Coated modified graphite negative electrode material, preparation method thereof, negative electrode and battery

Abstract

The invention discloses a coated modified graphite negative electrode material and a preparation method thereof, a negative electrode and a battery, the coated modified graphite negative electrode material comprises graphite particles and a nitrogen-doped carbon coating layer coating the surfaces of the graphite particles, the existence forms of nitrogen atoms in the nitrogen-doped carbon coating layer comprise heterocyclic nitrogen and graphite nitrogen, the heterocyclic nitrogen comprises one or two of pyridine nitrogen and pyrrole nitrogen; wherein in the nitrogen-doped carbon coating layer, the content of the heterocyclic nitrogen is gradually increased along the direction far away from the graphite particles; and / or, in the nitrogen-doped carbon coating layer, the content of graphite nitrogen is gradually reduced in the direction away from the graphite particles. The coated modified graphite negative electrode material disclosed by the invention has relatively good rate capability.

Description

Technical Field

[0001] This application relates to the field of new energy technology, and in particular to a coated modified graphite anode material, its preparation method, the anode, and the battery. Background Technology

[0002] Graphite anodes occupy the battery market due to their low charge / discharge plateau, good cycle stability, and low cost.

[0003] However, the rate performance of existing graphite anodes is poor and needs further improvement. Summary of the Invention

[0004] In view of this, this application provides a coated modified graphite anode material, its preparation method, anode, and battery.

[0005] This application provides a coated modified graphite anode material, comprising graphite particles and a nitrogen-doped carbon coating layer on the surface of the graphite particles. The nitrogen atoms in the nitrogen-doped carbon coating layer exist in heterocyclic nitrogen and graphitic nitrogen forms, wherein the heterocyclic nitrogen includes one or both of pyridine nitrogen and pyrrole nitrogen.

[0006] In the nitrogen-doped carbon coating layer, the content of heterocyclic nitrogen gradually increases along the direction away from the graphite particles; and / or

[0007] In the nitrogen-doped carbon coating layer, the content of graphite nitrogen gradually decreases along the direction away from the graphite particles.

[0008] Optionally, in some embodiments, the nitrogen-doped carbon coating layer has a porous structure in which the pore size gradually decreases along the direction away from the graphite particles; and / or

[0009] The density of the nitrogen-doped carbon coating gradually increases along the direction away from the graphite particles.

[0010] Optionally, in some embodiments, the nitrogen-doped carbon coating layer comprises a stacked inner layer and an outer layer, the inner layer being located between the graphite particles and the outer layer, the outer layer having a thickness of 10 nm, and the inner layer having a thickness of 10–90 nm, wherein:

[0011] The heterocyclic nitrogen content in the outer layer is higher than that in the inner layer, and the graphitic nitrogen content in the outer layer is lower than that in the inner layer; and / or

[0012] In the outer layer, the content of heterocyclic nitrogen is higher than the content of graphitic nitrogen; in the inner layer, the content of heterocyclic nitrogen is lower than the content of graphitic nitrogen; and / or

[0013] Based on an atomic percentage of 100% for all nitrogen atoms in the outer layer, the atomic percentage of heterocyclic nitrogen in the outer layer is 60-70 at%, and the atomic percentage of graphitic nitrogen is 30-40 at%; and / or

[0014] With the atomic percentage of all nitrogen atoms in the inner layer being 100%, the atomic percentage of heterocyclic nitrogen in the inner layer is 30-60 at, and the atomic percentage of graphite nitrogen is 40-70 at.

[0015] Optionally, in some embodiments, the thickness of the nitrogen-doped carbon coating layer is 20–100 nm; and / or

[0016] The D50 particle size of the graphite particles is 8–20 μm; and / or

[0017] The pore size range of the nitrogen-doped carbon coating is 2–20 nm.

[0018] Accordingly, this application also provides a method for preparing a coated modified graphite anode material, comprising the following steps:

[0019] Dopamine monomer was mixed with graphite particle dispersion and dried to obtain polydopamine-coated graphite particles.

[0020] The polydopamine-coated graphite particles are subjected to a first heat treatment and a second heat treatment in sequence, wherein the temperature of the second heat treatment is higher than that of the first heat treatment, to obtain a coated modified graphite anode material.

[0021] Optionally, in some embodiments, the first heat treatment includes: heating to a first temperature and then holding at that temperature for a first time, wherein:

[0022] The first temperature is 300℃~400℃; and / or

[0023] The first time is 0.5 to 1 hour; and / or

[0024] The heating rate to the first temperature is 0.5–2 °C / min.

[0025] Optionally, in some embodiments, the second heat treatment includes: a first sub-heat treatment, or a second sub-heat treatment, or sequentially performing a first sub-heat treatment and a second sub-heat treatment, wherein the temperature of the second sub-heat treatment is higher than the temperature of the first sub-heat treatment.

[0026] Optionally, in some embodiments, the first sub-heat treatment includes: heating to a second temperature and then holding at that temperature for a second time, wherein:

[0027] The second temperature is 500℃~700℃; and / or

[0028] The second time is 1 to 2 hours; and / or

[0029] The heating rate to the second temperature is 3-5 °C / min.

[0030] Optionally, in some embodiments, the second sub-heat treatment includes: heating to a third temperature and then holding at that temperature for a third time, wherein:

[0031] The third temperature is 800℃~900℃; and / or

[0032] The third time is 20–60 min; and / or

[0033] The heating rate to the third temperature is 5–10 °C / min.

[0034] Optionally, in some embodiments, the polydopamine-coated graphite particles are subjected to a first heat treatment and a second heat treatment in an inert atmosphere, wherein the inert gas in the inert atmosphere is one or more of nitrogen, helium, neon, argon, and xenon.

[0035] Optionally, in some embodiments, during the first heat treatment, the inert atmosphere further includes ammonia gas, wherein the volume content of the ammonia gas in the inert atmosphere ranges from 1% to 10%; and / or

[0036] The self-polymerization time is 12–36 hours; and / or

[0037] The dopamine monomer includes one or more of dopamine and dopamine hydrochloride; and / or

[0038] In the graphite particle dispersion, the solid-liquid ratio ranges from 1 g:(100 mL to 500 mL); and / or

[0039] The graphite particle dispersion comprises graphite particles and a buffer solution, wherein the buffer solution comprises Tris-HCl buffer; and / or

[0040] The mass ratio of graphite particles to dopamine hydrochloride in the graphite particle dispersion ranges from 1:(0.1 to 0.5).

[0041] Accordingly, embodiments of this application also provide a negative electrode, including the aforementioned coated modified graphite negative electrode material.

[0042] Accordingly, this application also provides a battery including the negative electrode.

[0043] The coated modified graphite anode material described in this application includes the graphite particles and the nitrogen-doped carbon coating layer. The nitrogen-doped carbon coating layer includes heterocyclic nitrogen and graphitic nitrogen, thus exhibiting better rate performance.

[0044] Furthermore, the modified graphite anode material described in this application includes the graphite particles and the nitrogen-doped carbon coating layer. In the nitrogen-doped carbon coating layer, the content of heterocyclic nitrogen gradually increases and the content of graphite nitrogen gradually decreases along the direction away from the graphite particles. Thus, the high graphite nitrogen content in the inner layer of the nitrogen-doped carbon coating layer (i.e., the side of the nitrogen-doped carbon coating layer closest to the graphite particles) is beneficial for improving electronic conductivity, and the enrichment of graphite nitrogen in the inner layer of the nitrogen-doped carbon coating layer allows more graphite nitrogen to directly contact / connect with the graphite particles, which is beneficial for reducing contact resistance. The heterocyclic nitrogen is a lithium-loving nitrogen, and the enrichment of heterocyclic nitrogen in the outer layer of the nitrogen-doped carbon coating layer (i.e., the side of the nitrogen-doped carbon coating layer away from the graphite particles) is beneficial for guiding uniform deposition. Furthermore, the gradual increase in the content of heterocyclic nitrogen along the direction away from the graphite particles in the modified graphite anode material can form a gradient of lithium affinity, which is beneficial for suppressing lithium dendrites, reducing polarization, and balancing ion transport and electronic conductivity. In this way, the coated modified graphite anode material can have high electrical properties such as high cycle performance and rate performance.

[0045] Furthermore, in the nitrogen-doped carbon coating layer of the modified graphite anode material described in this application, the pore size in the porous structure gradually decreases along the direction away from the graphite particles. Thus, the larger pore size in the inner layer of the nitrogen-doped carbon coating layer (i.e., the side of the nitrogen-doped carbon coating layer closer to the graphite particles) is beneficial for shortening the diffusion path of Li+, while the smaller pore size in the outer layer of the nitrogen-doped carbon coating layer (i.e., the side of the nitrogen-doped carbon coating layer away from the graphite particles) is beneficial for providing a uniform ion flow. Furthermore, the gradient pore size distribution is beneficial for forming "high-speed ion channels." The large pores in the inner layer (larger pore size) are beneficial for accelerating bulk phase transport, while the small pores in the outer layer (smaller pore size) are beneficial for optimizing the interface distribution. Moreover, the large pores in the inner layer are beneficial for buffering volume expansion, while the small pores in the outer layer can make the outer layer more compact, thereby helping to suppress electrolyte side reactions. Furthermore, if the pore size in the nitrogen-doped carbon coating is uniform (similar or the same) inside and out, the Li+ diffusion path will be long and easily blocked. Uniform pores are prone to stress concentration during fast charging, which can lead to cracking of the nitrogen-doped carbon coating. However, the pore size gradient distribution structure in the nitrogen-doped carbon coating described in this application is beneficial to dispersing mechanical stress, thereby helping to avoid local failure. Attached Figure Description

[0046] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0047] Figure 1 This is a flowchart of a method for preparing a coated modified graphite anode material provided in an embodiment of this application. Detailed Implementation

[0048] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application. Furthermore, it should be understood that the specific embodiments described herein are only for illustration and explanation of this application and are not intended to limit this application.

[0049] In this application, unless otherwise stated, directional terms such as "upper" and "lower" generally refer to the upper and lower positions of the device in its actual use or operating state, specifically the drawing directions in the accompanying drawings; while "inner" and "outer" refer to the outline of the device. Furthermore, in the description of this application, the term "comprising" means "including but not limited to". The terms first, second, third, etc., are used merely as illustrative purposes and do not impose numerical requirements or establish a numerical order.

[0050] In this application, "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural.

[0051] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or multiple items. For example, "at least one of a, b, or c," or "at least one of a, b, and c," can both mean: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple.

[0052] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, whichever applies. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the range referred to.

[0053] The technical solution of this application is as follows:

[0054] In a first aspect, embodiments of this application provide a coated modified graphite anode material, comprising graphite particles and a nitrogen-doped carbon coating layer covering the surface of the graphite particles. The nitrogen atoms in the nitrogen-doped carbon coating layer exist in the form of heterocyclic nitrogen and graphitic nitrogen, and the heterocyclic nitrogen includes one or both of pyridine nitrogen and pyrrole nitrogen.

[0055] It should be noted that, in this application, heterocyclic nitrogen refers to a nitrogen atom bonded to two carbon atoms to form a multi-membered ring structure, similar to the structure of a heterocyclic molecule. For example, pyridine nitrogen refers to a nitrogen atom bonded to two carbon atoms to form a six-membered ring structure, similar to the structure of a pyridine molecule; pyrrole nitrogen refers to a nitrogen atom bonded to two carbon atoms to form a five-membered ring structure, similar to the structure of a pyrrole molecule.

[0056] In some embodiments, in the nitrogen-doped carbon coating layer, the content of heterocyclic nitrogen gradually increases along the direction away from the graphite particles; for example, the content of pyridine nitrogen and / or pyrrole nitrogen gradually increases. In other words, in the nitrogen-doped carbon coating layer, the closer to the graphite particles, the lower the content of heterocyclic nitrogen; for example, the lower the content of pyridine nitrogen and / or pyrrole nitrogen.

[0057] In some embodiments, the content of graphitic nitrogen in the nitrogen-doped carbon coating layer gradually decreases along the direction away from the graphite particles. In other words, the content of graphitic nitrogen is higher closer to the graphite particles in the nitrogen-doped carbon coating layer.

[0058] In some embodiments, the nitrogen-doped carbon coating layer has a porous structure in which the pore size gradually decreases along the direction away from the graphite particles. In other words, the closer to the graphite particles in the nitrogen-doped carbon coating layer, the larger the pore size. This allows the porosity of the nitrogen-doped carbon coating layer to gradually increase along the direction away from the graphite particles, which is beneficial for gradually increasing the density of the nitrogen-doped carbon coating layer along the same direction, making the density of the outer layer of the nitrogen-doped carbon coating layer higher than that of the inner layer. Thus, the large pores in the inner layer of the nitrogen-doped carbon coating layer help buffer volume expansion, while the dense outer layer helps suppress electrolyte side reactions, thereby improving the chemical stability of the coated modified graphite anode material.

[0059] The coated modified graphite anode material described in this application includes the graphite particles and the nitrogen-doped carbon coating layer. The nitrogen-doped carbon coating layer includes heterocyclic nitrogen and graphitic nitrogen, thus exhibiting better rate performance.

[0060] The modified graphite anode material described in this application includes graphite particles and a nitrogen-doped carbon coating layer. In the nitrogen-doped carbon coating layer, the content of heterocyclic nitrogen gradually increases and the content of graphite nitrogen gradually decreases along the direction away from the graphite particles. Thus, the high graphite nitrogen content in the inner layer of the nitrogen-doped carbon coating layer (i.e., the side of the nitrogen-doped carbon coating layer closest to the graphite particles) is beneficial for improving electronic conductivity, and the enrichment of graphite nitrogen in the inner layer allows more graphite nitrogen to directly contact / connect with the graphite particles, which helps reduce contact resistance. The heterocyclic nitrogen is a lithium-loving nitrogen, and its enrichment in the outer layer of the nitrogen-doped carbon coating layer (i.e., the side of the nitrogen-doped carbon coating layer away from the graphite particles) helps guide uniform deposition. Furthermore, the gradual increase in the content of heterocyclic nitrogen along the direction away from the graphite particles in the modified graphite anode material can form a gradient of lithium affinity, which helps suppress lithium dendrites, reduce polarization, and balance ion transport and electronic conductivity. In this way, the coated modified graphite anode material can have high electrical properties such as high cycle performance and rate performance.

[0061] Furthermore, in the nitrogen-doped carbon coating layer of the modified graphite anode material described in this application, the pore size in the porous structure gradually decreases along the direction away from the graphite particles. Thus, the larger pore size in the inner layer of the nitrogen-doped carbon coating layer (i.e., the side of the nitrogen-doped carbon coating layer closer to the graphite particles) is beneficial for shortening the diffusion path of Li+, while the smaller pore size in the outer layer of the nitrogen-doped carbon coating layer (i.e., the side of the nitrogen-doped carbon coating layer away from the graphite particles) is beneficial for providing a uniform ion flow. Furthermore, the gradient pore size distribution is beneficial for forming "high-speed ion channels." The large pores in the inner layer (larger pore size) are beneficial for accelerating bulk phase transport, while the small pores in the outer layer (smaller pore size) are beneficial for optimizing the interface distribution. Moreover, the large pores in the inner layer are beneficial for buffering volume expansion, while the small pores in the outer layer can make the outer layer more compact, thereby helping to suppress electrolyte side reactions. Furthermore, if the pore size in the nitrogen-doped carbon coating is uniform (similar or the same) inside and out, the Li+ diffusion path will be long and easily blocked. Uniform pores are prone to stress concentration during fast charging, which can lead to cracking of the nitrogen-doped carbon coating. However, the pore size gradient distribution structure in the nitrogen-doped carbon coating described in this application is beneficial to dispersing mechanical stress, thereby helping to avoid local failure.

[0062] In some embodiments, the thickness of the nitrogen-doped carbon coating layer is 20–100 nm, for example, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, and any value or range between any two of the stated values.

[0063] The nitrogen-doped carbon coating has an inner surface bonded to the graphite particles and an outer surface located away from the graphite particles and opposite to the inner surface.

[0064] The nitrogen-doped carbon coating layer comprises a stacked inner layer and an outer layer, wherein the inner layer is located between the graphite particles and the outer layer, the outer layer has a thickness of 10 nm, and the inner layer has a thickness of 10–90 nm.

[0065] In some embodiments, the content of heterocyclic nitrogen in the outer layer is higher than the content of heterocyclic nitrogen in the inner layer, and the content of graphitic nitrogen in the outer layer is lower than the content of graphitic nitrogen in the inner layer.

[0066] In some embodiments, the content of heterocyclic nitrogen in the outer layer is higher than the content of graphitic nitrogen; and the content of heterocyclic nitrogen in the inner layer is lower than the content of graphitic nitrogen.

[0067] In some embodiments, with the atomic percentage of all nitrogen atoms in the outer layer being 100%, the atomic percentage of the heterocyclic nitrogen in the outer layer is 60-70 at%, for example, 60at%, 61at%, 62at%, 63at%, 64at%, 65at%, 66at%, 67at%, 68at%, 69at%, 70at%, and values ​​or ranges between any two of these values; the atomic percentage of the graphite nitrogen is 30-40at%, for example, 30at%, 31at%, 32at%, 33at%, 34at%, 35at%, 36at%, 37at%, 38at%, 39at%, 40at%, and values ​​or ranges between any two of these values.

[0068] In some embodiments, with the atomic percentage of all nitrogen atoms in the inner layer being 100%, the atomic percentage of heterocyclic nitrogen in the inner layer is 30-60 at%, for example, 30 at%, 40 at%, 50 at%, 60 at%, and any value or range between any two of these values; the atomic percentage of graphitic nitrogen is 40-70 at%, for example, 40 at%, 50 at%, 60 at%, 70 at%, and any value or range between any two of these values.

[0069] In at least some embodiments, with the atomic percentage of all nitrogen atoms in the inner layer being 100%, the atomic percentage of graphite nitrogen in the inner layer is 50-70 at, and the atomic percentage of heterocyclic nitrogen is 30-50 at.

[0070] In some embodiments, the pore size range of the nitrogen-doped carbon coating layer is 2 to 20 nm, for example, 2 nm, 4 nm, 5 nm, 10 nm, 12 nm, 14 nm, 15 nm, 16 nm, 18 nm, 20 nm, and any two of the above values ​​or the range thereof.

[0071] In some embodiments, the D50 particle size of the graphite particles is 8 to 20 μm, for example, 8 μm, 10 μm, 12 μm, 14 μm, 15 μm, 16 μm, 18 μm, 20 μm, and any value or range between any two of the stated values.

[0072] Secondly, please refer to Figure 1 This application also provides a method for preparing a coated modified graphite anode material, comprising the following steps:

[0073] Step S11: Mix dopamine monomer with graphite particle dispersion to allow dopamine monomer to self-polymerize, then dry to obtain polydopamine-coated graphite particles.

[0074] Step S12: The polydopamine-coated graphite particles are subjected to a first heat treatment and a second heat treatment in sequence. The temperature of the second heat treatment is higher than that of the first heat treatment, so as to perform gradient pyrolysis on the polydopamine-coated graphite particles to obtain the coated modified graphite anode material.

[0075] The polydopamine-coated graphite particles comprise graphite particles and a polydopamine coating layer covering the surface of the graphite particles. The polydopamine coating layer has an outer surface away from the graphite particles and an inner surface close to the graphite particles.

[0076] In the preparation method described in this application, the temperature of the second heat treatment is higher than that of the first heat treatment. This allows for gradient pyrolysis of the polydopamine-coated graphite particles. The first heat treatment decomposes the polydopamine near the outer surface of the polydopamine coating layer to form heterocyclic nitrogen. The second heat treatment decomposes the polydopamine near the graphite particles in the polydopamine coating layer to generate gas and form larger pores, and shrinks and densifies the region of the polydopamine coating layer away from the graphite particles. It also decomposes the polydopamine near the graphite particles in the polydopamine layer to form graphitic nitrogen. Through this gradient pyrolysis, the prepared nitrogen-doped carbon coating layer of the modified graphite anode material includes both heterocyclic nitrogen and graphitic nitrogen. The content of heterocyclic nitrogen gradually increases and the content of graphitic nitrogen gradually decreases along the direction away from the graphite particles, giving the nitrogen-doped carbon coating layer a porous structure, with the pore size gradually decreasing along the direction away from the graphite particles. This results in the prepared coated and modified graphite anode material having high electrical properties such as high cycle performance and rate performance.

[0077] The preparation method described in this application achieves nitrogen gradient and pore size gradient by combining dopamine self-polymerization with gradient pyrolysis, and the process is simple (single precursor) and does not require a template.

[0078] In step S11:

[0079] In some embodiments, the dopamine monomer includes one or more of dopamine and dopamine hydrochloride (DA).

[0080] In some embodiments, the graphite particle dispersion includes graphite particles and a buffer solution.

[0081] The graphite particles are as described above and will not be repeated here.

[0082] In some embodiments, the buffer solution includes, but is not limited to, Tris-HCl buffer solution.

[0083] In some embodiments, the solid-liquid ratio in the graphite particle dispersion ranges from 1g:(100mL to 500mL), for example, 1g:100mL, 1g:150mL, 1g:200mL, 1g:250mL, 1g:300mL, 1g:350mL, 1g:400mL, 1g:450mL, 1g:500mL, and any ratio or range between any two of the above.

[0084] In some embodiments, the pH of the buffer solution is 7 to 11, such as 7, 8, 9, 10, 11, and any two of the above values ​​or a range thereof.

[0085] In some embodiments, the mass ratio of the graphite particles to the dopamine monomer is in the range of 1:(0.1 to 0.5), for example, 1:0.1, 1:0.2, 1:0.3, 1:0.4, 1:0.5, and the ratio or range between any two of the above ratios.

[0086] In some embodiments, mixing the dopamine monomer with the graphite particle dispersion further includes stirring to promote the formation of a polydopamine coating layer on the surface of the graphite particles by the dopamine monomer.

[0087] The self-aggregation time is 12 to 36 hours, for example, 12 hours, 15 hours, 20 hours, 25 hours, 30 hours, 36 hours, and any value or range between any two of the stated values.

[0088] In some embodiments, the stirring rate is 100–200 rpm, for example, 100 rpm, 120 rpm, 150 rpm, 160 rpm, 180 rpm, 200 rpm, and any value or range between these two values. Within this stirring rate range, it is beneficial to avoid the generation of bubbles during the self-polymerization of dopamine monomers, and to achieve more uniform coating, thereby facilitating the preparation of coated modified graphite anode materials with better cycle performance and rate performance.

[0089] In some embodiments, the stirring can be magnetic stirring.

[0090] In some embodiments, the drying process may be vacuum drying.

[0091] In step S12:

[0092] In some embodiments, the first heat treatment includes: heating to a first temperature and then holding at that temperature for a first time.

[0093] In some embodiments, the first temperature is 300℃ to 400℃, for example, 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, 390℃, 400℃, and any value or range between any two of these values; the first time is 0.5 to 1 hour, for example, 0.5 hours, 0.6 hours, 0.7 hours, 0.8 hours, 0.9 hours, 1 hour, and any value or range between any two of these values. Within the temperature and time range, it is beneficial for the polydopamine near the outer surface of the polydopamine coating layer to decompose and form heterocyclic nitrogen, and for the content of heterocyclic nitrogen in the nitrogen-doped carbon coating layer of the prepared modified graphite anode material to gradually increase along the direction away from the graphite particles.

[0094] In some embodiments, the heating rate to the first temperature is 0.5 to 2 °C / min, for example, 0.5 °C / min, 1 °C / min, 1.5 °C / min, 2 °C / min, and any value or range between any two of the above values.

[0095] In some embodiments, the second heat treatment includes: a first sub-heat treatment, or a second sub-heat treatment, or sequentially performing a first sub-heat treatment and a second sub-heat treatment, wherein the temperature of the second sub-heat treatment is higher than the temperature of the first sub-heat treatment.

[0096] In some embodiments, the first sub-heat treatment includes: heating to a second temperature and then holding at that temperature for a second time.

[0097] In some embodiments, the second temperature is 500℃ to 700℃, for example, 500℃, 550℃, 600℃, 650℃, 700℃, and any value or range between these two values; the second time is 1 to 2 hours, for example, 1 hour, 1.5 hours, 2 hours, and any value or range between these two values. Within the temperature and time range, it is beneficial to decompose the polydopamine in the polydopamine coating layer near the graphite particles to form gas and larger pores, and to shrink and densify the region of the polydopamine coating layer away from the graphite particles. It also allows the polydopamine in the polydopamine layer near the graphite particles to decompose and form graphite nitrogen. It should be noted that within the second temperature and time range, it is more conducive to the decomposition of polydopamine in the polydopamine coating layer near the graphite particles to generate gas and form pores with larger diameters, and to the shrinkage and densification of the region in the polydopamine coating layer away from the graphite particles. This is conducive to the nitrogen-doped carbon coating layer having a porous structure, and the pore diameter in the porous structure gradually decreases along the direction away from the graphite particles.

[0098] In some embodiments, the heating rate to the second temperature is 3 to 5 °C / min, for example, 3 °C / min, 4 °C / min, 5 °C / min, and any value or range between any two of the stated values.

[0099] In some embodiments, the second sub-heat treatment includes: heating to a third temperature and then holding at that temperature for a third time.

[0100] In some embodiments, the third temperature is 800℃ to 900℃, for example, 800℃, 810℃, 820℃, 830℃, 840℃, 850℃, 860℃, 870℃, 880℃, 890℃, or 900℃; the third time is 20 to 60 minutes, for example, 20 minutes, 30 minutes, 40 minutes, 50 minutes, or 60 minutes, as well as any value or range between any two of the above values. Within the temperature and time range, it is beneficial to decompose the polydopamine in the polydopamine coating layer near the graphite particles to form gas and larger pores, and to shrink and densify the region of the polydopamine coating layer away from the graphite particles. It can also decompose the polydopamine in the polydopamine layer near the graphite particles to form graphite nitrogen. It should be noted that within the specified temperature and time range, it is more conducive to the decomposition of polydopamine in the polydopamine coating layer near the graphite particles to form graphite nitrogen, and to the gradual increase of the graphite nitrogen content in the nitrogen-doped carbon coating layer of the prepared coated modified graphite anode material along the direction close to the graphite particles.

[0101] In some embodiments, the heating rate to the third temperature is 5 to 10 °C / min, for example, 5 °C / min, 6 °C / min, 7 °C / min, 8 °C / min, 9 °C / min, 10 °C / min, and any value or range between any two of the above values.

[0102] It should be noted that when the second heat treatment includes both the first and second sub-heat treatments, the first sub-heat treatment is more conducive to the decomposition of polydopamine near the graphite particles in the polydopamine coating layer, forming gas and larger pores, and causing the region of the polydopamine coating layer away from the graphite particles to shrink and densify. This is beneficial for the nitrogen-doped carbon coating layer to have a porous structure, with the pore size gradually decreasing along the direction away from the graphite particles. The second sub-heat treatment is more conducive to the decomposition of polydopamine near the graphite particles in the polydopamine coating layer, forming graphitic nitrogen. This is beneficial for the content of graphitic nitrogen gradually increasing along the direction close to the graphite particles in the nitrogen-doped carbon coating layer of the prepared modified graphite anode material. Thus, the prepared modified graphite particles can simultaneously have a better gradient pore size distribution and a better gradient pore size distribution of graphitic nitrogen, thereby giving the prepared modified graphite particles higher cycle performance and rate performance, and other electrical properties.

[0103] In some embodiments, the polydopamine-coated graphite particles are subjected to a first heat treatment and a second heat treatment in an inert atmosphere. Further, in some embodiments, the inert gas in the inert atmosphere is one or more of nitrogen, helium, neon, argon, and xenon.

[0104] In some embodiments, the inert atmosphere during the first heat treatment also includes ammonia (NH3), which is beneficial for retaining the heterocyclic nitrogen formed by the decomposition of polydopamine near the outer surface of the polydopamine coating in the nitrogen-doped carbon coating.

[0105] In some embodiments, the volume content of ammonia in the inert atmosphere ranges from 1% to 10%, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, and any value or range between any two of the stated values.

[0106] Thirdly, embodiments of this application also provide a negative electrode, including a negative electrode current collector and an active material layer bonded to at least one surface of the negative electrode current collector, wherein the active material layer includes the coated modified graphite negative electrode material described above.

[0107] Fourthly, embodiments of this application also provide a battery, including a positive electrode, a separator, an electrolyte, and the negative electrode described above.

[0108] The battery can be a lithium-ion battery.

[0109] The negative electrode of the battery described in this application includes the above-mentioned coated modified graphite negative electrode material, thus exhibiting high cycle performance and good rate performance, among other electrical properties.

[0110] The present application will be specifically described below through specific embodiments. These embodiments are only some embodiments of the present application and are not intended to limit the present application. The materials or reagents used in the embodiments and comparative examples of this application are commercially available.

[0111] Example 1

[0112] The preparation method of the coated modified graphite anode material in this embodiment includes:

[0113] Step S11: Disperse 1g of graphite (D50 = 10μm) in 200mL Tris-HCl buffer (10mM, pH = 8.5), sonicate for 30 minutes, add 0.2g of dopamine hydrochloride (DA), and self-polymerize at 150rpm at room temperature for 24h. Collect the solid by vacuum filtration, wash it 3 times with deionized water, and then vacuum dry it at 60℃ for 12h to obtain polydopamine-coated graphite particles.

[0114] Step S12: In a tube furnace under an Ar atmosphere, the polydopamine-coated graphite particles are subjected to a first heat treatment, a first sub-heat treatment, and a second sub-heat treatment in sequence to perform gradient pyrolysis of the polydopamine-coated graphite particles. Then, the particles are naturally cooled to room temperature to obtain the coated modified graphite anode material, wherein:

[0115] The first heat treatment includes: heating to 400°C at a rate of 2°C / min and holding at that temperature for 1 hour;

[0116] The first heat treatment includes: heating to 700°C at a rate of 5°C / min and holding at that temperature for 2 hours;

[0117] The second sub-heat treatment includes: heating to 900°C at a rate of 10°C / min and holding at that temperature for 30 minutes.

[0118] The modified graphite anode material of this embodiment includes graphite particles and a nitrogen-doped carbon coating layer on the surface of the graphite particles. The nitrogen-doped carbon coating layer includes nitrogen-containing substances, and the nitrogen atoms in the nitrogen-containing substances exist in the form of heterocyclic nitrogen and graphitic nitrogen. The heterocyclic nitrogen includes one or both of pyridine nitrogen and pyrrole nitrogen.

[0119] The preparation methods of the coated modified graphite anode materials in Examples 2-9 and the coated modified graphite anode materials in Comparative Examples 1-4 are basically the same as the preparation method of the coated modified graphite anode material in Example 1. The main parameters are shown in Table 1 below.

[0120] Table 1:

[0121]

[0122]

[0123] The pore structure, heterocyclic nitrogen, and graphitic nitrogen distribution / content in the nitrogen-doped carbon coating layer of the coated modified graphite anode materials of Examples 1-9 and Comparative Examples 1-4 were detected, and the detection results are shown in Table 2.

[0124] The method for detecting the distribution of pore structure is as follows: the coated modified graphite anode material is cut and the SEM image of the cross section is observed to see the distribution of pore structure.

[0125] The distribution / content of heterocyclic nitrogen and graphitic nitrogen was determined using Ar... + Sputtering (energy 500 eV, rate 0.5 nm / s), XPS spectra were acquired every 5 nm of etching; N1s spectra were measured after each sputtering depth. Data analysis: Peak fitting of N1s spectra (binding energy range 395–405 eV): N-6 (pyridine nitrogen, approximately 398.5 eV), N-5 (pyrrole nitrogen, approximately 400.1 eV), NQ (graphite nitrogen, approximately 401.2 eV).

[0126] XPS N1s(10nm) represents the atomic percentage of all nitrogen atoms at a position 10nm from the outer surface of the nitrogen-doped carbon coating layer, as 100%, including the atomic percentage of heterocyclic nitrogen and graphitic nitrogen; XPS N1s(20nm) represents the atomic percentage of all nitrogen atoms at a position 20nm from the outer surface of the nitrogen-doped carbon coating layer, as 100%, including the atomic percentage of heterocyclic nitrogen and graphitic nitrogen; XPS N1s(50nm) represents the atomic percentage of all nitrogen atoms at a position 50nm from the outer surface of the nitrogen-doped carbon coating layer, as 100%, including the atomic percentage of heterocyclic nitrogen and graphitic nitrogen.

[0127] In the nitrogen-doped carbon coating layer, the ratio of the percentage content of N-5 to N-6 atoms in XPS N1s (10nm), XPS N1s (20nm), and XPS N1s (50nm) is 1:1.

[0128] Table 2:

[0129]

[0130]

[0131] From Table 1 and Table 2, we can see that:

[0132] In the nitrogen-doped carbon coating layers of the modified graphite anode materials in Examples 1-8, the proportions of pyridine nitrogen and pyrrole nitrogen decrease with increasing depth, while the proportion of graphite nitrogen increases; and the inner layer of the nitrogen-doped carbon coating layer is loose while the outer layer is dense.

[0133] Compared to the nitrogen-doped carbon coating layer of the modified graphite anode material in Example 8, the inner layer of the nitrogen-doped carbon coating layer of the modified graphite anode material in Example 1 has a higher content of graphite nitrogen. It can be seen that the higher the temperature, the more conducive it is to the formation of more graphite nitrogen in the inner layer.

[0134] Compared to the nitrogen-doped carbon coating layer of the modified graphite anode material in Example 1, the content of heterocyclic nitrogen in the outer layer of the nitrogen-doped carbon coating layer of the modified graphite anode material in Example 9 is higher. It can be seen that the presence of NH3 is beneficial to increasing the content of heterocyclic nitrogen in the outer layer of the nitrogen-doped carbon coating layer.

[0135] During the preparation of the coated modified graphite anode material in Comparative Example 1, there was no gradient temperature rise. The nitrogen-doped carbon coating layer did not form a gradient pore structure, and there was no gradient change in the content of heterocyclic nitrogen and graphitic nitrogen. The volumetric strain was concentrated, which affected the cycle stability. Furthermore, the ion transport and electron transport could not be balanced, which was not conducive to improving the rate performance.

[0136] In Comparative Example 2, the polymerization time and coating amount were too small during the preparation of the coated modified graphite anode material, resulting in a thin nitrogen-doped carbon coating layer with no obvious gradient.

[0137] In the preparation of the coated modified graphite anode material of Comparative Example 3, the heating rate of the first heat treatment was too fast, which was not conducive to the formation of heterocyclic nitrogen and the formation of gradient pore structure.

[0138] In Comparative Example 4, the temperature of the second heat treatment during the preparation of the coated modified graphite anode material was too high, which led to the conversion of heterocyclic nitrogen to graphitic nitrogen. This weakened the content gradient of heterocyclic nitrogen and graphitic nitrogen and caused more total nitrogen loss, thus affecting the nitrogen doping effect.

[0139] Coin cell lithium-ion batteries and pouch lithium-ion full cells were prepared using the coated modified graphite anode materials of Examples 1-9 and Comparative Examples 1-4, respectively. The reversible specific capacity, first efficiency, and charging window of the coin cell lithium-ion batteries were then tested, and the cycle performance of the pouch lithium-ion full cells was tested. The test results are shown in Table 3.

[0140] The preparation method of a coin cell lithium-ion battery includes: thoroughly mixing a modified graphite anode material, a conductive agent (Super P), a binder (SBR), and a thickener (CMC-Na) in an appropriate amount of deionized water at a mass ratio of 96.2:0.8:1.8:1.2 to form a uniform anode slurry; coating the anode slurry onto the surface of a copper foil current collector; and drying and cold pressing to obtain an anode sheet with a coating thickness of 200 μm. Using the above anode sheet, a lithium sheet is used as the counter electrode, a polyethylene (PE) film as the separator, and a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 1:1:1 as the electrolyte to assemble a coin cell lithium-ion battery.

[0141] Soft-pack lithium-ion full battery: LiNi nickel cobalt manganese oxide 0.5 Mn 0.3 Co 0.2 O2 (NMC532) material, conductive agent (Super P), and binder (PVDF) are thoroughly mixed in an appropriate amount of N-methylpyrrolidone (NMP) at a mass ratio of 96:2:2 to form a uniform positive electrode slurry. This slurry is then coated onto the surface of the positive electrode current collector aluminum foil. After drying and cold pressing, the positive electrode sheet is obtained. The coating surface density of the positive electrode sheet is 18.0 mg / cm³. 2 Modified graphite anode material, conductive agent (Super P), binder (SBR), and thickener (CMC-Na) were thoroughly mixed in an appropriate amount of deionized water at a mass ratio of 95.5:1.0:2.0:1.5 to form a uniform anode slurry. This slurry was then coated onto the surface of a copper foil current collector. After drying and cold pressing, the anode sheet was obtained, with a coating surface density of 9.0 mg / cm³. 2 Using the above-mentioned negative electrode, positive electrode, polyethylene (PE) film as separator, and a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 1:1:1 as electrolyte, a soft-pack lithium-ion full battery is assembled.

[0142] Reversible specific capacity test method: Place the coin cell lithium-ion battery in a constant temperature chamber of a 25℃ Blue Electric Test Cabinet (T-3002A-5V 1mA) and let the battery stand for 6 hours. Discharge the coin cell lithium-ion battery to 1.0mV at 0.1C, let it stand for 10 minutes, discharge it to 1.0mV at 0.01C, and then charge it to 1.5V at 0.05C. Record the capacity of the coin cell lithium-ion battery at this time, which is recorded as the reversible specific capacity.

[0143] The initial efficiency test method is as follows: The coin cell lithium-ion battery is placed in a constant temperature chamber (T-3002A-5V 1mA) at 25℃. The battery is allowed to stand for 6 hours. Then, the coin cell lithium-ion battery is discharged at 0.1C to 1.0mV, allowed to stand for 10 minutes, and then discharged at 0.01C to 1.0mV. The discharge capacity D is recorded. The battery is then charged at 0.05C to 1.5V, and the charging capacity at this point is recorded as the specific capacity C. The initial efficiency is calculated using the following formula: Initial efficiency (%) = C / D × 100%.

[0144] The charging window test method is as follows: The above-mentioned coin cell lithium-ion battery is charged and discharged for the first time at a current of 1C (i.e., the current value at which the theoretical capacity is completely discharged within 1 hour). Specifically, the battery is charged at a constant current rate of 1C to a voltage of 4.4V at 35℃, then charged at a constant voltage rate to a current ≤0.05C, left to stand for 5 minutes, and then discharged at a constant current rate of 0.33C to a voltage of 2.5V. Its actual capacity is recorded as C0. Then, the battery is charged sequentially at constant current of 1.0C0, 2.0C0, 3.0C0, 4C0, 5C0, and 6C0 until the full battery charging cutoff voltage of 4.4V or the negative electrode cutoff potential of 0V (whichever comes first). After each charging, it is discharged at 1C0 until the full battery discharge cutoff voltage of 2.5V. Record the charging rate range in which lithium plating does not occur on the negative electrode when charging to 100% SOC (State of Charge, where "SOC = 0" indicates that the battery is fully discharged and "SOC = 100%" indicates that the battery is fully charged).

[0145] Cyclic performance testing method: The soft-pack lithium-ion full battery was placed in a constant temperature chamber of the Xinwei test cabinet (BTS-5V6A) at 25℃. The battery was left to stand for 1 hour, charged at a constant current of 1C to 4.3V, then charged at a constant voltage to a current of 0.05C, left to stand for 5 minutes, and then discharged at a constant current of 1C to 2.8V. The initial capacity was recorded as C0. Then, the charge and discharge cycle was performed according to the above process, and the discharge capacity Cn of each cycle was recorded until the cycle capacity retention rate (Cn / C0×100%) was 80%, and the number of cycles was recorded.

[0146] Table 3:

[0147] Reversible specific capacity (mAh / g) First-efficacy (%) Loop (cycle) Charging window (C) Example 1 359 93.9％ ＞3000 1-6 Example 2 354 93.1％ ＞3000 1-6 Example 3 355 94.2％ ＞3000 1-6 Example 4 352 93.0％ ＞3000 1-6 Example 5 350 91.6％ ＞3000 1-5 Example 6 358 93.6％ ＞3000 1-6 Example 7 358 93.3％ ＞3000 1-5 Example 8 358 93.5％ ＞3000 1-6 Example 9 357 92.3％ ＞3000 1-8 Comparative Example 1 357 93.6％ ＜3000 1-3 Comparative Example 2 356 92.0％ ＜3000 1-3 Comparative Example 3 357 93.9％ ＜3000 1-3 Comparative Example 4 359 94.1％ ＞3000 1-3

[0148] As shown in Table 3:

[0149] Compared to the lithium-ion batteries of Comparative Examples 1 to 4, the lithium-ion batteries of Examples 1 to 9 can simultaneously have higher cycle performance and a wider charging window without affecting specific capacity and initial efficiency. That is, the lithium-ion batteries of Examples 1 to 9 can simultaneously have higher cycle performance and better rate performance without affecting specific capacity and initial efficiency.

[0150] The technical solutions provided by the embodiments of this application have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods 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 the specific implementation methods 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 coated modified graphite anode material, characterized in that, It includes graphite particles and a nitrogen-doped carbon coating layer covering the surface of the graphite particles. The nitrogen atoms in the nitrogen-doped carbon coating layer exist in the form of heterocyclic nitrogen and graphitic nitrogen, wherein the heterocyclic nitrogen includes one or both of pyridine nitrogen and pyrrole nitrogen; wherein, In the nitrogen-doped carbon coating layer, the content of heterocyclic nitrogen gradually increases along the direction away from the graphite particles; and / or In the nitrogen-doped carbon coating layer, the content of graphite nitrogen gradually decreases along the direction away from the graphite particles.

2. The coated modified graphite anode material as described in claim 1, characterized in that, The nitrogen-doped carbon coating has a porous structure, and the pore size in the porous structure gradually decreases along the direction away from the graphite particles; and / or The density of the nitrogen-doped carbon coating gradually increases along the direction away from the graphite particles.

3. The coated modified graphite anode material as described in claim 1, characterized in that, The nitrogen-doped carbon coating layer comprises a stacked inner layer and an outer layer, wherein the inner layer is located between the graphite particles and the outer layer, the outer layer has a thickness of 10 nm, and the inner layer has a thickness of 10–90 nm, wherein: The heterocyclic nitrogen content in the outer layer is higher than that in the inner layer, and the graphitic nitrogen content in the outer layer is lower than that in the inner layer; and / or In the outer layer, the content of heterocyclic nitrogen is higher than the content of graphitic nitrogen; in the inner layer, the content of heterocyclic nitrogen is lower than the content of graphitic nitrogen; and / or Based on an atomic percentage of 100% for all nitrogen atoms in the outer layer, the atomic percentage of heterocyclic nitrogen in the outer layer is 60-70 at%, and the atomic percentage of graphitic nitrogen is 30-40 at%; and / or With the atomic percentage of all nitrogen atoms in the inner layer being 100%, the atomic percentage of heterocyclic nitrogen in the inner layer is 30-60 at, and the atomic percentage of graphite nitrogen is 40-70 at.

4. The coated modified graphite anode material as described in claim 2, characterized in that, The thickness of the nitrogen-doped carbon coating layer is 20–100 nm; and / or The D50 particle size of the graphite particles is 8–20 μm; and / or The pore size range of the nitrogen-doped carbon coating is 2–20 nm.

5. A method for preparing a coated modified graphite anode material, characterized in that, Includes the following steps: Dopamine monomer was mixed with graphite particle dispersion and dried to obtain polydopamine-coated graphite particles. The polydopamine-coated graphite particles are subjected to a first heat treatment and a second heat treatment in sequence, wherein the temperature of the second heat treatment is higher than that of the first heat treatment, to obtain a coated modified graphite anode material.

6. The preparation method according to claim 5, characterized in that, The first heat treatment includes: heating to a first temperature, and then holding at that temperature for a first time, wherein: The first temperature is 300℃~400℃; and / or The first time is 0.5 to 1 hour; and / or The heating rate to the first temperature is 0.5–2 °C / min.

7. The preparation method according to claim 5, characterized in that, The second heat treatment includes: a first sub-heat treatment, or a second sub-heat treatment, or sequentially performing a first sub-heat treatment and a second sub-heat treatment, wherein the temperature of the second sub-heat treatment is higher than the temperature of the first sub-heat treatment.

8. The preparation method according to claim 7, characterized in that, The first sub-heat treatment includes: heating to a second temperature, and then holding at that temperature for a second time, wherein: The second temperature is 500℃~700℃; and / or The second time is 1 to 2 hours; and / or The heating rate to the second temperature is 3-5 °C / min.

9. The preparation method according to claim 7, characterized in that, The second sub-heat treatment includes: heating to a third temperature, and then holding at that temperature for a third time, wherein: The third temperature is 800℃~900℃; and / or The third time is 20–60 min; and / or The heating rate to the third temperature is 5–10 °C / min.

10. The preparation method according to claim 5, characterized in that, The polydopamine-coated graphite particles are subjected to a first heat treatment and a second heat treatment in an inert atmosphere, wherein the inert gas in the inert atmosphere is one or more of nitrogen, helium, neon, argon, and xenon.

11. The preparation method according to claim 10, characterized in that, During the first heat treatment, the inert atmosphere also includes ammonia gas, wherein the volume content of the ammonia gas in the inert atmosphere ranges from 1% to 10%; and / or The self-polymerization time is 12–36 hours; and / or The dopamine monomer includes one or more of dopamine and dopamine hydrochloride; and / or In the graphite particle dispersion, the solid-liquid ratio ranges from 1 g:(100 mL to 500 mL); and / or The graphite particle dispersion comprises graphite particles and a buffer solution, wherein the buffer solution comprises Tris-HCl buffer; and / or The mass ratio of graphite particles to dopamine monomer in the graphite particle dispersion ranges from 1:(0.1 to 0.5).

12. A negative electrode, characterized in that, It includes the coated modified graphite anode material as described in any one of claims 1 to 4, or the coated modified graphite anode material prepared by the preparation method described in any one of claims 5 to 11.

13. A battery, characterized in that, Includes the negative electrode as described in claim 12.

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