Anode materials, their preparation methods and applications

Abstract

This application provides an anode material, its preparation method, and its application. The anode material includes a carbon-based core and a carbon coating layer. The carbon-based core is doped with a first non-metallic element. The carbon coating layer covers the outer surface of the carbon-based core and is doped with a second non-metallic element. The anode material provided by this application not only has high ion mobility but also sufficient capacity, good rate performance, high first-pass coulombic efficiency, and good stability.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a negative electrode material, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries are one of the emerging types of rechargeable batteries. They offer high safety and low cost, and have a broad market prospect. Sodium and lithium belong to the same group, and sodium has a similar energy storage mechanism to lithium. However, because sodium ions have a larger radius than lithium ions, the electrode materials for sodium-ion batteries need to have a larger intercalation space to facilitate the insertion and extraction of sodium ions. Among the many anode materials for sodium-ion batteries, hard carbon is a superior anode material due to its low redox potential, high structural stability, low cost, and large interlayer spacing. However, when hard carbon is used as an anode material, sodium-ion batteries suffer from low initial coulombic efficiency and poor rate performance, limiting the overall performance improvement of sodium-ion batteries. Summary of the Invention

[0003] This application provides a negative electrode material, its preparation method, and its application.

[0004] In a first aspect, this application provides a negative electrode material, which includes a carbon-based material core and a carbon coating layer. The carbon-based material core is doped with a first non-metallic element. The carbon coating layer covers the outer surface of the carbon-based material core and is doped with a second non-metallic element.

[0005] In the negative electrode material provided in this application, on the one hand, the carbon coating layer can suppress the aggregation of negative electrode material particles, reduce the specific surface area of ​​the negative electrode material, improve the initial coulombic efficiency of the battery, and also improve the stability of the negative electrode material. On the other hand, a first non-metallic element and a second non-metallic element are respectively doped into the carbon-based material core and the carbon coating layer. The first non-metallic element is used to increase the interlayer spacing of the carbon-based material core, so as to facilitate the insertion and extraction of larger sodium ions, which is beneficial to improving ion mobility, thereby facilitating the fast charging / discharging process and improving the specific capacity of the negative electrode material. In addition, the second non-metallic element increases the attachment sites of sodium ions in the carbon coating layer, thereby improving the capacity of the negative electrode material.

[0006] In one embodiment, the distribution density of the first non-metallic element gradually decreases from the outer surface of the carbon-based material core to the core of the carbon-based material core. This structure provides a large number of active sites for sodium ion insertion, greatly improving the specific capacity of the material. On the other hand, the gradual decrease in the distribution density of the first non-metallic element from the outer surface of the carbon-based material core to the core of the carbon-based material core creates channels for sodium ion transport, effectively improving the transport kinetics of sodium ions and facilitating the transfer of sodium ions from the bulk phase to the surface, thus effectively improving the capacity retention and rate performance of the anode material 1.

[0007] In one embodiment, the distribution density of the second nonmetallic element gradually decreases from the outer surface to the inner surface of the carbon coating layer.

[0008] In one embodiment, the first nonmetallic element includes at least one of nitrogen, phosphorus, sulfur, and fluorine.

[0009] In one embodiment, the second nonmetallic element includes at least one of nitrogen, phosphorus, sulfur, and fluorine.

[0010] In one embodiment, the second nonmetallic element is the same as the first nonmetallic element.

[0011] In one embodiment, both the first non-metallic element and the second non-metallic element are one of nitrogen, phosphorus, sulfur, and fluorine. From the outer surface of the carbon coating layer to the core of the carbon-based material core, the distribution density of one of nitrogen, phosphorus, sulfur, and fluorine gradually decreases.

[0012] In one embodiment, both the second non-metallic element and the first non-metallic element are nitrogen elements, and the distribution density of nitrogen elements varies in a gradient from the outer surface of the carbon coating layer to the core of the carbon-based material core.

[0013] In one embodiment, the ratio of the total number of atoms of the first nonmetallic element and the second nonmetallic element to the number of atoms in the negative electrode material is 2%-15%.

[0014] In one embodiment, the ratio of the sum of the masses of the carbon coating layer, the first non-metallic element, and the second non-metallic element to the mass of the carbon-based material core is 2%-12%.

[0015] In one embodiment, the particle size of the negative electrode material is 2-50 micrometers.

[0016] In one embodiment, the particle size D50 of the negative electrode material is 5-10 micrometers.

[0017] In one embodiment, the specific surface area of ​​the negative electrode material is 5-10 m². 2 / g.

[0018] In one embodiment, the ratio of the number of hydrogen atoms in the negative electrode material to the total number of atoms in the negative electrode material is less than or equal to 4%.

[0019] In one embodiment, the porosity of the carbon-based material core is 10-70%.

[0020] In one embodiment, the carbon-based material core includes at least one of hard carbon, soft carbon, activated carbon, and graphite.

[0021] Secondly, this application provides a method for preparing a negative electrode material, the method comprising:

[0022] The non-metallic source is heated at a first temperature;

[0023] The carbon-based material core is heated at a second temperature;

[0024] A heated nonmetallic source is transported to the surface of a carbon-based material core by introducing a protective gas, and solid I is obtained after deposition on the surface of the carbon-based material core.

[0025] Solid I is heated at a third temperature to obtain the negative electrode material.

[0026] In one embodiment, the first temperature is greater than the second temperature and less than or equal to the third temperature.

[0027] In one embodiment, the first temperature is 500-1000°C.

[0028] In one embodiment, the second temperature is 100-300°C.

[0029] In one embodiment, the third temperature is 700-1200°C.

[0030] In one embodiment, the heating rate of the non-metallic source is 5-20°C / min, and the heating time of the non-metallic source at the first temperature is 15-60min.

[0031] In one embodiment, the heating rate of the carbon-based material core is 5-20°C / min.

[0032] In one embodiment, the flow rate of the protective gas is 10-100 mL / min, and the introduction time of the protective gas is 15-60 min.

[0033] In one embodiment, the heating rate of solid I is 5-20°C / min, and the heating time of solid I at the third temperature is 1-4h.

[0034] In one embodiment, the non-metallic source is a nitrogen source, which includes at least one of benzimidazole, melamine, urea, pyrrole, polypyrrole, pyrimidine, and aminopyrimidine.

[0035] In one embodiment, the mass fraction of nitrogen atoms in the nitrogen source is 13%-67%.

[0036] Thirdly, this application provides a negative electrode sheet, which includes the negative electrode material as described in any of the preceding claims, or a negative electrode material prepared by the method described above.

[0037] Fourthly, this application provides a secondary battery, which includes a positive electrode, a separator, and a negative electrode as described above.

[0038] In one embodiment, the secondary battery is a sodium-ion battery with an initial coulombic efficiency greater than or equal to 85%, a specific capacity greater than or equal to 280 mAh / g at 0.1C rate, and a specific capacity greater than or equal to 210 mAh / g at 10C rate. Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the embodiments of this application will be described below.

[0040] Figure 1 A schematic diagram of the negative electrode material provided in this application.

[0041] Figure 2 These are the first charge-discharge curves at 0.1C for the examples and comparative examples.

[0042] Figure 3 These are performance graphs of the embodiments and comparative examples at different magnifications.

[0043] Figure 4 This is a graph showing the change in nitrogen atom content on the surface of the material prepared in Example 3 as a function of X-ray energy spectrum etching depth. Detailed Implementation

[0044] The technical solutions of the embodiments of this application will be described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments.

[0045] In this document, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this application, unless otherwise stated, "multiple" means two or more.

[0046] Furthermore, in this article, directional terms such as "upper" and "lower" are defined relative to the orientation of the structure as shown in the attached drawings. It should be understood that these directional terms are relative concepts, used for relative description and clarification, and can change accordingly depending on the orientation of the structure.

[0047] For ease of understanding, the English abbreviations and related technical terms used in the embodiments of this application will be explained and described below.

[0048] SEI membrane: refers to Solid Electrolyte Interphase, i.e., solid electrolyte interface membrane.

[0049] Specific surface area: refers to the total area of ​​a unit mass of material.

[0050] Porosity: refers to the percentage of pore volume in a material to the total volume of the material in its natural state.

[0051] Hard carbon: refers to carbon materials that are difficult to graphitize, and usually cannot be graphitized above 2800℃.

[0052] Sodium-ion batteries are one of the emerging types of rechargeable batteries. They offer high safety and low cost, and have a broad market prospect. Sodium and lithium belong to the same group, and sodium has a similar energy storage mechanism to lithium. However, because sodium ions have a larger radius than lithium ions, the electrode materials for sodium-ion batteries need to have a larger intercalation space to facilitate the insertion and extraction of sodium ions. For example, among the many anode materials for sodium-ion batteries, hard carbon is a superior anode material due to its low redox potential, high structural stability, low cost, and large interlayer spacing. However, when hard carbon is used as an anode material, sodium-ion batteries suffer from low initial coulombic efficiency and poor rate performance, limiting the overall performance improvement of sodium-ion batteries.

[0053] This application provides a negative electrode material 1, which includes a carbon-based material core 10, a first non-metallic element 11, and a coating layer. The first non-metallic element 11 is distributed in the carbon-based material core 10 from the outer surface of the carbon-based material core 10 to the core of the carbon-based material core 10. The coating layer covers the outer surface of the carbon-based material core 10 and is used to isolate the carbon-based material core 10 from the outside world.

[0054] In one embodiment, the coating layer includes at least one of hard carbon, soft carbon, activated carbon, and graphite.

[0055] In one embodiment, the coating layer is a carbon coating layer 20.

[0056] This application provides a negative electrode material 1, which includes a carbon-based material core 10 and a carbon coating layer 20. The carbon-based material core 10 is doped with a first non-metallic element 11. The carbon coating layer covers the outer surface of the carbon-based material core 10, and the carbon coating layer 20 is doped with a second non-metallic element 21.

[0057] Among them, the negative electrode material 1 is in the form of particles. Figure 1The structure shown is the structure of one of the particles in the negative electrode material 1. The negative electrode material 1 can be used to prepare the negative electrode of a battery. The negative electrode material 1 is the carrier of ions and electrons during the charging and discharging process of the battery, determining energy storage and release. In one embodiment, the negative electrode material 1 is applied to a sodium-ion battery. The negative electrode material 1 has a core-shell structure, with a carbon-based material core 10 surrounded by a carbon coating layer 20. Carbon-based materials refer to materials primarily composed of carbon. The carbon-based material core 10 is approximately spherical, with the core of the carbon-based material core 10 referring to the center of the sphere.

[0058] In one embodiment, the carbon-based material core 10 includes at least one of hard carbon, soft carbon, activated carbon, and graphite.

[0059] In one embodiment, the carbon-based material core 10 is one of hard carbon, soft carbon, activated carbon, and graphite.

[0060] In one embodiment, the carbon-based material core 10 includes at least two of hard carbon, soft carbon, activated carbon, and graphite.

[0061] In one embodiment, the carbon-based material core 10 is hard carbon. Hard carbon possesses characteristics such as large interlayer spacing and high sodium storage capacity. Using hard carbon as the carbon-based material core 10 can improve the performance of the anode material 1.

[0062] In one embodiment, the carbon-based material core 10 is hard carbon, and the carbon coating layer 20 is activated carbon.

[0063] In one embodiment, both the carbon-based material core 10 and the carbon coating layer 20 are hard carbon.

[0064] The first non-metallic element 11 is doped into the carbon-based material core 10. The first non-metallic element 11 is used to increase the interlayer spacing of the carbon-based material core 10, so as to facilitate the insertion and extraction of larger sodium ions, which is beneficial to improving ion mobility, thereby facilitating the fast charging / discharging process and improving the specific capacity of the negative electrode material 1.

[0065] A carbon coating layer 20 is applied to the outer surface of the carbon-based material core 10. Since the first non-metallic element 11 is distributed within the carbon-based material core 10, the carbon coating layer 20 also coats the outer side of the first non-metallic element 11. By providing the carbon coating layer 20 in the negative electrode material 1, on the one hand, the carbon coating layer 20 can suppress the aggregation of particles in the negative electrode material 1, reducing the specific surface area of ​​the negative electrode material 1. Since a solid electrolyte interphase (SEI) film forms on the surface of the battery negative electrode material 1 during the first charge and discharge process, the formation of the SEI film consumes sodium ions and electrolyte in the battery. In this application, because the specific surface area of ​​the negative electrode material 1 is relatively small, the amount of sodium ions and electrolyte consumed by the negative electrode material 1 during the formation of the SEI film is small, thus improving the initial coulombic efficiency of the battery. On the other hand, the carbon coating layer 20 isolates the carbon-based material core 10 and the first non-metallic element 11 from the electrolyte, reducing the occurrence of side reactions. The carbon coating layer 20 improves the stability of the first non-metallic element 11 and the carbon-based material core 10, which is conducive to giving full play to the synergistic effect of the first non-metallic element 11 and the carbon-based material core 10, so as to improve the performance and cycle stability of the battery.

[0066] In one embodiment, the carbon coating layer 20 completely covers the outer surface of the carbon-based material core 10, meaning the carbon coating layer 20 covers the entire outer surface of the carbon-based material core 10. In this case, both the carbon-based material core 10 and the first non-metallic element 11 are located within the carbon coating layer 20. In another embodiment, the carbon coating layer 20 covers a portion of the outer surface of the carbon-based material core 10. In this case, a portion of the outer surface of the carbon-based material core 10 is covered by the carbon coating layer 20, while another portion of the outer surface of the carbon-based material core 10 is exposed and not covered by the carbon coating layer 20.

[0067] In one embodiment, the carbon coating layer 20 is deposited onto the outer surface of the carbon-based material core 10 via vapor deposition. This coating method can further reduce the specific surface area of ​​the negative electrode material 1, thereby improving the initial coulombic efficiency of the battery.

[0068] In this application, the negative electrode material 1 further includes a second non-metallic element 21, which is distributed in the carbon coating layer 20. Doping the carbon coating layer 20 with the second non-metallic element 21 increases the attachment sites for sodium ions in the carbon coating layer 20, thereby improving the capacity of the negative electrode material 1.

[0069] The negative electrode material 1 provided in this application has the following advantages: on the one hand, the carbon coating layer 20 in the negative electrode material 1 reduces the specific surface area of ​​the negative electrode material 1 and improves the first coulombic efficiency of the negative electrode material 1; on the other hand, both the carbon-based material core 10 and the carbon coating layer 20 are doped with non-metallic elements, which can provide more sodium storage sites, thereby improving the specific capacity of the battery.

[0070] In one embodiment, the distribution density of the first non-metallic element 11 varies in a gradient from the outer surface of the carbon-based material core 10 to the core of the carbon-based material core 10.

[0071] The first non-metallic element 11 is non-uniformly dispersed within the carbon-based material core 10. From the outer surface of the carbon-based material core 10 to its core, the distribution density of the first non-metallic element 11 exhibits a gradient change. This gradient change means that the distribution density of the first non-metallic element 11 varies at different locations from the outer surface to the core of the carbon-based material core 10; that is, within the carbon-based material core 10, the distribution density of the first non-metallic element 11 differs at locations at different distances from either the outer surface or the core. This gradient change can be linear or step-like. Linear changes include both linear and curvilinear variations.

[0072] The negative electrode material 1 provided in this application has a gradient distribution density of the first non-metallic element 11 from the outer surface of the carbon-based material core 10 to the core of the carbon-based material core 10. This can improve the transport dynamics of sodium ions, facilitate the transfer of sodium ions from the bulk phase to the surface, and improve the capacity retention and rate performance of the negative electrode material 1.

[0073] Please see Figure 1 , Figure 1 A schematic diagram of the negative electrode material 1 provided in this application. Figure 1 In the coordinate system, the horizontal axis represents the length of the negative electrode material 1 along the horizontal axis direction, and the vertical axis represents the distribution density of the first non-metallic element 11. In one embodiment, the distribution density of the first non-metallic element 11 gradually decreases from the outer surface of the carbon-based material core 10 to the core of the carbon-based material core 10 (e.g., ...). Figure 1 (As shown). The negative electrode material 1 has a structure in which the distribution density of the first non-metallic element 11 gradually decreases from the outer surface of the carbon-based material core 10 to the core of the carbon-based material core 10. This structure provides a large number of active sites for sodium ion insertion, greatly improving the specific capacity of the material. On the other hand, the gradual decrease in the distribution density of the first non-metallic element 11 from the outer surface of the carbon-based material core 10 to the core of the carbon-based material core 10 creates channels for sodium ion transport, effectively improving the transport dynamics of sodium ions and facilitating the transfer of sodium ions from the bulk phase to the surface, thus effectively improving the capacity retention and rate performance of the negative electrode material 1.

[0074] The negative electrode material 1 provided in this application not only has a large intercalation space to facilitate the intercalation and deintercalation of large sodium ions, resulting in high sodium ion mobility, but also has sufficient capacity, good rate performance, high first coulombic efficiency, and good stability.

[0075] In one embodiment, within the carbon-based material core 10, the smaller the distance between the first non-metallic element 11 and the outer surface of the carbon-based material core 10, the greater the distribution density of the first non-metallic element 11; conversely, the greater the distance between the first non-metallic element 11 and the outer surface of the carbon-based material core 10, the smaller the distribution density of the first non-metallic element 11. The distance between the first non-metallic element 11 and the outer surface of the carbon-based material core 10 refers to the minimum distance between the first non-metallic element 11 and the outer surface of the carbon-based material core 10 along its radial direction.

[0076] In one embodiment, in the carbon-based material core 10, the smaller the distance between the first non-metallic element 11 and the core of the carbon-based material core 10, the smaller the distribution density of the first non-metallic element 11; the larger the distance between the first non-metallic element 11 and the core of the carbon-based material core 10, the larger the distribution density of the first non-metallic element 11.

[0077] In one embodiment, the distribution density of the first non-metallic element 11 can also gradually increase from the outer surface of the carbon-based material core 10 to the core of the carbon-based material core 10.

[0078] In one embodiment, the second non-metallic element 21 is uniformly distributed in the carbon coating layer 20, that is, the distribution density of the second non-metallic element 21 is the same at different positions in the carbon coating layer 20.

[0079] In one embodiment, the second non-metallic element 21 is non-uniformly distributed in the carbon coating layer 20. For example, the distribution density of the second non-metallic element 21 exhibits a gradient distribution from the outer surface to the inner surface of the carbon coating layer 20. It is worth noting that the non-uniform distribution in this application includes, but is not limited to, a gradient distribution.

[0080] In one embodiment, the distribution density of the second non-metallic element 21 gradually decreases from the outer surface to the inner surface of the carbon coating layer 20. This gradual decrease in the distribution density of the second non-metallic element 21 from the outer surface to the inner surface of the carbon coating layer 20 creates a channel for sodium ion transport, effectively improving the transport kinetics of sodium ions and facilitating the transfer of sodium ions from the bulk phase to the surface. This effectively improves the capacity retention and rate performance of the negative electrode material 1.

[0081] In one embodiment, the distribution density of the second nonmetallic element 21 gradually increases from the outer surface of the carbon coating layer 20 to the inner surface of the carbon coating layer 20.

[0082] In one embodiment, the first non-metallic element 11 includes at least one of nitrogen, phosphorus, sulfur, and fluorine. Nitrogen, phosphorus, sulfur, and fluorine can better expand the interlayer spacing or porosity of the carbon-based material core 10, improve the insertion and migration rate of sodium ions, and facilitate the fast charging / discharging process of the battery.

[0083] In one embodiment, the first non-metallic element 11 is selected from nitrogen, phosphorus, sulfur, and fluorine. The first non-metallic element 11 is nitrogen, phosphorus, sulfur, or fluorine. Selecting a single element as the first non-metallic element 11 facilitates the preparation of the negative electrode material 1.

[0084] In one embodiment, the first non-metallic element 11 is nitrogen, and the distribution density of nitrogen gradually decreases from the outer surface of the carbon-based material core 10 to the core of the carbon-based material core 10.

[0085] In one embodiment, the first nonmetallic element 11 includes at least two of nitrogen, phosphorus, sulfur, and fluorine. For example, in one embodiment, the first nonmetallic element 11 includes nitrogen and phosphorus. In another embodiment, the first nonmetallic element 11 includes sulfur and fluorine. In yet another embodiment, the first nonmetallic element 11 includes nitrogen, sulfur, and phosphorus. In yet another embodiment, the first nonmetallic element 11 includes nitrogen, sulfur, and fluorine. In yet another embodiment, it includes nitrogen, sulfur, phosphorus, and fluorine.

[0086] In one embodiment, the first non-metallic element 11 includes two elements selected from nitrogen, phosphorus, sulfur, and fluorine. The distribution density of both elements varies stepwise from the outer surface of the carbon-based material core 10 to its core. For example, the first non-metallic element 11 includes nitrogen and sulfur. The distribution density of nitrogen gradually decreases from the outer surface of the carbon-based material core 10 to its core, and the distribution density of sulfur also gradually decreases from the outer surface of the carbon-based material core 10 to its core.

[0087] In one embodiment, the first non-metallic element 11 includes two elements selected from nitrogen, phosphorus, sulfur, and fluorine. The distribution density of one element varies gradually from the outer surface of the carbon-based material core 10 to its core, while the other element is uniformly distributed throughout the carbon-based material core 10. For example, the first non-metallic element 11 includes nitrogen and fluorine, wherein the distribution density of nitrogen gradually decreases from the outer surface of the carbon-based material core 10 to its core, while fluorine is uniformly distributed throughout the carbon-based material core 10.

[0088] In one embodiment, the second non-metallic element 21 includes at least one of nitrogen, phosphorus, sulfur, and fluorine. Nitrogen, phosphorus, sulfur, and fluorine facilitate the insertion and migration of sodium ions, thereby increasing the capacity of the negative electrode material 1.

[0089] In one embodiment, the second non-metallic element 21 is selected from nitrogen, phosphorus, sulfur, and fluorine. The second non-metallic element 21 may be nitrogen, phosphorus, sulfur, or fluorine. Selecting a single element for the second non-metallic element 21 facilitates the preparation of the negative electrode material 1.

[0090] In one embodiment, the second nonmetallic element 21 includes at least two of nitrogen, phosphorus, sulfur and fluorine.

[0091] In one embodiment, the second nonmetallic element 21 is the same as the first nonmetallic element 11. This means that the element types and the number of element types in the second nonmetallic element 21 are the same as those in the first nonmetallic element 11. For example, in one embodiment, the second nonmetallic element 21 contains only nitrogen, and the first nonmetallic element 11 also contains only nitrogen. In another embodiment, the second nonmetallic element 21 contains both nitrogen and phosphorus, and the first nonmetallic element 11 also contains both nitrogen and phosphorus.

[0092] In this application, the second non-metallic element 21 in the negative electrode material 1 is the same as the first non-metallic element 11. When the negative electrode material 1 is prepared, the first non-metallic element 11 doped in the carbon-based material core 10 and the second non-metallic element 21 doped in the carbon coating layer 20 can use the same non-metallic source. This simplifies the preparation steps of the negative electrode material 1, reduces the preparation difficulty of the negative electrode material 1, and also improves the first coulombic efficiency and rate performance of the negative electrode material 1.

[0093] In one embodiment, both the first non-metallic element 11 and the second non-metallic element 21 are one of nitrogen, phosphorus, sulfur, and fluorine. From the outer surface of the carbon coating layer 20 to the core of the carbon-based material core 10, the distribution density of one of the nitrogen, phosphorus, sulfur, and fluorine elements gradually decreases. This gradual decrease in the distribution density of non-metallic elements from the outer surface of the carbon coating layer 20 to the core of the carbon-based material core 10 creates a channel for sodium ion transport, effectively improving the sodium ion transport kinetics and facilitating the transfer of sodium ions from the bulk phase to the surface. This effectively improves the capacity retention and rate performance of the negative electrode material 1.

[0094] In one embodiment, both the second non-metallic element 21 and the first non-metallic element 11 in the negative electrode material 1 are nitrogen elements, and the distribution density of nitrogen elements varies in a gradient from the outer surface of the carbon coating layer 20 to the core of the carbon-based material core 10.

[0095] In one embodiment, both the second non-metallic element 21 and the first non-metallic element 11 in the negative electrode material 1 are nitrogen elements, and the distribution density of nitrogen elements gradually decreases from the outer surface of the carbon coating layer 20 to the core of the carbon-based material core 10.

[0096] In one embodiment, the second non-metallic element 21 and the first non-metallic element 11 in the negative electrode material 1 are both fluorine elements, and the distribution density of fluorine elements gradually decreases from the outer surface of the carbon coating layer 20 to the core of the carbon-based material core 10.

[0097] In one embodiment, the second nonmetallic element 21 is different from the first nonmetallic element 11. This difference means that the element type or the number of element types in the second nonmetallic element 21 is different from that in the first nonmetallic element 11. For example, in one embodiment, the element type in the second nonmetallic element 21 is different from that in the first nonmetallic element 11; for instance, the second nonmetallic element 21 is nitrogen, and the first nonmetallic element 11 is sulfur. In another embodiment, the number of element types in the second nonmetallic element 21 is different from that in the first nonmetallic element 11; for instance, the second nonmetallic element 21 includes both nitrogen and sulfur, while the first nonmetallic element 11 includes only sulfur.

[0098] In one embodiment, the ratio of the total number of atoms of the first non-metallic element 11 and the second non-metallic element 21 to the total number of atoms in the negative electrode material 1 is 2%-15%. The proportion of the number of atoms of non-metallic elements in the negative electrode material 1 will affect the performance of the negative electrode material 1.

[0099] If the proportion of non-metallic elements in anode material 1 is too low, for example, less than 2%, the active sites for sodium ion insertion provided by non-metallic elements are few and cannot provide effective sodium ion transport channels. A small amount of non-metallic element doping has no significant promoting effect on the capacity, rate performance, and initial coulombic efficiency of anode material 1. If the proportion of non-metallic elements in anode material 1 is too high, for example, greater than 15%, excessive non-metallic elements excessively increase the pore size of anode material 1, making it prone to collapse. Furthermore, excessive non-metallic elements may occupy sodium ion insertion sites, reducing the capacity of anode material 1.

[0100] In this application, the ratio of the total number of atoms of the first non-metallic element 11 and the second non-metallic element 21 to the number of atoms in the negative electrode material 1 is controlled to be greater than or equal to 2% and less than or equal to 15%. The proportion of the number of atoms of non-metallic elements in the negative electrode material 1 is moderate, so that the first non-metallic element 11 and the second non-metallic element 21 can not only improve the capacity, rate performance and first coulombic efficiency of the negative electrode material 1, but also ensure the stability of the negative electrode material 1.

[0101] Furthermore, in this application, the distribution density of the first non-metallic element 11 varies gradient from the outer surface of the carbon-based material core 10 to its core. This distribution pattern of the first non-metallic element 11 can construct channels more conducive to sodium ion transport. Compared to arbitrary distribution patterns of the first non-metallic element 11, the anode material 1 in this application can achieve the same sodium ion transport rate even with a smaller atomic percentage of the first non-metallic element 11. The density distribution pattern of the first non-metallic element 11 provided in this application can save on the amount of the first non-metallic element 11 used. For example, if the distribution density of the first non-metallic element 11 in the anode material 1 is not gradient-varying, a 10% proportion of the first non-metallic element 11 can achieve the preset sodium ion transport rate. However, if the distribution density of the first non-metallic element 11 in the anode material 1 is gradient-varying, then only a 2% proportion of the first non-metallic element 11 is needed to achieve the preset sodium ion transport rate.

[0102] Similarly, compared to the arbitrary distribution of the first non-metallic element 11, the anode material 1 in this application can achieve a higher sodium ion transport rate under the condition that the atomic proportion of the first non-metallic element 11 is the same.

[0103] In one embodiment, the ratio of the total number of atoms of the first nonmetallic element 11 and the second nonmetallic element 21 to the number of atoms in the negative electrode material 1 is 2%-12%.

[0104] In one embodiment, the ratio of the total number of atoms of the first non-metallic element 11 and the second non-metallic element 21 to the number of atoms in the negative electrode material 1 is 2%-10%.

[0105] In one embodiment, the ratio of the total number of atoms of the first non-metallic element 11 and the second non-metallic element 21 to the number of atoms in the negative electrode material 1 is 2%-8%.

[0106] In one embodiment, the ratio of the total number of atoms of the first nonmetallic element 11 and the second nonmetallic element 21 to the number of atoms in the negative electrode material 1 is 2%-6%.

[0107] In one embodiment, the ratio of the total number of atoms of the first non-metallic element 11 and the second non-metallic element 21 to the number of atoms in the negative electrode material 1 is 2%-4%.

[0108] In one embodiment, the ratio of the total number of atoms of the first non-metallic element 11 and the second non-metallic element 21 to the number of atoms in the negative electrode material 1 is 5%-15%.

[0109] In one embodiment, the ratio of the total number of atoms of the first nonmetallic element 11 and the second nonmetallic element 21 to the number of atoms in the negative electrode material 1 is 8%-15%.

[0110] In one embodiment, the ratio of the total number of atoms of the first non-metallic element 11 and the second non-metallic element 21 to the number of atoms in the negative electrode material 1 is 10%-15%.

[0111] In one embodiment, the ratio of the total number of atoms of the first nonmetallic element 11 and the second nonmetallic element 21 to the number of atoms in the negative electrode material 1 is 12%-15%.

[0112] In one embodiment, the ratio of the total number of atoms of the first nonmetallic element 11 and the second nonmetallic element 21 to the number of atoms in the negative electrode material 1 is 5%-10%.

[0113] In one embodiment, the ratio of the total number of atoms of the first nonmetallic element 11 and the second nonmetallic element 21 to the number of atoms in the negative electrode material 1 is 6%-8%.

[0114] In one embodiment, the ratio of the total number of atoms of the first non-metallic element 11 and the second non-metallic element 21 to the number of atoms in the negative electrode material 1 can be 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%.

[0115] In one embodiment, the sum of the masses of the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21 is 2%-12% of the mass of the carbon-based material core 10. This application modifies the carbon-based material core 10 to improve the performance of the negative electrode material 1. The weight gain of the modified carbon-based material core 10, i.e., the content of the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21, affects the performance of the negative electrode material 1.

[0116] If the sum of the masses of the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21 is less than 2% of the mass ratio of the carbon-based material core 10, the content of the carbon coating layer 20 is too small to improve the stability and initial coulombic efficiency of the negative electrode material 1. If the content of the first non-metallic element 11 and the second non-metallic element 21 is too small, it has no significant promoting effect on the capacity, rate performance, and initial coulombic efficiency of the negative electrode material 1. If the sum of the masses of the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21 is greater than 12% of the mass ratio of the carbon-based material core 10, the larger proportion of the carbon coating layer mass increases the processing difficulty of the negative electrode material 1 and affects the energy density of the battery, thus reducing the specific capacity. At the same time, excessive non-metallic elements excessively increase the pore size of the negative electrode material 1, making it prone to collapse. Furthermore, excessive non-metallic elements may occupy sodium ion intercalation sites, reducing the capacity of the negative electrode material 1.

[0117] In this application, the sum of the masses of the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21 is greater than or equal to 2% and less than or equal to 12% of the mass of the carbon-based material core 10. The contents of the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21 are controlled within a suitable range, so that the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21 can not only improve the capacity, rate performance, and first coulombic efficiency of the negative electrode material 1, but also ensure the stability of the negative electrode material 1 and enhance the bonding force between the carbon coating layer 20 and the carbon-based material core 10.

[0118] In one embodiment, the sum of the masses of the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21 is 2%-10% of the mass of the carbon-based material core 10.

[0119] In one embodiment, the sum of the masses of the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21 is 2%-8% of the mass of the carbon-based material core 10.

[0120] In one embodiment, the sum of the masses of the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21 is 2%-6% of the mass of the carbon-based material core 10.

[0121] In one embodiment, the sum of the masses of the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21 is 2%-4% of the mass of the carbon-based material core 10.

[0122] In one embodiment, the sum of the masses of the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21 is 4%-12% of the mass of the carbon-based material core 10.

[0123] In one embodiment, the sum of the masses of the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21 is 6%-12% of the mass of the carbon-based material core 10.

[0124] In one embodiment, the sum of the masses of the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21 is 8%-12% of the mass of the carbon-based material core 10.

[0125] In one embodiment, the sum of the masses of the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21 is 10%-12% of the mass of the carbon-based material core 10.

[0126] In one embodiment, the sum of the masses of the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21 is 5%-10% of the mass of the carbon-based material core 10.

[0127] In one embodiment, the sum of the masses of the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21 is 6%-8% of the mass of the carbon-based material core 10.

[0128] In one embodiment, the mass ratio of the sum of the carbon coating layer 20, the first non-metallic element 11, and the second non-metallic element 21 to the mass of the carbon-based material core 10 can be 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5%, 10%, 10.5%, 11%, 11.5%, or 12%.

[0129] In one embodiment, the particle size of the negative electrode material 1 is 2-50 micrometers. If the particle size of the negative electrode material 1 is too large or too small, it will affect the viscosity of the slurry during the preparation of the negative electrode sheet, thus affecting the preparation of the negative electrode sheet. Simultaneously, if the particle size of the negative electrode material 1 is too large or too small, it will also affect the packing density of the negative electrode material 1, thus affecting the improvement of energy density. In this application, the particle size of the negative electrode material 1 is greater than or equal to 2 micrometers and less than or equal to 50 micrometers, that is, the maximum particle size of all particles in the negative electrode material 1 is 50 micrometers, and the minimum particle size is 2 micrometers. This is beneficial for the preparation of the negative electrode sheet while also improving the energy density of the negative electrode material 1.

[0130] In one embodiment, the particle size of the negative electrode material 1 is 2-50 micrometers.

[0131] In one embodiment, the particle size of the negative electrode material 1 is 5-40 micrometers.

[0132] In one embodiment, the particle size of the negative electrode material 1 is 5-30 micrometers.

[0133] In one embodiment, the particle size of the negative electrode material 1 is 10-20 micrometers.

[0134] In one embodiment, the particle size D50 of the negative electrode material 1 is 5-10 micrometers. Here, particle size D50 refers to the particle size corresponding to a cumulative particle size distribution percentage of 50% in the negative electrode material 1; D50 is also called the median diameter or median particle size. In this application, controlling the particle size D50 of the negative electrode material 1 to 5-10 micrometers is beneficial for preparing the negative electrode sheet and for improving the energy density of the negative electrode material 1.

[0135] In one embodiment, the particle size D50 of the negative electrode material 1 is 5-8 micrometers.

[0136] In one embodiment, the particle size D50 of the negative electrode material 1 is 8-10 micrometers.

[0137] In one embodiment, the particle size D50 of the negative electrode material 1 is 6-8 micrometers.

[0138] In one embodiment, the particle size D50 of the negative electrode material 1 is 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5 or 10 micrometers.

[0139] In one embodiment, the specific surface area of ​​the negative electrode material 1 is 5-10 m². 2 / g. In one embodiment, the specific surface area is tested using the BET (Brunauer-Emmett-Teller) specific surface area test method, and the specific surface area is the BET specific surface area. If the specific surface area of ​​the negative electrode material 1 is too large, the consumption of sodium ions and electrolyte and other active materials during the formation of the SEI film is large, resulting in a low initial coulombic efficiency of the battery. If the specific surface area of ​​the negative electrode material 1 is too small, it is not conducive to the insertion and extraction of sodium ions, affecting the capacity utilization of the negative electrode material 1 and leading to a decrease in the capacity of the negative electrode material 1. In this application, the specific surface area of ​​the negative electrode material 1 is controlled at 5-10 μm. 2 Within the range of / g, the capacity of the negative electrode material 1 can be increased while ensuring the improvement of the initial coulombic efficiency of the battery.

[0140] In one embodiment, the specific surface area of ​​the negative electrode material 1 is 5-9 m². 2 / g.

[0141] In one embodiment, the specific surface area of ​​the negative electrode material 1 is 5-8 m². 2 / g.

[0142] In one embodiment, the specific surface area of ​​the negative electrode material 1 is 5-7 m². 2 / g.

[0143] In one embodiment, the specific surface area of ​​the negative electrode material 1 is 5-6 m². 2 / g.

[0144] In one embodiment, the specific surface area of ​​the negative electrode material 1 is 6-10 m². 2 / g.

[0145] In one embodiment, the specific surface area of ​​the negative electrode material 1 is 7-10 m². 2 / g.

[0146] In one embodiment, the specific surface area of ​​the negative electrode material 1 is 8-10 m². 2 / g.

[0147] In one embodiment, the specific surface area of ​​the negative electrode material 1 is 9-10 m². 2 / g.

[0148] In one embodiment, the specific surface area of ​​the negative electrode material 1 is 6-9 m². 2 / g.

[0149] In one embodiment, the specific surface area of ​​the negative electrode material 1 is 7-8 m². 2 / g.

[0150] In one embodiment, the specific surface area of ​​the negative electrode material 1 is 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.4, or 10 m². 2 / g.

[0151] In one embodiment, the ratio of hydrogen atoms to the total number of atoms in the negative electrode material 1 is less than or equal to 4%. The hydrogen atom content in the negative electrode material 1 is related to the degree of carbonization of the negative electrode material 1. When the ratio of hydrogen atoms to the total number of atoms in the negative electrode material 1 is too high, the carbonization of the negative electrode material 1 is insufficient, meaning that the pyrolysis of the raw materials is incomplete during the preparation of the negative electrode material 1, resulting in a decrease in the capacity of the negative electrode material 1. In this application, controlling the ratio of hydrogen atoms to the total number of atoms in the negative electrode material 1 to be no higher than 4% can improve the capacity of the negative electrode material 1.

[0152] In one embodiment, the ratio of the number of hydrogen atoms in the negative electrode material 1 to the total number of atoms in the negative electrode material is less than or equal to 3%.

[0153] In one embodiment, the ratio of the number of hydrogen atoms in the negative electrode material 1 to the total number of atoms in the negative electrode material is less than or equal to 2%.

[0154] In one embodiment, the ratio of the number of hydrogen atoms in the negative electrode material 1 to the total number of atoms in the negative electrode material is less than or equal to 1%.

[0155] In one embodiment, the negative electrode material 1 does not contain hydrogen atoms.

[0156] In one embodiment, the carbon-based material core 1 has pores. Setting the carbon-based material core 1 as a porous structure facilitates the insertion and extraction of sodium ions, increases the migration rate of sodium ions, and improves the specific capacity of the negative electrode material 1.

[0157] In one embodiment, the porosity of the carbon-based material core 10 is 10-70%. If the porosity of the carbon-based material core 10 is too high, its strength is low, and it is prone to collapse. If the porosity of the carbon-based material core 10 is too low, the effect of creating pores on improving the migration rate of sodium ions is not significant. In this application, controlling the porosity of the carbon-based material core 10 within the range of 10-70% not only ensures that the carbon-based material core 10 has high structural strength but also effectively improves the migration rate of sodium ions.

[0158] In one embodiment, the porosity of the carbon-based material core 10 is 10-60%.

[0159] In one embodiment, the porosity of the carbon-based material core 10 is 10-70%.

[0160] In one embodiment, the porosity of the carbon-based material core 10 is 10-50%.

[0161] In one embodiment, the porosity of the carbon-based material core 10 is 10-40%.

[0162] In one embodiment, the porosity of the carbon-based material core 10 is 10-20%.

[0163] In one embodiment, the porosity of the carbon-based material core 10 is 15-40%.

[0164] In one embodiment, the porosity of the carbon-based material core 10 is 10%, 15%, 20%, 25%, 30%, 36%, 40%, 45%, 50%, 55%, 60%, 65%, or 70%.

[0165] This application provides a method for preparing a negative electrode material 1. The method for preparing the negative electrode material 1 includes steps S1, S2, S3 and S4, and the specific steps are as follows:

[0166] Step S1: Heat the non-metallic source at a first temperature;

[0167] Step S2: Heat the carbon-based material core 10 at a second temperature;

[0168] Step S3: A protective gas is introduced to deliver the heated non-metallic source to the surface of the carbon-based material core 10 and solid I is obtained after deposition on the surface of the carbon-based material core 10;

[0169] Step S4: Heat solid I at a third temperature to obtain negative electrode material 1.

[0170] Heating the non-metallic source causes it to decompose into small molecules, which include non-metallic elements. These small molecules are easily transported by a protective gas to the surface of the carbon-based material core 10 and deposited there, forming a carbon coating layer 20 rich in non-metallic elements. Heating the carbon-based material core 10 ensures that the small molecules formed by the decomposition of the non-metallic source are uniformly deposited on its surface. Due to the concentration difference of non-metallic elements between the carbon-based material core 10 and the carbon coating layer 20 in solid I, non-metallic elements can diffuse from the carbon coating layer 20 to the carbon-based material core 10. Heating solid I accelerates this diffusion rate. The portion of non-metallic elements that diffuses into the carbon-based material core 10 is the first non-metallic element 11, while the portion remaining in the carbon coating layer 20 is the second non-metallic element 21. At this time, in the obtained negative electrode material 1, from the core of the carbon-based material core 10 to the outer surface of the carbon-based material core 10, the content of the first non-metallic element 11 gradually increases, that is, from the outer surface of the carbon-based material core 10 to the core of the carbon-based material core 10, the distribution density of the first non-metallic element 11 gradually decreases.

[0171] In this application, small molecules formed by the pyrolysis of a non-metallic source are coated onto the outer surface of a carbon-based material core 10 via vapor deposition. The resulting carbon coating layer 20 reduces the surface area of ​​the negative electrode material 1 and simultaneously enriches the non-metallic elements in the specific surface area of ​​the carbon-based material core 10, allowing for the control of the distribution density of non-metallic elements in subsequent steps. If the non-metallic source and the carbon-based material core 10 are directly mixed and sintered, not only will the specific surface area of ​​the negative electrode material 1 increase, but the distribution of non-metallic elements cannot be controlled.

[0172] It is worth noting that the negative electrode material is not prepared strictly in the order of steps S1, S2, S3 and S4. In this application, the preparation steps can be interchanged or performed simultaneously.

[0173] In one embodiment, the carbon-based material core 10 is a hard carbon core, and the preparation method of the negative electrode material 1 includes steps S1-1, S2-1, S3-1 and S4-1, the specific steps of which are as follows:

[0174] Step S1-1: Heat the non-metallic source at a first temperature;

[0175] Step S2-1: Heat the hard carbon core at a second temperature;

[0176] Step S3-1: A protective gas is introduced to deliver the heated non-metallic source to the surface of the hard carbon core and solid I is obtained after deposition on the surface of the hard carbon core;

[0177] Step S4-1: Heat solid I at a third temperature to obtain negative electrode material 1.

[0178] In one embodiment, the protective gas includes at least one of nitrogen, helium, neon, argon, krypton, xenon, and radon.

[0179] In one embodiment, the method for preparing the negative electrode material further includes: heating a carbon source to obtain a carbon-based material core 10. The carbon source includes at least one of phenolic resin, epoxy resin, polyfurfuryl alcohol, asphalt, cellulose, lignin, and starch.

[0180] In one embodiment, the first temperature is greater than the second temperature and less than or equal to the third temperature. The higher first temperature in step S1, which heats the non-metallic source, facilitates its complete decomposition into small molecules. The lower second temperature in step S2, which heats the carbon-based material core 10, allows the small molecules formed from the decomposition of the non-metallic source to be readily deposited on the outer surface of the carbon-based material core 10. Increasing the third temperature of the heated solid I in step S4 accelerates the diffusion of non-metallic elements into the carbon-based material core 10. Simultaneously, the higher temperature also increases the bonding force between the carbon coating layer 20 and the carbon-based material core 10.

[0181] In one embodiment, the first temperature is 500-1000°C. Heating the non-metallic source at 500-1000°C facilitates its complete decomposition. In one embodiment, the first temperature can be 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, or 1000°C.

[0182] In one embodiment, the second temperature is 100-300°C. If the second temperature is higher than 300°C, the small molecules formed by the cracking of the non-metallic source are difficult to deposit. If the second temperature is lower than 100°C, the small molecules formed by the cracking of the non-metallic source are deposited too quickly, resulting in uneven coating of the carbon coating layer 20. In this application, setting the second temperature within the range of 100-300°C ensures that the small molecules formed by the cracking of the non-metallic source can be deposited smoothly and uniformly on the outer surface of the carbon-based material core 10.

[0183] In one embodiment, the second temperature is 100, 150, 200, 250 or 300°C.

[0184] In one embodiment, the third temperature is 700-1200°C. Heating solid I at 700-1200°C can accelerate the diffusion of non-metallic elements into the carbon-based material core 10. At the same time, the higher temperature can also increase the bonding force between the carbon coating layer 20 and the carbon-based material core 10.

[0185] In one embodiment, the heating rate of the non-metallic source is 5-20 °C / min. In another embodiment, the heating rate of the non-metallic source is 5, 6, 8, 10, 12, 14, 16, 18, or 20 °C / min.

[0186] In one embodiment, the heating time of the non-metallic source at the first temperature is 15-60 minutes. That is, after the temperature of the non-metallic source is heated to the first temperature, it is kept at the first temperature for another 15-60 minutes to allow the non-metallic source to fully decompose. In one embodiment, the heating time of the non-metallic source at the first temperature is 15, 20, 25, 30, 35, 40, 45, 50, 55, or 60 minutes.

[0187] In one embodiment, the heating rate of the carbon-based material core 10 is 5-20°C / min. In another embodiment, the heating rate of the carbon-based material core 10 is 5, 6, 8, 10, 12, 14, 16, 18, or 20°C / min.

[0188] In one embodiment, the flow rate of the protective gas is 10-100 mL / min. A flow rate that is too high or too low is detrimental to the deposition of small molecules formed by the pyrolysis of the non-metallic source. In this application, setting the flow rate of the protective gas within the range of 10-100 mL / min ensures that the small molecules formed by the pyrolysis of the non-metallic source can be deposited smoothly and uniformly on the outer surface of the carbon-based material core 10.

[0189] In one embodiment, the flow rate of the protective gas is 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100 mL / min.

[0190] In one embodiment, the protective gas is introduced for 15-60 minutes. That is, the deposition time for small molecules formed by the cracking of the non-metallic source is 15-60 minutes. The thickness of the carbon coating layer 20 in the negative electrode material 1 is controlled by adjusting the protective gas introduction time according to product requirements. In one embodiment, the protective gas introduction time is 15, 20, 25, 30, 35, 40, 45, 50, 55, or 60 minutes.

[0191] In one embodiment, after the carbon-based material core 10 is heated to a second temperature, a protective gas is introduced, and the carbon-based material core 10 is kept at the second temperature for 15-60 minutes after the protective gas is introduced.

[0192] In one embodiment, the heating rate of solid I is 5-20 °C / min. In another embodiment, the heating rate of solid I is 5, 6, 8, 10, 12, 14, 16, 18, or 20 °C / min.

[0193] In one embodiment, the heating time of solid I at the third temperature is 1-4 hours. In another embodiment, the heating time of solid I at the third temperature is 1, 1.5, 2, 2.5, 3, 3.5, or 4 hours.

[0194] In one embodiment, the non-metallic source is a nitrogen source, which includes at least one of benzimidazole, melamine, urea, pyrrole, polypyrrole, pyrimidine, and aminopyrimidine. These nitrogen sources—benzimidazole, melamine, urea, pyrrole, polypyrrole, pyrimidine, and aminopyrimidine—are easily carbonized and activated to form a nitrogen-doped carbon coating layer 20 loaded on the surface of the carbon-based material core 10. Furthermore, some of the nitrogen generated during carbonization and decomposition easily penetrates into the interior of the carbon-based material core 10, forming a gradient nitrogen-doped carbon-based material core 10. Simultaneously, nitrogen loss is low and utilization is high, making it suitable for industrial development.

[0195] In one embodiment, the mass fraction of nitrogen atoms in the nitrogen source is 13%-67%. Selecting a nitrogen source with a high nitrogen content facilitates the preparation of nitrogen-doped anode material 1.

[0196] In one embodiment, the mass fraction of nitrogen atoms in the nitrogen source is 10%-65%.

[0197] In one embodiment, the mass fraction of nitrogen atoms in the nitrogen source is 20%-60%.

[0198] In one embodiment, the mass fraction of nitrogen atoms in the nitrogen source is 30%-50%.

[0199] In one embodiment, the mass fraction of nitrogen atoms in the nitrogen source is 40%-67%.

[0200] In one embodiment, the mass fraction of nitrogen atoms in the nitrogen source is 50%-67%.

[0201] In one embodiment, the method for preparing the negative electrode material 1 includes the following steps:

[0202] The asphalt was sintered at 1200℃ for 4 hours and then ball-milled to obtain the carbon-based material core 10.

[0203] Weigh 10g of carbon-based material core 10 and place it at the far right end of the three-zone tube furnace, and weigh 0.04mol of melamine and place it at the far left end of the three-zone tube furnace.

[0204] Argon gas was introduced at a flow rate of 200 ml / min for 10 minutes to purge the air from the furnace, and then the gas supply was stopped.

[0205] The left-end tube furnace is heated to 900℃ at a heating rate of 10℃ / min and held for 30min. When the left end begins to hold, the right-end tube furnace is heated to 300℃ at the same rate. After reaching the set temperature, argon gas is introduced at a flow rate of 50mL / min and held for 30min.

[0206] The right-end tube furnace was then heated to 900℃ at a rate of 10℃ / min and held for 2 hours to obtain 11.2g of negative electrode material 1.

[0207] This application provides a negative electrode sheet, which includes the negative electrode material 1 as described above or the negative electrode material 1 prepared by the preparation method of the negative electrode material 1 as described above.

[0208] This application provides a secondary battery, which includes a positive electrode, a separator, and a negative electrode as described above.

[0209] In one embodiment, the secondary battery is a sodium-ion battery with an initial coulombic efficiency greater than or equal to 85%, a specific capacity greater than or equal to 280 mAh / g at 0.1C rate, and a specific capacity greater than or equal to 210 mAh / g at 10C rate.

[0210] To illustrate the beneficial effects of the method of this application, the following embodiments and comparative examples are also provided.

[0211] Example 1

[0212] This embodiment provides a negative electrode material, including a hard carbon core and a carbon coating layer. Both the hard carbon core and the carbon coating layer are doped with the non-metallic element N, and the non-metallic element N is uniformly doped in the hard carbon core and the carbon coating layer.

[0213] The preparation method of the negative electrode material includes the following steps: (1) Sintering asphalt at 1200℃ for 4h and then ball milling it to obtain hard carbon material;

[0214] (2) Weigh 10g of the above-mentioned hard carbon material and mix it evenly with 0.005mol of melamine;

[0215] (3) The mixture was sintered at 900°C for 4 hours to obtain the above-mentioned negative electrode material.

[0216] The negative electrode material prepared by the above method has a total N-doped atom count ratio of 3.7% to the total number of atoms in the negative electrode material, and a specific surface area of ​​37 m². 2 / g, with a particle size D50 of 11μm.

[0217] Example 2

[0218] This embodiment provides a negative electrode material, including a hard carbon core and a carbon coating layer. Both the hard carbon core and the carbon coating layer are doped with the non-metallic element N. In the hard carbon core, the distribution density of the doped element N gradually decreases from the outer surface of the hard carbon core to the core of the hard carbon core.

[0219] The preparation method of this negative electrode material includes the following steps:

[0220] (1) Hard carbon material was obtained by sintering asphalt at 1200℃ for 4 hours and then ball milling it.

[0221] (2) Weigh 10g of the above hard carbon material and place it at the rightmost end of the three-zone tube furnace, and weigh 0.005mol of melamine and place it at the leftmost end of the three-zone tube furnace.

[0222] (3) Introduce argon gas at a flow rate of 200 ml / min for 10 min to purge the air from the furnace, and then stop the gas flow;

[0223] (4) The left end tube furnace is heated to 900℃ at a heating rate of 10℃ / min and held for 30min. When the left end starts to be held, the right end tube furnace is heated to 300℃ at the same rate. After reaching the set temperature, argon gas is introduced at a flow rate of 50mL / min and held for 30min.

[0224] (5) The right end of the tubular furnace was then heated to 900°C at a heating rate of 10°C / min and held for 2 hours to obtain the above-mentioned negative electrode material.

[0225] The negative electrode material prepared by the above method has a total N-doped atom count ratio of 2% to the total number of atoms in the negative electrode material, and a specific surface area of ​​6 m². 2 / g, with a particle size D50 of 7μm.

[0226] Example 3

[0227] This embodiment provides a negative electrode material, including a hard carbon core and a carbon coating layer. Both the hard carbon core and the carbon coating layer are doped with the non-metallic element N. In the hard carbon core, the distribution density of the doped element N gradually decreases from the outer surface of the hard carbon core to the core of the hard carbon core.

[0228] The preparation method of this negative electrode material includes the following steps:

[0229] (1) Hard carbon material was obtained by sintering asphalt at 1200℃ for 4 hours and then ball milling it.

[0230] (2) Weigh 10g of the above hard carbon material and place it at the rightmost end of the three-zone tube furnace, and weigh 0.01mol of melamine and place it at the leftmost end of the three-zone tube furnace.

[0231] (3) Introduce argon gas at a flow rate of 200 ml / min for 10 min to purge the air from the furnace, and then stop the gas flow;

[0232] (4) The left end tube furnace is heated to 900℃ at a heating rate of 10℃ / min and held for 30min. When the left end starts to be held, the right end tube furnace is heated to 300℃ at the same rate. After reaching the set temperature, argon gas is introduced at a flow rate of 50mL / min and held for 30min.

[0233] (5) Subsequently, the right-end tube furnace was heated to 900°C at a heating rate of 10°C / min and held for 2 hours to obtain the above-mentioned negative electrode material. The negative electrode material provided in this embodiment has a basically the same structure as the negative electrode material provided in Example 2, the difference being that: the ratio of the total number of N-doped atoms to the number of atoms in the negative electrode material is 8%, the specific surface area is 6 m2 / g, and the particle size D50 is 7 μm.

[0234] Example 4

[0235] This embodiment provides a negative electrode material, including a hard carbon core and a carbon coating layer. Both the hard carbon core and the carbon coating layer are doped with the non-metallic element N. In the hard carbon core, the distribution density of the doped element N gradually decreases from the outer surface of the hard carbon core to the core of the hard carbon core.

[0236] The preparation method of this negative electrode material includes the following steps:

[0237] (1) Hard carbon material was obtained by sintering asphalt at 1200℃ for 4 hours and then ball milling it.

[0238] (2) Weigh 10g of the above hard carbon material and place it at the rightmost end of the three-zone tube furnace, and weigh 0.04mol of melamine and place it at the leftmost end of the three-zone tube furnace.

[0239] (3) Introduce argon gas at a flow rate of 200 ml / min for 10 min to purge the air from the furnace, and then stop the gas flow;

[0240] (4) The left end tube furnace is heated to 900℃ at a heating rate of 10℃ / min and held for 30min. When the left end starts to be held, the right end tube furnace is heated to 300℃ at the same rate. After reaching the set temperature, argon gas is introduced at a flow rate of 50mL / min and held for 30min.

[0241] (5) The right end of the tubular furnace was then heated to 900°C at a heating rate of 10°C / min and held for 2 hours to obtain the above-mentioned negative electrode material.

[0242] The negative electrode material provided in this embodiment has a basically the same structure as the negative electrode material provided in Example 2. The difference is that the ratio of the total number of N-doped atoms to the number of atoms in the negative electrode material is 15%, the specific surface area is 8 m2 / g, and the particle size D50 is 8 μm.

[0243] Example 5

[0244] This embodiment provides a negative electrode material, including a hard carbon core and a carbon coating layer. Both the hard carbon core and the carbon coating layer are doped with the non-metallic element P. In the hard carbon core, the distribution density of the doped element P gradually decreases from the outer surface of the hard carbon core to the core of the hard carbon core.

[0245] The preparation method of this negative electrode material includes the following steps:

[0246] (1) Hard carbon material was obtained by sintering asphalt at 1200℃ for 4 hours and then ball milling it.

[0247] (2) Weigh 10g of the above hard carbon material and place it at the rightmost end of the three-zone tube furnace, and weigh 0.02mol of 3-p-phenylmethylphosphine and place it at the leftmost end of the three-zone tube furnace.

[0248] (3) Introduce argon gas at a flow rate of 200 ml / min for 10 min to purge the air from the furnace, and then stop the gas flow;

[0249] (4) The left end tube furnace is heated to 900℃ at a heating rate of 10℃ / min and held for 30min. When the left end starts to be held, the right end tube furnace is heated to 300℃ at the same rate. After reaching the set temperature, argon gas is introduced at a flow rate of 50mL / min and held for 30min.

[0250] (5) The right end of the tubular furnace was then heated to 900°C at a heating rate of 10°C / min and held for 2 hours to obtain the above-mentioned negative electrode material.

[0251] The negative electrode material provided in this embodiment has a basically the same structure as the negative electrode material provided in embodiment 2. The difference is that the non-metallic element doped in this embodiment is P, the ratio of the total number of P atoms to the number of atoms in the negative electrode material is 1.7%, the specific surface area is 6 m2 / g, and the particle size D50 is 8 μm.

[0252] Comparative Example 1

[0253] The difference between the negative electrode material provided in this comparative example and the negative electrode material provided in Example 1 is that neither the hard carbon core nor the carbon coating layer is doped with non-metallic elements.

[0254] The negative electrode materials provided in Examples 1 to 5 and the negative electrode material provided in Comparative Example 1 were assembled into negative electrode sheets and sodium-ion batteries respectively according to the following methods:

[0255] Negative electrode sheet: The negative electrode material is mixed with carboxymethyl cellulose (CMC), SBR and SP in a mass ratio of 95.8:1.2:2:1 and ball-milled to obtain a negative electrode slurry. The negative electrode slurry is coated on the surface of copper foil and vacuum dried at 110°C overnight to obtain a negative electrode sheet.

[0256] Positive electrode: Sodium metal sheet;

[0257] Electrolyte: Ethyl carbonate and methyl ethyl carbonate are mixed in a volume ratio of 3:7, and NaPF6 is added to form an electrolyte with a NaPF6 concentration of 1 mol / L.

[0258] Diaphragm: Polypropylene microporous diaphragm;

[0259] Sodium-ion battery assembly: Assemble button sodium-ion batteries in an inert atmosphere glove box according to the assembly sequence of negative electrode plate - separator - electrolyte - positive electrode plate.

[0260] The electrochemical performance of each sodium-ion battery assembled in the above sodium-ion battery examples was tested as shown in Table 1. The test results are shown in Table 1 below:

[0261] Table 1 Performance Test Results

[0262]

[0263] Comparing Example 1 and Comparative Example 1 in Table 1, it can be seen that after coating and doping, the first-stage efficiency and capacity of the anode material are improved. This is evident from... Figure 2 The first charge-discharge curves clearly show that the rate performance is not significantly different. However, with similar nitrogen doping levels, gradient doping is more beneficial to improving the rate performance of the anode material than uniform doping. This can be seen by comparing Examples 1-2, which is precisely the improvement brought about by the ion transport channels formed by gradient doping.

[0264] As can be seen from the test results of Examples 2-4 in Table 1, the specific capacity of the anode material increases with the increase of nitrogen doping content. This indicates that nitrogen doping provides sodium ion binding sites, thereby improving the capacity. However, Example 3 has the highest initial efficiency and the best capacity retention rate at 10C high rate, indicating that the nitrogen doping content needs to be kept at an appropriate level.

[0265] Furthermore, comparing Examples 2 and 5 reveals that, at similar doping levels, the capacity improvement brought by phosphorus doping and nitrogen doping is not significantly different, but the rate performance is slightly better with phosphorus doping. Figure 3As can be seen, with the increase of charge / discharge rate, the capacity retention rate of Example 5 is higher than that of Example 2. This may be due to the fact that the radius of phosphorus atoms is larger than that of nitrogen atoms, which increases the width of sodium ion transport channels and thus improves their transport efficiency. Taking Example 3 as another example, the negative electrode material prepared in Example 3 was etched with X-rays, and its energy spectrum was analyzed. Figure 4 As can be seen, the number of N atoms gradually decreases with increasing etching depth, indicating that N atoms do indeed exhibit gradient doping in the anode material.

[0266] The foregoing has provided a detailed description of the negative electrode material, its preparation method, and its application provided in the embodiments of this application. Specific examples have been used to illustrate the principles and embodiments of this application. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of this application. At the same time, for those skilled in the art, there will be changes in specific embodiments and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A negative electrode material, characterized in that, The negative electrode material is used in sodium-ion batteries, and the negative electrode material comprises: The core of the carbon-based material is doped with the first non-metallic element; A carbon coating layer is applied to the outer surface of the carbon-based material core, and the carbon coating layer is doped with a second non-metallic element. From the outer surface of the carbon-based material core to the core of the carbon-based material core, the distribution density of the first non-metallic element gradually decreases; from the outer surface of the carbon coating layer to the inner surface of the carbon coating layer, the distribution density of the second non-metallic element gradually decreases.

2. The negative electrode material according to claim 1, characterized in that, The first nonmetallic element includes at least one of nitrogen, phosphorus, sulfur, and fluorine; and / or The second nonmetallic element includes at least one of nitrogen, phosphorus, sulfur, and fluorine.

3. The negative electrode material according to claim 2, characterized in that, The first nonmetallic element is the same as the second nonmetallic element; and / or, Both the first non-metallic element and the second non-metallic element are one of nitrogen, phosphorus, sulfur, and fluorine. From the outer surface of the carbon coating layer to the core of the carbon-based material core, the distribution density of one of the nitrogen, phosphorus, sulfur, and fluorine elements gradually decreases.

4. The negative electrode material according to claim 1, characterized in that, The ratio of the total number of atoms of the first non-metallic element and the second non-metallic element to the number of atoms in the negative electrode material is 2%-15%.

5. The negative electrode material according to claim 1, characterized in that, The particle size of the negative electrode material is 2-50 micrometers; and / or The particle size D50 of the negative electrode material is 5-10 micrometers; and / or The specific surface area of ​​the negative electrode material is 5-10 m². 2 / g; and / or The ratio of the number of hydrogen atoms in the negative electrode material to the total number of atoms in the negative electrode material is less than or equal to 4%; and / or The porosity of the carbon-based material core is 10-70%.

6. A method for preparing a negative electrode material, characterized in that, The preparation method is used to prepare the negative electrode material according to any one of claims 1-5, wherein the preparation method of the negative electrode material includes: The non-metallic source is heated at a first temperature; The carbon-based material core is heated at a second temperature; A heated non-metallic source is delivered to the surface of the carbon-based material core by introducing a protective gas, and solid I is obtained after deposition on the surface of the carbon-based material core; The solid I is heated at a third temperature to obtain the negative electrode material.

7. The method for preparing the negative electrode material according to claim 6, characterized in that, The first temperature is greater than the second temperature and less than or equal to the third temperature.

8. A negative electrode sheet, characterized in that, It includes the negative electrode material as described in any one of claims 1-5, or the negative electrode material prepared by the method described in claim 6 or 7.

9. A secondary battery, characterized in that, It includes a positive electrode, a separator, and a negative electrode as described in claim 8.

10. The secondary battery according to claim 9, characterized in that, The secondary battery is a sodium-ion battery, the initial coulombic efficiency of the sodium-ion battery is greater than or equal to 85%, the specific capacity of the sodium-ion battery at 0.1C rate is greater than or equal to 280mAh / g, and the specific capacity at 10C rate is greater than or equal to 210mAh / g.

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