Carbon-coated sodium ferrous fluorophosphate positive electrode material and preparation method thereof

Abstract

The invention discloses a carbon-coated sodium ferrous fluorophosphate positive electrode material and a preparation method thereof. A phosphorus source, an iron source, an organic carbon source, a sodium source and a surfactant are mixed, stirred and heated, a fluorine source solution is added after reaction, a conductive carbon source is added after continuous stirring, and the sodium ferrous fluorophosphate positive electrode material is obtained through dispersion, drying and high-temperature sintering. Compared with an existing synthesis method, the method provided by the invention is small in pollution and simple in process, the conductivity of the prepared positive electrode material is greatly improved after the positive electrode material is subjected to carbon coating modification, and the specific capacity can reach 80% or above of a theoretical value.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium ion batteries, and particularly relates to a carbon-coated sodium iron fluorophosphate cathode material and a preparation method thereof. Background Art

[0002] Compared with the widely used lithium ion batteries today, although the charge-discharge mechanism is the same, sodium ion batteries have higher safety. The synthesis process can basically follow the process of lithium ion batteries. Under such advantages, coupled with the emergence of hard carbon anode materials, people have carried out in-depth research on various sodium ion battery cathode materials. Sodium ion battery cathode materials are mainly divided into three categories: transition metal oxides, Prussian blue, and polyanions. Among them, polyanion materials can be divided into phosphates, fluorophosphates, pyrophosphates, mixed phosphates, sulfates, etc., which have the characteristics of simple synthesis process, low environmental requirements, high structural stability, and excellent electrochemical performance, and are the most promising category for practical applications.

[0003] NaZar et al. [Nazar, et al. Chemistry of Materials, 2010, 22: 105 - 1070.] first reported the sodium iron fluorophosphate material, which has a theoretical specific capacity of 124 mAh / g and an average working voltage of 3.5 V (vs. Li / Li + )). In its framework structure, Fe 2 O 7 F 2 The double octahedral structural unit consists of two coplanar FeO 4 F 2 regular octahedrons, which form a chain under the connection of fluorine atoms and form a layer through the connection of PO 4 tetrahedrons, forming a two-dimensional channel for sodium ion diffusion. At the same time, this material has the structural stability of polyanions, and the charge-discharge volume change rate is only 3.7%. However, due to poor electronic conductivity, there is usually no obvious plateau region in the charge-discharge curve, and the rate performance of the material is poor.

[0004] Like other polyanion materials, there are currently the following methods for the preparation of sodium iron fluorophosphate: solid-phase mixing method, sol-gel method, high-temperature carbon reduction method, etc. There are some reports on the synthesis and improvement of sodium iron fluorophosphate, such as:

[0005] In the preparation method of a sodium iron fluorophosphate cathode material for a hybrid ion battery (publication number: CN102306772A), sodium iron fluorophosphate material was prepared by solid-phase mixing and carbothermal reduction using iron oxide as the iron source. The solid-phase mixing method has become mature in the research of lithium iron phosphate materials. However, due to limited mixing uniformity, iron oxide impurity phases appeared in the finished product, resulting in a first-cycle discharge specific capacity of only 73 mAh / g for the material, and the use of a relatively large proportion of conductive carbon increased the preparation cost.

[0006] In the method for solvent-thermal preparation of carbon-coated nanorod-shaped sodium iron fluorophosphate material in Chinese Patent (publication number: CN106058249A), the synthesis method of sodium iron fluorophosphate is as follows: using ethylene glycol as the reaction solvent, dissolving soluble iron source, sodium fluoride and antioxidant, adding a mixed solution of sodium source and phosphorus source, adjusting the pH of the formed suspension to alkaline, carrying out high-temperature hydrothermal reaction in a reaction kettle, washing and drying to obtain a precursor. This patent adopts the solvent-thermal method to synthesize sodium iron fluorophosphate material without adding additional conductive carbon material, and the first-cycle discharge specific capacity reaches 94 mAh / g. However, this method is time-consuming and energy-consuming (the reaction kettle needs to be heated above 200 °C for several hours), and soluble iron salts are likely to introduce impurity ions into the system, affecting sintering and the performance of the finished product, and it is not conducive to environmental protection at the same time.

[0007] In the carbon-coated sodium iron fluorophosphate material, its preparation method and application in Chinese Patent (publication number: CN114242968A), the synthesis steps of the sodium iron fluorophosphate material are as follows: weighing iron phosphate, sodium fluoride, sodium carbonate (molar ratio 1:1:0.5), and adding glucose which is 1 / 2 of the molar amount of iron phosphate as a reducing agent and a conductive agent, and mixing the above materials evenly by high-energy ball milling under argon protection to obtain a precursor powder. The precursor is pressed tightly, sintered once at 300 °C under argon, and then sintered twice at 700 °C after ball milling and pulverization to obtain the finished sodium iron fluorophosphate material. The discharge capacity of the cathode material synthesized by this method reaches 123 mAh / g, approaching the theoretical value of sodium iron fluorophosphate. Using traditional solid-phase mixing and high-temperature carbothermal reduction, the synthesis cost is low, the performance is excellent, and the operability is strong. However, solid-phase mixing has the disadvantages of long time consumption, high energy consumption, and limited uniformity.

[0008] Regarding the above-mentioned deficiencies of sodium iron fluorophosphate in terms of precursor mixing and intrinsic electronic conductivity, there is currently no advantageous solution. Summary of the Invention

[0009] The purpose of the present invention is to overcome the deficiencies of the prior art, and provide a carbon-coated sodium iron fluorophosphate cathode material and its preparation method, which solve the problems in the above-mentioned background technology.

[0010] One of the technical solutions adopted by the present invention to solve its technical problems is: providing a preparation method of a carbon-coated sodium iron fluorophosphate cathode material, comprising the following steps:

[0011] (1) Mix a phosphorus source with an iron source, an organic carbon source, and a sodium source to obtain a suspension and stir and heat it;

[0012] (2) Completely dissolve a fluorine source and a surfactant in water to obtain a mixed solution; wherein, the molar ratio of sodium, iron, phosphorus, and fluorine elements is 2:1:1:1, satisfying the finished product chemical formula Na 2 FePO 4 F;

[0013] (3) Dropwise add the suspension obtained in step (1) into the mixed solution obtained in step (2), and continuously stir until completely mixed;

[0014] (4) Add a conductive carbon source and stir to disperse it to obtain a dispersed mixed solution;

[0015] (5) Dry the mixed solution obtained in step (4) to obtain a sodium iron fluorophosphate precursor;

[0016] (6) High-temperature sinter the sodium iron fluorophosphate precursor obtained in step (5) under an inert atmosphere to obtain a carbon-coated sodium iron fluorophosphate cathode material.

[0017] In a preferred embodiment of the present invention, in step (1), the phosphorus source includes phosphoric acid or a phosphate compound, and the mass concentration of the phosphoric acid is 10% - 60%;

[0018] The iron source is an iron-containing compound or iron element without heteroatoms such as S and Cl, and the iron-containing compound includes one or a mixture of two or more of iron acetate, ferrous oxalate, iron phosphate, ferrous phosphate, iron citrate, sodium iron oxalate, iron oxide, magnetite, iron powder, iron hydroxide, and ferrous oxide;

[0019] The sodium source is a sodium compound without heteroatoms such as S and Cl, including one or a mixture of two or more of sodium acetate, sodium formate, sodium carbonate, sodium oxalate, sodium citrate, sodium gluconate, sodium glycolate, sodium tartrate, sodium bicarbonate, sodium hydroxide, and sodium peroxide;

[0020] The organic carbon source includes one or a mixture of two or more of citric acid, malic acid, tartaric acid, sub-tartaric acid, gluconic acid, glycolic acid, lactic acid, oxalic acid, glucose, sucrose, starch, and fructose; the addition amount of the organic carbon source is 10% - 40% of the total mass of the solid materials. Preferably, the addition amount of the organic carbon source is 20% - 40%, taking into account both the mixing uniformity and the subsequent drying efficiency.

[0021] In a preferred embodiment of the present invention, in step (1), the heating temperature is 40-100 °C, preferably 60-80 °C; the stirring time is 0.5-5 h, preferably 1-3 h.

[0022] In a preferred embodiment of the present invention, in step (2), the surfactant includes sodium dodecyl sulfonate, span, pluronic, polysorbate, polyethylene glycol, isopropanol, polyvinylpyrrolidone or polyethyleneimine; the fluorine source includes sodium fluoride, and the mass concentration of the sodium fluoride solution is 0.01-0.1 g / mL, preferably 0.02-0.05 g / ml.

[0023] In a preferred embodiment of the present invention, in step (3), the dropping frequency is 1-200 drops / min, preferably 30-90 drops / min.

[0024] In a preferred embodiment of the present invention, in step (4), the conductive carbon source includes one of single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, Super P, and Ketjen black, and the added mass of the conductive carbon source is 0.1%-5% of the total mass of the solid materials, preferably 0.5%-2.5%.

[0025] In a preferred embodiment of the present invention, in step (4), the dispersion method is ultrasonic, and the ultrasonic time is 0.5-2 h; the continuous stirring time is 0.5-3 h, preferably 1-2 h.

[0026] In a preferred embodiment of the present invention, in step (5), the drying method includes hot air drying, vacuum drying, evaporation drying, spray drying and freeze drying; among them, the inlet temperature of the spray drying is 150-250 °C, preferably 180-220 °C.

[0027] In a preferred embodiment of the present invention, in step (6), the sodium iron fluorophosphate precursor is placed in a tube furnace, heated to 500-600 °C at a rate of 3-5 °C / min, and heated for 8-12 hours, and the sintering atmosphere is argon atmosphere or hydrogen-argon atmosphere.

[0028] The second technical solution adopted by the present invention to solve its technical problems is: to provide a carbon-coated sodium iron fluorophosphate cathode material, which is prepared by the above method. The material shapes include ellipsoids with shriveled and sunken surfaces (plum-like), spheres with smooth surfaces and their fragments, mainly ellipsoids (plum-like), and the morphologies are uniform; the diameters of the ellipsoids or spheres are distributed from hundreds of nanometers to 10 microns, and they have a core-shell structure, where the core is sodium iron fluorophosphate and the shell layer is a conductive carbon layer. The shriveled and sunken surfaces on the sphere surface increase the contact area between the materials and between the materials and the electrolyte, which is beneficial to the rapid progress of the electrochemical reaction.

[0029] Compared with the background technology, this technical solution has the following advantages:

[0030] 1. In the preparation method of the present invention, an organic carbon source is used to stabilize the suspension system and the dispersion effect of the surfactant is utilized to highly and uniformly mix the raw materials, reducing particle agglomeration; an iron source with high atomic utilization rate and low pollution is selected, and a surfactant and an organic carbon source are added to form a stable system. The organic carbon source has multiple functions such as a reducing agent, a stabilizer, and a conductive agent; a conductive carbon material is added to modify it, significantly improving the conductivity and electrochemical performance of the material.

[0031] 2. The present invention combines the solvothermal method and spray drying. During the raw material mixing stage, a conductive carbon slurry is incorporated and the uniformity of the precursor particles is improved through spray drying, synthesizing a uniform sodium iron fluorophosphate precursor powder; solving the problem of the synthesis rate of the traditional high-energy ball milling or sol-gel method in the background technology, meeting the requirements of large-scale preparation, and having practical operability and feasibility.

[0032] 3. No impurity elements are introduced during the preparation process of the present invention. The prepared cathode material has high morphological uniformity, good carbon coating effect, and high crystal phase purity. After being made into a pole piece by slurry coating and assembling a half-cell for evaluation, the discharge specific capacity at 0.1C can reach 114 mAh / g, which is 92% of the theoretical specific capacity. Description of the Drawings

[0033] Figure 1 is the X-ray powder diffraction pattern of the sodium iron fluorophosphate cathode material prepared in Example 1.

[0034] Figure 2A and 2B is the scanning electron microscope image of the sodium iron fluorophosphate cathode material prepared in Example 1.

[0035] Figures 3A to 3C are the charge-discharge curves of the button cells made of the materials prepared in Example 1, Example 2, and the comparative example respectively. Detailed Description of the Invention

[0036] Example 1

[0037] A preparation method of a carbon-coated sodium iron fluorophosphate cathode material in this example includes the following steps:

[0038] (1) Weigh a certain mass of iron phosphate as both the phosphorus source and the iron source, add water for dispersion, and the mass of water is 10 times that of the iron phosphate. Add 80% of the mass of the iron phosphate of oxalic acid, preheat and stir for 40 min, and then slowly add sodium carbonate and sucrose to the system. The molar ratio of sodium carbonate to iron phosphate satisfies 0.5:1, and sucrose is used as the reducing agent of the system, and the added mass is the same as that of oxalic acid. Continue to heat and stir for 6 h until a stable semi-suspension system is formed.

[0039] (2) Weigh sodium fluoride and sodium dodecyl sulfate, add them to the solvent water, and dissolve them to form a transparent solution under heating and stirring. The molar ratio of sodium fluoride to iron phosphate satisfies 1:1, and the added mass of the surfactant is 0.5% of the mass of iron phosphate.

[0040] (3) Add the iron phosphate semi-suspension to the sodium fluoride transparent solution at a rate of 10 drops / min, wash the wall of the container with deionized water 3 times, and pour the washing water into the sodium fluoride solution.

[0041] (4) Continue stirring for 30 min to make the mixture homogeneous, and then add the conductive carbon slurry. The mass of the slurry is 1% of the total mass of the solid raw materials.

[0042] (5) After ultrasonic dispersion for 30 min, introduce it into a spray drying device under stirring to obtain a grayish-black powder precursor.

[0043] (6) Anneal the precursor in an argon atmosphere by heating it to 600 °C at a rate of 5 °C / min for 10 h, and then pulverize it to obtain the finished sodium iron fluorophosphate cathode material.

[0044] The X-ray powder diffraction pattern of the carbon-coated sodium iron fluorophosphate cathode material prepared in this example is as Figure 1 , and the characteristic peaks of sodium iron fluorophosphate are shown in the figure. The material shapes include ellipsoids with shriveled and sunken surfaces (plum-like), spheres with smooth surfaces and their fragments, mainly ellipsoids (plum-like), and the morphology is uniform; the diameter of the ellipsoids or spheres ranges from hundreds of nanometers to 10 microns, and it has a core-shell structure, where the core is sodium iron fluorophosphate and the shell layer is a conductive carbon layer. Figure 2A , and Figure 2B shows the scanning electron microscope image of the sodium iron fluorophosphate material prepared by the present invention, and the morphology and particle size of iron phosphate are shown in the figure.

[0045] Example 2

[0046] (1) Weigh a certain mass of phosphoric acid, dilute it with water to a mass concentration of 40%. Add iron acetate and stir it with water to disperse, and control the total mass of water to be 10 times the mass of iron acetate. Add citric acid with a mass 1.6 times that of iron acetate, preheat and stir for 40 min, and then slowly add sodium carbonate and sucrose to the system. The molar ratio of sodium carbonate, iron acetate, and phosphoric acid satisfies 0.5:1:1, and sucrose is used as the reducing agent in the system, and the added mass is 0.1 times that of citric acid. Continue heating and stirring for 6 h until a stable semi-suspension system is formed.

[0047] (2) Weigh sodium fluoride and polyvinylpyrrolidone, add them to the solvent water, and dissolve them to form a transparent solution under heating and stirring. The molar ratio of sodium fluoride to iron phosphate satisfies 1:1, and the added mass of the surfactant is 0.6% of the mass of iron phosphate.

[0048] (3) Add the iron phosphate semi-suspension into the sodium fluoride transparent solution at a rate of 10 drops / min, wash the wall of the vessel with deionized water 3 times and pour the washing water into the sodium fluoride solution.

[0049] (4) Continue stirring for 30 min to make the mixture homogeneous, then add the conductive carbon slurry, and the mass of the slurry is 1% of the total mass of the solid raw materials.

[0050] (5) After ultrasonic dispersion for 30 min, introduce it into the spray drying equipment under stirring to obtain a grayish-black powder precursor.

[0051] (6) Anneal the precursor in an argon atmosphere by heating it to 550 °C at a rate of 5 °C / min for 10 h, and obtain the finished sodium iron fluorophosphate cathode material after pulverization.

[0052] Comparative example

[0053] The difference between the preparation method of the comparative example and that of Example 1 is that after adding the iron phosphate suspension into the sodium fluoride transparent solution, stir and disperse it evenly, and directly introduce it into the spray drying equipment to obtain a yellowish-green precursor powder, that is, the sodium iron fluorophosphate material without conductive carbon coating.

[0054] Half-cell assembly test:

[0055] (1) According to the mass ratio, cathode material (materials prepared in Examples 1-2 and comparative example respectively): conductive carbon (acetylene black): PVDF = 8:1:1, mix and stir the substances into a slurry, and use N-methylpyrrolidone as the solvent to make the electrode coating slurry;

[0056] (2) Coat the prepared slurry on the current collector carbon-coated aluminum foil and dry it for 8 hours to remove the solvent;

[0057] (3) Use the 2016 model battery case, sodium metal negative electrode, Whatman glass fiber separator, 1M NaClO 4 (pure PC + 5% FEC) electrolyte and the positive electrode sheet prepared in step (2) to assemble the battery;

[0058] (4) The standing time is 6 h, set a charge-discharge program, the magnification of the first 3 cycles is 0.1C, the magnification of the 4th to 200th cycles is 1C, and the voltage range is 1.5V - 4.2V.

[0059] The test results show that: as Figure 3A , the discharge specific capacities of the sodium iron fluorophosphate material prepared in Example 1 at 0.1C and 1C are 114.1 and 105.4 mAh / g respectively; as Figure 3B , the charge-discharge curve of the material in Example 2, the first-cycle discharge specific capacity at 0.1C is 105.3 mAh / g. The test results of the positive electrode material prepared in the comparative example are as Figure 3C, the charge-discharge curve shows that the discharge specific capacity at 0.1C in the first cycle is only 88.5 mAh / g, indicating that carbon coating modification has a great improvement effect on the electrochemical performance. Moreover, the carbon coating amount in the embodiment is only 1% of the total raw material mass, having high cost-effectiveness and practical operability.

[0060] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a carbon-coated sodium ferrous fluorophosphate positive electrode material, characterized in that: The steps include: (1) mixing a phosphorus source with an iron source, an organic carbon source, and a sodium source to obtain a suspension and stirring and heating the suspension; (2) adding water to completely dissolve the fluorine source and the surfactant to obtain a mixed solution; wherein the molar ratio of sodium, iron, phosphorus and fluorine elements is 2:1:1:1; (3) adding the suspension of step (1) dropwise to the mixed solution of step (2), and stirring continuously until the mixture is completely mixed; (4) adding a conductive carbon source and stirring and dispersing the mixture to obtain a dispersed mixed solution; (5) drying the mixed solution of step (4) to obtain a sodium ferrous fluorophosphate precursor; (6) Sintering the sodium ferrous fluorophosphate precursor of step (5) at high temperature under an inert atmosphere to obtain a carbon-coated sodium ferrous fluorophosphate positive electrode material.

2. The method for preparing a carbon-coated sodium ferrous fluorophosphate positive electrode material according to claim 1, characterized in that: In step (1), the phosphorus source includes phosphoric acid or a phosphate compound, and the mass concentration of the phosphoric acid is 10% to 60%; the iron source is an iron-containing compound or an iron element, and the iron-containing compound includes one or a mixture of two or more of ferric acetate, ferrous oxalate, ferric phosphate, ferrous phosphate, ferric citrate, sodium ferric oxalate, ferric oxide, ferric oxide, iron powder, ferric hydroxide, and ferrous oxide; the sodium source includes one or a mixture of two or more of sodium acetate, sodium formate, sodium carbonate, sodium oxalate, sodium citrate, sodium gluconate, sodium glycolate, sodium tartrate, sodium bicarbonate, sodium hydroxide, and sodium peroxide; the organic carbon source includes one or a mixture of two or more of citric acid, malic acid, tartaric acid, tartaric acid, gluconic acid, glycolic acid, lactic acid, oxalic acid, glucose, sucrose, starch, and fructose; and the amount of the organic carbon source added is 10% to 40% of the total mass of the solid material.

3. The method for preparing a carbon-coated sodium ferrous fluorophosphate positive electrode material according to claim 1, characterized in that: In step (1), the heating temperature is 40 to 100° C. and the stirring time is 0.5 to 5 h.

4. The method for preparing a carbon-coated sodium ferrous fluorophosphate positive electrode material according to claim 1, characterized in that: In step (2), the surfactant includes sodium dodecyl sulfate, Span, Pluronic, polysorbate, polyethylene glycol, isopropyl alcohol, polyvinyl pyrrolidone or polyethyleneimine; the fluorine source includes sodium fluoride, and the mass concentration of the sodium fluoride solution is 0.01-0.1 g / mL.

5. The method for preparing a carbon-coated sodium ferrous fluorophosphate positive electrode material according to claim 1, characterized in that: In step (3), the dropping frequency is 1 to 200 drops / min.

6. The method for preparing a carbon-coated sodium ferrous fluorophosphate positive electrode material according to claim 1, characterized in that: In step (4), the conductive carbon source includes one of single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, Super P, and Ketjen black, and the added mass of the conductive carbon source is 0.1% to 5% of the total mass of the solid material.

7. The method for preparing a carbon-coated sodium ferrous fluorophosphate positive electrode material according to claim 1, characterized in that: In step (5), the drying method includes forced air drying, vacuum drying, evaporative drying, spray drying and freeze drying; wherein the spray inlet temperature of the spray drying is 150-250°C.

8. The method for preparing a carbon-coated sodium ferrous fluorophosphate positive electrode material according to claim 1, characterized in that: In step (6), the sodium ferrous fluorophosphate precursor is placed in a tube furnace, heated to 500-600° C. at a rate of 3-5° C. / min, and heated for 8-12 hours. The sintering atmosphere is an argon atmosphere or a hydrogen-argon atmosphere.

9. A carbon-coated sodium ferrous fluorophosphate positive electrode material, characterized in that: The method is prepared according to any one of claims 1 to 8.

10. The carbon-coated sodium ferrous fluorophosphate positive electrode material according to claim 9, characterized in that: The material shapes include ellipsoids with shriveled and concave surfaces, spheres with smooth surfaces and fragments thereof; the ellipsoids or spheres have a core-shell structure, wherein the core is sodium ferrous fluorophosphate and the shell is a conductive carbon layer.

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