Intermediate phase hard carbon composite material, preparation method thereof and sodium ion battery

Abstract

The embodiment of the invention provides an intermediate phase hard carbon composite material, a preparation method thereof and a sodium ion battery. The preparation method of the intermediate phase hard carbon composite material comprises the following steps: S1, mixing asphalt, a pore-forming agent and phosphorus salt, and then carrying out polymerization reaction to obtain a first product; s2, extracting the first product, and then performing activation reaction to obtain a second product; s3, introducing oxygen mixed gas into the second product, and carrying out a cross-linking reaction to obtain phosphorus-doped mesophase porous hard carbon; and S4, dispersing sodium salt and phosphorus-doped mesophase porous hard carbon in a sodium carboxymethyl cellulose solution, and performing spray drying to obtain the mesophase hard carbon composite material. According to the embodiment of the invention, the specific capacity and the cycle performance can be improved, and expansion is reduced.

Description

Technical Field

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

[0002] The working principle of a sodium ion battery is similar to that of a lithium ion battery, and the storage and release of electrical energy are realized based on the insertion and extraction process of sodium ions between the positive and negative electrodes. The positive electrode materials of sodium ion batteries include layered transition metal oxides, polyanion compounds, Prussian blue compounds, etc.

[0003] The negative electrode materials used in sodium ion batteries are mainly hard carbon. Hard carbon materials have good cycle stability, but have defects such as low specific capacity and low tap density. At the same time, the hard carbon materials have poor electronic conductivity and many internal defects, thus reducing their fast charging performance and first efficiency, making it impossible to meet the requirements of high energy density fast charging sodium ion batteries. There are many measures to improve the energy density and fast charging of hard carbon, mainly including doping non-metallic materials such as nitrogen, metals such as silver, and compound materials, or coating on the surface to reduce defects. However, these generally improve the expansion, cycle performance, etc. of the materials. Summary of the Invention

[0004] To overcome at least some of the defects and deficiencies in the prior art, embodiments of the present invention provide a mesophase hard carbon composite material, a preparation method thereof, and a sodium ion battery, which can reduce the specific capacity, improve the cycle performance, and increase the specific capacity.

[0005] The first aspect of the embodiments of the present invention provides a preparation method of a mesophase hard carbon composite material, including the following steps:

[0006] S1: Mix asphalt, a pore-forming agent, and a phosphorus salt, and then carry out a polymerization reaction to obtain a first product;

[0007] S2: Extract the first product, and then carry out an activation reaction to obtain a second product;

[0008] S3: Pass an oxygen mixture into the second product to carry out a cross-linking reaction to obtain phosphorus-doped mesophase porous hard carbon;

[0009] S4: Disperse a sodium salt and the phosphorus-doped mesophase porous hard carbon in a sodium carboxymethyl cellulose solution, and then carry out spray drying to obtain the mesophase hard carbon composite material.

[0010] In some embodiments, in the step S1, the temperature of the polymerization reaction is 350°C to 450°C, and the time of the polymerization reaction is 2h to 6h; and / or, the polymerization reaction is carried out at a rotation speed of 5rpm to 20rpm.

[0011] In some embodiments, in the step S2, the activation reaction specifically includes introducing steam for activation. The temperature of the activation reaction is 1000°C to 1200°C, and the time of the activation reaction is 1h to 6h.

[0012] In some embodiments, in the step S3, the temperature of the cross-linking reaction is 300°C to 500°C, and the time of the cross-linking reaction is 0.5h to 2h.

[0013] In some embodiments, in the step S1, the pore-forming agent is one of zinc chloride, zinc bromide, magnesium chloride, and magnesium bromide; and / or, in the step S1, the phosphate salt is one of ammonium hydrogen phosphate, ammonium phosphate, diammonium hydrogen phosphate, and diammonium dihydrogen phosphate.

[0014] In some embodiments, in the step S2, the extractant used for the extraction is at least one of n-heptane, pyridine, tetrahydrofuran, toluene, or N,N-dimethylformamide.

[0015] In some embodiments, in the step S4, the sodium salt is one of sodium acetate, sodium malate, and sodium citrate.

[0016] A second aspect of the embodiments of the present invention further provides a mesophase hard carbon composite material, which is prepared by the preparation method of the mesophase hard carbon composite material as described in any one of the above.

[0017] In some embodiments, the mesophase hard carbon composite material has a core-shell structure, the inner core is phosphorus-doped mesophase porous hard carbon, and the outer shell is sodium salt-doped amorphous carbon; calculated according to the mass ratio of the mesophase hard carbon composite material of 100%, the mass ratio of the outer shell is 1wt% to 5wt%.

[0018] A third aspect of the embodiments of the present invention further provides a sodium-ion battery, and the negative electrode of the sodium-ion battery includes the mesophase hard carbon composite material as described in any one of the above.

[0019] As can be seen from the above, the above technical solutions have at least one or more of the following beneficial effects:

[0020] In the embodiment of the present invention, a pore former and a phosphorus salt are added to asphalt and a polymerization reaction is carried out for phosphorus doping to improve the specific capacity; and activation and pore formation are carried out in step S2 to form a porous structure, thereby further improving the specific capacity of the subsequent obtained mesophase hard carbon composite; then oxidative crosslinking is carried out in step S3 to optimize the pore structure of the material and obtain a porous phosphorus-doped mesophase porous hard carbon; moreover, relying on the isotropic carbon structure formed after activation of asphalt, the expansion is reduced and the cycle performance is improved; in step S4, sodium salt is added to the sodium carboxymethyl cellulose solution to coat the surface of the phosphorus-doped mesophase porous hard carbon, thereby forming sodium amorphous carbon-coated phosphorus-doped mesophase porous hard carbon, which can reduce defects and improve the first efficiency; and, the phosphorus-doped mesophase porous hard carbon is also coated with a sodium ion compound, which can improve the intercalation and deintercalation rate of sodium ions during charge and discharge, thereby improving the rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The following will specifically describe the embodiments of the present invention in detail with reference to the drawings.

[0022] Figure 1 It is a schematic flowchart of a preparation method of a mesophase hard carbon composite in an embodiment of the present invention.

[0023] Figure 2 It is an SEM diagram of the mesophase hard carbon composite in Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0024] In order to make the above objects, features and advantages of the present invention more obvious and understandable, the following will specifically describe the embodiments of the present invention with reference to the drawings.

[0025] In order to enable those of ordinary skill in the art to better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0026] It should be noted that the terms "first", "second", etc. in the description, claims and above-mentioned drawings of the present invention are used to distinguish similar objects, and do not necessarily describe a specific order or sequence. It should be understood that such terms can be interchanged under appropriate circumstances, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "comprising" and "having" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device comprising a series of steps or units does not necessarily limit to those clearly listed steps or units, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0027] It should also be noted that the division of multiple embodiments in the present invention is only for the convenience of description and should not constitute a special limitation. The features in various embodiments can be combined and cross-referenced without contradiction.

[0028] See Figure 1 , an embodiment of the present invention provides a method for preparing a mesophase hard carbon composite material, comprising the following steps:

[0029] S1: Mix asphalt, pore former and phosphate salt, and then carry out a polymerization reaction to obtain a first product;

[0030] S2: Extract the first product, and then carry out an activation reaction to obtain a second product;

[0031] S3: Pass an oxygen mixture gas into the second product to carry out a cross-linking reaction to obtain a phosphorus-doped mesophase porous hard carbon;

[0032] S4: Disperse sodium salt and phosphorus-doped mesophase porous hard carbon in a sodium carboxymethylcellulose solution, and then carry out spray drying to obtain a mesophase hard carbon composite material.

[0033] In the embodiment of the present invention, in step S1, a pore former and phosphate salt are added to asphalt and a polymerization reaction is carried out for phosphorus doping, thereby improving the specific capacity; and in step S2, activation pore formation is carried out to form a porous structure, thereby improving the specific capacity of the subsequent obtained mesophase hard carbon composite material; then in step S3, oxidative cross-linking is carried out to optimize the pore structure of the material to obtain a porous structure of phosphorus-doped mesophase porous hard carbon; and relying on the isotropic carbon structure formed after activation of asphalt, the expansion is reduced and the cycle performance is improved; in step S4, sodium salt is added to the sodium carboxymethylcellulose solution to coat the surface of the phosphorus-doped mesophase porous hard carbon, and then sodium amorphous carbon-coated phosphorus-doped mesophase porous hard carbon is formed, thereby being able to reduce defects and improve the first efficiency; and, the phosphorus-doped mesophase porous hard carbon is also coated with a sodium ion compound, which can improve the intercalation and deintercalation rate of sodium ions during the charge and discharge process, thereby improving the rate performance.

[0034] Specifically, in step S1, the mass ratio of asphalt, pore former, and phosphate salt is 100:1-5:1-5. If the proportion of the phosphate salt is too high, the tapped density of the mesophase hard carbon composite will be reduced; while if the proportion of the phosphate salt is too low, the improvement of the specific capacity of the mesophase hard carbon composite will be limited.

[0035] In some specific embodiments, in step S1, the temperature of the polymerization reaction is 350°C - 450°C, and the time of the polymerization reaction is 2h - 6h. If the temperature of the polymerization reaction is too high, that is, greater than 450°C, the nano-pores will be too small, resulting in poor kinetics of the mesophase hard carbon composite; while if the temperature of the polymerization reaction is too low, that is, less than 350°C, the pores of the material will be too large, making it difficult to store sodium, and thus reducing the specific capacity and compaction density of the mesophase hard carbon composite.

[0036] Specifically, in step S1, the polymerization reaction is carried out at a rotation speed of 5 rpm - 20 rpm.

[0037] In some specific embodiments, in step S1, the pore former is one of zinc chloride, zinc bromide, magnesium chloride, and magnesium bromide;

[0038] In some specific embodiments, in step S1, the phosphate salt is one of ammonium hydrogen phosphate, ammonium phosphate, diammonium hydrogen phosphate, and diammonium dihydrogen phosphate. The phosphate salt used has low activity, can reduce gas production, and has good compatibility with the electrolyte, so the storage performance of the mesophase hard carbon composite can be improved.

[0039] In some embodiments, in step S2, the activation reaction specifically includes introducing steam for activation. The temperature of the activation reaction is 1000°C - 1200°C, and the time of the activation reaction is 1h - 6h. When the temperature of the activation reaction is too low, that is, less than 1000°C, the activation degree of the material will be low, and thus the specific capacity of the mesophase hard carbon composite will be low; while if the temperature of the activation reaction is too high, that is, greater than 1200°C, there will be more closed pores in the material, resulting in poor kinetics of the mesophase hard carbon composite. If the time of the activation reaction is too short, that is, less than 1h, the structural stability of the pores of the material will be poor; if the time of the activation reaction is too long, that is, greater than 6h, it will cause energy waste.

[0040] In some embodiments, in step S2, the extractant used for extraction is at least one of n-heptane, pyridine, tetrahydrofuran, toluene, or N, N-dimethylformamide.

[0041] In some specific embodiments, in step S3, the temperature of the cross-linking reaction is 300°C - 500°C, and the time of the cross-linking reaction is 0.5h - 2h, to obtain phosphorus-doped mesophase porous hard carbon.

[0042] Specifically, the oxygen mixture is a mixture of oxygen:nitrogen with a volume ratio of 3:10.

[0043] In some specific embodiments, in step S4, the sodium salt is one of sodium acetate, sodium malate, and sodium citrate. Compared with inorganic sodium salts, the organic sodium salts used can have lower impurity content, and the organic sodium salts have better compatibility with the electrolyte; at the same time, the organic sodium salts used contain at least one of hydroxyl and carboxyl groups. Therefore, after spray drying, the surface of the material will contain hydrophilic groups, which can improve the processing performance of the mesophase hard carbon composite material.

[0044] In some specific embodiments, step S4 specifically includes:

[0045] S41: Disperse the sodium salt evenly in the sodium carboxymethylcellulose solution to obtain a first solution;

[0046] S42: Disperse the phosphorus-doped mesophase porous hard carbon evenly in the first solution to obtain a second solution;

[0047] S43: Spray-dry the second solution to obtain a mesophase hard carbon composite material.

[0048] Specifically, the solvent of the sodium carboxymethylcellulose solution is deionized water, and the mass concentration of the sodium carboxymethylcellulose solution is 0.5 wt% - 2 wt%.

[0049] Specifically, the mass ratio of the sodium salt, sodium carboxymethylcellulose, and phosphorus-doped mesophase porous hard carbon is 1 - 5:1 - 5:100.

[0050] The embodiment of the present invention also provides a mesophase hard carbon composite material, which is prepared by the preparation method of the mesophase hard carbon composite material described in any one of the above.

[0051] In some embodiments, the mesophase hard carbon composite material has a core-shell structure, the inner core is phosphorus-doped mesophase porous hard carbon, and the outer shell is sodium salt-doped amorphous carbon; calculated according to the mass ratio of 100% of the mesophase hard carbon composite material, the mass ratio of the outer shell is 1 wt% - 5 wt%.

[0052] The embodiment of the present invention also provides a sodium ion battery, and the negative electrode of the sodium ion battery includes the mesophase hard carbon composite material described in any one of the above.

[0053] Example 1

[0054] This embodiment provides a preparation method of a mesophase hard carbon composite material, including the following steps:

[0055] Step S1: Add 100 g of asphalt, 3 g of zinc chloride, and 3 g of ammonium hydrogen phosphate into a reaction kettle, heat up to 400 °C, and carry out a polymerization reaction at a rotation speed of 10 rpm for 3 h to obtain a first product;

[0056] Step S2: Add the first product into 800 g of n - heptane extractant for extraction and filtration to obtain a filter residue. Heat the filter residue up to 1100 °C, and then pass water vapor into the filter residue for activation for 3 h to obtain a second product;

[0057] Step S3: Cool the second product down to 400 °C, and then pass an oxygen - containing mixed gas into the second product for a cross - linking reaction for 1 h to obtain phosphorus - doped mesophase porous hard carbon. The oxygen - containing mixed gas is a mixed gas with an oxygen: nitrogen volume ratio of 3:10.

[0058] Step S4: Add 3 g of sodium carboxymethylcellulose into 300 g of deionized water and disperse evenly to prepare a solution with a mass concentration of 1 wt%. Then add 3 g of sodium acetate into the solution and disperse evenly. Then add 100 g of phosphorus - doped mesophase porous hard carbon into the solution and disperse evenly. Then carry out spray drying to obtain a mesophase hard carbon composite material. The inlet temperature of spray drying is 220 °C, the outlet temperature is 80 °C, the flow rate is 0.2 kg / h, and the spray drying time is 2 h.

[0059] Example 2

[0060] This example provides a preparation method of a mesophase hard carbon composite material, including the following steps:

[0061] Step S1: Add 100 g of asphalt, 1 g of zinc bromide, and 1 g of ammonium phosphate into a reaction kettle, heat up to 350 °C, and carry out a polymerization reaction at a rotation speed of 5 rpm for 2 h to obtain a first product;

[0062] Step S2: Add the first product into 800 g of pyridine extractant for extraction and filtration to obtain a filter residue. Heat the filter residue up to 1000 °C, and then pass water vapor into the filter residue for activation for 6 h to obtain a second product;

[0063] Step S3: Cool the second product down to 300 °C, and then pass an oxygen - containing mixed gas into the second product for a cross - linking reaction for 2 h to obtain phosphorus - doped mesophase porous hard carbon. The oxygen - containing mixed gas is a mixed gas with an oxygen: nitrogen volume ratio of 3:10.

[0064] Step S4: Add 1 g of sodium carboxymethylcellulose to 200 g of deionized water and disperse evenly to prepare a solution with a mass concentration of 0.5 wt%. Then add 3 g of sodium malate to the solution and disperse evenly. Next, add 100 g of phosphorus-doped mesophase porous hard carbon to the solution and disperse evenly. Then perform spray drying to obtain a mesophase hard carbon composite. The inlet temperature of the spray drying is 220 °C, the outlet temperature is 80 °C, the flow rate is 0.2 kg / h, and the spray drying time is 2 h.

[0065] Example 3

[0066] This example provides a method for preparing a mesophase hard carbon composite, which includes the following steps:

[0067] Step S1: Add 100 g of pitch, 5 g of magnesium chloride, and 5 g of ammonium dihydrogen phosphate to a reaction kettle, heat up to 450 °C, and carry out a polymerization reaction at a rotation speed of 20 rpm for 6 h to obtain a first product;

[0068] Step S2: Add the first product to 800 g of tetrahydrofuran extractant for extraction and filtration to obtain a filter residue. Heat the filter residue to 1200 °C, and then pass water vapor through the filter residue for activation for 1 h to obtain a second product;

[0069] Step S3: Cool the second product to 500 °C, and then pass an oxygen mixture gas through the second product for a cross-linking reaction for 0.5 h to obtain phosphorus-doped mesophase porous hard carbon. The oxygen mixture gas is a mixture of oxygen and nitrogen with a volume ratio of oxygen: nitrogen = 3:10.

[0070] Step S4: Add 5 g of sodium carboxymethylcellulose to 250 g of deionized water and disperse evenly to prepare a solution with a mass concentration of 2 wt%. Then add 5 g of sodium acetate to the solution and disperse evenly. Next, add 100 g of phosphorus-doped mesophase porous hard carbon to the solution and disperse evenly. Then perform spray drying to obtain a mesophase hard carbon composite. The inlet temperature of the spray drying is 220 °C, the outlet temperature is 80 °C, the flow rate is 0.2 kg / h, and the spray drying time is 2 h.

[0071] Comparative Example 1:

[0072] The difference between this comparative example and Example 1 is that zinc chloride and ammonium hydrogen phosphate are not added in Step S1, and the others are the same as in Example 1.

[0073] Comparative Example 2:

[0074] The difference between this comparative example and Example 1 is that water vapor activation is not carried out in Step S2, and oxygen mixture gas is not passed through for cross-linking reaction in Step S3, and the others are the same as in Example 1.

[0075] Comparative Example 3:

[0076] This comparative example is different from Example 1 in that step S4 is not implemented, that is, the phosphorus-doped mesophase porous hard carbon in step S3 is used as the negative electrode, and the others are the same as in Example 1.

[0077] Performance test

[0078] The mesophase hard carbon composite materials provided in Examples 1 to 3 and Comparative Examples 1 to 3 were subjected to relevant tests.

[0079] (1) Morphology characterization:

[0080] The mesophase hard carbon composite material prepared in Example 1 was subjected to SEM testing, and the results are as Figure 2 shown. It can be seen from Figure 2 that the mesophase hard carbon composite material presents a granular structure with uniform size distribution, a small amount of microporous structure on the surface, and the particle size is between 10 μm and 15 μm.

[0081] (2) Physical and chemical properties and coin cell testing:

[0082] The specific surface area, specific capacity, and first efficiency of the mesophase hard carbon composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3 were tested. The test method refers to the standard: GBT-43114-2023 "Hard Carbon". At the same time, the diffusion coefficient of the material was tested by GITT.

[0083] The mesophase hard carbon composite materials obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were respectively assembled into coin cells. The assembly method is as follows: Add a binder, a conductive agent, and a solvent to the negative electrode material, stir to make a slurry, coat it on a copper foil, and obtain it through drying and rolling. The binder used is LA133 binder, the conductive agent is Super P, the negative electrode materials are the mesophase hard carbon composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3 respectively, the solvent is secondary distilled water, and the specific content is: the mesophase hard carbon composite material, Super P, LA133, and secondary distilled water are 90 g, 3 g, 7 g, and 220 mL respectively, and a negative electrode sheet is prepared; the electrolyte is NaPF6 / (EC+DEC) (volume ratio 1:1, concentration 1.1 mol / L), the metal sodium sheet is the counter electrode, the separator uses a polypropylene membrane, the coin cell is assembled in a glove box filled with argon, and the electrochemical performance of the coin cell is tested on a Wuhan Blue Electric CT2001A battery tester, with the charge-discharge voltage range of 0.00 V to 2.0 V and the charge-discharge rate of 0.1 C. At the same time, the rate performance (1C / 0.1C) and full charge expansion of the coin cell were tested.

[0084] The above test results are shown in Table 1

[0085]

[0086] As can be seen from Table 1, the initial discharge capacities of the mesophase hard carbon composite materials provided in Examples 1 to 3 are significantly better than those in Comparative Examples 1 to 3. The reason is that phosphorus doping of the material and activation to form a porous structure can both improve the specific capacity of the material; the initial efficiency of the mesophase hard carbon composite materials provided in Examples 1 to 3 is significantly better than that in Comparative Examples 1 to 3. The reason is that the outer shell formed by sodium salts coating the core reduces the irreversible capacity of the material; the diffusion coefficients and rate performances of the mesophase hard carbon composite materials provided in Examples 1 to 3 are significantly better than those in Comparative Examples 1 to 3. The reason is that pore formation in the core and phosphorus doping can improve the sodium storage function, diffusion coefficient and rate performance of the material, and the outer shell formed by sodium salts coating the core can also improve the intercalation and deintercalation efficiency of sodium ions during charge and discharge, and improve the diffusion coefficient of sodium ions; the specific surface area of the mesophase hard carbon composite materials provided in Examples 1 to 3 is significantly larger than that in Comparative Examples 1 to 3. The reason is that there are more pores in the mesophase hard carbon composite materials in Examples 1 to 3.

[0087] The initial discharge capacity of the mesophase hard carbon composite material provided in Example 1 is better than that in Comparative Example 1. The reason is that phosphorus doping of pitch can improve the specific capacity of the material.

[0088] The full charge expansion of the mesophase hard carbon composite material provided in Example 1 is lower than that in Comparative Example 2. The reason is that the isotropic carbon structure formed after activation of pitch can reduce expansion; the initial discharge capacity, diffusion coefficient and rate performance of the mesophase hard carbon composite material provided in Example 1 are higher than those in Comparative Example 2. The reason is that the porous structure formed by activation to form pores can improve the sodium storage function of the material, thereby improving the diffusion coefficient and rate performance, and the porous structure can also improve the specific capacity.

[0089] The initial discharge capacity, diffusion coefficient, rate performance and initial efficiency of the mesophase hard carbon composite material provided in Example 1 are higher than those in Comparative Example 3. The reason is that the outer shell formed by sodium salts coating the core can improve the intercalation and deintercalation efficiency of sodium ions during charge and discharge, improve the diffusion coefficient of sodium ions, improve the rate performance, and can also reduce the irreversible capacity and improve the specific capacity.

[0090] (3) Soft-pack battery test

[0091] The mesophase hard carbon composite materials in Examples 1 to 3 and Comparative Examples 1 to 3 were used as the negative electrode, and were slurried and coated to prepare a negative electrode sheet. A layered oxide (NaFe 1 / 3 Mn 1 / 3 Ni 1 / 3 O2) was used as the positive electrode, NaPF6 with an electrolyte concentration of 1.3 mol / L (the solvent is a mixed solvent of EC + DEC with a volume ratio of 1:1) was used as the electrolyte, and celegard2400 was used as the separator to prepare a 2 Ah soft-pack battery.

[0092] High-temperature storage performance test:

[0093] Test method: At 60 °C, the capacity of the soft-pack battery in a fully charged state is X1. After placing it at 60 °C for 30 days, the capacity of the soft-pack battery is tested again as X2, and the charge retention is calculated as (X2 / X1) × 100%; then the soft-pack battery is fully charged to the fully charged state (100% SOC), the capacity of its battery is tested as X3, and the recovery capacity is calculated as (X3 / X1) × 100%;

[0094] Cycling performance test:

[0095] The test temperature is 25 °C, the test voltage range is 1.5 V to 3.95 V, the current density is 1C / 1C, and the number of charge-discharge cycles is 500 times;

[0096] Rate performance test:

[0097] Constant current + constant voltage charging is carried out at a rate of 2C + 0.1C to 3.95 V, and then the constant current ratio of the battery is calculated by the constant current ratio = 2C constant current capacity / (2C constant current capacity + 0.1C constant voltage capacity).

[0098] The test results are shown in Table 2

[0099] Example Charge retention rate Capacity recovery rate Cycling performance Constant current ratio Example 1 95.1％ 97.2％ 92.6％ 85.4％ Example 2 94.6％ 96.7％ 91.1％ 84.9％ Example 3 95.7％ 97.8％ 93.3％ 86.8％ Comparative example 1 91.4％ 92.6％ 86.3％ 74.3％ Comparative example 2 90.1％ 91.2％ 85.7％ 75.5％ Comparative example 3 89.3％ 90.4％ 875％ 73.1％

[0100] As can be seen from Table 2, the mesophase hard carbon composite materials provided in Examples 1-3 have a high diffusion coefficient, improve the rate performance, and also have improved cycling performance; at the same time, the sodium salt coated on the shell reduces the contact between the electrolyte and the core material, thereby reducing the occurrence of side reactions, so the high-temperature storage performance of the mesophase hard carbon composite material is improved.

[0101] In summary, the mesophase hard carbon composite material provided in the embodiments of the present invention is obtained by mixing asphalt with a pore-forming agent and a phosphate salt and carrying out a polymerization reaction, doping phosphorus into the asphalt, forming a porous structure through activation pore formation and oxidation, obtaining phosphorus-doped mesophase porous hard carbon, which can reduce swelling, improve the specific capacity and cycling performance, and coat a sodium salt shell on the core of the phosphorus-doped mesophase porous hard carbon to reduce defects, improve the first efficiency, increase the insertion and extraction rate of sodium ions during the charge-discharge process, and improve the rate performance.

[0102] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Although the present invention has been disclosed above with the preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some changes or modifications to equivalent embodiments by using the above-disclosed technical content within the scope of the technical solution of the present invention. However, as long as it does not depart from the content of the technical solution of the present invention, any simple modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a mesophase hard carbon composite material, characterized in that: The steps include: S1: mixing asphalt, pore-forming agent and phosphate salt, and then performing polymerization reaction to obtain a first product; S2: extracting the first product, and then performing an activation reaction to obtain a second product; S3: introducing an oxygen mixed gas into the second product to perform a cross-linking reaction to obtain a phosphorus-doped mesophase porous hard carbon; S4: dispersing the sodium salt and the phosphorus-doped mesophase porous hard carbon in a sodium carboxymethyl cellulose solution, and then spray-drying to obtain the mesophase hard carbon composite material.

2. The preparation method according to claim 1, characterized in that: In the step S1, the polymerization reaction temperature is 350° C. to 450° C., the polymerization reaction time is 2 h to 6 h; and / or the polymerization reaction is performed at a rotation speed of 5 rpm to 20 rpm.

3. The preparation method according to claim 1, characterized in that: In the step S2, the activation reaction specifically includes introducing water vapor for activation, the temperature of the activation reaction is 1000° C. to 1200° C., and the time of the activation reaction is 1 hour to 6 hours.

4. The preparation method according to claim 1, characterized in that: In the step S3, the temperature of the cross-linking reaction is 300° C. to 500° C., and the time of the cross-linking reaction is 0.5 h to 2 h.

5. The preparation method according to claim 1, characterized in that: In the step S1, the pore-forming agent is one of zinc chloride, zinc bromide, magnesium chloride and magnesium bromide; and / or, in the step S1, the phosphate salt is one of ammonium hydrogen phosphate, ammonium phosphate, diammonium hydrogen phosphate and diammonium dihydrogen phosphate.

6. The preparation method according to claim 1, characterized in that: In step S2, the extraction agent used for the extraction is at least one of n-heptane, pyridine, tetrahydrofuran, toluene or N,N-dimethylformamide.

7. The preparation method according to claim 1, characterized in that: In step S4, the sodium salt is one of sodium acetate, sodium malate and sodium citrate.

8. A mesophase hard carbon composite material, characterized in that: The intermediate phase hard carbon composite material is prepared by the preparation method of any one of claims 1 to 7.

9. The mesophase hard carbon composite material according to claim 8, characterized in that: The mesophase hard carbon composite material is a core-shell structure, wherein the core is phosphorus-doped mesophase porous hard carbon and the shell is sodium salt-doped amorphous carbon; based on the mass ratio of the mesophase hard carbon composite material being 100%, the mass ratio of the shell is 1wt% to 5wt%.

10. A sodium ion battery, characterized in that: The negative electrode of the sodium ion battery comprises the intermediate phase hard carbon composite material as described in any one of claims 8 to 9.

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