Composite carbon-coated ferric sodium pyrophosphate, preparation method and sodium ion battery

Abstract

The invention relates to composite carbon coated ferric sodium pyrophosphate, a preparation method and a sodium ion battery. The preparation method comprises the following steps: performing first mixing on a sodium source, an iron source and a phosphorus source in water to obtain a precursor solution; carrying out second mixing reaction on the precursor solution, a first additive and a second additive to obtain precursor sol; adding a carbon source dispersion liquid into the precursor sol to obtain a composite precursor sol; performing vacuum drying; sintering in an inert atmosphere to obtain composite carbon-coated ferric sodium pyrophosphate; a carbon source in the carbon source dispersion liquid comprises a carbon nanotube and graphene oxide; the first additive comprises monobasic acid and / or polybasic acid; the second additive comprises a monohydric alcohol and / or a polyhydric alcohol. On the basis of a sol-gel process, a composite carbon coating layer jointly formed by carbon nanotubes, reduced graphene oxide and amorphous carbon is formed on the surface of sodium ferric phosphate pyrophosphate in situ, so that the rate performance and the cycle performance of sodium ferric phosphate pyrophosphate are synchronously improved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of sodium ion batteries, in particular to a sodium ion battery positive electrode material, and especially to a composite carbon-coated sodium iron pyrophosphate phosphate, a preparation method and a sodium ion battery. BACKGROUND

[0002] With the in-depth development of sodium ion batteries, sodium iron pyrophosphate phosphate positive electrode materials with high theoretical capacity, excellent thermal stability and low cost advantages have become one of the research hotspots in the field of sodium ion batteries. In order to solve the problem of poor intrinsic electronic conductivity of sodium iron pyrophosphate phosphate, the existing technology generally adopts a carbon-coated modification strategy to build an electronic transmission channel and reduce the interface charge transfer impedance, thereby greatly improving the capacity retention rate of the battery during the charging and discharging process, laying a foundation for its practical application.

[0003] CN118183678A discloses a carbon-coated sodium iron pyrophosphate phosphate and a preparation method thereof, which comprises the following steps: mixing dodecyl phosphate with water and anhydrous ethanol, and performing first stirring under water bath conditions to obtain a self-assembly solution; dissolving an iron source, a sodium source and a phosphorus source in deionized water respectively, and adding them into the self-assembly solution in sequence, and performing second stirring under water bath conditions to obtain a sodium iron pyrophosphate phosphate precursor; drying and grinding the sodium iron pyrophosphate phosphate precursor; and sintering the treated sodium iron pyrophosphate phosphate precursor under vacuum conditions to obtain the carbon-coated sodium iron pyrophosphate phosphate. The method can ensure uniform distribution of various elements, more uniform morphology of the sodium iron pyrophosphate phosphate, higher phase purity, more stable structure, better carbon-coating effect, higher surface activity and good electrical conductivity, and the sodium iron pyrophosphate phosphate has excellent cycle stability when used as a positive electrode material.

[0004] CN117886295A discloses a plurality of carbon-coated sodium iron pyrophosphate phosphate positive electrode materials prepared by a full solid-phase reaction, and a preparation method thereof, which comprises the following steps: (1) uniformly mixing an iron source, a phosphoric acid source, a carbon source material and a sodium source material according to a proportion, and processing the mixture by using a ball mill to obtain a precursor; and (2) placing the precursor into a tube furnace, and obtaining a final product, i.e. the plurality of carbon-coated sodium iron pyrophosphate phosphate, by high-temperature calcination under the protection of an inert gas. The application improves the electronic conductivity and sodium ion diffusion capacity of the sodium iron pyrophosphate phosphate positive electrode material through in-situ carbon coating, and thus the battery has excellent performance. The method adopts a full solid-phase path, and has the advantages of simple preparation, controllable process, short synthesis period, suitability for batch production, significant practical value and good application prospect.

[0005] CN115974033A discloses a preparation method of nitrogen-doped mesoporous carbon-coated sodium iron pyrophosphate phosphate composite material, which comprises preparing a template organic solution; adding compounds containing pyrophosphate, phosphate, sodium source, nitrogen source and carbon source into the template organic solution; preparing an iron source solution; adding the iron source solution into the mixed solution to obtain a suspension; heating and stirring the suspension to dry, and obtaining a nano positive electrode precursor after drying; and performing solid phase sintering on the nano positive electrode precursor to obtain the nitrogen-doped mesoporous carbon-coated sodium iron pyrophosphate phosphate composite material; the invention simultaneously utilizes a soft template agent to play a role in the generation of the positive electrode material and the nitrogen-doped carbon material, and obtains a nitrogen-doped mesoporous carbon-coated spherical sodium iron pyrophosphate phosphate composite material with uniform particle size and nanometerization, which exhibits excellent performance in the high-rate discharge of a sodium ion battery.

[0006] Therefore, it is of great significance to provide a composite carbon-coated sodium iron pyrophosphate phosphate positive electrode material and a preparation method thereof, so as to further improve the rate performance and cycle performance of sodium iron pyrophosphate phosphate. SUMMARY

[0007] In view of the deficiencies of the prior art, the purpose of the present application is to provide a composite carbon-coated sodium iron pyrophosphate phosphate, a preparation method and a sodium ion battery. Based on the sol-gel process, the present application forms a composite carbon coating layer composed of carbon nanotubes-reduced graphene oxide-amorphous carbon on the surface of sodium iron pyrophosphate phosphate in situ, wherein the carbon nanotubes and the reduced graphene oxide are dispersedly distributed in the amorphous carbon, effectively improving the conductivity and sodium ion transmission rate of sodium iron pyrophosphate phosphate, and the composite carbon coating layer can fully absorb the stress generated by the volume strain of sodium iron pyrophosphate phosphate during charging and discharging, improving the structural stability of the material, thereby realizing the synchronous improvement of the rate performance and cycle performance of sodium iron pyrophosphate phosphate.

[0008] To achieve the purpose of the present application, the following technical solutions are adopted:

[0009] In a first aspect, the present application provides a preparation method of a composite carbon-coated sodium iron pyrophosphate phosphate, which comprises:

[0010] According to the stoichiometric ratio, a sodium source, an iron source and a phosphorus source are first mixed in water to obtain a precursor solution; the precursor solution, a first additive and a second additive are secondly mixed and reacted to obtain a precursor sol; a carbon source dispersion liquid is added into the precursor sol to obtain a composite precursor sol; vacuum drying is performed to obtain a composite precursor gel; and the composite precursor gel is sintered to obtain the composite carbon-coated sodium iron pyrophosphate phosphate; the carbon source in the carbon source dispersion liquid comprises a combination of carbon nanotubes and graphene oxide; the first additive comprises a monobasic acid and / or a polybasic acid; and the second additive comprises a monohydric alcohol and / or a polyhydric alcohol.

[0011] The present application is based on a sol-gel process, a first additive comprising -COOH and a second additive comprising -OH are added to the precursor solution, the first additive acts as a chelating agent and a reducing agent, and at the same time the first additive and the second additive undergo a certain degree of esterification reaction, effectively dispersing the sodium pyrophosphate precursor to form a precursor sol, and further adding a carbon source dispersion liquid comprising carbon nanotubes and graphene oxide to the precursor sol, the uniform dispersion of the carbon source and the sodium pyrophosphate iron precursor in the sol, after vacuum drying, forming a preliminary structure of the carbon source and the sodium pyrophosphate iron precursor uniformly dispersed in the gel matrix. In the subsequent sintering process, the gel matrix is carbonized in situ to form amorphous carbon, optimizing the interface between the carbon nanotubes and the reduced graphene oxide and the sodium pyrophosphate iron, the graphene oxide is reduced to reduced graphene oxide, forming a porous structure, and forming a composite carbon coating layer composed of carbon nanotubes-reduced graphene oxide-amorphous carbon on the surface of sodium pyrophosphate iron, providing a fast transmission channel for sodium ions, effectively improving the conductivity of sodium pyrophosphate iron and the transmission rate of sodium ions. At the same time, the composite carbon coating layer can fully absorb the stress generated by the volume strain of sodium pyrophosphate during charging and discharging, improving the structural stability of the material, thereby realizing the synchronous improvement of the rate performance and cycle performance of sodium pyrophosphate iron.

[0012] Preferably, the mass of the carbon source is 1.5wt%-2.5wt% of the total mass of the sodium source, iron source and phosphorus source in the precursor solution.

[0013] Preferably, the total mass of the first additive and the second additive is 3wt%-5wt% of the total mass of the sodium source, iron source and phosphorus source in the precursor solution.

[0014] Preferably, in the carbon source, the mass ratio of carbon nanotubes to graphene oxide is 1:(0.5-10).

[0015] Preferably, the mass ratio of the first additive to the second additive is 1:(0.4-0.8).

[0016] Preferably, the first additive comprises any one or a combination of at least two of formic acid, acetic acid, propionic acid, oxalic acid, malonic acid or citric acid.

[0017] Preferably, the second additive comprises any one or a combination of at least two of methanol, ethanol, propanol, ethylene glycol, propylene glycol or glycerol.

[0018] Preferably, the preparation method of the carbon source dispersion liquid comprises: dispersing graphene oxide in water, first ultrasonic treatment to obtain a graphene oxide dispersion liquid; adding carbon nanotubes to the graphene oxide dispersion liquid, second ultrasonic treatment to obtain the carbon source dispersion liquid.

[0019] Preferably, the second mixing reaction includes: first mixing the precursor solution with the first additive, adjusting the pH to 4-5, and then adding the second additive.

[0020] Preferably, the temperature of the second mixing reaction is 65°C to 85°C.

[0021] Preferably, the vacuum drying temperature is 55℃~75℃.

[0022] Preferably, the sintering includes a first stage of sintering and a second stage of sintering.

[0023] Preferably, the sintering temperature of the first stage is 300℃~400℃.

[0024] Preferably, the sintering time of the first stage is 1h to 3h.

[0025] Preferably, the sintering temperature of the second stage is 600℃~750℃.

[0026] Preferably, the sintering time for the second stage is 4h to 8h.

[0027] Preferably, the atmosphere for the first sintering stage includes nitrogen and / or an inert gas.

[0028] Preferably, the atmosphere for the second sintering stage includes an H2 / Ar mixture with a volume percentage of 3 vol% to 8 vol%.

[0029] Preferably, the heating rates of the first sintering stage and the second sintering stage are each independently 3℃ / min to 5℃ / min.

[0030] In a second aspect, the present invention provides a composite carbon-coated sodium ferric pyrophosphate, wherein the composite carbon-coated sodium ferric pyrophosphate is prepared by the preparation method described in the first aspect; the composite carbon-coated sodium ferric pyrophosphate comprises a sodium ferric pyrophosphate matrix and a composite carbon coating layer coated on the surface of the sodium ferric pyrophosphate matrix; the thickness of the composite carbon coating layer is 10 nm to 30 nm.

[0031] Thirdly, the present invention provides a sodium-ion battery comprising, as described in the second aspect, a composite carbon-coated sodium iron pyrophosphate.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] The application is based on a sol-gel process, and a composite carbon coating layer composed of carbon nanotubes, reduced graphene oxide and amorphous carbon is formed in situ on the surface of sodium iron phosphate pyrophosphate, wherein the carbon nanotubes and the reduced graphene oxide are dispersedly distributed in the amorphous carbon, effectively improving the conductivity and sodium ion transmission rate of the sodium iron phosphate pyrophosphate, and the composite carbon coating layer can fully absorb the stress generated by the volume strain of the sodium iron phosphate pyrophosphate during the charging and discharging process, thereby improving the structural stability of the material, and achieving the synchronous improvement of the rate performance and cycle performance of the sodium iron phosphate pyrophosphate. DETAILED DESCRIPTION

[0034] The technical solutions of the application will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only used to help understand the application, and should not be regarded as specific limitations on the application.

[0035] The "range" disclosed in the application can be limited in the form of lower limit and upper limit, a given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a specific range. The range limited in this way can include or not include the end value, either end value can be independently included or not included, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if the ranges of 60-120 and 80-110 are listed for a specific parameter, it is understood that the ranges of 60-110 and 80-120 are also expected. In addition, if the minimum range values of 1 and 2 are listed, and if the maximum range values of 3, 4 and 5 are also listed, the following ranges can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the application, unless otherwise stated, the numerical range "a-b" represents a shorthand representation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0-5" represents that all real numbers between "0-5" have been listed herein, and "0-5" is only a shorthand representation of these numerical combinations. In addition, when it is stated that a parameter is an integer ≥2, it is equivalent to listing the parameter as, for example, integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For example, when it is stated that a parameter is an integer selected from "2-10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9 and 10.

[0036] In the application, "combination of at least two" is referred to, which, unless otherwise specified, means greater than or equal to 2 in quantity. For example, "any one or a combination of at least two" means one or more than or equal to two. It can be understood that when "combination of at least two" is referred to, it means a combination of any suitable number of items, that is, a combination of "at least two" items in a manner that does not conflict and can implement the application.

[0037] All embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions if there is no special description.

[0038] The term "embodiment" mentioned in the present application means that the specific features, structures or properties described in connection with the embodiment can be included in at least one embodiment or implementation of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment to other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described in the present application can be combined with other embodiments.

[0039] Those skilled in the art can understand that in the method of each embodiment, the writing order of each step does not mean a strict execution order, and the detailed execution order of each step should be determined by its function and possible internal logic. If there is no special description, all steps of the present application can be performed in sequence, or randomly, and can be preferably performed in sequence. For example, the method comprises step (a) and step (b), which means that the method can comprise step (a) and step (b) performed in sequence, or step (b) and step (a) performed in sequence. For example, the method can also comprise step (c), which means that step (c) can be added to the method in any order, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0040] In the present application, the open technical features or technical solutions described by the words such as "include" and the like, if there is no other description, do not exclude additional members from the listed members, which can be regarded as providing both the closed features or technical solutions composed of the listed members, and the open features or solutions including additional members in addition to the listed members. For example, A includes a1, a2 and a3, if there is no other description, it can also include other members, or it can not include additional members, which can be regarded as providing the technical features or technical solutions of "A is composed of a1, a2 and a3" or "A is selected from a1, a2 and a3", and also providing the technical features or technical solutions of "A not only includes a1, a2 and a3, but also includes other members".

[0041] In the present application, “and / or” corresponds to any one of two or more related listed items, or any and all combinations of the related listed items, including any two related listed items, any more related listed items, or all related listed items, unless otherwise specified. For example, “A and / or B” represents a group consisting of A, B, and a combination of A and B. Wherein “including A and / or B” can mean “including A, including B, and including A and B”, or “including A, including B, or including A and B”, which can be properly understood according to the sentence.

[0042] In the present application, the terms “first”, “second”, “third” and the like in “first aspect”, “second aspect”, “third aspect” and the like are only for description purposes, and cannot be understood as indicating or implying relative importance or quantity, nor can they be understood as implying the importance or quantity of the indicated technical features. Moreover, “first”, “second”, “third” and the like only serve the purpose of non-exhaustive enumeration description, and should be understood as not constituting a closed limitation on the quantity.

[0043] In the present application, “optional” means optional, i.e. selected from either of the two parallel schemes “yes” or “no”. If there are multiple “optional” in a technical solution, each “optional” is independent of each other, unless otherwise specified, and there is no contradiction or mutual restriction.

[0044] In the present application, “room temperature” generally refers to 4℃-35℃, and can refer to 20℃±5℃. In some embodiments of the present application, room temperature refers to 20℃-30℃.

[0045] In one specific embodiment, the present application provides a preparation method of composite carbon-coated sodium iron pyrophosphate, which comprises:

[0046] According to the stoichiometric ratio, a sodium source, an iron source and a phosphorus source are first mixed in water to obtain a precursor solution; the precursor solution, a first additive and a second additive are secondly mixed to obtain a precursor sol; a carbon source dispersion liquid is added to the precursor sol to obtain a composite precursor sol; vacuum drying is performed to obtain a composite precursor gel; and the composite precursor gel is sintered to obtain the composite carbon-coated sodium iron pyrophosphate; the carbon source in the carbon source dispersion liquid comprises a combination of carbon nanotubes and graphene oxide; the first additive comprises a monobasic acid and / or a polybasic acid; and the second additive comprises a monohydric alcohol and / or a polyhydric alcohol.

[0047] The present application is based on a sol-gel process, adding a first additive including -COOH and a second additive including -OH to the precursor solution, the first additive acts as a chelating agent and a reducing agent, while the first additive and the second additive undergo a certain degree of esterification reaction, effectively dispersing the sodium pyrophosphate precursor to form a precursor sol, further adding a carbon source dispersion liquid including carbon nanotubes and graphene oxide to the precursor sol, uniform dispersion of the carbon source and the sodium pyrophosphate iron precursor in the sol, after vacuum drying, a preliminary structure of the carbon source and the sodium pyrophosphate iron precursor uniformly dispersed in the gel matrix is formed. In the subsequent sintering process, the gel matrix is carbonized in situ to form amorphous carbon, optimizing the interface between the carbon nanotubes and the reduced graphene oxide and the sodium pyrophosphate iron, the graphene oxide is reduced to reduced graphene oxide, forming a porous structure, and an in-situ formed composite carbon coating layer composed of carbon nanotubes-reduced graphene oxide-amorphous carbon on the surface of sodium pyrophosphate iron, providing a fast transmission channel for sodium ions, effectively improving the conductivity of sodium pyrophosphate iron and the transmission rate of sodium ions. At the same time, the composite carbon coating layer can fully absorb the stress generated by the volume strain of sodium pyrophosphate during charging and discharging, improving the structural stability of the material, thereby realizing the synchronous improvement of the rate performance and cycle performance of sodium pyrophosphate iron.

[0048] In some embodiments, the mass of the carbon source is 1.5wt%-2.5wt% of the total mass of the sodium source, the iron source and the phosphorus source in the precursor solution, for example, it can be 1.5wt%, 1.75wt%, 2wt%, 2.25wt% or 2.5wt%.

[0049] In some embodiments, the total mass of the first additive and the second additive is 3wt%-5wt% of the total mass of the sodium source, the iron source and the phosphorus source in the precursor solution, for example, it can be 3wt%, 3.5wt%, 4wt%, 4.5wt% or 5wt%.

[0050] In some embodiments, the mass ratio of carbon nanotubes to graphene oxide in the carbon source is 1:(0.5-10), for example, it can be 1:0.5, 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9 or 1:10.

[0051] In some embodiments, the mass ratio of the first additive to the second additive is 1:(0.4-0.8), for example, it can be 1:0.4, 1:0.45, 1:0.5, 1:0.55, 1:0.6, 1:0.65, 1:0.7, 1:0.75 or 1:0.8.

[0052] In some embodiments, the first additive comprises any one or a combination of at least two of formic acid, acetic acid, propionic acid, oxalic acid, malonic acid, or citric acid, typically but not limitedly a combination of acetic acid and propionic acid, a combination of oxalic acid and malonic acid, or a combination of acetic acid and citric acid.

[0053] In some embodiments, the second additive comprises any one or a combination of at least two of methanol, ethanol, propanol, ethylene glycol, propylene glycol, or glycerol, typically but not limitedly a combination of methanol and ethanol, a combination of propanol and ethylene glycol, or a combination of propylene glycol and glycerol.

[0054] In the present application, the types of sodium source, iron source, and phosphorus source are not particularly limited, and exemplarily, the sodium source comprises any one or a combination of at least two of sodium sulfate, sodium bicarbonate, sodium phosphate, monosodium phosphate, or disodium phosphate; the iron source comprises any one or a combination of at least two of iron sulfate, iron chloride, or iron nitrate; and the phosphorus source comprises any one or a combination of at least two of phosphoric acid, sodium phosphate, sodium hydrogen phosphate, or disodium hydrogen phosphate.

[0055] In some embodiments, the preparation method of the carbon source dispersion liquid comprises: dispersing graphene oxide in water, first ultrasonic treatment, to obtain a graphene oxide dispersion liquid; adding carbon nanotubes to the graphene oxide dispersion liquid, second ultrasonic treatment, to obtain the carbon source dispersion liquid.

[0056] In some embodiments, the carbon nanotubes are oxidized carbon nanotubes.

[0057] In the present application, the preparation method of the oxidized carbon nanotubes is not particularly limited, and exemplarily, the oxidized carbon nanotubes are prepared by the following method:

[0058] The carbon nanotubes are placed in concentrated nitric acid, stirred and refluxed at 70°C to 80°C to introduce carboxyl groups, washed after cooling, and vacuum dried at 50°C to 70°C, to obtain the oxidized carbon nanotubes.

[0059] In some embodiments, the second mixing reaction is performed by first mixing the precursor solution with the first additive, adjusting the pH to 4 to 5, for example, 4, 4.2, 4.4, 4.6, 4.8, or 5, and then adding the second additive.

[0060] In some embodiments, the temperature of the second mixing reaction is 65°C to 85°C, for example, 65°C, 70°C, 75°C, 80°C, or 85°C.

[0061] In some embodiments, the temperature of the vacuum drying is 55°C to 75°C, for example, 55°C, 60°C, 65°C, 70°C, or 75°C.

[0062] In some embodiments, the sintering comprises a first-stage sintering and a second-stage sintering.

[0063] In some embodiments, the temperature of the first-stage sintering is 300-400°C, for example, can be 300°C, 320°C, 340°C, 360°C, 380°C or 400°C.

[0064] In some embodiments, the time of the first-stage sintering is 1-3h, for example, can be 1h, 1.5h, 2h, 2.5h or 3h.

[0065] In some embodiments, the temperature of the second-stage sintering is 600-750°C, for example, can be 600°C, 650°C, 675°C, 700°C, 725°C or 750°C.

[0066] In some embodiments, the time of the second-stage sintering is 4-8h, for example, can be 4h, 5h, 6h, 7h or 8h.

[0067] In some embodiments, the atmosphere of the first-stage sintering comprises nitrogen and / or an inert gas, wherein the inert gas comprises helium and / or argon.

[0068] In some embodiments, the atmosphere of the second-stage sintering comprises H2 / Ar mixed gas with H2 volume percentage of 3-8vol%, for example, the volume percentage of H2 in the mixed gas can be 3vol%, 4vol%, 5vol%, 6vol%, 7vol% or 8vol%.

[0069] In some embodiments, the heating rate of the first-stage sintering and the second-stage sintering is independently 3-5°C / min, for example, can be 3°C / min, 3.5°C / min, 4°C / min, 4.5°C / min or 5°C / min.

[0070] In another specific embodiment, the present application provides a composite carbon-coated sodium iron pyrophosphate phosphate, which is prepared by the preparation method described in the aforementioned one specific embodiment; the composite carbon-coated sodium iron pyrophosphate phosphate comprises a sodium iron pyrophosphate phosphate matrix and a composite carbon coating layer coated on the surface of the sodium iron pyrophosphate phosphate matrix; the thickness of the composite carbon coating layer is 10-30nm, for example, can be 10nm, 15nm, 20nm, 25nm or 30nm.

[0071] In still another specific embodiment, the present application provides a sodium ion battery, which comprises the composite carbon-coated sodium iron pyrophosphate phosphate as described in the aforementioned still another specific embodiment.

[0072] The numerical ranges recited herein include all values from and including the lower and upper values. This is true even if the upper and lower values are stated to be limited. Rounding according to significant figures has been employed. Where a numerical range is recited, it is intended to include all individual values within that range, including the recited values.

[0073] Example 1

[0074] The embodiment provides a preparation method of composite carbon-coated sodium iron pyrophosphate phosphate, and the preparation method comprises the following steps:

[0075] (1) according to the stoichiometric ratio, the first mixing of sodium carbonate, iron sulfate and phosphoric acid in water to obtain a precursor solution; the precursor solution is heated to 80 DEG C, citric acid is added into the precursor solution, the pH is adjusted to 4.5, ethylene glycol is added, the second mixing reaction is carried out, and the precursor sol is obtained; the mass ratio of citric acid and ethylene glycol is 1:0.5, and the total mass of citric acid and ethylene glycol is 4wt% of the total mass of sodium carbonate, iron sulfate and phosphoric acid;

[0076] (2) the graphene oxide is dispersed in water, and ultrasonic treatment is carried out to obtain an oxidized graphene dispersion solution; the carbon nanotube is added into the graphene oxide dispersion solution, and ultrasonic treatment is carried out to obtain the carbon source dispersion solution; wherein the mass ratio of carbon nanotube and graphene oxide is 1:2;

[0077] (3) the carbon source dispersion solution obtained in step (2) is slowly added into the precursor sol obtained in step (1) to obtain a composite precursor sol, wherein the total mass of carbon source is 2wt% of the total mass of sodium carbonate, iron sulfate and phosphoric acid; 60 DEG C vacuum drying is carried out to obtain a composite precursor gel;

[0078] (4) the composite precursor gel prepared in step (3) is crushed, placed in an argon tube furnace, heated to 350 DEG C at 4 DEG C / min, and kept for 2h; then 5vol% H2 / Ar mixed gas is introduced, and the temperature is continuously increased to 650 DEG C at 3 DEG C / min, and kept for 6h, to obtain composite carbon-coated sodium iron pyrophosphate phosphate with a composite carbon-coated layer thickness of 22nm.

[0079] Example 2

[0080] The embodiment provides a preparation method of composite carbon-coated sodium iron pyrophosphate phosphate, and the preparation method comprises the following steps:

[0081] (1) according to the stoichiometric ratio, the first mixing of sodium carbonate, iron sulfate and phosphoric acid in water to obtain a precursor solution; the precursor solution is heated to 65 DEG C, citric acid is added into the precursor solution, the pH is adjusted to 4, ethylene glycol is added, the second mixing reaction is carried out, and the precursor sol is obtained; the mass ratio of citric acid and ethylene glycol is 1:0.4, and the total mass of citric acid and ethylene glycol is 3wt% of the total mass of sodium carbonate, iron sulfate and phosphoric acid;

[0082] (2) dispersing graphene oxide in water, ultrasonic treatment to obtain a graphene oxide dispersion; adding carbon nanotubes to the graphene oxide dispersion, ultrasonic treatment to obtain the carbon source dispersion; wherein the mass ratio of carbon nanotubes to graphene oxide is 1:1;

[0083] (3) slowly adding the carbon source dispersion obtained in step (2) into the precursor sol obtained in step (1) to obtain a composite precursor sol, wherein the total mass of the carbon source is 1.5wt% of the total mass of sodium carbonate, iron sulfate and phosphoric acid; vacuum drying at 55℃ to obtain a composite precursor gel;

[0084] (4) crushing the composite precursor gel prepared in step (3), placing it in an argon tube furnace, heating to 300℃ at a rate of 3℃ / min, and keeping the temperature for 1h; then introducing H2 / Ar mixed gas with a volume percentage of H2 of 3vol%, and continuing to heat to 600℃ at a rate of 3.5℃ / min, and keeping the temperature for 4h to obtain composite carbon-coated sodium iron pyrophosphate phosphate with a carbon-coated layer thickness of 10nm.

[0085] Example 3

[0086] The embodiment provides a preparation method of composite carbon-coated sodium iron pyrophosphate phosphate, and the preparation method comprises the following steps:

[0087] (1) mixing sodium carbonate, iron sulfate and phosphoric acid in water according to the stoichiometric ratio to obtain a precursor solution; heating the precursor solution to 85℃, adding citric acid to the precursor solution to adjust the pH to 5, and adding ethylene glycol to perform a second mixing reaction to obtain a precursor sol; the mass ratio of citric acid to ethylene glycol is 1:0.8, and the total mass of citric acid and ethylene glycol is 5wt% of the total mass of sodium carbonate, iron sulfate and phosphoric acid;

[0088] (2) dispersing graphene oxide in water, ultrasonic treatment to obtain a graphene oxide dispersion; adding carbon nanotubes to the graphene oxide dispersion, ultrasonic treatment to obtain the carbon source dispersion; wherein the mass ratio of carbon nanotubes to graphene oxide is 1:10;

[0089] (3) slowly adding the carbon source dispersion obtained in step (2) into the precursor sol obtained in step (1) to obtain a composite precursor sol, wherein the total mass of the carbon source is 2.5wt% of the total mass of sodium carbonate, iron sulfate and phosphoric acid; vacuum drying at 75℃ to obtain a composite precursor gel;

[0090] (4) The composite precursor gel prepared in step (3) is crushed and placed in an argon tube furnace, heated to 400℃ at a rate of 5℃ / min, and kept for 3h; then 5vol% H2 / Ar mixed gas is introduced, and the temperature is continuously increased to 750℃ at a rate of 4℃ / min, and kept for 8h, to obtain the composite carbon-coated sodium iron pyrophosphate phosphate with a carbon coating layer thickness of 30nm.

[0091] Example 4

[0092] The present example provides a preparation method of composite carbon-coated sodium iron pyrophosphate phosphate, which is the same as that of Example 1, except that the total mass of citric acid and ethylene glycol in step (1) is 7.0wt% of the total mass of sodium carbonate, iron sulfate and phosphoric acid.

[0093] Example 5

[0094] The present example provides a preparation method of composite carbon-coated sodium iron pyrophosphate phosphate, which is the same as that of Example 1, except that the total mass of citric acid and ethylene glycol in step (1) is 7.0wt% of the total mass of sodium carbonate, iron sulfate and phosphoric acid.

[0095] Example 6

[0096] The present example provides a preparation method of composite carbon-coated sodium iron pyrophosphate phosphate, which is the same as that of Example 1, except that the total mass of carbon source in step (3) is 1.0wt% of the total mass of sodium carbonate, iron sulfate and phosphoric acid.

[0097] Example 7

[0098] The present example provides a preparation method of composite carbon-coated sodium iron pyrophosphate phosphate, which is the same as that of Example 1, except that the total mass of carbon source in step (3) is 3.0wt% of the total mass of sodium carbonate, iron sulfate and phosphoric acid.

[0099] Comparative Example 1

[0100] The present comparative example provides a preparation method of composite carbon-coated sodium iron pyrophosphate phosphate, which is the same as that of Example 1, except that an equal mass of citric acid is used to replace ethylene glycol in step (1).

[0101] Comparative Example 2

[0102] The present comparative example provides a preparation method of composite carbon-coated sodium iron pyrophosphate phosphate, which is the same as that of Example 1, except that an equal mass of graphene oxide is used to replace carbon nanotubes in step (2), and no carbon nanotubes are added, and the carbon source dispersion liquid only includes graphene oxide.

[0103] Comparative Example 3

[0104] The comparative example provides a preparation method of composite carbon-coated sodium iron pyrophosphate phosphate, wherein step (2) is replaced by using equal mass of carbon nanotubes instead of graphene oxide, and the carbon source dispersion liquid only includes carbon nanotubes.

[0105] Performance test:

[0106] The composite carbon-coated sodium iron pyrophosphate phosphate provided by all the above examples and comparative examples is dispersed in NMP according to a mass ratio of 94:3:3 with PVDF and conductive carbon black to obtain a positive electrode slurry, the positive electrode slurry is coated on the surface of an aluminum foil to prepare a positive electrode sheet; a sodium ion battery is assembled by taking metal sodium as a negative electrode, and the rate performance and cycle performance of the battery are tested, as follows:

[0107] At 25 DEG C, in the voltage range of 1.6V-3.8V, the above-prepared sodium ion battery is charged at 0.2C and discharged at 0.2C, 0.5C, 1C, 2C, 3C and 5C, respectively, the rate capacity retention rate of 0.5C / 0.2C, 1C / 0.2C, 2C / 0.2C, 3C / 0.2C and 5C / 0.2C is calculated, and the test results are shown in Table 1.

[0108] At 25 DEG C, in the voltage range of 1.6V-3.8V, the above-prepared sodium ion battery is charged and discharged at 0.2C, and the cycle capacity retention rate is tested after 500 cycles, and the test results are shown in Table 1.

[0109] Table 1

[0110]

[0111] In summary, based on the sol-gel process, the composite carbon coating layer composed of carbon nanotubes-reduced graphene oxide-amorphous carbon is formed in situ on the surface of sodium iron pyrophosphate phosphate, wherein the carbon nanotubes and reduced graphene oxide are dispersedly distributed in the amorphous carbon, which effectively improves the conductivity of sodium iron pyrophosphate phosphate and the sodium ion transmission rate, and the composite carbon coating layer can fully absorb the stress generated by the volume strain of sodium iron pyrophosphate phosphate during charging and discharging, thereby improving the structural stability of the material, so as to realize the synchronous improvement of the rate performance and cycle performance of sodium iron pyrophosphate phosphate.

[0112] According to the test results of Example 1, Example 4, Example 5 and Comparative Example 1, if the total amount of the first additive and the second additive in the precursor solution is low or high, or even any one of the additives is absent, a stable precursor colloidal solution cannot be formed, which greatly reduces the coating effect of the composite carbon source and the uniformity of the subsequent sintered material, seriously damages the structural stability of the composite carbon-coated sodium iron pyrophosphate phosphate during the charging and discharging process, and causes the initial capacity, rate performance and cycle performance of the composite carbon-coated sodium iron pyrophosphate phosphate to decrease.

[0113] According to the test results of Example 1 and Example 6 and Example 7, the mass percentage content of the total amount of carbon sources affects the conductivity of the composite carbon-coated sodium iron pyrophosphate phosphate. If the mass percentage content of the total amount of carbon sources is too low, the conductivity of the composite carbon-coated sodium iron pyrophosphate phosphate is poor, which affects the electron transmission efficiency, increases the electrode polarization, accelerates the deterioration of the composite carbon-coated sodium iron pyrophosphate phosphate, and causes the rate performance and cycle performance of the composite carbon-coated sodium iron pyrophosphate phosphate to decrease. If the mass percentage content of the total amount of carbon sources is too high, it hinders the transmission of sodium ions and occupies too much proportion of active material, thereby reducing the specific capacity.

[0114] According to the test results of Example 1 and Comparative Example 2 and Comparative Example 3, if graphene oxide is absent in the composite carbon coating layer, the coating integrity of the carbon nanotube will be greatly reduced, and if carbon nanotubes are absent, the coating toughness and stability of graphene oxide will be reduced. Therefore, the absence of any one of graphene oxide or carbon nanotubes will affect the stability of the composite carbon coating structure, and further cause the rate performance and cycle performance of the composite carbon-coated sodium iron pyrophosphate phosphate to decrease.

[0115] The applicant declares that the above description is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto. It should be understood by those skilled in the art that any changes or replacements within the technical scope disclosed by the present application can be easily thought of by those skilled in the art, and all fall within the protection scope and disclosure scope of the present application.

Claims

1. A method for preparing a composite carbon-coated sodium iron pyrophosphate phosphate, characterized in that, The preparation method comprises: According to a stoichiometric ratio, a sodium source, an iron source and a phosphorus source are first mixed in water to obtain a precursor solution; the precursor solution, a first additive and a second additive are second mixed to obtain a precursor sol; a carbon source dispersion liquid is added to the precursor sol to obtain a composite precursor sol; vacuum drying is performed to obtain a composite precursor gel; and the composite precursor gel is sintered to obtain the composite carbon-coated sodium iron pyrophosphate; The carbon source in the carbon source dispersion liquid comprises a combination of carbon nanotubes and graphene oxide; The first additive comprises a monobasic acid and / or a polybasic acid; The second additive comprises a monohydric alcohol and / or a polyhydric alcohol.

2. The production method according to claim 1, wherein The mass of the carbon source is 1.5wt%-2.5wt% of the total mass of the sodium source, the iron source and the phosphorus source in the precursor solution; And / or, the total mass of the first additive and the second additive is 3wt%-5wt% of the total mass of the sodium source, the iron source and the phosphorus source in the precursor solution; And / or, in the carbon source, the mass ratio of carbon nanotubes to graphene oxide is 1:(0.5-10); And / or, the mass ratio of the first additive to the second additive is 1:(0.4-0.8).

3. The production method according to claim 1 or 2, characterized by, The first additive comprises any one or a combination of at least two of formic acid, acetic acid, propionic acid, oxalic acid, malonic acid or citric acid; And / or, the second additive comprises any one or a combination of at least two of methanol, ethanol, propanol, ethylene glycol, propylene glycol or glycerol.

4. The production method according to any one of claims 1 to 3, wherein The preparation method of the carbon source dispersion liquid comprises: The graphene oxide is dispersed in water, first ultrasonic treatment is performed to obtain a graphene oxide dispersion liquid; carbon nanotubes are added to the graphene oxide dispersion liquid, second ultrasonic treatment is performed to obtain the carbon source dispersion liquid.

5. The production method according to any one of claims 1 to 4, wherein The second mixing reaction comprises: The precursor solution and the first additive are first mixed, the pH is adjusted to 4-5, and then the second additive is added.

6. The production method according to any one of claims 1 to 5, wherein The temperature of the second mixing reaction is 65°C-85°C; And / or, the temperature of the vacuum drying is 55°C-75°C.

7. The production method according to any one of claims 1 to 6, wherein The sintering comprises first-stage sintering and second-stage sintering.

8. The production method according to claim 7, wherein The temperature of the first-stage sintering is 300°C-400°C; And / or, the time of the first-stage sintering is 1h-3h; And / or, the temperature of the second-stage sintering is 600°C-750°C; And / or, the time of the second-stage sintering is 4h-8h; And / or, the atmosphere of the first-stage sintering comprises nitrogen and / or inert gas; And / or, the atmosphere of the second-stage sintering comprises H2 / Ar mixed gas with a H2 volume percentage of 3vol%-8vol%; And / or, the heating rate of the first-stage sintering and the second-stage sintering is independently 3°C / min-5°C / min.

9. A composite carbon-coated sodium iron pyrophosphate phosphate characterized by, The composite carbon-coated sodium iron pyrophosphate is prepared by the preparation method of any one of claims 1-8; The composite carbon-coated sodium iron pyrophosphate comprises a sodium iron pyrophosphate matrix and a composite carbon coating layer coated on the surface of the sodium iron pyrophosphate matrix; The thickness of the composite carbon coating layer is 10nm-30nm.

10. A sodium-ion battery, characterized in that, The sodium-ion battery comprises the composite carbon-coated sodium iron pyrophosphate as claimed in claim 9.

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