, there are fewer active sites, and it often participates in the shaping of the carbon layer skeleton. In the present application, the asphalt with a relatively low softening point is used, which is conducive to the formation of hard carbon.

[0038] Further, the asphalt can be asphalt particles or asphalt powder, so that the asphalt can be more fully contacted with nitric acid, the contact area between nitric acid and asphalt is increased, and the nitration treatment is more fully carried out. Specifically, the average particle size of the asphalt can be 10-100 μm.

[0039] In an embodiment, the nitration treatment of the asphalt using nitric acid specifically includes: providing a mixed solution containing nitric acid and asphalt, and carrying out a mixing reaction in an inert atmosphere.

[0040] In a specific embodiment, before the providing of the mixed solution containing nitric acid and asphalt, the method further includes: mixing an aqueous nitric acid solution with the asphalt.

[0041] Specifically, the mass fraction of the aqueous nitric acid solution can be 30%-65%, i.e., the mass percentage of nitric acid in the aqueous nitric acid solution is 30%-65%, and specifically can be 30%, 40%, 50%, 60%, 65%, etc.

[0042] Specifically, the mass-volume ratio of the asphalt and the aqueous nitric acid solution can be 1:1-1:15, i.e., the mass of the asphalt to the volume of the aqueous nitric acid solution is 1:1-1:15, and specifically can be 1:1-1:2, 1:2-1:3, 1:3-1:4, etc.

[0043] In an embodiment, sulfuric acid is also added to the aqueous nitric acid solution. The use of sulfuric acid can promote the protonation of nitric acid and accelerate the reaction rate of the nitration reaction. At the same time, sulfuric acid can also act as a dehydrating agent, and timely removal of water can accelerate the reaction.

[0044] In a specific embodiment, the aqueous nitric acid solution can be prepared by mixing concentrated nitric acid and concentrated sulfuric acid. Specifically, the mass ratio of the concentrated nitric acid and the concentrated sulfuric acid can be 3:1-5:1, and specifically can be 3:1, 4:1, 5:1, etc.

[0045] In this step, the essence of nitration treatment is the decomposition of concentrated nitric acid into electrophilic NO 2+ attack aromatic ring, and then deprotonate. The reaction environment requires that the nitric acid has a high concentration, i.e. the mass fraction of the nitric acid aqueous solution is 30% to 65%; when the concentration of the nitric acid aqueous solution is low, more oxidation of the high-reactivity aliphatic side chain of asphalt generates carbonyl or carboxyl groups. Considering comprehensively, the nitric acid aqueous solution with a mass fraction of 65% is selected as the preferred scheme.

[0046] It can be understood that the inert atmosphere can include an atmosphere formed by one or more of nitrogen, helium, neon, argon, and xenon. In a specific embodiment, a flow atmosphere of an inert atmosphere can be achieved using aeration, such as by aerating the inert gas at a flow rate of 12 to 20 ml / min.

[0047] In an embodiment, the temperature of the mixed reaction during the nitration treatment can be 0 to 20°C, and can be specifically 0°C, 5°C, 10°C, 15°C, 20°C, etc. In a specific embodiment, the above-mentioned temperature can be controlled by a water bath method, such as an ice water bath. The time of the mixed reaction can be 12 to 24 h, and can be specifically 12 h, 18 h, 24 h, etc.

[0048] It can be understood that the mixed reaction can be performed by a mixing method known in the art, such as one or more of stirring, ultrasonic, etc.

[0049] Further, after the mixed reaction is completed, solid-liquid separation and drying are performed to obtain a powder of the nitration asphalt. Further, after the solid-liquid separation, deionized water can be used to wash and adjust the pH value to neutral. Specifically, the drying temperature can be 80 to 120°C, such as 80°C, 100°C, 120°C, etc.; and the drying time can be 8 to 20 h, such as 8 h, 12 h, 15 h, 20 h, etc.

[0050] In an embodiment, before the step S1, a step of asphalt doping can be further included, wherein the doping element can be N, P, B, etc. The N element can be mixed with asphalt by using a nitrogen-containing compound such as urea.

[0051] In the step S2:

[0052] The oxygen-containing atmosphere can be a pure oxygen atmosphere, an air atmosphere, or a mixed atmosphere formed by oxygen and an inert gas. In a specific embodiment, the oxygen-containing atmosphere can be an air atmosphere.

[0053] The holding temperature of the oxidation heat treatment can be 150-400℃, specifically 150-200℃, 200-250℃, 250-300℃, 300-350℃, 350-400℃, etc. The holding time of the oxidation heat treatment can be 2-8h, specifically 2-4h, 4-6h, 6-8h, etc. In a specific embodiment, the heating rate to the holding temperature can be 3-5℃ / min. The holding temperature of the oxidation heat treatment is higher than the softening point of the pitch, which melts and oxidizes the nitration pitch, and introduces oxygen-containing functional groups such as carboxyl and carbonyl into the molecular structure of the pitch to induce the formation of cross-linked structure; at the same time, part of the nitro groups react with oxygen to form small molecular nitrogen oxides to form defect sites in the pitch molecules.

[0054] In an embodiment, after the holding of the oxidation heat treatment, the temperature is lowered to room temperature, and crushing or pulverization is performed to obtain a powder of the oxidized pitch, and then subsequent steps are performed.

[0055] In the step S3:

[0056] The inert atmosphere can include an atmosphere formed by one or more of nitrogen, helium, neon, argon, and xenon.

[0057] The temperature of the carbonization heat treatment can be 1100-1700℃, specifically 1100-1200℃, 1200-1300℃, 1300-1500℃, 1500-1700℃, etc.; and the time can be 2-8h, specifically 2-4h, 4-6h, 6-8h, etc.

[0058] In an embodiment, the carbonization heat treatment can further include a pre-carbonization treatment, and the temperature of the pre-carbonization treatment can be 600-900℃, specifically 600-700℃, 700-800℃, 800-900℃, 650-750℃, 750-850℃, etc. The temperature of the carbonization treatment can be 1-5h, specifically 1-2h, 2-4h, 4-5h, etc.

[0059] In a specific embodiment, the pre-carbonization treatment and the carbonization heat treatment can be realized by continuous heating, i.e., first heating to the temperature of the pre-carbonization treatment, then holding for a corresponding time, then continuously heating to the temperature of the pre-carbonization treatment, and then holding for a corresponding time.

[0060] In another specific embodiment, the material is cooled to room temperature after the pre-carbonization treatment, and then the carbonization heat treatment is performed. Further, before the carbonization heat treatment, a step of crushing or pulverization can be performed to scatter the material into particles or powder, or to crush or pulverize the material into smaller particles or powder.

[0061] In an embodiment, after step S3, a coating process is further included, and the coating material can be one or more of coating asphalt, tar, etc.

[0062] The application provides a hard carbon negative electrode material prepared by the preparation method provided by the application. In an embodiment, the median particle size of the hard carbon negative electrode material is 7-20 μm, and specifically can be 7-10 μm, 10-25 μm, 15-20 μm, etc.

[0063] The application further provides a use of the nitration asphalt hard carbon negative electrode material as a negative electrode material of a sodium ion battery.

[0064] The sodium ion battery assembled by using the nitration asphalt hard carbon as a negative electrode material has excellent electrochemical performance, effectively solves the problems of low reversible capacity and low voltage platform capacity caused by the formation of soft carbon after carbonization of the current asphalt-based hard carbon. The hard carbon negative electrode material still has good electrochemical performance at a high rate.

[0065] The technical solutions and technical effects of the application are described in detail below through specific embodiments, comparative examples and experimental examples. The following embodiments are only part of the embodiments of the application, and do not specifically limit the application. Unless otherwise specified, the raw materials, reagents, equipment, etc. used in the following embodiments can be purchased through a conventional market channel or prepared by a conventional procedure under a conventional condition.

[0066] Embodiment 1

[0067] The embodiment provides a method for preparing a hard carbon negative electrode material by using nitration asphalt, which comprises the following steps:

[0068] S1, concentrated nitric acid with a mass fraction of 65% is slowly added to a three-necked flask containing 30 g of asphalt powder along a glass rod, high-purity argon is introduced into the flask, the gas flow is controlled in the range of 12-20 ml / min, and the state is maintained for 10 minutes to ensure that the air in the flask is completely discharged, a stirring device is started, and the stirring device is placed in a water bath containing ice water, so that the mixture is completely immersed in the water level, the reaction temperature is controlled in the range of 0-5 ℃, and stirring is performed for 12 h; wherein the asphalt powder is petroleum fraction asphalt with a softening point of 200 ℃ and an ash content of <0.1%.

[0069] During the entire reaction process, it is observed that the powdered asphalt in the flask is dissolved and yellow smoke is overflowed, which proves that the nitration reaction occurs between the nitric acid and the asphalt and nitrogen oxides are generated.

[0070] S2, the molten mixture in step S1 is cooled, and solid-liquid separation is performed by using a suction filtration device, the material is continuously washed with deionized water during the process, the pH value is adjusted to neutral, and then the nitration asphalt is placed in a vacuum drying oven for drying, and the drying time is 12 h at 100 ℃.

[0071] S3, the dried nitration pitch in step S2 is taken out, crushed, and placed in a muffle furnace in an air atmosphere for heat treatment, the heating rate of this step is set to 3-5°C / min, the heating temperature is set to 250°C, and the holding time is set to 4h, to obtain the pre-oxidized nitration pitch.

[0072] S4, the pre-oxidized nitration pitch in step S3 is cooled and finely crushed to obtain the pre-oxidized modified pre-carbonization powder.

[0073] S5, the pre-oxidized powder in step S4 is placed in a tube furnace in a high-purity argon atmosphere for high-temperature carbonization, the heating rate of this step is set to 3°C / min, the gas flow rate in the tube furnace is set to 20ml / min, the heating temperature is set to 1300°C, and the holding time is set to 2h, to obtain the pitch-based hard carbon negative electrode material after high-temperature carbonization treatment; the high-temperature carbonization treatment rearranges the carbon layer of the nitration pitch, dehydrogenates and condenses the saturated carbon-hydrogen components, and causes part of the nitrogen and oxygen functional groups to escape in the form of gaseous small molecules, and forms a large number of pore defects, which is beneficial to the deintercalation of sodium ions.

[0074] S6, the pitch-based hard carbon negative electrode material prepared in this embodiment is prepared into a negative electrode sheet, and 80mg of pitch-based hard carbon material, 10mg of conductive carbon black (superp), and 500ul of a 20mg / ml sodium alginate solution are weighed according to a mass ratio of 8:1:1, uniformly mixed and stirred for 10min to form a black slurry, which is uniformly and evenly coated on a carbon-coated aluminum foil using a 200μm doctor blade, then it is placed in a 80°C air oven for drying for 1h to remove the water in the sodium alginate binder, and then it is placed in a 105°C vacuum oven for drying for 6h, and the dried electrode sheet is cut into a circular negative electrode sheet using a slicer, and then it is randomly transferred into a glove box for standby.

[0075] S7, the pitch-based hard carbon negative electrode material prepared in this embodiment is assembled into a sodium ion battery, the battery is assembled in an Ar atmosphere-filled glove box, the pitch-based hard carbon electrode sheet in step S6 is used as the electrode, a sodium sheet is used as the counter electrode, and a commercial 1.0M NaPF6 in DIGLYME=100Vol% is used as the electrolyte, and a button cell is assembled.

[0076] Example 2:

[0077] Compared with Example 1, the difference lies in that the pitch powder with a softening point of 250°C is used in step S1.

[0078] Example 3:

[0079] Compared with Example 1, the difference lies in that the softening point of the asphalt powder used in step S1 is 250℃, and the mass fraction of the nitric acid used is adjusted to 30%.

[0080] Comparative Example 1:

[0081] Compared with Example 1, the difference lies in that the 65wt% of nitric acid in step S1 is replaced by hydrogen peroxide.

[0082] Comparative Example 2:

[0083] Compared with Example 1, the difference lies in that steps S1 and S2 are omitted, and the pure asphalt powder with a softening point of 200℃ without nitration is directly placed in a muffle furnace for pre-oxidation, and then placed in a tube furnace for high-temperature carbonization.

[0084] Comparative Example 3:

[0085] Compared with Example 1, the difference lies in that steps S1 and S2 are omitted, and the pure asphalt powder with a softening point of 250℃ without nitration is directly placed in a muffle furnace for pre-oxidation, and then placed in a tube furnace for high-temperature carbonization.

[0086] Comparative Example 4:

[0087] Compared with Example 1, the difference lies in that steps S3 and S4 are omitted, and the nitration powder with a softening point of 200℃ after nitration is directly placed in a tube furnace for high-temperature carbonization.

[0088] Comparative Example 5:

[0089] Compared with Example 1, the difference lies in that steps S3 and S4 are omitted, and the nitration powder with a softening point of 250℃ after nitration is directly placed in a tube furnace for high-temperature carbonization.

[0090] The charge-discharge performance of the button cells of the above examples and comparative examples was tested. The battery test method used constant current charge-discharge, and the charge-discharge test was carried out at a current density of 0.1C, the discharge cut-off voltage was 0V, the charge cut-off voltage was 2.5V, and the nominal specific capacity was 200mAh / g.

[0091] The rate performance of the button cells of the above examples and comparative examples was tested. The battery test used a new battery test system, and the test method used constant current charge-discharge. The charge-discharge test was carried out at current densities of 0.1C, 0.5C, 1C, 2.5C, 5C, 10C, 20C, and 0.1C, respectively, in turn, the discharge cut-off voltage was 0V, and the charge cut-off voltage was 2.5V.

[0092] The test results are shown in Table 1 and Figures 2-7 . Among them, Figure 2 are the charge-discharge curves of Example 1 and Comparative Example 1, Figure 3Charge-discharge curves of Example 1, Comparative Example 2, Comparative Example 4, Figure 4 Charge-discharge curves of Example 2, Comparative Example 3, Comparative Example 5, Figure 5 Rate graph of Example 1 and Comparative Example 1, Figure 6 Rate graph of Example 1 and Comparative Example 2, Figure 7 Rate graph of Example 1 and Comparative Example 4.

[0093] Table 1

[0094]

[0095] From the charge-discharge curves and rate graph of Example 1, it can be seen that the first circle discharge specific capacity of the sodium ion battery assembled with the hard carbon negative electrode material prepared in this example is 365.19 mAh / g, the first circle charge specific capacity is 294.07 mAh / g, the charge specific capacity below 0.1 V is 176 mAh / g, and the charge specific capacities at current densities of 10 C (2 A / g) and 20 C (4 A / g) are 231.46 mAh / g and 160.12 mAh / g, respectively, showing excellent capacity and rate performance.

[0096] From Table 1 and Figures 2-7 It can be seen that:

[0097] Example 1 and Example 2 have the best reversible capacity and rate performance, because they are both treated with concentrated nitric acid before pre-oxidation, and the degree of nitration reaction is the highest, and they have excellent ion conductivity at a large current density. Comparative Example 3 and Example 2 only reduce the concentration of nitric acid, which shows that the nitration reaction between asphalt powder and nitric acid requires a relatively high concentration of nitric acid. The ability of low-concentration nitric acid to decompose into NO2+ and asphalt to undergo electrophilic aromatic substitution decreases. At this time, dilute nitric acid only plays a role in promoting the oxidation of asphalt. It is noted that Example 3 and Comparative Example 3 have similar electrochemical properties, which also shows that low-concentration nitric acid does not have a good nitration modification effect on the raw material.

[0098] Comparative Example 1 and Example 1 are compared, and the nitric acid is replaced by hydrogen peroxide, which has poor electrochemical performance and is at the same level as the asphalt without nitration in Comparative Example 2. This shows that the role of nitric acid in modifying the asphalt before pre-oxidation is not only as a strong oxidizing agent to promote the oxidation of asphalt, but also as an electrophilic reagent to introduce a nitro functional group through electrophilic substitution with asphalt.

[0099] Comparative examples 2 and 3 are un-nitrated bitumen, after pre-oxidation and carbonization process, high softening point bitumen (i.e. comparative example 3) has better reversible capacity and rate performance, which is due to the high softening point bitumen itself contains less small molecular components such as saturated alkanes, more aromatic components, can better react with oxygen at the same temperature, there are more oxygen-containing functional groups in the macromolecule, and the carbonization process forms a higher degree of disorder, more defects of hard carbon.

[0100] Comparative examples 4 and 5 are different softening point bitumen after nitrification, without pre-oxidation, directly carbonized, compared with examples 1 and 2, the synergistic process of nitrification followed by pre-oxidation has more excellent electrochemical performance than single nitrification treatment.

[0101] The hard carbon negative electrode material and the preparation method thereof and the sodium ion battery provided by the embodiments of the present application are described in detail above, and the principles and implementation modes of the present application are described by applying specific examples; the above examples are only used to help understand the method and the core idea of the present application; at the same time, for those skilled in the art, according to the idea of the present application, the specific implementation mode and the application range will be changed; in view of the above, the content of the specification should not be understood as a limitation of the present application.

Claims

1. A method for preparing a hard carbon negative electrode material, characterized by, The preparation method comprises the following steps: nitration treatment is performed on the asphalt by using nitric acid to obtain nitration asphalt; oxidation heat treatment is performed on the nitration asphalt in an oxygen-containing atmosphere to obtain oxidation asphalt; carbonization heat treatment is performed on the oxidation asphalt in an inert atmosphere to obtain the hard carbon negative electrode material; the nitration treatment performed on the asphalt by using nitric acid comprises the following steps: after the nitric acid aqueous solution and the asphalt are mixed, mixing reaction is performed in an inert atmosphere, and the temperature of the mixing reaction is 0-20 ℃.

2. The preparation method according to claim 1, wherein the mass percentage of nitric acid in the nitric acid aqueous solution is 30%-65%; and / or the ratio of the mass of the asphalt to the volume of the nitric acid aqueous solution is 1:1-1:15; and / or the nitric acid aqueous solution comprises sulfuric acid; and / or the inert atmosphere comprises an atmosphere formed by one or more of nitrogen, helium, neon, argon and xenon; and / or the time of the mixing reaction is 12-24 h.

3. The preparation method according to claim 1, characterized in that, after the mixing reaction, solid-liquid separation and drying are further performed to obtain the powder of the nitration asphalt, wherein the temperature of the drying is 80-120 ℃.

4. The method of claim 1, wherein, the asphalt comprises one or more of petroleum asphalt and coal pitch; and / or the softening point of the asphalt is 100 ℃-350 ℃; and / or the asphalt is asphalt particles or asphalt powder; and / or the average particle size of the asphalt is 10-100 μm.

5. The preparation method according to claim 1, wherein the oxygen-containing atmosphere is pure oxygen atmosphere, air atmosphere or mixed atmosphere formed by oxygen and inert gas; and / or the holding temperature of the oxidation heat treatment is 150-400 ℃; and / or the holding time of the oxidation heat treatment is 2-8 h; and / or the heating rate of the oxidation heat treatment is 3-5 ℃ / min.

6. The preparation method according to claim 1, wherein the inert atmosphere in the carbonization heat treatment comprises an atmosphere formed by one or more of nitrogen, helium, neon, argon and xenon; and / or the temperature of the carbonization heat treatment is 1100-1700 ℃; and / or the time of the carbonization heat treatment is 2-8 h; and / or the carbonization heat treatment further comprises pre-carbonization treatment before the carbonization heat treatment, and the temperature of the pre-carbonization treatment is 600-900 ℃.

7. A hard carbon negative electrode material, characterized in that, the hard carbon negative electrode material is prepared by the preparation method according to any one of claims 1-6. 8.The hard carbon negative electrode material of claim 7, characterized in that, the median particle size of the hard carbon negative electrode material is 7-20 μm.

9. A sodium-ion battery, characterized in that, the hard carbon negative electrode material according to claim 7 or 8.

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