CN113285072A - Pyrophosphate composite material, sodium ion battery anode, sodium ion battery cathode and sodium ion battery - Google Patents

Pyrophosphate composite material, sodium ion battery anode, sodium ion battery cathode and sodium ion battery Download PDF

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CN113285072A
CN113285072A CN202110840248.9A CN202110840248A CN113285072A CN 113285072 A CN113285072 A CN 113285072A CN 202110840248 A CN202110840248 A CN 202110840248A CN 113285072 A CN113285072 A CN 113285072A
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pyrophosphate
sodium
ion battery
composite material
sodium ion
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CN113285072B (en
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王芳
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Nanjie Zhihui Shenzhen Technology Co ltd
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    • H—ELECTRICITY
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    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/362—Composites
    • H01M4/366—Composites as layered products
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
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    • H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
    • H01M4/136—Electrodes based on inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy
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    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
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    • H—ELECTRICITY
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Abstract

The invention relates to the technical field of sodium ion batteries, and particularly provides a pyrophosphate composite material, a sodium ion battery anode, a sodium ion battery cathode and a sodium ion battery. The pyrophosphate composite material has a core-shell structure, the core is vanadium sodium pyrophosphate, the shell is carbon, and the vanadium sodium pyrophosphate is in a hollow spherical or hollow sphere-like structure. The pyrophosphate composite material provided by the invention has good sodium ion diffusion characteristics, and when the pyrophosphate composite material is used as a positive electrode active material or a negative electrode active material of a sodium ion battery, the sodium ion battery has good cycle stability and rate characteristics, and has higher energy density.

Description

Pyrophosphate composite material, sodium ion battery anode, sodium ion battery cathode and sodium ion battery
Technical Field
The invention belongs to the technical field of sodium ion batteries, and particularly relates to a pyrophosphate composite material, a sodium ion battery anode, a sodium ion battery cathode and a sodium ion battery.
Background
Because of the abundant reserve of sodium, sodium ion batteries have a great cost advantage compared with lithium ion batteries, and therefore have good development prospects in the field of large-scale energy storage battery application.
The common cathode material of the existing sodium ion battery comprises a layered oxide, Prussian blue, a phosphate system and the like, wherein the layered oxide such as sodium transition metal oxide has specific capacity close to that of the layered oxide as the cathode of the lithium ion battery; prussian blue has lower cost; the phosphate system has higher structural stability. However, the high capacity layered oxide positive electrode material has poor cycle stability; the Prussian blue with low cost has low specific capacity; the phosphate system with stable structure has low electronic conductivity and poorer rate performance.
The cathode materials commonly used in the existing sodium ion battery comprise hard carbon, soft carbon and the like, for example, the hard carbon-metal oxide-soft carbon composite material has the advantages of large reversible capacity, good cycle performance and the like. However, negative electrode materials such as hard carbon and soft carbon also have problems such as low coulombic efficiency and poor rate capability.
Therefore, the development of a novel positive active material and a novel negative active material is critical to the development of sodium ion batteries.
Disclosure of Invention
The first aspect of the embodiments of the present invention is to provide a pyrophosphate composite material, so as to solve the problems of poor cycle performance, poor rate performance, etc. of an anode active material of an existing sodium ion battery, or low coulombic efficiency, poor rate performance, etc. of a cathode active material.
In order to achieve the purpose, the technical scheme of the invention is as follows:
the pyrophosphate composite material has a core-shell structure, the core is vanadium sodium pyrophosphate, the shell is carbon, and the vanadium sodium pyrophosphate is in a hollow spherical or hollow sphere-like structure.
Further, the general formula of the sodium vanadium pyrophosphate is Na7±xV3±y(P2O7)4Wherein x is more than or equal to 0 and less than or equal to 0.45; y is more than or equal to 0 and less than or equal to 0.4;
and/or the wall body of the sodium vanadium pyrophosphate is in a porous structure.
Further, the sodium vanadium pyrophosphate comprises Na6.55V3.3(P2O7)4、Na6.9V2.8(P2O7)4、Na7.4V2.6(P2O7)4At least one of (1).
Further, the particle size of the pyrophosphate composite primary particles is 50 nm-300 nm; the particle size of the pyrophosphate composite secondary particles is 1-20 mu m;
and/or the pyrophosphate composite material is prepared according to the following preparation method:
mixing vanadium pentoxide with a template agent and a solvent, carrying out heat treatment at 200-250 ℃, and then carrying out treatment at 280-350 ℃ in an air atmosphere to obtain V2O5Micron hollow spheres;
the V is put into2O5Mixing the micron hollow spheres and sodium salt in a molar ratio which is excessive than that of a target product of the pyrophosphate composite material to obtain a first mixed material;
mixing and stirring the first mixed material and a carbon source in deionized water, uniformly mixing, and evaporating the deionized water until the deionized water is dried to obtain precursor powder;
placing the precursor powder in an argon atmosphere at 250-400 ℃ for 2-5 h, then sintering at 600-700 ℃ for 8-20 h, cooling, and washing in a water bath at 40-100 ℃ to obtain the pyrophosphate composite material;
wherein the template agent comprises any one of oxalic acid and citric acid; the solvent includes any one of a mixed solvent of water and methanol and a mixed solvent of water and ethanol.
Further, the carbon content in the pyrophosphate composite material is 0.1 wt.% to 15 wt.%.
In a second aspect, the embodiment of the present invention further provides a positive electrode of a sodium ion battery, where the positive electrode of the sodium ion battery includes a positive electrode active layer, and the positive electrode active layer contains the pyrophosphate composite material.
Furthermore, in the active layer of the positive electrode, the mass content of the pyrophosphate composite material is more than or equal to 70%.
In a third aspect, the embodiment of the invention further provides a sodium ion battery negative electrode, which includes a negative active layer, and the negative active layer contains the pyrophosphate composite material.
Further, in the negative active layer, the mass content of the pyrophosphate composite material is more than or equal to 70%.
In a fourth aspect, embodiments of the present invention further provide a sodium ion battery, where the sodium ion battery includes a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode for separating the positive electrode and the negative electrode, and the positive electrode is the positive electrode of the sodium ion battery;
or, the negative electrode is the negative electrode of the sodium-ion battery.
The invention has the beneficial effects that:
the pyrophosphate composite material, the positive electrode and the negative electrode of the sodium ion battery have high sodium ion diffusion coefficient, so that the pyrophosphate composite material has good sodium ion de-intercalation capability and can show good rate characteristics.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the embodiments will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art that other drawings can be obtained according to these drawings without creative efforts.
FIG. 1 is an XRD spectrum of a pyrophosphate composite material in example 1 of the present invention;
FIG. 2 is an SEM image of a pyrophosphate composite material in example 1 of the present invention;
FIG. 3 is a graph of the cycling performance of application example 1 of the present invention and comparative application example 1;
fig. 4 is a cycle performance curve of application example 2 of the present invention and comparative application example 2.
Detailed Description
In order to make the technical problems, technical solutions and advantageous effects solved by the present invention more clearly apparent, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
The noun explains:
primary particles: refers to particles of material in an unstacked state;
secondary particles: refers to a stacked particle, which is formed by stacking a plurality of primary particles.
The pyrophosphate composite material provided by the embodiment of the invention comprises vanadium sodium pyrophosphate and carbon coated on the surface of the vanadium sodium pyrophosphate, namely the pyrophosphate composite material has a core-shell structure, the core part is the vanadium sodium pyrophosphate, the shell part is the carbon, wherein the vanadium sodium pyrophosphate is in a spherical structure or a sphere-like structure, and the wall body of the vanadium sodium pyrophosphate is provided with a porous structure.
Specifically, the general formula of the hollow spherical sodium vanadium pyrophosphate is Na7±xV3±y(P2O7)4Wherein x is more than or equal to 0 and less than or equal to 0.45; 0. ltoreq. y.ltoreq.0.4, for example, Na6.55V3.3(P2O7)4、Na6.9V2.8(P2O7)4、Na7.4V2.6(P2O7)4May be carbon-coated Na, the pyrophosphate composite material thus obtained may be a carbon-coated Na6.55V3.3(P2O7)4Carbon-coated Na6.9V2.8(P2O7)4And carbon-coated Na7.4V2.6(P2O7)4At least one of (1). In some embodiments, the primary pyrophosphate composite particles have a particle size of 50 nm to 300 nm; the secondary particles have a particle size of 1 to 20 [ mu ] m, and in some embodiments, the pyrophosphate composite has a carbon content of 0.1 to 15 wt.%.
The pyrophosphate composite material provided by the embodiment of the invention can be prepared by the following method:
firstly, preparing V by adopting a template method2O5Micron hollow spheres. Specifically, vanadium pentoxide (V)2O5) Mixing the template agent and the solvent, and thenThen the V is treated by heat at the temperature of 200-250 ℃ and then treated at the temperature of 280-350 ℃ in air atmosphere to obtain the V2O5Micron hollow spheres. Wherein the template agent comprises any one of oxalic acid and citric acid; the solvent includes any one of a mixed solvent of water and methanol and a mixed solvent of water and ethanol.
Secondly, V is mixed2O5And (3) mixing the micro hollow spheres and sodium salt according to a molar ratio which is excessive than the target product amount of the pyrophosphate composite material to obtain a first mixed material. Wherein the sodium salt comprises anhydrous sodium acetate ((CH)3COO) Na), ammonium dihydrogen phosphate (NH)4H2PO4) Any one of the above. In this step, V2O5The molar excess of the micro hollow spheres and the sodium salt to the target product of the pyrophosphate composite material is 0.2-1.5, which is mainly because the pyrophosphate composite material is a stable monoclinic material, and more impurities can be eluted in a water bath process.
Mixing and stirring the first mixed material and a carbon source in deionized water, uniformly mixing, and evaporating the deionized water until the deionized water is dried to obtain precursor powder; the carbon source may be an organic substance such as citric acid or glucose.
And finally, placing the precursor powder in an argon atmosphere at 250-400 ℃ for 2-5 h at constant temperature, then sintering at 600-700 ℃ for 8-20 h at constant temperature, cooling, washing by a water bath at 40-100 ℃, and drying to obtain the pyrophosphate composite material.
In the preparation process, the structure of the obtained pyrophosphate is relatively stable, and other heterogeneous phases can be washed away by water bath, so that the preparation method can obtain the target product pyrophosphate composite material without particularly controlling the mixing ratio of the first mixed material and the carbon source.
The pyrophosphate composite material provided by the invention has good particle characteristics, a high sodium ion diffusion coefficient and a high energy density, and therefore, can be used as a positive electrode active material or a negative electrode active material of sodium ions. Based on the basis, the embodiment of the invention also provides a sodium-ion battery positive electrode taking the pyrophosphate composite material as a positive electrode active material, wherein the sodium-ion battery positive electrode comprises a positive electrode current collector and a positive electrode active layer stacked on the surface of the positive electrode current collector, and the positive electrode active layer comprises a positive electrode conductive agent, a positive electrode binder and the pyrophosphate composite material. Wherein the mass content of the pyrophosphate composite material in the active layer of the positive electrode is more than or equal to 70 percent, and preferably 80 percent or more. The positive electrode conductive agent and the positive electrode binder are commonly used materials in the field of secondary batteries, and thus, detailed descriptions thereof are omitted.
The embodiment of the invention also provides a sodium ion battery cathode based on the pyrophosphate composite material as a cathode active material, the sodium ion battery cathode comprises a cathode current collector and a cathode active layer stacked on the surface of the cathode current collector, and the cathode active layer comprises a cathode conductive agent, a cathode binder and the pyrophosphate composite material. Wherein the mass content of the pyrophosphate composite material in the negative electrode active layer is not less than 70%, preferably 80% or more. The negative electrode conductive agent and the negative electrode binder are commonly used materials in the field of secondary batteries, and therefore, the description thereof is omitted.
The embodiment of the invention further provides a sodium ion battery, which comprises a positive electrode, a negative electrode and a diaphragm arranged between the positive electrode and the negative electrode and used for isolating the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode of the sodium ion battery; or, the negative electrode is the negative electrode of the sodium-ion battery. When the positive active material adopted by the positive electrode in the sodium ion battery is a pyrophosphate composite material, the active material of the negative electrode is not suitable for the pyrophosphate composite material, and at the moment, the negative active material can be suitable for any one of sodium metal, hard carbon or soft carbon; similarly, when the negative active material used in the negative electrode of the sodium ion battery is a pyrophosphate composite, the active material of the positive electrode is not suitable for the pyrophosphate composite, and in this case, the positive active material may be suitable for the layered transition metal oxide.
In order to better illustrate the embodiments of the present invention, the following examples are further illustrative.
Example 1
A pyrophosphate composite material and a preparation method thereof, wherein the preparation method of the pyrophosphate composite material comprises the following steps:
(1) vanadium pentoxide (V)2O5) With oxalic acid (H)2C2O4) According to a molar ratio of 1:3 in a volume ratio of 1:3, then carrying out heat treatment at 200 ℃ for 0.5 h to evaporate the mixed solvent to dryness, and then keeping the temperature at 300 ℃ for 0.5 h in an air atmosphere to obtain V2O5Micron hollow spheres.
(2) Anhydrous sodium acetate ((CH)3COO) Na), V obtained in step (1)2O5Micron hollow ball, ammonium dihydrogen phosphate (NH)4H2PO4) The molar ratio of the raw materials is 14: 1: 8 to obtain a first mixture, and then mixing the first mixture, 2 g of citric acid and 2 g of glucose in 150 mL of deionized water to be mixed uniformly; obtaining a first mixed solution, and evaporating water of the first mixed solution until the first mixed solution is dried to obtain precursor powder;
(3) and placing the precursor powder in an argon atmosphere at the constant temperature of 300 ℃ for 2 h, then sintering at the constant temperature of 600 ℃ for 10 h, cooling, and washing in a water bath at 40 ℃ to obtain a sample.
The obtained sample was subjected to X-ray diffraction to obtain an XRD spectrum as shown in fig. 1.
As can be seen from FIG. 1, the structure of the sample and monoclinic Na7V3(P2O7)4The structures are consistent. Combining with Inductively Coupled Plasma (ICP) test, the chemical formula of the sample is Na6.9V2.8(P2O7)4。
And observing the obtained sample by an electron microscope to obtain the micro morphology shown in figure 2.
As can be seen from fig. 2, some of the samples were hollow spherical particles, some were hollow spheroidal particles, and the surface was coated with carbon. Here, the average particle size of the primary particles is less than 100 nm, and the average particle size of the secondary particles is about 5 μm. The carbon content of the sample was 10 wt.% obtained by thermogravimetric analysis.
In summary, the pyrophosphate composite material prepared in this example was mixed with Na6.9V2.8(P2O7)4Composite material with hollow sphere or quasi-sphere as core and carbon as shell, and Na6.9V2.8(P2O7)4Surface of porous structure, Na6.9V2.8(P2O7)4The average grain diameter of the hollow spheres is 100 nm; the average particle size of the pyrophosphate composite was 5 μm, and the carbon content of the pyrophosphate composite was 10 wt.%.
Comparative example 1
Carbon-coated Na6.9V2.8(P2O7)4Micron solid spheres prepared as follows:
anhydrous sodium acetate ((CH)3COO) Na), vanadium pentoxide (V)2O5) Phosphoric acid diamine (NH)4H2PO4) According to a molar ratio of 14: 1: 8 to obtain a first mixture, and then mixing the first mixture, 2 g of citric acid and 2 g of glucose into 150 mL of deionized water to obtain a first mixed solution;
evaporating the water of the first mixed solution, and drying to obtain precursor powder;
placing the precursor powder in an argon atmosphere at the constant temperature of 300 ℃ for 2 h, then sintering at the temperature of 600 ℃ for 10 h, cooling, washing by a water bath at the temperature of 40 ℃ to obtain a sample, and performing X-ray diffraction, ICP test analysis and microscopic morphology observation to obtain the sample which is the carbon-coated solid porous vanadium sodium pyrophosphate microspheres, namely the carbon-coated Na6.9V2.8(P2O7)4Micron solid spheres.
Example 2
A pyrophosphate composite material and a preparation method thereof, wherein the preparation method of the pyrophosphate composite material comprises the following steps:
(1) vanadium pentoxide (V)2O5) With oxalic acid (H)2C2O4) Push buttonAccording to a molar ratio of 1:3 in a volume ratio of 1:3, then carrying out heat treatment at 200 ℃ for 0.5 h to evaporate the mixed solvent to dryness, and then keeping the temperature at 300 ℃ for 0.5 h in an air atmosphere to obtain V2O5Micron hollow spheres.
(2) Anhydrous sodium acetate ((CH)3COO) Na), V obtained in step (1)2O5Micron hollow ball, ammonium dihydrogen phosphate (NH)4H2PO4) The molar ratio of the raw materials is 12: 1: 8 to obtain a first mixture, and then mixing the first mixture, 3 g of citric acid and 3 g of glucose in 150 mL of deionized water to be mixed uniformly; obtaining a first mixed solution, and evaporating water of the first mixed solution until the first mixed solution is dried to obtain precursor powder;
(3) and placing the precursor powder in an argon atmosphere at the constant temperature of 300 ℃ for 2 h, then sintering at the constant temperature of 650 ℃ for 10 h, cooling, and washing in a water bath at the temperature of 80 ℃ to obtain a sample.
XRD and ICP tests show that the obtained sample is carbon-coated hollow spherical Na6.55V3.3(P2O7)4The crystal structure of the pyrophosphate composite material is a monoclinic structure, and the content of carbon in a sample is determined to be 14.8 wt.% through thermogravimetric analysis, so that the obtained sample is the target product pyrophosphate composite material.
Example 3
A pyrophosphate composite material and a preparation method thereof, wherein the preparation method of the pyrophosphate composite material comprises the following steps:
(1) vanadium pentoxide (V)2O5) And citric acid according to a molar ratio of 1:3 in a volume ratio of 1:3, then carrying out heat treatment at 200 ℃ for 0.5 h to evaporate the mixed solvent to dryness, and then keeping the temperature at 300 ℃ for 0.5 h in an air atmosphere to obtain V2O5Micron hollow spheres.
(2) Anhydrous sodium acetate ((CH)3COO) Na), V obtained in step (1)2O5Micron hollow ball, ammonium dihydrogen phosphate (NH)4H2PO4) The molar ratio of the raw materials is 16:1: 8 to obtain a first mixture, and then mixing the first mixture, 1 g of citric acid and 1 g of glucose in 150 mL of deionized water to be mixed uniformly; obtaining a first mixed solution, and evaporating water of the first mixed solution until the first mixed solution is dried to obtain precursor powder;
(3) and placing the precursor powder in an argon atmosphere at the constant temperature of 300 ℃ for 2 h, then sintering at the constant temperature of 700 ℃ for 10 h, cooling, and washing in a water bath at the temperature of 60 ℃ to obtain a sample.
XRD and ICP tests show that the obtained sample is carbon-coated hollow spherical Na7.4V2.6(P2O7)4The crystal structure of the pyrophosphate-based composite material is a monoclinic structure, and the content of carbon in a sample is determined to be 5.0 wt.% through thermogravimetric analysis, so that the obtained sample is the target product pyrophosphate composite material.
Application example 1
This application example demonstrates the sample obtained in example 1 as a positive electrode active material for a sodium ion battery.
In this embodiment, the preparation method of the sodium ion battery (half cell) includes the following operation steps:
the pyrophosphate composite material obtained in example 1 is used as a positive electrode active material, and is mixed with conductive carbon black and polyvinylidene fluoride (PVDF) according to the mass ratio of 8:1:1, the mixture is uniformly mixed, dissolved in N-methyl pyrrolidone (NMP) to prepare slurry, and the slurry is coated on an aluminum foil, dried and cut to obtain a positive electrode sheet.
Assembling the obtained positive plate, sodium metal and a diaphragm into a battery shell, injecting electrolyte, standing for 24 hours, and obtaining a sodium-ion battery (half battery) after the voltage is stable; wherein, in the electrolyte, the solvent is a mixture of Ethylene Carbonate (EC) and Propylene Carbonate (PC) according to the volume ratio of 1:1, and the solute is 1M NaClO4And 5 wt.% diethyl carbonate (FEC).
Comparative application example 1
In this comparative application example, a sodium ion battery (half cell) was prepared by using the sample obtained in comparative example 1 as a positive electrode active material and referring to the preparation method of application example 1.
Application example 2
This application example demonstrates the sample obtained in example 1 as a negative electrode active material for a sodium ion battery.
In this embodiment, the preparation method of the sodium ion battery (half cell) includes the following operation steps:
the pyrophosphate composite material obtained in the example 1 is used as a negative electrode active material, mixed with conductive carbon black and polyvinylidene fluoride (PVDF) according to the mass ratio of 8:1:1, uniformly mixed, dissolved in N-methyl pyrrolidone (NMP) to prepare slurry, coated on a copper foil, dried and cut to obtain a negative electrode sheet.
Assembling the obtained negative plate, sodium metal and a diaphragm into a battery shell, injecting electrolyte, standing for 24 hours, and obtaining a sodium-ion battery (half battery) after the voltage is stable; wherein, in the electrolyte, the solvent is a mixture of Ethylene Carbonate (EC) and Propylene Carbonate (PC) according to the volume ratio of 1:1, and the solute is 1M NaClO4And 5 wt.% diethyl carbonate (FEC).
Comparative application example 2
In this comparative application example, a sodium ion battery (half cell) was prepared by using the sample obtained in comparative example 1 as a negative electrode active material and referring to the preparation method of application example 2.
Application example 3
This application example demonstrates the sample obtained in example 2 as a positive electrode active material for a sodium ion battery. For a specific preparation method, reference is made to application example 1, which is not described herein again.
Application example 4
This application example demonstrates the sample obtained in example 3 as a positive electrode active material for a sodium ion battery. For a specific preparation method, reference is made to application example 1, which is not described herein again.
Application example 5
This application example demonstrates the sample obtained in example 2 as a negative electrode active material for a sodium ion battery. The specific preparation method refers to application example 2, and details are not repeated herein.
Application example 6
This application example demonstrates the sample obtained in example 3 as a negative electrode active material for a sodium ion battery. The specific preparation method refers to application example 2, and details are not repeated herein.
Performance testing
1. Sodium ion diffusion coefficient test
The results of measuring the diffusion coefficient of sodium ions by a constant current Intermittent Titration method (GITT) for the positive electrode of example 1, the positive electrode of comparative application example 1, the negative electrode of application example 2, the negative electrode of comparative application example 2, the positive electrode of application example 3, the positive electrode of application example 4, the negative electrode of application example 5, and the negative electrode of application example 6 are shown in table 1.
TABLE 1 sodium ion diffusion coefficient of each application example and comparative application example
Figure 108761DEST_PATH_IMAGE002
As can be seen from table 1, the positive electrodes of application example 1, application example 3, and application example 4 have sodium ion diffusion coefficients 2.75 times, 3.125 times, and 3.475 times, respectively, that are superior to those of comparative application example 1 in sodium vanadium pyrophosphate coated carbon having a hollow spherical porous structure, and thus have superior sodium ion diffusion ability to solid spherical porous sodium vanadium pyrophosphate coated carbon. The negative electrodes of application examples 2, 5 and 6 had sodium ion diffusion coefficients 3.4375 times, 3.625 times and 4.1875 times, respectively, that of comparative application example 2; it was demonstrated that the negative electrode obtained by coating carbon on the surface of the hollow spherical porous vanadium sodium pyrophosphate was more excellent in the sodium ion diffusion ability than the negative electrode obtained by coating carbon on the surface of the solid spherical porous vanadium sodium pyrophosphate. It was also demonstrated that the vanadium sodium pyrophosphate having a hollow spherical porous structure coated with carbon had a sodium ion diffusing ability more excellent as a negative electrode than as a positive electrode.
It can be shown that the pyrophosphate composite materials provided by the embodiments 1 to 3 of the present invention have a faster sodium ion deintercalation rate, and the electrodes made of the pyrophosphate composite materials have more excellent large current discharge capability, higher power density and better high rate performance than the electrodes made of the carbon-coated solid spherical porous vanadium sodium pyrophosphate microspheres.
2. Electrochemical performance test
(1) And charge and discharge cycle tests were performed on the corresponding application example 1 and the comparative application example 1. The specific test mode is as follows: the results of the charge and discharge cycle test at a current density of 0.5C at 2.5V to 4.35V are shown in FIG. 3.
As can be seen from fig. 3, the pyrophosphate composite material of example 1 of the present invention is used as a sodium ion battery (application example 1) assembled by a positive active material of the sodium ion battery, the initial discharge specific capacity is 62.5 mAh/g, the discharge specific capacity is 61.0 mAh/g after 100 cycles, and the discharge capacity retention rate reaches 97.6%, while the carbon-coated vanadium sodium pyrophosphate solid microspheres of comparative example 1 are used as a sodium ion battery (comparative application example 1) assembled by a positive active material of the sodium ion battery, the initial discharge specific capacity is 51.8 mAh/g, the discharge specific capacity is 48.6 mAh/g and the discharge capacity retention rate is 93.8% after 100 cycles. Therefore, compared with carbon-coated solid porous vanadium sodium pyrophosphate microspheres, when the pyrophosphate composite material of embodiment 1 of the invention is used as a positive electrode active material of a sodium ion battery, the initial specific discharge capacity is improved by more than 20.6%, higher specific discharge capacity can be exerted, and the cycling stability is higher.
(2) And carrying out charge-discharge cycle test on the corresponding application example 2 and the comparative application example 2, wherein the specific test mode is as follows: the results of the charge and discharge cycle test at a current density of 2C at 0.5V to 2.5V are shown in FIG. 4.
As can be seen from fig. 4, the pyrophosphate composite material of example 1 of the present invention is used as a sodium ion battery (application example 2) assembled by using the negative active material of the sodium ion battery, the initial charging capacity is 61.0 mAh/g, the specific charging capacity is 58.5 mAh/g after 100 cycles, and the capacity retention rate reaches 95.9%, while the carbon-coated solid porous vanadium sodium pyrophosphate microspheres of comparative example 1 are used as a sodium ion battery (comparative application example 2) assembled by using the negative active material of the sodium ion battery, the initial charging specific capacity is 54.9 mAh/g, and the specific charging capacity is 51.8 mAh/g and the specific charging capacity retention rate is 94.4% after 100 cycles. Therefore, compared with carbon-coated solid porous vanadium sodium pyrophosphate microspheres, when the pyrophosphate composite material of embodiment 1 of the invention is used as a negative electrode active material of a sodium ion battery, the initial charge specific capacity is improved by nearly 11.1%, higher charge specific capacity can be exerted, and the cycling stability is higher.
From the cycle performance tests of the sodium ion batteries of the application example 1 and the comparative application example 1 in combination with the sodium ion diffusion coefficient data in the table 1, it can be seen that the sodium ion diffusion coefficient of the application example 1 is lower than the sodium ion diffusion coefficients of the application example 2, the application example 3, the application example 4, the application example 5 and the application example 6, and has higher specific discharge capacity, specific charge capacity, cycle stability and rate characteristics, and from the electrode dynamics, the Fick second law, the electrode ion deintercalation performance rule and the like, the sodium ion batteries of the application example 2, the application example 3, the application example 4, the application example 5 and the application example 6 also have higher specific discharge capacity, specific charge capacity, cycle stability and rate characteristics.
In conclusion, the pyrophosphate composite material provided by the embodiment of the invention has good sodium ion de-intercalation capability, and is beneficial to improving electrochemical properties such as rate characteristic, cycling stability and the like of a sodium ion battery taking the pyrophosphate composite material as an active material.
The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents and improvements made within the spirit and principle of the present invention are intended to be included within the scope of the present invention.

Claims (10)

1. The pyrophosphate composite material is characterized in that the pyrophosphate composite material has a core-shell structure, the core is vanadium sodium pyrophosphate, the shell is carbon, and the vanadium sodium pyrophosphate is in a hollow spherical or hollow sphere-like structure.
2. The pyrophosphate composite material of claim 1 wherein said sodium vanadium pyrophosphate has the formula Na7±xV3±y(P2O7)4Wherein x is more than or equal to 0 and less than or equal to 0.45; 0 is less than or equal toy≤0.4；
And/or the wall body of the sodium vanadium pyrophosphate is in a porous structure.
3. The pyrophosphate composite of claim 1 wherein said sodium vanadium pyrophosphate comprises Na6.55V3.3(P2O7)4、Na6.9V2.8(P2O7)4、Na7.4V2.6(P2O7)4At least one of (1).
4. The pyrophosphate composite according to any one of claims 1 to 3, wherein the primary particles of the pyrophosphate composite have a particle size of 50 nm to 300 nm; the particle size of the pyrophosphate composite secondary particles is 1-20 mu m;
and/or the pyrophosphate composite material is prepared according to the following preparation method:
mixing vanadium pentoxide with a template agent and a solvent, carrying out heat treatment at 200-250 ℃, and then carrying out treatment at 280-350 ℃ in an air atmosphere to obtain V2O5Micron hollow spheres;
the V is put into2O5Mixing the micron hollow spheres and sodium salt in a molar ratio which is excessive than that of a target product of the pyrophosphate composite material to obtain a first mixed material;
mixing and stirring the first mixed material and a carbon source in deionized water, uniformly mixing, and evaporating the deionized water until the deionized water is dried to obtain precursor powder;
placing the precursor powder in an argon atmosphere at 250-400 ℃ for 2-5 h, then sintering at 600-700 ℃ for 8-20 h, cooling, and washing in a water bath at 40-100 ℃ to obtain the pyrophosphate composite material;
wherein the template agent comprises any one of oxalic acid and citric acid; the solvent includes any one of a mixed solvent of water and methanol and a mixed solvent of water and ethanol.
5. The pyrophosphate composite according to any one of claims 1 to 3, wherein the carbon content of the pyrophosphate composite is from 0.1 wt.% to 15 wt.%.
6. A positive electrode for a sodium-ion battery, comprising a positive electrode active layer containing the pyrophosphate composite material according to any one of claims 1 to 5.
7. The positive electrode for sodium-ion batteries according to claim 6, characterized in that the mass content of the pyrophosphate composite material in the active layer of the positive electrode is not less than 70%.
8. A sodium ion battery negative electrode comprising a negative electrode active layer containing the pyrophosphate composite material according to any one of claims 1 to 5.
9. The sodium-ion battery cathode as claimed in claim 8, wherein the pyrophosphate composite material has a mass content of 70% or more in the cathode active layer.
10. A sodium ion battery comprising a positive electrode and a negative electrode, and a separator provided between the positive electrode and the negative electrode for separating the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode for a sodium ion battery according to any one of claims 6 to 7;
alternatively, the negative electrode is a sodium ion battery negative electrode as defined in any one of claims 8 to 9.
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