Modified graphene coated ferric sodium pyrophosphate, preparation method and sodium ion battery

Abstract

The invention relates to modified graphene coated ferric sodium pyrophosphate, a preparation method and a sodium ion battery. The modified graphene coated ferric sodium phosphate pyrophosphate comprises a ferric sodium phosphate pyrophosphate matrix and modified graphene coated on the surface of the ferric sodium phosphate pyrophosphate matrix; the modified graphene comprises nitrogen-doped graphene and a copper plating layer formed on the surface of the nitrogen-doped graphene through in-situ reduction. The nitrogen doping and copper plating combined double-strategy modified graphene is adopted to coat the ferric sodium pyrophosphate, a copper plating layer on the surface of the modified graphene and the nitrogen doped graphene form a double-conductive network, the rate capability is improved, and meanwhile, the flexible support of the graphene, the rigid protection of the copper plating layer and the synergistic effect of the double shell layers are utilized, so that the performance of the lithium ion battery is improved. The structural stability of the ferric sodium phosphate pyrophosphate in the charge-discharge process is maintained, and the cycle performance of the ferric sodium phosphate pyrophosphate is improved.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to sodium-ion cathode materials, and more particularly to a modified graphene-coated sodium iron pyrophosphate, its preparation method, and a sodium-ion battery. Background Technology

[0002] With the rapid development of new energy vehicles and large-scale energy storage, the demand for secondary batteries has increased dramatically. As limited lithium resources are being consumed in large quantities, sodium-ion batteries, which are rich in material resources and have low cost, have been widely studied. Sodium iron pyrophosphate material has become a hot research topic in the field of sodium-ion batteries due to its good cycle stability.

[0003] However, sodium iron pyrophosphate has poor electronic conductivity, resulting in insufficient rate performance. Although existing technologies have coated it with a carbon layer, the conductivity of the carbon layer is limited, and ion diffusion is hindered at high rates, so the rate performance cannot be significantly improved. Furthermore, the bonding force between the carbon layer and the active material is weak, and it is easy to fall off after long-term cycling, resulting in a decrease in cycle retention.

[0004] CN119079962A discloses a method for preparing a sodium iron pyrophosphate cathode material with a double-layer carbon structure. By coating different carbon sources separately, a double carbon layer can be formed, and different carbon layers have different structural types, which can make the material have higher phase stability and excellent ionic conductivity. Specifically, a two-stage spraying method is used. In the first spraying, a polymer is coated to form an inner carbon layer, and in the second spraying, a low-molecular-weight organic compound is coated to form an outer carbon layer, forming a multi-layer coating effect. The dense and compact inner carbon layer can avoid excessive side reactions between the electrolyte and the cathode material, thereby improving cycle performance. The outer carbon layer is used to fill the defects of the inner carbon layer, further preventing contact with the electrolyte, and at the same time, it serves as an ion transport channel between particles to improve conductivity.

[0005] CN120389037A discloses a carbon-coated sodium iron pyrophosphate cathode material and its preparation method, including a Na source, an Fe source, an aluminum acetate source, and a P source. This invention is based on gel, which allows the material to form a three-dimensional carbon conductive network during sintering. This three-dimensional carbon conductive network improves the conductivity of the material, and the in-situ coated carbon layer effectively inhibits the growth of primary particles and reduces the transport distance of sodium ions. The secondary carbon coating is a method of small-scale uniform fluidized bed vapor deposition (CVD) on the basis of the first carbon coating, to coat the material surface with a nanoscale conductive carbon film again. With relatively good temperature treatment, the coated carbon layer has a high degree of graphitization, thereby improving the electronic conductivity of the material.

[0006] CN120573673A discloses a sodium iron pyrophosphate-based composite material with a core-shell structure, comprising a matrix material, a heterogeneous interface layer, and a coating layer from the inside out. The heterogeneous interface layer consists of iron-based phosphide nanoparticles distributed on the surface of the matrix material, and the coating layer is a carbon layer. The iron-based phosphide includes at least one of FeP and Fe2P. This invention significantly improves the electronic and ion transport properties of the material and enhances its rate performance by generating iron-based phosphide nanoparticles in situ on the surface of the sodium iron pyrophosphate material through in-situ reduction, forming a tightly bound heterogeneous interface network.

[0007] Therefore, it is of great significance to provide sodium iron pyrophosphate that can adapt to high-rate application scenarios and has excellent cycle performance, as well as its preparation method. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention aims to provide a modified graphene-coated sodium iron pyrophosphate, its preparation method, and a sodium-ion battery. This invention employs a dual-strategy of nitrogen-doped and copper-plated modified graphene to coat sodium iron pyrophosphate. The copper plating layer on the modified graphene surface and the nitrogen-doped graphene form a dual conductive network, improving rate performance. Simultaneously, the flexible support of the graphene and the rigid protection of the copper plating layer work synergistically to maintain the structural stability of sodium iron pyrophosphate during charge and discharge, thereby improving its cycle performance.

[0009] To achieve this objective, the present invention adopts the following technical solution:

[0010] In a first aspect, the present invention provides a modified graphene-coated sodium ferric pyrophosphate, wherein the modified graphene-coated sodium ferric pyrophosphate comprises a sodium ferric pyrophosphate matrix and modified graphene coated on the surface of the sodium ferric pyrophosphate matrix; the modified graphene comprises nitrogen-doped graphene and a copper plating layer formed in situ on the surface of the nitrogen-doped graphene.

[0011] This invention employs a dual strategy of nitrogen doping combined with copper plating to coat sodium iron pyrophosphate. The copper plating layer on the surface of the modified graphene forms a dual conductive network with the nitrogen-doped graphene, improving rate performance. At the same time, the flexible support of the graphene and the rigid protection of the copper plating layer work together to maintain the structural stability of sodium iron pyrophosphate during charge and discharge, thereby improving the cycle performance of sodium iron pyrophosphate.

[0012] Preferably, in the modified graphene-coated sodium iron pyrophosphate, the mass percentage of modified graphene is 1 wt.% to 3 wt.%.

[0013] Preferably, the copper plating layer in the modified graphene has a mass percentage content of 30 wt.% to 40 wt.%.

[0014] In a second aspect, the present invention provides a method for preparing modified graphene-coated sodium ferric pyrophosphate as described in the first aspect, the method comprising:

[0015] According to the mass ratio, graphene oxide and sodium iron pyrophosphate precursor are dispersed in water to obtain a graphene-coated precursor; the graphene-coated precursor is dispersed in a reducing solution to obtain a precursor dispersion; a copper salt solution is added to the precursor dispersion to obtain a precursor slurry; the precursor slurry is mixed with a nitrogen-containing compound and subjected to a hydrothermal reaction to prepare a modified graphene-coated precursor; the modified graphene-coated precursor is calcined to prepare the modified graphene-coated sodium iron pyrophosphate.

[0016] Preferably, the reducing agent in the reducing solution includes any one or a combination of at least two of hydrazine hydrate, ascorbic acid, or formaldehyde.

[0017] Preferably, the nitrogen-containing compound includes any one or a combination of at least two of urea, melamine, or thiourea.

[0018] Preferably, the copper salt includes any one or a combination of at least two of copper nitrate, copper chloride, copper sulfate, or copper acetate.

[0019] Preferably, the concentration of the reducing agent in the reducing solution is 0.5 mol / L to 1.5 mol / L.

[0020] Preferably, the graphene-coated precursor in the precursor dispersion has a solid content of 1 wt.% to 10 wt.%.

[0021] Preferably, the concentration of the copper salt solution is 0.05 mol / L to 0.2 mol / L.

[0022] Preferably, the mass of the nitrogen-containing compound is 3 wt.% to 6 wt.% of the mass of the graphene-coated precursor.

[0023] Preferably, the dispersion method includes ultrasonic treatment.

[0024] Preferably, the temperature of the hydrothermal reaction is 120℃~180℃.

[0025] Preferably, the hydrothermal reaction time is 1h to 10h.

[0026] Preferably, the roasting includes first heating to a first temperature, holding at that temperature for a first time, then heating to a second temperature and holding at that temperature for a second time.

[0027] Preferably, the first temperature is 300℃~400℃.

[0028] Preferably, the first time is 3h to 8h.

[0029] Preferably, the second temperature is 500℃~700℃.

[0030] Preferably, the second time is 8h to 15h.

[0031] Preferably, the calcination atmosphere comprises an Ar / H2 mixture with a H2 volume fraction of 3 vol% to 8 vol%.

[0032] Preferably, the preparation method of the sodium iron pyrophosphate precursor includes: dispersing sodium source, iron source and phosphorus source in water according to stoichiometric ratio, spray drying to obtain the sodium iron pyrophosphate precursor.

[0033] Thirdly, the present invention provides a sodium-ion battery, the sodium-ion battery comprising the modified graphene-coated sodium iron pyrophosphate as described in the first aspect, or comprising the modified graphene-coated sodium iron pyrophosphate prepared by the preparation method described in the second aspect.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] This invention employs a dual strategy of nitrogen doping combined with copper plating to coat sodium iron pyrophosphate. The copper plating layer on the surface of the modified graphene forms a dual conductive network with the nitrogen-doped graphene, improving rate performance. At the same time, the flexible support of the graphene and the rigid protection of the copper plating layer work together to maintain the structural stability of sodium iron pyrophosphate during charge and discharge, thereby improving the cycle performance of sodium iron pyrophosphate. Detailed Implementation

[0036] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0037] The "range" disclosed in this invention can be defined in the form of a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the specific range. This type of range definition can include or exclude endpoints; any endpoint can be independently included or excluded, and they can be arbitrarily combined, meaning any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60~120 and 80~110 are listed for specific parameters, it is understood that ranges of 60~110 and 80~120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and maximum range values ​​3, 4, and 5 are also listed, then the following ranges are all expected: 1~3, 1~4, 1~5, 2~3, 2~4, and 2~5. In this invention, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0" and "5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer ≥2, it is equivalent to listing integers such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For instance, when a parameter is described as an integer selected from "2~10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0038] In this invention, "a combination of at least two" refers to a quantity greater than or equal to two, unless otherwise specified. For example, "any combination of one or at least two" means one or more or more items. It can be understood that when referring to "a combination of at least two," it refers to any suitable combination of multiple items, that is, a combination of "at least two" items carried out in a manner that does not conflict with and enables the implementation of this invention.

[0039] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0040] The term "embodiment" as used in this invention means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment or implementation of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described in this invention can be combined with other embodiments.

[0041] Those skilled in the art will understand that the order in which the steps are written in the methods of the various embodiments does not imply a strict execution order. The detailed execution order of each step should be determined by its function and possible internal logic. Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, but are preferably performed sequentially. For example, if the method includes steps (a) and (b), it means that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the method may also include step (c), meaning that step (c) can be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0042] In this invention, open-ended technical features or solutions described using terms such as "comprising" do not exclude additional members beyond those listed unless otherwise specified. They can be considered as providing both closed-ended features or solutions comprised of the listed members and open-ended features or solutions that include additional members beyond the listed members. For example, A includes a1, a2, and a3. Unless otherwise specified, it may also include other members or exclude additional members. This can be considered as providing both technical features or solutions where "A is composed of a1, a2, and a3" or "A is selected from a1, a2, and a3," and technical features or solutions where "A includes not only a1, a2, and a3, but also other members."

[0043] In this invention, unless otherwise specified, the features or solutions corresponding to "and / or" include any one of two or more of the related listed items, as well as any and all combinations of the related listed items. These arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. For example, "A and / or B" represents a group consisting of A, B, and "a combination of A and B". "Containing A and / or B" can mean "containing A, containing B, and containing A and B", or "containing A, containing B, or containing A and B", and can be appropriately understood according to the context.

[0044] In this invention, the terms "first aspect," "second aspect," "third aspect," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, "first," "second," "third," etc., serve only as a non-exhaustive enumeration and should be understood not to constitute a closed limitation on the quantity.

[0045] In this invention, "optional" means that something is optional, that is, it refers to any one of the two parallel solutions of "having" or "not having". If there are multiple "optional" options in a technical solution, unless otherwise specified, and there are no contradictions or mutual constraints, then each "optional" option is independent.

[0046] In this invention, "room temperature" generally refers to 4℃~35℃, and can refer to 20℃±5℃. In some embodiments of this invention, room temperature refers to 20℃~30℃.

[0047] In one specific embodiment, the present invention provides a modified graphene-coated sodium ferric pyrophosphate, wherein the modified graphene-coated sodium ferric pyrophosphate comprises a sodium ferric pyrophosphate matrix and modified graphene coated on the surface of the sodium ferric pyrophosphate matrix; the modified graphene comprises nitrogen-doped graphene and a copper plating layer formed in situ on the surface of the nitrogen-doped graphene.

[0048] This invention employs a dual strategy of nitrogen doping combined with copper plating to coat sodium iron pyrophosphate. The copper plating layer on the surface of the modified graphene forms a dual conductive network with the nitrogen-doped graphene, improving rate performance. At the same time, the flexible support of the graphene and the rigid protection of the copper plating layer work together to maintain the structural stability of sodium iron pyrophosphate during charge and discharge, thereby improving the cycle performance of sodium iron pyrophosphate.

[0049] In some embodiments, the modified graphene-coated sodium iron pyrophosphate contains 1 wt.% to 3 wt.% of modified graphene, for example, 1 wt.%, 1.5 wt.%, 2 wt.%, 2.5 wt.%, or 3 wt.%.

[0050] In some embodiments, the copper plating layer in the modified graphene has a mass percentage content of 30 wt.% to 40 wt.%, for example, it can be 30 wt.%, 32 wt.%, 34 wt.%, 36 wt.%, 38 wt.%, or 40 wt.%.

[0051] In another specific embodiment, the present invention provides a method for preparing modified graphene-coated sodium ferric pyrophosphate as described in one of the foregoing specific embodiments, the preparation method comprising:

[0052] According to the mass ratio, graphene oxide and sodium iron pyrophosphate precursor are dispersed in water to obtain a graphene-coated precursor; the graphene-coated precursor is dispersed in a reducing solution to obtain a precursor dispersion; a copper salt solution is added to the precursor dispersion to obtain a precursor slurry; the precursor slurry is mixed with a nitrogen-containing compound and subjected to a hydrothermal reaction to prepare a modified graphene-coated precursor; the modified graphene-coated precursor is calcined to prepare the modified graphene-coated sodium iron pyrophosphate.

[0053] In the preparation method provided by the present invention, the graphene-coated precursor is first mixed with a reducing agent solution. The graphene oxide is reduced under the action of the reducing agent to form a porous reduced graphene oxide containing defects, which can be further tightly coated on the surface of the sodium iron pyrophosphate precursor. At the same time, the porous structure is beneficial to the diffusion rate of sodium ions during battery charging and discharging. Furthermore, when a copper salt solution is added subsequently, copper ions are more likely to adhere to the surface of the porous reduced graphene oxide containing defects, forming a uniform copper plating layer.

[0054] In some embodiments, the reducing agent in the reducing solution includes any one or a combination of at least two of hydrazine hydrate, ascorbic acid, or formaldehyde. Typical but non-limiting combinations include a combination of hydrazine hydrate and ascorbic acid, a combination of formaldehyde and hydrazine hydrate, or a combination of ascorbic acid and formaldehyde.

[0055] In this invention, nitrogen-containing compounds are used to modify graphene with nitrogen through a hydrothermal reaction, thereby further improving the conductivity of graphene.

[0056] In some embodiments, the nitrogen-containing compound includes any one or a combination of at least two of urea, melamine, or thiourea. Typical but non-limiting combinations include a combination of urea and melamine, a combination of thiourea and urea, or a combination of melamine and thiourea.

[0057] In some embodiments, the copper salt includes any one or a combination of at least two of copper nitrate, copper chloride, copper sulfate, or copper acetate.

[0058] In some embodiments, the concentration of the reducing agent in the reducing solution is 0.5 mol / L to 1.5 mol / L, for example, it can be 0.5 mol / L, 0.75 mol / L, 1 mol / L, 1.25 mol / L or 1.5 mol / L.

[0059] In some embodiments, the graphene-coated precursor in the precursor dispersion has a solid content of 1 wt.% to 10 wt.%, for example, it can be 1 wt.%, 2 wt.%, 3 wt.%, 4 wt.%, 5 wt.%, 6 wt.%, 7 wt.%, 8 wt.%, 9 wt.%, or 10 wt.%.

[0060] In some embodiments, the concentration of the copper salt solution is 0.05 mol / L to 0.2 mol / L, for example, it can be 0.05 mol / L, 0.075 mol / L, 0.1 mol / L, 0.125 mol / L, 0.15 mol / L, 0.175 mol / L or 0.2 mol / L.

[0061] In some embodiments, the mass of the nitrogen-containing compound is 3 wt.% to 6 wt.% of the mass of the graphene-coated precursor, for example, it can be 3 wt.%, 3.5 wt.%, 4 wt.%, 4.5 wt.%, 5 wt.%, 5.5 wt.% or 6 wt.%.

[0062] In some embodiments, the dispersion method includes ultrasonic treatment.

[0063] In this invention, graphene oxide and sodium iron pyrophosphate precursor are uniformly dispersed in water by ultrasonic treatment. Under the action of surface tension, graphene oxide self-assembles and coats the surface of sodium iron pyrophosphate precursor particles to form a graphene-coated precursor.

[0064] In some embodiments, the temperature of the hydrothermal reaction is 120°C to 180°C, for example, 120°C, 130°C, 140°C, 150°C, 160°C, 170°C or 180°C.

[0065] In some embodiments, the hydrothermal reaction time is 1h to 10h, for example, it can be 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h or 10h.

[0066] In some embodiments, the calcination includes first heating to a first temperature, holding at that temperature for a first time, then heating to a second temperature and holding at that temperature for a second time.

[0067] In this invention, the temperature is first raised to a first temperature for pre-sintering to remove inorganic matter, and then the temperature is raised to a second temperature to realize the transformation of sodium iron pyrophosphate precursor into sodium iron pyrophosphate.

[0068] In some embodiments, the first temperature is 300°C to 400°C, for example, it can be 300°C, 320°C, 340°C, 360°C, 380°C or 400°C.

[0069] In some implementations, the first time is 3h to 8h, for example, it can be 3h, 4h, 5h, 6h, 7h or 8h.

[0070] In some embodiments, the second temperature is 500°C to 700°C, for example, it can be 500°C, 550°C, 600°C, 650°C or 700°C.

[0071] In some implementations, the second time is 8h to 15h, for example, it can be 8h, 10h, 11h, 13h or 15h.

[0072] In some embodiments, the calcination atmosphere comprises an Ar / H2 mixture with an H2 volume fraction of 3 vol% to 8 vol%, where the H2 volume fraction may be, for example, 3 vol%, 4 vol%, 5 vol%, 6 vol%, 7 vol%, or 8 vol%.

[0073] In some embodiments, the preparation method of the sodium iron pyrophosphate precursor includes: dispersing a sodium source, an iron source and a phosphorus source in water according to a stoichiometric ratio, and spray drying to obtain the sodium iron pyrophosphate precursor.

[0074] Specifically, the preparation method of the sodium iron pyrophosphate precursor includes the following steps:

[0075] According to the stoichiometric ratio, sodium source, iron source and phosphorus source are dispersed in water to obtain a precursor dispersion with a solid content of 10wt%~40wt%, for example, 10wt%, 20wt%, 30wt% or 40wt%; the inlet temperature is set to 200℃~250℃, for example, 200℃, 210℃, 220℃, 230℃, 240℃ or 250℃, and the outlet temperature is set to 80℃~120℃, for example, 80℃, 90℃, 100℃, 110℃ or 120℃, and spray drying is performed to prepare the sodium iron pyrophosphate phosphate precursor.

[0076] In this invention, the types of sodium, iron, and phosphorus sources are not particularly limited. Exemplarily, the sodium source includes any one or a combination of at least two of sodium sulfate, sodium bicarbonate, sodium phosphate, sodium monohydrogen phosphate, or sodium dihydrogen phosphate; the iron source includes any one or a combination of at least two of ferric sulfate, ferric chloride, or ferric nitrate; and the phosphorus source includes any one or a combination of at least two of phosphoric acid, sodium phosphate, sodium hydrogen phosphate, or sodium dihydrogen phosphate.

[0077] In yet another embodiment, the present invention provides a sodium-ion battery comprising the modified graphene-coated sodium pyrophosphate as described in one of the preceding embodiments, or the modified graphene-coated sodium pyrophosphate prepared by the preparation method described in another of the preceding embodiments.

[0078] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0079] Example 1

[0080] This embodiment provides a modified graphene-coated sodium ferric phosphate pyrophosphate, comprising a sodium ferric phosphate pyrophosphate matrix and modified graphene with a mass percentage of 2 wt.% coated on the surface of the sodium ferric phosphate pyrophosphate matrix; the modified graphene comprises nitrogen-doped graphene and a copper plating layer formed in situ on the surface of the nitrogen-doped graphene, wherein the copper plating layer has a mass percentage of 35 wt.% in the modified graphene.

[0081] The preparation method of the modified graphene-coated sodium iron pyrophosphate includes:

[0082] (1) Sodium nitrate, ferric nitrate and sodium hydrogen phosphate were dispersed in water according to the stoichiometric ratio to obtain a precursor dispersion with a solid content of 25 wt%; the inlet temperature was set to 220 ℃ and the outlet temperature to 100 ℃, and the precursor was prepared by spray drying.

[0083] (2) According to the mass ratio, graphene oxide and the sodium iron pyrophosphate precursor prepared in step (1) are ultrasonically dispersed in water to obtain a graphene-coated precursor; the graphene-coated precursor is dispersed in a 1 mol / L hydrazine hydrate solution to obtain a precursor dispersion; the solid content of the graphene-coated precursor in the precursor dispersion is 5 wt.%.

[0084] (3) A 0.1 mol / L copper sulfate solution was added dropwise to the precursor dispersion to obtain a precursor slurry; the precursor slurry was mixed with urea and subjected to a hydrothermal reaction at 150°C for 5 h to prepare a modified graphene-coated precursor; the mass of the urea was 5 wt.% of the mass of the graphene-coated precursor.

[0085] (4) The modified graphene-coated precursor is calcined in an Ar / H2 mixture with a volume fraction of 5 vol% by heating, specifically by first heating to 350°C and holding for 6 h, then heating to 600°C and holding for 12 h to prepare the modified graphene-coated sodium pyrophosphate.

[0086] Example 2

[0087] This embodiment provides a modified graphene-coated sodium ferric phosphate pyrophosphate, comprising a sodium ferric phosphate pyrophosphate matrix and modified graphene with a mass percentage of 1 wt.% coated on the surface of the sodium ferric phosphate pyrophosphate matrix; the modified graphene comprises nitrogen-doped graphene and a copper plating layer formed in situ on the surface of the nitrogen-doped graphene, wherein the copper plating layer has a mass percentage of 30 wt.% in the modified graphene.

[0088] The preparation method of the modified graphene-coated sodium iron pyrophosphate includes:

[0089] (1) Sodium sulfate, ferric nitrate and phosphoric acid were dispersed in water according to the stoichiometric ratio to obtain a precursor dispersion with a solid content of 10 wt%; the inlet temperature was set to 200 ℃ and the outlet temperature to 80 ℃, and the precursor was prepared by spray drying.

[0090] (2) According to the mass ratio, graphene oxide and the sodium iron pyrophosphate precursor prepared in step (1) are ultrasonically dispersed in water to obtain a graphene-coated precursor; the graphene-coated precursor is dispersed in a 0.5 mol / L ascorbic acid solution to obtain a precursor dispersion; the solid content of the graphene-coated precursor in the precursor dispersion is 1 wt.%.

[0091] (3) A 0.05 mol / L copper nitrate solution was added dropwise to the precursor dispersion to obtain a precursor slurry; the precursor slurry was mixed with thiourea and subjected to a hydrothermal reaction at 120°C for 1 h to prepare a modified graphene-coated precursor; the mass of the thiourea was 3 wt.% of the mass of the graphene-coated precursor.

[0092] (4) The modified graphene-coated precursor is calcined in an Ar / H2 mixture with a volume fraction of 3 vol% by heating, specifically by first heating to 300°C and holding for 3 h, then heating to 500°C and holding for 8 h to prepare the modified graphene-coated sodium pyrophosphate.

[0093] Example 3

[0094] This embodiment provides a modified graphene-coated sodium ferric phosphate pyrophosphate, comprising a sodium ferric phosphate pyrophosphate matrix and modified graphene with a mass percentage of 3 wt.% coated on the surface of the sodium ferric phosphate pyrophosphate matrix; the modified graphene comprises nitrogen-doped graphene and a copper plating layer formed in situ on the surface of the nitrogen-doped graphene, wherein the copper plating layer has a mass percentage of 40 wt.% in the modified graphene.

[0095] The preparation method of the modified graphene-coated sodium iron pyrophosphate includes:

[0096] (1) Sodium nitrate, ferric nitrate and sodium hydrogen phosphate were dispersed in water according to the stoichiometric ratio to obtain a precursor dispersion with a solid content of 40 wt%; the inlet temperature was set to 250 ℃ and the outlet temperature to 120 ℃, and the precursor was prepared by spray drying.

[0097] (2) According to the mass ratio, graphene oxide and the sodium iron pyrophosphate precursor prepared in step (1) are ultrasonically dispersed in water to obtain a graphene-coated precursor; the graphene-coated precursor is dispersed in a 1.5 mol / L formaldehyde solution to obtain a precursor dispersion; the solid content of the graphene-coated precursor in the precursor dispersion is 10 wt.%.

[0098] (3) A 0.2 mol / L copper sulfate solution was added dropwise to the precursor dispersion to obtain a precursor slurry; the precursor slurry was mixed with melamine and subjected to a hydrothermal reaction at 180°C for 10 h to prepare a modified graphene-coated precursor; the mass of the melamine was 6 wt.% of the mass of the graphene-coated precursor.

[0099] (4) The modified graphene-coated precursor is calcined in an Ar / H2 mixture with a volume fraction of 6 vol% by heating, specifically by first heating to 400°C and holding for 8 h, then heating to 700°C and holding for 15 h to prepare the modified graphene-coated sodium pyrophosphate.

[0100] Example 4

[0101] This embodiment provides a modified graphene-coated iron phosphate sodium pyrophosphate. Except that the mass percentage of modified graphene in the modified graphene-coated iron phosphate sodium pyrophosphate is 0.5 wt.%, all other aspects are the same as in Example 1.

[0102] Example 5

[0103] This embodiment provides a modified graphene-coated iron phosphate sodium pyrophosphate. Except that the modified graphene content in the modified graphene-coated iron phosphate sodium pyrophosphate is 3.5 wt.%, all other aspects are the same as in Example 1.

[0104] Example 6

[0105] This embodiment provides a modified graphene-coated sodium iron pyrophosphate. Except that the copper plating layer in the modified graphene has a mass percentage content of 25 wt.%, the rest is the same as in Example 1.

[0106] Example 7

[0107] This embodiment provides a modified graphene-coated sodium iron pyrophosphate. Except that the copper plating layer in the modified graphene has a mass percentage content of 45 wt.%, the rest is the same as in Example 1.

[0108] Comparative Example 1

[0109] This comparative example provides a modified graphene-coated sodium iron pyrophosphate. Except that no copper plating layer is provided in the modified graphene, and in the preparation method, step (3) does not add copper sulfate solution to the precursor dispersion, but directly mixes the precursor dispersion with urea for hydrothermal reaction, the rest is the same as in Example 1.

[0110] Comparative Example 2

[0111] This comparative example provides a modified graphene-coated sodium iron pyrophosphate. Except that the modified graphene is not nitrogen-doped, and the preparation method is the same as Example 1 except that step (3) does not involve mixing the precursor slurry with urea for hydrothermal reaction.

[0112] Performance testing:

[0113] The composite carbon-coated sodium iron pyrophosphate, PVDF, and conductive carbon black provided in all the above embodiments and comparative examples were dispersed in NMP at a mass ratio of 92:4:4 to obtain a positive electrode slurry. The positive electrode slurry was coated on the surface of aluminum foil to prepare a positive electrode sheet. A sodium-ion battery was assembled using metallic sodium as the negative electrode. The rate performance and cycle performance of the battery were tested as follows:

[0114] The sodium-ion battery prepared above was charged at 0.2C and discharged at 0.2C, 1C, 3C, 5C, 8C and 10C respectively within a voltage range of 1.5V to 4V at 25℃. The rate capacity retention rate at 1C / 0.2C, 3C / 0.2C, 5C / 0.2C, 8C / 0.2C and 10C / 0.2C is calculated. The test results are shown in Table 1.

[0115] The sodium-ion battery prepared above was cycled for 200 cycles at a charge-discharge rate of 1C at 25℃ and within a voltage range of 1.5V to 4V. The cycle capacity retention rate was tested, and the test results are shown in Table 1.

[0116] Table 1

[0117]

[0118] In summary, this invention employs a dual strategy of nitrogen doping combined with copper plating to coat sodium iron pyrophosphate with modified graphene. The copper plating layer on the surface of the modified graphene forms a dual conductive network with the nitrogen-doped graphene, improving rate performance. At the same time, the flexible support of the graphene and the rigid protection of the copper plating layer work synergistically to maintain the structural stability of sodium iron pyrophosphate during charge and discharge, thereby improving the cycle performance of sodium iron pyrophosphate.

[0119] According to the test results of Example 1, Comparative Example 1, and Comparative Example 2, if the graphene is not simultaneously nitrogen-doped and in-situ reduced to form a copper plating layer, it is impossible to effectively construct a dual conductive network and maintain the structural stability of sodium iron pyrophosphate during charging and discharging, resulting in a decrease in the rate performance and cycle performance of the material.

[0120] Based on the test results of Examples 1, 4, and 5, the mass percentage of modified graphene affects the coating thickness of modified graphene on the surface of the cathode material. If the mass percentage of modified graphene is too low, the modified graphene coating on the cathode material surface is insufficient, simultaneously reducing the copper plating content and failing to effectively construct a dual conductive network and maintain the structural stability of sodium iron pyrophosphate during charging and discharging. If the mass percentage of modified graphene is too high, the modified graphene coating on the cathode material surface is too thick, simultaneously increasing the copper plating content, leading to Na... + Diffusion lag, and the excessive Cu doping disrupts the original stable composite structure, leading to a decrease in the rate performance and cycle performance of the material.

[0121] Based on the test results of Examples 1, 6, and 7, the mass percentage of the copper plating layer in the modified graphene affects the structural stability of the modified graphene during the charge-discharge cycle of the cathode material. If the mass percentage of the copper plating layer is too low, the rigid protection of the copper plating layer is insufficient, leading to easy detachment of the modified graphene during the charge-discharge cycle. If the mass percentage of the copper plating layer is too high, the excessive doping of Cu atoms causes lattice distortion, weakening the original stable composite structure of the cathode material and the modified graphene, resulting in a decrease in the structural stability of the material and a reduction in Na. + Diffusion kinetics lead to a decrease in the rate performance and cycling performance of materials.

[0122] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A modified graphene-coated sodium iron pyrophosphate phosphate, characterized by, The modified graphene coated sodium iron pyrophosphate phosphate comprises a sodium iron pyrophosphate phosphate matrix and modified graphene coated on the surface of the sodium iron pyrophosphate phosphate matrix. The modified graphene comprises nitrogen-doped graphene and a copper plating layer formed in situ on the surface of the nitrogen-doped graphene.

2. The modified graphene coated sodium ferric pyrophosphate of claim 1, wherein, In the modified graphene coated sodium iron pyrophosphate phosphate, the mass percentage of the modified graphene is 1wt.% to 3wt.%. And / or, in the modified graphene, the mass percentage of the copper plating layer is 30wt.% to 40wt.%.

3. A method for producing the modified graphene-coated sodium iron phosphate pyrophosphate of claim 1 or 2, characterized by, The preparation method comprises: According to the mass ratio, graphene oxide and sodium iron pyrophosphate phosphate precursor are dispersed in water to obtain a graphene coated precursor; the graphene coated precursor is dispersed in a reducing solution to obtain a precursor dispersion liquid; a copper salt solution is added to the precursor dispersion liquid to obtain a precursor slurry; the precursor slurry and a nitrogen-containing compound are mixed for hydrothermal reaction to prepare a modified graphene coated precursor; and the modified graphene coated precursor is calcined to prepare the modified graphene coated sodium iron pyrophosphate phosphate.

4. The production method according to claim 3, wherein The reducing agent in the reducing solution comprises any one or a combination of at least two of hydrazine hydrate, ascorbic acid or formaldehyde; And / or, the nitrogen-containing compound comprises any one or a combination of at least two of urea, melamine or thiourea; And / or, the copper salt comprises any one or a combination of at least two of copper nitrate, copper chloride, copper sulfate or copper acetate.

5. The production method according to claim 3 or 4, characterized by, The concentration of the reducing agent in the reducing solution is 0.5mol / L to 1.5mol / L; And / or, in the precursor dispersion liquid, the solid content of the graphene coated precursor is 1wt.% to 10wt.%; And / or, the concentration of the copper salt solution is 0.05mol / L to 0.2mol / L; And / or, the mass of the nitrogen-containing compound is 3wt.% to 6wt.% of the mass of the graphene coated precursor.

6. The production method according to any one of claims 3 to 5, wherein The dispersion mode comprises ultrasonic treatment; And / or, the temperature of the hydrothermal reaction is 120°C to 180°C; And / or, the time of the hydrothermal reaction is 1h to 10h.

7. The production method according to any one of claims 3 to 6, wherein The calcination comprises first increasing the temperature to a first temperature, maintaining the temperature for a first time, then continuously increasing the temperature to a second temperature, and maintaining the temperature for a second time.

8. The production method according to claim 7, wherein The first temperature is 300°C to 400°C, and the first time is 3h to 8h; And / or, the second temperature is 500°C to 700°C, and the second time is 8h to 15h; And / or, the atmosphere of the calcination comprises Ar / H2 mixed gas with a H2 volume fraction of 3vol% to 8vol%.

9. The production method according to any one of claims 3 to 8, wherein The preparation method of the sodium iron pyrophosphate phosphate precursor comprises: According to the stoichiometric ratio, a sodium source, an iron source and a phosphorus source are dispersed in water, and spray drying is performed to obtain the sodium iron pyrophosphate phosphate precursor.

10. A sodium-ion battery, characterized in that, The sodium ion battery comprises the modified graphene coated sodium iron pyrophosphate phosphate according to claim 1 or 2, or the modified graphene coated sodium iron pyrophosphate phosphate prepared by the preparation method according to any one of claims 3 to 9.

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