Hard carbon material and preparation method therefor, negative electrode, sodium-ion secondary battery and electric device

Abstract

Disclosed in the present application are a hard carbon material and a preparation method therefor, a negative electrode, a sodium-ion secondary battery and an electric device. The sodium-ion secondary battery comprises: a positive electrode, a negative electrode and a separator located between the positive electrode and the negative electrode, wherein the negative electrode comprises a hard carbon material, and the specific surface area of the hard carbon material is less than or equal to 10 m2 / g.

Description

Hard carbon material, preparation method, negative electrode, sodium-ion secondary battery and electric device TECHNICAL FIELD

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

[0002] Sodium-ion batteries are considered as a competitive alternative to lithium-ion batteries due to the use of sodium element which is more cost-effective and more abundant in resources, and have gained more and more attention. Since the type of ion used for energy storage has changed, it is necessary to develop negative electrode materials suitable for storing sodium ions. The graphite negative electrode material commonly used in lithium-ion batteries is not suitable for sodium-ion battery systems due to its low sodium storage capacity. In contrast, hard carbon material is used as a negative electrode material for sodium-ion batteries due to its long and highly reversible platform region. However, the current hard carbon material still has unreasonable pore structure, resulting in low initial efficiency of sodium-ion batteries using hard carbon material as negative electrode, and this problem is more significant in high-capacity hard carbon negative electrode.

[0003] Therefore, the current hard carbon material, preparation method, negative electrode, sodium-ion secondary battery and electric device still need to be improved. SUMMARY

[0004] In view of the above problems, the present application provides a hard carbon material, a preparation method, a negative electrode, a secondary battery and an electric device, which can adjust the amount of micropores by processing the material, thereby improving the problem of low initial efficiency of the battery due to unreasonable pore structure.

[0005] In one aspect of the present application, a method for preparing a hard carbon material is provided. The method comprises: providing a carbon-based material, at least part of the pores in the porous structure have a pore size of micropores; and performing closed pore sintering treatment on the carbon-based material in the presence of a non-gaseous carbon source to decompose the non-gaseous carbon source and obtain the hard carbon material.

[0006] In the technical solution of the present application, closed pore sintering treatment is performed in the presence of a non-gaseous carbon source, so that the process similar to chemical vapor deposition (CVD) carbon growth can be realized in the sintering process, the closed pores in the porous structure are closed, the specific surface area is increased due to poor closed pore effect, and the problem of low initial efficiency caused by the increase of side reactions on the surface of the material is alleviated.

[0007] In some embodiments, the specific surface area BC2 of the hard carbon material is reduced by at least 10% relative to the specific surface area BC1 of the carbon-based material.

[0008] In some embodiments, the specific surface area of the hard carbon material is BC2, the specific surface area of the carbon-based material is BC1, B C1 > B C2 , and the value of (B C1 -B C2 ) / B C1 is ≥10%. When the decrease in the specific surface area of the obtained hard carbon material relative to the specific surface area of the raw carbon-based material is within the above range, the closed pores can be better achieved.

[0009] In some embodiments, the temperature of the closed pore sintering treatment is 600-1500°C. When the sintering temperature is within the above range, the non-gaseous carbon source can be better decomposed into small molecules during the sintering process and grow at the pores, achieving the closed pores.

[0010] In some embodiments, the non-gaseous carbon source includes a part of the carbon-based material and / or organic carbon. In this way, the decomposition of the non-gaseous carbon source and the carbon growth can be better achieved, thereby improving the closed pore effect of the closed pore sintering treatment.

[0011] In some embodiments, the material for forming the carbon-based material includes at least one of a biomass raw material and a resin, the non-gaseous carbon source is a part of the carbon-based material, the closed pore sintering treatment includes intermediate sintering and final sintering, the temperature of the intermediate sintering is 600-1000°C, and the temperature of the final sintering is greater than 1000°C. The intermediate sintering is beneficial to improving the decomposition effect of the non-gaseous carbon source.

[0012] In some embodiments, the temperature of the intermediate sintering is 650-900°C, and the temperature of the final sintering is 1200-1400°C. In this way, the closed pore effect of the closed pore sintering treatment can be further improved, and the battery initial efficiency of the obtained material when used in a secondary battery can be improved.

[0013] In some embodiments, the providing of the carbon-based material includes pre-treatment and pore-forming treatment of the material for forming the carbon-based material, the pre-treatment includes first sintering treatment, and the temperature of the first sintering treatment is lower than the temperature of the intermediate sintering. In this way, the non-gaseous carbon source that can be decomposed can be better decomposed during the intermediate sintering, thereby further improving the closed pore effect of the closed pore sintering treatment.

[0014] In some embodiments, the organic carbon is selected from at least one of a resin, a monosaccharide, a disaccharide, and a polysaccharide, and the method includes mixing the organic carbon and the carbon-based material and performing the closed pore sintering treatment, and the temperature of the closed pore sintering treatment is 1200-1400°C. In this way, the closed pore effect of the closed pore sintering treatment can be further improved, and the battery initial efficiency of the obtained material when used in a secondary battery can be improved.

[0015] In some embodiments, the non-gaseous carbon source has a mass of 0.01-20% of the mass of the carbon-based material. Thereby, the closed-pore effect after decomposition of the non-gaseous carbon source is further enhanced.

[0016] In some embodiments, the sintering process satisfies at least one of the following conditions: the sintering process has a temperature increasing rate of 5-20℃ / min; the sintering process has a time of 1-10h. Thereby, the closed-pore effect of the sintering process is further enhanced.

[0017] In another aspect of the present application, the present application provides a hard carbon material having a pore structure, the specific surface area of the hard carbon material being ≤10m2 / g. The hard carbon material has a reasonable pore structure, and has the advantage of improving the initial efficiency of the battery when used to form an electrode.

[0018] In some embodiments, the hard carbon material is obtained based on a closed-pore sintering process on a carbon-based material, the specific surface area of the hard carbon material being BC2, the specific surface area of the carbon-based material being BC1, B C1 >B C2 , and the value of (B C1 -B C2 ) / B C1 is ≥10%.

[0019] The specific surface area BC2 of the hard carbon material is reduced by at least 10% compared to the specific surface area BC1 of the carbon-based material.

[0020] In some embodiments, the hard carbon material is prepared by the method described above.

[0021] In yet another aspect of the present application, the present application provides a negative electrode for a sodium-ion battery. A current collector, and a negative electrode active layer on the current collector, the negative electrode active layer comprising a hard carbon material having a pore structure, the specific surface area of the hard carbon material being ≤10m2 / g.

[0022] In some embodiments, the hard carbon material is obtained based on a closed-pore sintering process on a carbon-based material, the specific surface area of the hard carbon material being BC2, the specific surface area of the carbon-based material being BC1, B C1 >B C2 , and the value of (B C1 -B C2 ) / B C1 is ≥10%.

[0023] The specific surface area BC2 of the hard carbon material is reduced by at least 10% compared to the specific surface area BC1 of the carbon-based material.

[0024] In some embodiments, the negative electrode comprises the hard carbon material described above.

[0025] In yet another aspect of the present application, the present application provides a sodium-ion secondary battery. The sodium-ion secondary battery comprises: a positive electrode, a negative electrode, and a separator between the positive electrode and the negative electrode, the negative electrode comprising a hard carbon material, the hard carbon material having a pore structure, the specific surface area of the hard carbon material being ≤10 m2 / g.

[0026] In some embodiments, the hard carbon material is treated by a closed pore sintering process,

[0027] In some embodiments, the sodium-ion secondary battery comprises the above-mentioned negative electrode.

[0028] In yet another aspect of the present application, the present application provides an electric device. The electric device comprises the above-mentioned sodium-ion secondary battery, the sodium-ion secondary battery being used to provide electric energy. BRIEF DESCRIPTION OF DRAWINGS

[0029] Various other advantages and benefits will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments. The accompanying drawings are intended to depict only preferred embodiments of the application, and therefore should not be considered to limit the scope of the application in any way. In the drawings:

[0030] FIG. 1 is a graph showing the results of the first efficiency test of Comparative Example 1 of the present application;

[0031] FIG. 2 is a graph showing the results of the first efficiency test of Example 1 of the present application;

[0032] FIG. 3 is a graph showing the results of the first efficiency test of Example 5 of the present application;

[0033] FIG. 4 is a graph showing the results of the first efficiency test of Comparative Example 1 of the present application;

[0034] FIG. 5 is a graph showing the results of the first efficiency test of Example 19 of the present application;

[0035] FIG. 6 is a graph showing the results of the first efficiency test of Example 22 of the present application;

[0036] FIG. 7 is a graph showing the results of the first efficiency test of Example 25 of the present application;

[0037] FIG. 8 is a graph showing the results of the rate performance test of Comparative Example 1 of the present application;

[0038] FIG. 9 is a graph showing the results of the rate performance test of Comparative Example 2 of the present application;

[0039] FIG. 10 is a graph showing the results of the rate performance test of Comparative Example 4 of the present application;

[0040] FIG. 11 is a graph showing the results of the rate performance test of Comparative Example 6 of the present application;

[0041] Figure 12 is a graph of the initial efficiency and rate capability test results of some embodiments of the application and Control Group 4;

[0042] Figure 13 is a graph of the initial efficiency and rate capability test results of some other embodiments of the application and Control Group 4;

[0043] Figure 14 is a graph of the initial efficiency and rate capability test results of yet other embodiments of the application and Control Group 4. DETAILED DESCRIPTION

[0044] The embodiments of the technical solutions of the present application will be described in detail below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present application, and therefore only serve as examples, and cannot limit the protection scope of the present application.

[0045] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the present application; the terms "include" and "have" and any variations thereof in the specification and claims of the present application are intended to cover non-exclusive inclusion.

[0046] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is two or more, unless otherwise explicitly and specifically limited.

[0047] Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the present application. The appearance of the phrase in various places in the specification does not necessarily all refer to the same embodiment, nor is it necessarily mutually exclusive or alternative to other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0048] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the three cases of A alone, A and B together, and B alone. In addition, the character " / " in this paper generally represents that the front and rear associated objects are a "or" relationship.

[0049] In the description of the embodiments of the present application, the term "multiple" refers to two or more (including two), and similarly, "multiple groups" refers to two or more groups (including two groups), and "multiple pieces" refers to two or more pieces (including two pieces).

[0050] Unless otherwise indicated, the terms used in the present application have the commonly understood meanings as understood by those skilled in the art. Unless otherwise indicated, the values of the parameters mentioned in the present application can be measured by various measuring methods commonly used in the art (for example, can be tested according to the methods given in the examples of the present application).

[0051] At present, from the development of market situation, the application of power battery is more and more widely. Power battery is not only applied to energy storage power supply system of hydropower, thermal power, wind power and solar power station, but also widely used in electric bicycles, electric motorcycles, electric vehicles and other electric vehicles, military equipment, aerospace and other fields. With the continuous expansion of the application field of power battery, the demand of its market is also increasing.

[0052] As mentioned above, sodium-ion batteries are considered to be a competitive alternative to lithium-ion batteries due to their similar working principles and cost advantages. However, sodium ions have a larger radius than lithium ions, so it is necessary to develop corresponding negative electrode materials. The commonly used negative electrode materials for sodium-ion batteries include hard carbon negative electrode. Hard carbon refers to carbon materials that are difficult to graphitize. Hard carbon materials used in sodium-ion batteries are mostly derived from the thermal decomposition of high molecular polymers, have stable structure, and have long cycle life.

[0053] The capacity of hard carbon negative electrode is related to the amount of micropores in the material. In the process of preparing high-capacity negative electrodes, a mode of first forming pores and then closing pores is often used to form hard carbon negative electrode materials. However, as the amount of pore formation increases, the self-closing effect during the sintering process of the material will become worse and worse, resulting in that the pore structure of the material cannot be realized according to the expected design. In this case, the electrolyte enters part of the pores, leading to an increase in side reactions, and thus causing the initial efficiency of the battery to be low. This problem is more pronounced in high-capacity hard carbon negative electrode materials. Although the above problems can be alleviated to some extent by, for example, increasing the maximum sintering temperature and time, increasing the sintering temperature will on the one hand affect the rate performance of the material, leading to an increase in sodium precipitation during the cycle process, and on the other hand will significantly increase the requirements of the processing conditions on the equipment, leading to an increase in the cost of the equipment. Another strategy to alleviate the poor sintering and pore closing effect is to close the surface pores through solid / liquid / gas phase coating treatment. However, the coating strategy will on the one hand increase the cost, and on the other hand some coating materials may react abnormally at the interface between the electrode material and the electrolyte during the cycle process due to the action of additives such as adhesives. In addition, the increased coating layer on the surface of the material will also affect the original particle size distribution, leading to an increase in the gap between particles, thereby affecting the powder compaction density.

[0054] For the convenience of understanding, the hard carbon material, the preparation method and the negative electrode proposed in the present application will be described in detail below.

[0055] The hard carbon material and the preparation method provided in the present application realize the closure of part of the microporous structure by using the decomposable carbon source of the material itself or an external carbon source to realize a carbon growth process similar to chemical vapor deposition (CVD) in the closed pore sintering process. Therefore, the hard carbon material obtained has a relatively reasonable pore structure, which can alleviate or even avoid the problem of the increase of side reactions caused by the entry of electrolyte into part of the pore channel, thereby affecting the initial efficiency of the battery. Moreover, since a non-gas carbon source is used, the safety performance is better than that of the alkane gas carbon source commonly used in the CVD process. Therefore, the process has lower requirements for the vacuum degree of the equipment, which is conducive to realizing carbon growth and closure under relatively mild conditions.

[0056] The preparation method and the hard carbon material disclosed in the embodiments of the present application can be used in, for example, a sodium ion battery, for example, can be loaded on a battery pole piece to form a negative electrode. The electrode and the battery provided in the present application can be used in an electric device such as a vehicle, a ship or an aircraft. The power supply system of the electric device can be composed of the battery monomer and the battery disclosed in the present application.

[0057] According to some embodiments of the present application, the method provided in the present application includes the operations of providing a carbon-based material and performing a closed pore sintering treatment on the carbon-based material. Specifically, the carbon-based material has a pore structure, for example, at least part of the pore diameter of the pore structure is microporous. The closed pore sintering treatment includes treating the carbon-based material in the presence of a non-gas carbon source to decompose the non-gas carbon source and obtain the hard carbon material. The method can adjust the porous structure of the carbon-based structure and improve the initial efficiency of the obtained hard carbon material when used in a secondary battery.

[0058] Initial efficiency

[0059] The initial efficiency (or simply referred to as the initial efficiency) refers to the ratio of the discharge capacity to the charge capacity of the battery in the first charge and discharge process, and the ratio is less than 100%. Specifically, in the process of forming a solid electrolyte interface (SEI) film, part of the energy storage ions such as lithium ions or sodium ions will be consumed. Therefore, the first discharge capacity will not be equal to the first charge capacity, and the battery capacity will be lost to a certain extent. Factors affecting the initial efficiency of the full battery include the specific surface area of the negative electrode. When the specific surface area is large, the area of the SEI film formed will be large, and more energy storage ions will be consumed.

[0060] In a hard carbon negative electrode, especially a high-capacity hard carbon negative electrode material, the capacity is usually improved by providing more porous structure, but more porous structure also leads to the difficulty in realizing the closure of the material in the preparation process according to the expected strategy, thereby increasing the specific surface area of the material and the contact area between the electrolyte and the material, and losing more energy storage ions in the first charge and discharge process.

[0061] The method provided in the present application utilizes a closed pore sintering process to decompose the non-gaseous carbon source in the reaction system, provides raw materials for carbon growth in the closed pore sintering process, thereby realizing a carbon growth process similar to that in a chemical vapor deposition process, and realizing the closure (closed pore) of part of the pores through carbon growth, which can adjust the porous structure in the carbon-based material.

[0062] In some embodiments, the non-gaseous carbon source in the system can be derived from the carbon-based material itself. For example, there can be part of carbon in the carbon-based material that is not completely decomposed in the previous treatment process, which can be decomposed into substances with smaller molecular weights in the closed pore sintering process, thereby serving as a carbon source similar to that in the chemical vapor deposition process in the closed pore sintering process.

[0063] In other embodiments, the non-gaseous carbon source in the system can also be added additionally, for example, organic carbon can be added, in particular, the organic carbon can be selected from at least one of a resin, a monosaccharide, a disaccharide, and a polysaccharide as the non-gaseous carbon source. The resin can include a phenolic resin and the like, the monosaccharide can be glucose and the like, the disaccharide can be sucrose and the like, and the polysaccharide can be selected from biomass substances such as cellulose and the like. The above-mentioned substances can be better decomposed into small molecular weight substances in the closed pore sintering process, and the decomposed carbon can better grow on the surface of the carbon-based material in the closed pore sintering process, thereby having a better closed pore effect.

[0064] Therefore, the method has good universality, and the raw materials of the method can be simply and conveniently adjusted according to the specific circumstances of the raw materials, thereby realizing a good closed pore effect: when the raw material (carbon-based material) contains decomposable substances, the raw material itself can be utilized to realize “secondary growth” and promote pore closure through closed pore sintering treatment of the raw material. When the content of decomposable substances in the raw material as the carbon-based material is insufficient to meet the closed pore requirement, a non-gaseous carbon source can be added in the system before the closed pore sintering treatment. For example, the above-mentioned resin, monosaccharide, disaccharide, and polysaccharide, and the like can be prepared into a solution and mixed with the carbon-based material, or added in a solid form.

[0065] In some embodiments, in order to further improve the effect of the closed pore sintering treatment, when the carbon-based material contains carbon substances that can be decomposed, a resin, a monosaccharide, a disaccharide, and a polysaccharide, and the like can also be appropriately added as a non-gaseous carbon source. For example, when the content of carbon substances that can be decomposed in the carbon-based material is small, a part of the resin, the monosaccharide, the disaccharide, and the polysaccharide, and the like can also be added as raw materials to improve the closed pore effect of the closed pore sintering treatment.

[0066] In some embodiments, the specific surface area B C2 of the hard carbon material is lower than the specific surface area B C1 of the carbon-based material, and the specific surface area B C2 of the hard carbon material is less than 10% of the specific surface area B C1decreased by at least 10%. Namely: (B C1 -B C2 ) / B C1 ≥10%. When the decrease of the specific surface area of the obtained hard carbon material relative to the specific surface area of the raw carbon-based material is within the above range, the pore closure can be better achieved, the pore structure of the obtained hard carbon material can be optimized, and the increase of side reactions caused by the electrolyte entering the pore structure can be alleviated.

[0067] For example, in some examples, the specific surface area B C1 of the carbon-based material can be 8-10 m 2 / g, and the specific surface area B C2 of the hard carbon material obtained after the treatment can be 6-9 m 2 / g, B C2 is less than B C1 . In other examples, the specific surface area B C1 of the carbon-based material can be 6-9 m 2 / g, and the specific surface area B C2 of the hard carbon material obtained after the treatment can be 4-7 m 2 / g, B C2 is less than B C1 . When the decrease of the specific surface area of the hard carbon material and the carbon-based material is within the above range, the pore closure effect can be better improved, and the first efficiency of the battery can be better.

[0068] The specific surface area can be determined by investigating the adsorption amount of a sample to a gas, for example, the adsorption-desorption curve of the sample to an inert gas such as nitrogen can be determined. In the present application, the specific surface area can be determined by the test method in GB / T 19587-2017.

[0069] In some embodiments, the temperature of the pore closure sintering treatment is 600-1500°C. Within the above temperature range, the non-gaseous carbon source can be effectively decomposed, and the small molecule carbon substances generated by the decomposition can grow "in situ" in the subsequent pore closure sintering process, so as to achieve the closure of part of the pores. For example, the temperature of the pore closure sintering treatment can be 70-1400°C. In this way, the first efficiency of the battery when the hard carbon material obtained by the method is used as an electrode material can be improved. Moreover, when the pore closure sintering treatment is performed within the above temperature range, the first efficiency can be improved while the rate performance of the material is maintained.

[0070] In some embodiments, the pore closure sintering treatment can be performed at a certain heating rate, so as to raise the material from the ambient temperature to a predetermined temperature. For example, the heating rate of the pore closure sintering treatment can be 5-20°C / min, for example, specifically, 5-10°C / min. When the pore closure sintering treatment is performed at a controlled heating rate, the decomposition efficiency of the non-gaseous carbon source can be improved.

[0071] In some embodiments, the closed pore sintering process can be performed for a certain time. For example, the sintering can be performed for 1-10 hours, such as 1.5 hours or more. More specifically, the sintering can be performed for 1.5-8 hours, 2-6 hours, or the like. The closed pore sintering process for a certain time can sufficiently decompose the non-gaseous carbon source, thereby improving the in-situ growth effect of the method as a whole.

[0072] In some embodiments, the closed pore sintering process is performed in an inert atmosphere. The sintering process can save the production cost of using a high-cost production site such as a Class A plant, and can save the use of components such as explosion-proof components, since the sintering process does not involve flammable and explosive gaseous carbon sources such as methane. Since the present application uses a relatively safe non-gaseous carbon source, the safety is higher than that of gaseous carbon sources such as methane, and the risk of the production process can be reduced.

[0073] In some embodiments, the carbon-based material can include at least one of a biomass raw material and a resin. For example, the biomass raw material can include coconut shell, starch, bamboo charcoal, or the like, which is a carbon-containing material that can be obtained from nature or artificially separated or synthesized in a biological process. The biomass raw material has the advantages of high carbon content, certain molecular structure, and low cost, and is suitable for pore forming treatment to obtain a porous structure. The resin material is low in cost and easy to obtain, and different batches of resin materials have good repeatability, which is beneficial to maintaining the consistency of different batches of products.

[0074] The carbon-based material using the biomass raw material and / or the resin as the raw material can use a part of the carbon-based material as the non-gaseous carbon source. In some embodiments, the closed pore sintering process can be two-stage sintering, such as intermediate sintering and final sintering. The intermediate sintering can cause the part of the carbon-based material that can be decomposed to serve as the non-gaseous carbon source to be sufficiently decomposed, and the final sintering can cause the small molecule substances after decomposition to better in-situ growth, thereby realizing closed pores.

[0075] The temperature of the intermediate sintering and the final sintering can be adjusted according to the specific composition of the carbon-based material, and the requirement of the battery for the first efficiency when the hard carbon material formed is used for the battery electrode. For example, the temperature of the intermediate sintering can be 600-1000°C, and the temperature of the final sintering can be greater than 1000°C. In some examples, the temperature of the final sintering can be greater than the temperature of the intermediate sintering. In this way, the decomposition of the aforementioned non-gaseous carbon source and the in-situ growth process of the closed pores can be more fully realized.

[0076] In some embodiments, the intermediate sintering can also be performed at a certain heating rate from room temperature to a predetermined temperature. For example, the heating rate of the intermediate sintering can be 5-20°C / min, such as 5-10°C / min. In order to sufficiently decompose the decomposable non-gaseous carbon source in the intermediate sintering, the intermediate sintering can be performed for a certain time, such as 1.5-2 hours.

[0077] In some embodiments, the temperature of the intermediate sintering can be 650-900°C, and the temperature of the final sintering can be 1200-1400°C. The above temperature ranges can effectively decompose the non-gaseous carbon source in the carbon-based material based on the biomass raw material, and achieve efficient in-situ growth under relatively low temperature (lower than 1500°C).

[0078] In some embodiments, providing the carbon-based material can include an operation of treating the biomass raw material and / or the resin. For example, it can include pre-treatment and pore-forming treatment of the carbon source such as biomass, resin raw material, etc. for forming the carbon-based material. The pre-treatment includes a first sintering treatment, and the temperature of the first sintering treatment is lower than the temperature of the intermediate sintering. For example, specifically, the first sintering treatment can be performed under an inert atmosphere, the heating rate of the first sintering treatment can be 5-10°C / min, and the sintering temperature can be 500-800°C. For example, the biomass raw material can be subjected to the first sintering treatment at 500-650°C, and the resin raw material can be subjected to the first sintering treatment at 600-800°C. The first sintering treatment can be kept for a period of time, for example, 2 hours, and then naturally cooled to room temperature. The sintered material can be washed, deashed and dried by a low-concentration acid to reduce impurities in the material.

[0079] In some embodiments, the pore-forming treatment can be performed by a pore-forming method familiar to those skilled in the art. For example, the raw material subjected to the first sintering treatment and ZnCl2 powder can be mixed in a certain proportion, for example, a mass ratio of about 1:5, and then sintered to 500-800°C under an inert atmosphere and kept for a period of time, and then naturally cooled to room temperature. The obtained product can be washed and dried by a weak acid to remove impurities to obtain the carbon-based material, and then subjected to the aforementioned closed pore sintering treatment.

[0080] Since the method proposed in the present application is subjected to closed pore sintering treatment of the carbon-based material, the control of the pore-forming treatment conditions can be appropriately relaxed during the pore-forming treatment. The pore structure obtained by the pore-forming treatment can be adjusted during the subsequent closed pore sintering treatment.

[0081] In some embodiments, when the raw material of the carbon-based material is selected from hard carbon negative electrode materials, especially from hard carbon negative electrode materials that have been subjected to pore-forming and sintering treatment, the content of the decomposable non-gaseous carbon source remaining in the carbon-based material can be insufficient to achieve effective closed pore effect during the closed pore sintering treatment. Therefore, at this time, a non-gaseous carbon source can be additionally added to the system.

[0082] In other embodiments, when the carbon-based material is selected from at least one of biomass raw material and resin, a small amount of organic carbon can also be selected and added as a non-gaseous carbon source according to the components of the carbon-based material after the pore-forming treatment.

[0083] In some embodiments, the added organic carbon can be selected from at least one of resin, monosaccharide, disaccharide, and polysaccharide. The polysaccharide can be selected from cellulose, chitin, and the like, and the monosaccharide can be selected from glucose and the like. The resin can be selected from at least one of phenol-formaldehyde resin, epoxy resin, furfural resin, polyurethane resin, and polyamide resin. The above non-gaseous carbon source can be decomposed into small molecules in the aforementioned closed pore sintering process and grown in-situ at the pores of the carbon-based material to achieve closed pores.

[0084] In some embodiments, the organic carbon can be prepared into a solution and mixed with the carbon-based material and subjected to closed pore sintering. For example, a solution containing the non-gaseous carbon source can be sprayed on the surface of the carbon-based material, or the organic carbon can be placed at the bottom of a container (e.g., a magnetic boat) for closed pore sintering, and the carbon-based material can be placed above the non-gaseous carbon source. In this way, the non-gaseous carbon source and the carbon-based material can be mixed well.

[0085] In some embodiments, the amount of the non-gaseous carbon source in the system can be small, so as to achieve closed pores and avoid the formation of a coating on the surface of the carbon-based material by the remaining carbon source, which can affect the performance of the final hard carbon material. Specifically, the amount of the added non-gaseous carbon source can be 0.01-20% of the mass of the carbon-based material. For example, it can be 0.5%, 1%, 2%, 3%, 5%, 8%, 10%, 15%, or 20%. The above non-gaseous carbon source can be part of the carbon-based material or added organic carbon in the system. When the mass of the non-gaseous carbon source and the mass of the carbon-based material are within the above ratio range, the excess pores in the carbon-based material can be better closed.

[0086] In some embodiments, the temperature of the closed pore sintering process with additional non-gaseous carbon source can be slightly higher, for example, it can be 1200-1400°C. When the closed pore sintering temperature is within the above range, the effect of closed pores can be further improved. In the case of additional non-gaseous carbon source, the closed pore sintering process can be performed without segmentation, i.e., the mixture of the non-gaseous carbon source and the carbon-based material can be heated from room temperature to 1200-1400°C at a certain heating rate (e.g., 5-10°C / min) in an inert atmosphere, and then held for 1-2h. In this way, the effect of the closed pore sintering process can be further improved.

[0087] In summary, the method can be used to treat various carbon-based materials, and the excess pore structure in the carbon-based material can be closed by in-situ chemical vapor deposition-like process, so as to adjust the porous structure and improve the first efficiency of the material. Moreover, since the method decomposes non-gaseous carbon source, the requirement for environmental pressure in the closed pore sintering process is also low, the safety is high, and the production equipment is more friendly.

[0088] In another aspect of the present application, the present application provides a hard carbon material. The hard carbon material is subjected to a closed pore sintering treatment, the hard carbon material has a pore structure, and the specific surface area of the hard carbon material is ≤10 m 2 / g. The hard carbon material has the advantages of being able to improve the first efficiency of the battery when used in a secondary battery electrode.

[0089] In some embodiments, the specific surface area of the hard carbon material is ≤10 m 2 / g, for example, can be 2-9.5 m 2 / g. The hard carbon material subjected to the closed pore sintering treatment has a lower specific surface area, which can further alleviate the problem of low first efficiency of the battery due to interface side reactions.

[0090] In some embodiments, the closed pore sintering treatment is carried out in the presence of a non-gaseous carbon source, which includes the part of the carbon-based material that can be decomposed during the closed pore sintering treatment, or an additional organic carbon. In this way, the safety of the closed pore sintering treatment can be improved, the requirements for equipment during the closed pore sintering treatment can be reduced, and the adjustment of the pore structure can be achieved in a more moderate environment. Moreover, the above-mentioned non-gaseous carbon source can be well decomposed into small molecular substances that can effectively grow on the surface of the hard carbon material and achieve closed pores during the closed pore sintering treatment, thereby further improving the effect of the closed pore sintering treatment.

[0091] In some embodiments, the hard carbon material is prepared by the method described above. Therefore, the hard carbon material has all the features and advantages of the material prepared by the method described above. For example, the hard carbon material has a pore structure, and the specific surface area of the hard carbon material is ≤10 m 2 / g. The hard carbon material has a relatively low cost, and the electrode formed therefrom can improve the first efficiency of the battery.

[0092] In yet another aspect of the present application, the present application provides a negative electrode for a sodium-ion battery. The negative electrode comprises the hard carbon material described above. The negative electrode has the beneficial effect of being able to improve the first efficiency of the battery.

[0093] In yet another aspect of the present application, the present application provides a secondary battery. The secondary battery comprises the electrode described above. The secondary battery can be a sodium-ion secondary battery, which has the advantage of having a high first efficiency.

[0094] According to embodiments of the present application, the sodium-ion secondary battery can comprise a positive electrode, a negative electrode, and a separator between the positive electrode and the negative electrode. The negative electrode is the negative electrode described above, or comprises a hard carbon material. The hard carbon material can be the hard carbon material described above, for example, can be obtained by a closed pore sintering treatment. The hard carbon material has a pore structure, and the specific surface area of the hard carbon material is ≤10 m 2 / g.

[0095] In yet another aspect of the present application, the present application provides a power consuming device. The power consuming device comprises the battery as described above, which is configured to provide electric power. The battery can be a sodium-ion secondary battery.

[0096] In the present application, the power consuming device can be, for example, a mobile phone, a tablet, a notebook computer, an electric toy, an electric tool, an electric vehicle, an electric automobile, a ship, a spacecraft, etc. The electric toy can include a stationary or mobile electric toy, such as a game console, an electric automobile toy, an electric ship toy, an electric aircraft toy, etc. The spacecraft can include an airplane, a rocket, a space shuttle, a spacecraft, etc.

[0097] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are intended to explain the present application only, and should not be understood as limiting the present application. In the embodiments, the specific techniques or conditions not mentioned are performed according to the techniques or conditions described in the literature in the art or according to the product manual. The reagents or instruments not mentioned by the manufacturer are all conventional products that can be obtained commercially.

[0098] 1. Preparation of hard carbon material based on biomass raw material

[0099] The coconut shell was first sintered under an inert atmosphere, and after 2 hours of heat preservation, it was naturally cooled to room temperature at a rate of 5°C / min. The sintered material was crushed and then washed with low-concentration acid to remove ash and dried to reduce impurities. The sintered material and ZnCl2 powder were uniformly mixed (at a ratio of 1:5) and then sintered under an inert atmosphere to 600°C for 2 hours of heat preservation, and then naturally cooled to room temperature at a rate of 5°C / min for pore forming treatment. The obtained sample was washed with acid and dried to remove impurities to obtain material A, and then subjected to closed pore sintering treatment. The sintering conditions are shown in Table 1 and Table 2 below. The final sintering heat preservation time of the examples and comparative examples not specially marked is 2h.

[0100] 2. Preparation of hard carbon material based on resin

[0101] The phenolic resin was first sintered under an inert atmosphere, and after 2 hours of heat preservation, it was naturally cooled to room temperature at a rate of 5°C / min. The sintered material was washed with low-concentration acid to remove ash and dried to reduce impurities. The sintered material and ZnCl2 powder were uniformly mixed (at a ratio of 1:5) and then sintered under an inert atmosphere to 600°C for 2 hours of heat preservation, and then naturally cooled to room temperature at a rate of 5°C / min for pore forming treatment. The obtained sample was washed with acid and dried to remove impurities to obtain material B, and then subjected to closed pore sintering treatment. The sintering conditions are shown in Table 1 and Table 2 below. The final sintering heat preservation time of the examples and comparative examples not specially marked is 2h.

[0102] 3. Preparation of hard carbon material based on hard carbon raw material

[0103] The hard carbon material subjected to the closed pore treatment was used as material C, a non-gaseous carbon source was added (the amount of addition was based on the weight of material C), and a closed pore sintering treatment was performed to obtain the same. The sintering conditions are shown in Table 3 below.

[0104] A battery was prepared using the samples obtained in Tables 1-3 as the negative electrode material. The battery was prepared as follows:

[0105] Preparation of the negative electrode:

[0106] The hard carbon negative electrode material, a binder styrene-butadiene rubber (SBR), a thickening agent sodium carboxymethyl cellulose (CMC-Na), and a conductive agent carbon black were mixed in deionized water in a mass ratio of 96.2:1.8:1.2:0.8 to form a uniform negative electrode slurry. The negative electrode slurry was uniformly coated on the surface of a negative electrode current collector copper foil, and after cold pressing and cutting, a negative electrode sheet was obtained.

[0107] Preparation of the positive electrode sheet:

[0108] After the surface oxide layer of the metal sodium was removed, the sodium sheet was rolled and cut to obtain a circular thin sodium sheet.

[0109] Separator:

[0110] A glass fiber membrane was used as the separator.

[0111] Electrolyte:

[0112] Vinyl carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent, and then NaPF6 was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0113] Battery preparation:

[0114] The button cell was prepared according to the conventional CR2430 type button cell assembly method.

[0115] The first efficiency of the secondary battery, the reversible specific capacity at a current density of 10 mA / g, and the reversible specific capacity at a current density of 50 mA / g were tested.

[0116] Test conditions:

[0117] First efficiency: the button cell was discharged at a current density of 10 mA / g to 5 mV, and the specific capacity during the discharge process was recorded as the first cycle discharge specific capacity; after 5 minutes of rest, the button cell was charged to 2 V at a current density of 10 mA / g, and the specific capacity during the charge process was recorded as the first cycle charge specific capacity; the first coulombic efficiency = first cycle charge specific capacity / first cycle discharge specific capacity x 100%.

[0118] Reversible specific capacity at 10 mA / g: the coin cell was discharged to 5 mV at a current density of 10 mA / g; after 5 minutes of rest at the end of discharge, the coin cell was charged to 2 V at a current density of 10 mA / g, and the specific capacity during the charging process was recorded as the first cycle charging specific capacity, which was the reversible specific capacity.

[0119] Reversible specific capacity at 50 mA / g: the coin cell was discharged to 5 mV at a current density of 50 mA / g; after 5 minutes of rest at the end of discharge, the coin cell was charged to 2 V at a current density of 50 mA / g, and the specific capacity during the charging process was recorded as the first cycle charging specific capacity, which was the reversible specific capacity.

[0120] The test results are as follows:

[0121] Table 1

[0122] Table 2

[0123] Table 3

[0124] To further illustrate the performance of the materials obtained in the above examples, four groups of control groups were set in Tables 1-3 above. The control groups were subjected to the same pore-forming treatment as the corresponding examples, but did not use the closed pore sintering treatment proposed in the present application, but used the conventional sintering treatment to close the pores. Control groups 1-3 could not play the role of decomposing the non-gaseous carbon source in the system during the conventional sintering treatment, and control group 4 did not additionally add a decomposable non-gaseous carbon source during the sintering treatment.

[0125] Reversible specific capacity (mAh / g) tests at a current density of 10 mA / g were performed on control group 4 and examples 8-17 in Table 4 above, and the test results of the reversible specific capacity (Specific Capacity (mAh / g)) are shown in Figures 12-14. The Coulumbic Efficiency (%) of the batteries of the examples is also shown in Figures 12-14. Among them, Figure 12 is a test result graph of control group 4 (with an additional carbon source addition amount of 0%) and examples 8-10 (with an additional carbon source addition amount of 1-10%); Figure 13 is a test result graph of control group 4 (with an additional carbon source addition amount of 0%) and examples 11-13 (with an additional carbon source addition amount of 1-10%); and Figure 14 is a test result graph of control group 4 (with an additional carbon source addition amount of 0%) and examples 14-17 (with an additional carbon source addition amount of 1-20%).

[0126] As can be seen from the test results in Tables 1-3 and Figs. 1-14, the material obtained according to the embodiments of the present application has a reasonable porous structure, effective pore closure is achieved in the closed pore sintering process, and thus the first charging efficiency is improved.

[0127] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application, and they should be covered in the scope of the claims and the description of the present application. Especially, as long as there is no structural conflict, each technical feature mentioned in each embodiment can be combined in any way. The present application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A sodium-ion secondary battery, characterized by, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material. A positive electrode, a negative electrode, and a separator between the positive electrode and the negative electrode, the negative electrode including a hard carbon material, the hard carbon material having a specific surface area < 10 m 2 / g.

2. The sodium-ion secondary battery according to claim 1, characterized in that, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material.

3. The sodium-ion secondary battery according to claim 2, characterized in that, The hard carbon material is obtained based on a closed pore sintering treatment of a carbon-based material, the specific surface area of the hard carbon material being BC2, the specific surface area of the carbon-based material being BC1, B C1 > B C2 , and (B C1 -B C2 ) / B C1 the value of is ≥ 10%.

4. A negative electrode for a sodium-ion battery, characterized by, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, 5. The negative electrode according to claim 4, characterized by The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The specific surface area of the hard carbon material is BC2, the specific surface area of the carbon-based material is BC1, B C1 > B C2 , and (B C1 -B C2 ) / B C1 the value of is ≥ 10%.

6. A hard carbon material characterized in that, The specific surface area of the hard carbon material is ≤ 10 m 2 / g.

7. The hard carbon material of claim 6, wherein, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The specific surface area of the hard carbon material is BC2, the specific surface area of the carbon-based material is BC1, B C1 > B C2 , and (B C1 -B C2 ) / B C1 the value of is ≥ 10%.

8. A method of producing a hard carbon material, characterized by, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, 9. The method of claim 8, wherein, The specific surface area of the hard carbon material is BC2, the specific surface area of the carbon-based material is BC1, B C1 > B C2 , and (B C1 -B C2 ) / B C1 the value of is ≥ 10%.

10. The method according to claim 8 or 9, characterized in that, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, 11. The method according to any one of claims 8-10, characterized in that, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, 12. The method of claim 11, wherein, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, 13. The method of claim 12, wherein, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, 14. The method of claim 12, wherein, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, 15. The method of claim 11, wherein, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, 16. The method according to any one of claims 8-15, characterized in that, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, 17. The method according to any one of claims 8-16, characterized in that, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, 18. An electrical device, comprising: The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based material, The hard carbon material is obtained based on a closed pore sintering treatment on a carbon-based 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