Preparation method and application of carbon-coated cobalt-doped ferric sodium pyrophosphate positive electrode material

Abstract

The invention relates to the field of battery materials, and discloses a preparation method and application of a carbon-coated cobalt-doped ferric sodium pyrophosphate positive electrode material. The preparation method comprises the following steps: (1) preparing a clear solution from an iron source, a phosphorus source, a sodium source, a cobalt source and a carbon source; (2) converting the settled solution into precursor powder by using a spray drying method; (3) calcining the precursor powder in an inert atmosphere; and (4) crushing and grinding the calcined product to obtain the carbon-coated cobalt-doped sodium ferric pyrophosphate positive electrode material. The sodium ferric phosphate pyrophosphate positive electrode material is doped and modified through a spray drying method, the electrochemical performance of the positive electrode material is improved, and the material is high in purity, controllable in particle morphology and easy to commercially produce.

Description

Technical Field

[0001] The present invention relates to the field of battery materials, and in particular to a preparation method and application of a carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material. Background Art

[0002] As the consumption of fossil fuels increases, the world faces an energy crisis and climate change challenges. Electrochemical energy storage technologies such as lithium-ion batteries are seen as a solution to reduce dependence on oil and natural gas. However, the reserves of lithium resources on the earth are relatively limited and unevenly distributed. The scarcity and geographical concentration of such resources may lead to supply chain fragility and price fluctuations.

[0003] Sodium-ion batteries are considered to be one of the most promising alternatives to lithium-ion batteries due to their abundant sodium resources, low cost and high safety performance. Sodium-ion battery cathode materials occupy a core position in sodium-ion battery technology, and their performance directly affects key parameters such as battery energy density, cycle stability, charge and discharge rate and safety performance. So far, the cathode materials of sodium-ion batteries mainly include three categories: transition metal layered oxides, Prussian blue compounds and polyanion compounds. Among the polyanion compound cathode materials, iron-based polyanion cathode materials have attracted attention due to their strong structural stability, excellent cycle performance, low cost and good safety.

[0004] Among them, Na4Fe3(PO4)2P2O7 has a higher theoretical specific capacity (128.9 mAh g -1 ), stable polyanion system, and open framework structure, and thus have good application prospects. However, Na4Fe3(PO4)2P2O7 also has some disadvantages: (1) its electronic conductivity is relatively low, which leads to an increase in the internal resistance of the battery, thereby affecting the charge and discharge performance of the battery; (2) the weak electronegativity of the phosphate group makes the polyanion group have a weak inductive effect, resulting in a lower operating voltage; (3) the inactive polyanion has a large molecular weight, which leads to a lower specific capacity and poor rate performance of the battery.

[0005] In view of the above shortcomings, researchers have improved its overall performance through various means such as ion doping, structural regulation, carbon coating, and composite materials. For example, patent CN118811792A discloses a method for preparing sodium iron phosphate pyrophosphate / C composite materials by mechanical ball milling, wherein the raw materials are mixed by ball milling and then calcined at high temperature to obtain a carbon-coated sodium iron phosphate pyrophosphate positive electrode material. Although the preparation method is simple and easy to operate, the solid-phase method for preparing sodium iron phosphate pyrophosphate is prone to agglomeration, which makes the specific surface area of ​​the product powder small and the utilization rate of the electrode material low; in addition, although long-term ball milling can reduce the relative particle size of the product, it also increases the preparation time, which brings difficulties to the application of large-scale production. Patent CN118062822A discloses a method for preparing manganese-doped sodium iron phosphate pyrophosphate derivatives by bimetallic coprecipitation. The introduction of manganese element increases the average working voltage, plays a role in improving structural stability and reducing interface side reactions, but due to the disproportionation reaction of trivalent manganese ions and the Jahn-Teller effect, it will cause the initial specific capacity to decrease and the stability of long cycles is poor. Patent CN118630202A discloses a method for preparing niobium-doped sodium iron phosphate pyrophosphate by spray drying. The high-valent metal niobium oxalate occupies some active iron sites in the sodium iron phosphate pyrophosphate, thereby improving the electrochemical properties of the sodium iron phosphate pyrophosphate. However, niobium oxalate is insoluble in water and difficult to form a uniform solution. The suspension needs to be pre-treated by sand grinding before spraying. In addition, niobium oxalate, as a niobium source, is expensive, and the preparation time and cost are high.

[0006] In summary, it is of great significance to find a simple, efficient, safe and low-cost method to prepare and modify sodium ferrous pyrophosphate materials. Summary of the invention

[0007] The present invention provides a preparation method and application of a carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material to solve the problems of low specific capacity, unsatisfactory cycle and rate performance of existing sodium iron phosphate pyrophosphate positive electrode materials. The present invention adopts a spray drying method to dope the sodium iron phosphate pyrophosphate positive electrode material with cobalt. 2+ The ionic radius is larger than that of Fe 2+ The ionic radius is high, and Co is doped in sodium iron pyrophosphate 2+ After that, the Fe-O bond grows and the lattice expands, which is more favorable for Na + Active sites for transport; moreover, exogenous metal ions can act as lattice columns in the material, preventing lattice collapse, reducing adverse phase transitions, and inhibiting the generation of electrically inert impurity phase NaFePO4.

[0008] First, the present invention provides a method for preparing a carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material, which comprises the following steps:

[0009] Step 1: Dissolve an iron source, a sodium source, a phosphorus source, a carbon source and a cobalt source in water to obtain a uniform clear solution.

[0010] Step 2: Prepare the precursor powder from the clear solution by spray drying.

[0011] Step 3: calcining the precursor powder under an inert atmosphere.

[0012] Step 4: Crush and grind the calcined product obtained in step 3 to obtain a carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material.

[0013] The present invention adopts a spray drying method to prepare a carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material, and the doped Co 2+ Later, due to Co 2+ The ionic radius is larger than that of Fe 2+ The ionic radius is high, the lattice expands after doping, the Fe-O bond grows, the lattice opens, and it is more favorable for Na + Active sites for transport. Exogenous metal ions can act as lattice columns in the material, preventing lattice collapse, reducing undesirable phase changes, and inhibiting the generation of electrically inert impurity phase NaFePO4. In addition, the spray drying method can quickly convert liquid materials into powder, and the uniformity and particle size distribution of the product particles can be maintained during the drying process. The materials prepared by spray drying have a high specific surface area, which is beneficial to improving electrochemical activity. Compared with the hydrothermal method, the spray drying method can achieve continuous production, high efficiency and suitability for large-scale production, and can accurately control the size and shape of the particles, which is conducive to customizing materials that meet specific requirements.

[0014] Preferably, in step 1, the iron source is ferric nitrate and / or ferric phosphate; and the carbon source is citric acid.

[0015] The present invention found that in the preparation process of the specific process combination of "spray drying + calcination" of the present invention, not any combination of iron source and carbon source can successfully produce high-performance carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode materials. In other words, the type of iron source and carbon source in the method of the present invention is very important. For example, when the present invention team tried to use iron phosphate as the iron source and oxalic acid as the carbon source, they found that introducing Co into the oxalic acid-iron phosphate solution system 2+ Will destroy Fe 3+ The metal complex formed with oxalic acid turns the solution into a suspension. Precipitated particles with larger particle sizes tend to agglomerate during the spraying process, and uneven heating and incomplete redox occur during the calcination process, which results in the generation of impurities and a decrease in capacity. For another example, when the present invention team tried to use ascorbic acid as a carbon source in combination with ferric nitrate, they found that the strong reducing property of ascorbic acid would reduce Fe in advance before calcination. 3+Finally, the present invention found that under the above-mentioned combination of iron source and carbon source, the process combination of "spray drying + calcination" is suitable, and finally a high-performance carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material can be prepared.

[0016] Preferably, in step 1, the sodium source is one or more of sodium carbonate, sodium dihydrogen phosphate, sodium bicarbonate, sodium pyrophosphate, disodium dihydrogen pyrophosphate, sodium citrate, and sodium phosphate; the phosphorus source is one or more of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, pyrophosphoric acid, and sodium pyrophosphate; and the cobalt source is one or more of cobalt acetate, potassium cobalt cyanide, cobalt sulfate, cobalt chloride, and cobalt nitrate.

[0017] Most preferably, in step 1, the iron source is ferric nitrate; the carbon source is citric acid; the sodium source and phosphorus source are sodium dihydrogen phosphate; and the cobalt source is cobalt nitrate.

[0018] Preferably, the carbon source is citric acid; the molar ratio of iron, sodium, phosphorus, carbon and cobalt in the clear solution is (2-3): (3.5-4.5): (3.5-4.5): (1-4): (0.01-1.0); further preferably, the molar ratio of iron, sodium, phosphorus, carbon and cobalt is (2.35-2.45): (3.5-4.5): (3.5-4.5): (1-4): (0.4-0.8).

[0019] The present invention finds that the above-mentioned ratio of each element has a significant effect on the performance of the product. Excessive doping of elements will change the structure and properties of the material, which can easily cause the collapse of the structure, resulting in a decrease in material performance and requiring more raw materials and energy, increasing production costs. On the contrary, if the doping amount is not enough, vacancies cannot be generated, which is not conducive to the diffusion and migration of sodium ions, and the improper ratio of iron source and cobalt source will also introduce inert miscellaneous NaFePO4, resulting in a decrease in specific capacity and an increase in capacity decay at high rates.

[0020] Preferably, in step 3, the inlet temperature of the spray drying is 100-250°C, the outlet temperature is 100-180°C, and the feed rate is 0.5-20%; further preferably, the inlet temperature of the spray drying is 200-230°C, the outlet temperature is 100-180°C, and the feed rate is 0.5-15%.

[0021] The present invention finds that the spray drying process has a great influence on the performance of the product. Specifically: a higher inlet air temperature can promote the grain growth and morphology control of the material, help to form more microporous structures and nano-scale particles, thereby increasing the specific surface area of ​​the electrode material. However, too high an inlet air temperature may cause excessive heat treatment or sintering of the electrode material, causing changes in the material structure, and even causing the material to sinter or melt, thereby affecting the battery performance. On the contrary, too low an inlet air temperature will cause the material to be dried incompletely or unevenly, and then cause the material to agglomerate and reduce its specific surface area. In addition, a too fast feed rate may cause the solution to be unable to be fully dispersed when entering the spray dryer, forming larger particles, and increase the tendency of the particles to agglomerate when they are formed, causing the particles to combine into a larger block structure.

[0022] Preferably, in step 4, the inert gas is one of nitrogen, argon and argon-hydrogen mixed gas.

[0023] Preferably, in step 4, the calcination temperature is 500-800° C. and the calcination time is 5-15 hours.

[0024] During the calcination process, the heat in the furnace provided by the too low temperature can easily make the binding energy too low, unable to reach the thermal energy to generate the final product, thereby generating inert intermediate phases NaFePO4 and Na2FeP2O7, resulting in a sharp drop in capacity.

[0025] Secondly, the present invention provides the use of the carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material prepared by the above preparation method in a sodium ion battery.

[0026] Preferably, the assembly method of the sodium ion battery includes: first mixing the carbon-coated cobalt-doped sodium phosphate iron pyrophosphate positive electrode material with a conductive agent and a binder, and then adding an organic solvent and mixing evenly to obtain a positive electrode slurry of the sodium ion battery; uniformly coating the positive electrode slurry on the positive electrode collector, rolling and drying to obtain the positive electrode, using a sodium metal sheet as a negative electrode, a diethyl carbonate / ethylene carbonate solution of sodium perchlorate as an electrolyte, and glass fiber as a battery separator for assembly.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] (1) The present invention adopts a spray drying method to prepare a carbon-coated cobalt-doped sodium iron phosphate positive electrode material, doped with Co 2+ Later, due to Co 2+ The ionic radius is larger than that of Fe 2+ The ionic radius is high, the lattice expands after doping, the Fe-O bond grows, the lattice opens, and it is more favorable for Na + Exogenous metal ions can act as lattice columns in the material, preventing lattice collapse, reducing adverse phase transitions, and inhibiting the generation of electrically inert impurity phase NaFePO4.

[0029] (2) The spray drying method of the present invention can quickly convert liquid materials into powder, and can maintain the uniformity and particle size distribution of the product particles during the drying process. The material prepared by the spray drying method has a higher specific surface area, which is beneficial to improving the electrochemical activity. Compared with the hydrothermal method, the spray drying method can achieve continuous production, high efficiency and is suitable for large-scale production, and can accurately control the size and shape of the particles, which is conducive to customizing materials that meet specific requirements. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 X-ray diffraction (XRD) patterns of carbon-coated cobalt-doped Na4Fe3(PO4)2P2O7 and carbon-coated Na4Fe3(PO4)2P2O7 positive electrode materials prepared in Example 1 and Comparative Example 1;

[0031] Figure 2 This is the SEM spectrum of the carbon-coated cobalt-doped Na4Fe3(PO4)2P2O7 positive electrode material prepared in Example 1;

[0032] Figure 3 It is a battery rate performance diagram of the carbon-coated cobalt-doped Na4Fe3(PO4)2P2O7 and Na4Fe3(PO4)2P2O7 positive electrode materials prepared in Example 1 and Comparative Example 1;

[0033] Figure 4 This is a cycle curve diagram of the carbon-coated cobalt-doped Na4Fe3(PO4)2P2O7 positive electrode material prepared in Example 2 at a current density of 5C. DETAILED DESCRIPTION

[0034] The present invention is further specifically described below by way of examples, but the present invention is not limited to the following cases.

[0035] Example 1

[0036] Preparation of Carbon-Coated Cobalt-Doped Na4Fe 2.4 Co 0.6 (PO4)2(P2O7) cathode material

[0037] Step 1: Dissolve ferric nitrate, sodium dihydrogen phosphate, citric acid and cobalt nitrate in 50 mL of deionized water at a molar ratio of 2.4:4:1.5:0.6 to obtain a clear solution.

[0038] Step 2: spray drying the solution obtained in step 1 at an inlet air rate of 100%, an inlet air temperature of 220° C., an outlet air temperature of 160° C., and a feed rate of 10% to obtain a precursor product.

[0039] Step 3: Grind the precursor evenly, add the powder obtained after grinding into a tubular furnace filled with nitrogen, heat it at a heating rate of 10°C / min, and calcine it at 550°C for 5h to obtain the carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material.

[0040] Example 2 (Compared with Example 1, the ratio of iron source is changed)

[0041] Preparation of carbon-coated cobalt-doped Na4Fe with different molar ratios of iron source 2.4 Co 0.6 (PO4)2(P2O7) cathode material

[0042] Step 1: Dissolve ferric nitrate, sodium dihydrogen phosphate, citric acid and cobalt nitrate in 50 mL of deionized water at a molar ratio of 2.33:4:1.5:0.6 to obtain a clear solution.

[0043] Step 2: spray drying the solution obtained in step 1 at an inlet air rate of 100%, an inlet air temperature of 220° C., an outlet air temperature of 160° C., and a feed rate of 10% to obtain a precursor product.

[0044] Step 3: Grind the precursor product evenly, add the powder obtained after grinding into a tubular furnace filled with nitrogen, heat it at a heating rate of 10°C / min, and calcine it at 550°C for 5h to obtain the carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material.

[0045] Example 3 (Compared with Example 1, the ratio of iron source is changed)

[0046] Preparation of carbon-coated cobalt-doped Na4Fe with different molar ratios of iron source 2.4 Co 0.6 (PO4)2(P2O7) cathode material

[0047] Step 1: Dissolve ferric nitrate, sodium dihydrogen phosphate, citric acid and cobalt nitrate in 50 mL of deionized water at a molar ratio of 2.47:4:1.5:0.6 to obtain a clear solution.

[0048] Step 2: spray drying the solution obtained in step 1 at an inlet air rate of 100%, an inlet air temperature of 220° C., an outlet air temperature of 160° C., and a feed rate of 10% to obtain a precursor product.

[0049] Step 3: Grind the precursor product evenly, add the powder obtained after grinding into a tubular furnace filled with nitrogen, heat it at a heating rate of 10°C / min, and calcine it at 550°C for 5h to obtain the carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material.

[0050] Example 4 (compared with Example 1, the cobalt doping amount is increased)

[0051] Preparation of carbon-coated cobalt-doped Na4Fe 2.1 Co 0.9 (PO4)2(P2O7) cathode material

[0052] Step 1: Dissolve ferric nitrate, sodium dihydrogen phosphate, citric acid and cobalt nitrate in 50 mL of deionized water at a molar ratio of 2.1:4:1.5:0.9 to obtain a clear solution.

[0053] Step 2: spray drying the solution obtained in step 1 at an inlet air rate of 100%, an inlet air temperature of 220° C., an outlet air temperature of 160° C., and a feed rate of 10% to obtain a precursor product.

[0054] Step 3: Grind the precursor product evenly, add the powder obtained after grinding into a tubular furnace filled with nitrogen, heat it at a heating rate of 10°C / min, and calcine it at 550°C for 5h to obtain the carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material.

[0055] Example 5 (Cobalt doping amount reduced compared to Example 1)

[0056] Preparation of carbon-coated cobalt-doped Na4Fe 2.7 Co 0.3 (PO4)2(P2O7) cathode material

[0057] Step 1: Dissolve ferric nitrate, sodium dihydrogen phosphate, citric acid and cobalt nitrate in 50 mL of deionized water at a molar ratio of 2.7:4:1.5:0.3 to obtain a clear solution.

[0058] Step 2: spray drying the solution obtained in step 1 at an inlet air rate of 100%, an inlet air temperature of 220° C., an outlet air temperature of 160° C., and a feed rate of 10% to obtain a precursor product.

[0059] Step 3: Grind the precursor product evenly, add the powder obtained after grinding into a tubular furnace filled with nitrogen, heat it at a heating rate of 10°C / min, and calcine it at 550°C for 5h to obtain the carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material.

[0060] Comparative Example 1 (Compared with Example 1, without cobalt doping)

[0061] Preparation of carbon-coated Na4Fe3(PO4)2(P2O7) cathode material

[0062] Step 1: Dissolve ferric nitrate, sodium dihydrogen phosphate, and citric acid in 50 mL of deionized water at a molar ratio of 3.0:4:1.5 to obtain a clear solution.

[0063] Step 2: spray drying the solution obtained in step 1 at an inlet air rate of 100%, an inlet air temperature of 220° C., an outlet air temperature of 160° C., and a feed rate of 10% to obtain a precursor product.

[0064] Step 3: Grind the precursor product evenly, add the powder obtained after grinding into a tubular furnace filled with nitrogen, heat it at a heating rate of 10°C / min, and calcine it at 550°C for 5h to obtain the carbon-coated sodium iron phosphate pyrophosphate positive electrode material.

[0065] Comparative Example 2 (Compared with Example 1, the type of carbon source was changed)

[0066] Preparation of carbon-coated cobalt-doped Na4Fe 2.4 Co 0.6 (PO4)2(P2O7) cathode material

[0067] Step 1: Dissolve ferric nitrate, sodium dihydrogen phosphate, ascorbic acid and cobalt nitrate in 50 mL of deionized water at a molar ratio of 2.4:4:1.5:0.6 to obtain a clear solution.

[0068] Step 2: spray drying the solution obtained in step 1 at an inlet air rate of 100%, an inlet air temperature of 220° C., an outlet air temperature of 160° C., and a feed rate of 10% to obtain a precursor product.

[0069] Step 3: Grind the precursor product evenly, add the powder obtained after grinding into a tubular furnace filled with nitrogen, heat it at a heating rate of 10°C / min, and calcine it at 550°C for 5h to obtain the carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material.

[0070] Comparative Example 3 (Compared with Example 1, mixed carbon source was used)

[0071] Preparation of carbon-coated cobalt-doped Na4Fe 2.4 Co 0.6 (PO4)2(P2O7) cathode material

[0072] Step 1: Dissolve ferric nitrate, sodium dihydrogen phosphate, citric acid, glucose and cobalt nitrate in 50 mL of deionized water at a molar ratio of 2.4:4:1.0:0.5:0.6 to obtain a clear solution.

[0073] Step 2: spray drying the solution obtained in step 1 at an inlet air rate of 100%, an inlet air temperature of 220° C., an outlet air temperature of 160° C., and a feed rate of 10% to obtain a precursor product.

[0074] Step 3: Grind the precursor product evenly, add the powder obtained after grinding into a tubular furnace filled with nitrogen, heat it at a heating rate of 10°C / min, and calcine it at 550°C for 5h to obtain the carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material.

[0075] Comparative Example 4 (Compared with Example 1, the type of iron source was changed)

[0076] Preparation of Carbon-coated Cobalt-doped Na4Fe with Different Iron Sources 2.4 Co 0.6 (PO4)2(P2O7) cathode material

[0077] Step 1: Dissolve ferric phosphate, sodium dihydrogen phosphate, sodium carbonate, citric acid, oxalic acid, and cobalt nitrate in 50 mL of deionized water at a molar ratio of 4.8:2.25:3:2:12:1.2, and heat and stir to obtain a clear solution.

[0078] Step 2: spray drying the solution obtained in step 1 at an inlet air rate of 100%, an inlet air temperature of 220° C., an outlet air temperature of 160° C., and a feed rate of 10% to obtain a precursor product.

[0079] Step 3: Grind the precursor product evenly, add the powder obtained after grinding into a tubular furnace filled with nitrogen, heat it at a heating rate of 10°C / min, and calcine it at 550°C for 5h to obtain the carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material.

[0080] Comparative Example 5 (lowering the spray inlet temperature compared with Example 1)

[0081] Preparation of Cobalt-doped Carbon-coated Na4Fe with Different Spray Inlet Temperatures 2.4 Co 0.6 (PO4)2(P2O7) cathode material

[0082] Step 1: Dissolve ferric nitrate, sodium dihydrogen phosphate, citric acid and cobalt nitrate in 50 mL of deionized water at a molar ratio of 2.4:4:1.5:0.6 to obtain a clear solution.

[0083] Step 2: spray drying the solution obtained in step 1 at an air inlet rate of 100%, an air inlet temperature of 180° C., an air outlet temperature of 140° C., and a feed rate of 10% to obtain a precursor product.

[0084] Step 3: Grind the precursor product evenly, add the powder obtained after grinding into a tubular furnace filled with nitrogen, heat it at a heating rate of 10°C / min, and calcine it at 550°C for 5h to obtain the sodium iron pyrophosphate positive electrode material.

[0085] Comparative Example 6 (increasing the spray inlet temperature compared with Example 1)

[0086] Preparation of Cobalt-doped Carbon-coated Na4Fe with Different Spray Inlet Temperatures 2.4 Co 0.6 (PO4)2(P2O7) cathode material

[0087] Step 1: Dissolve ferric nitrate, sodium dihydrogen phosphate, citric acid and cobalt nitrate in 50 mL of deionized water at a molar ratio of 2.4:4:1.5:0.6 to obtain a clear solution.

[0088] Step 2: spray drying the solution obtained in step 1 at an air inlet rate of 100%, an air inlet temperature of 250° C., an air outlet temperature of 190° C., and a feed rate of 10% to obtain a precursor product.

[0089] Step 3: Grind the precursor product evenly, add the powder obtained after grinding into a tubular furnace filled with nitrogen, heat it at a heating rate of 10°C / min, and calcine it at 550°C for 5h to obtain the sodium iron pyrophosphate positive electrode material.

[0090] Comparative Example 7 (Compared with Example 1, the spray feed rate was changed)

[0091] Preparation of Spray Feed Rate Cobalt-doped Carbon-coated Na4Fe 2.4 Co 0.6 (PO4)2(P2O7) cathode material

[0092] Step 1: Dissolve ferric nitrate, sodium dihydrogen phosphate, citric acid and cobalt nitrate in 50 mL of deionized water at a molar ratio of 2.4:4:1.5:0.6 to obtain a clear solution.

[0093] Step 2: spray drying the solution obtained in step 1 at an air inlet rate of 100%, an air inlet temperature of 180° C., an air outlet temperature of 140° C., and a feed rate of 20% to obtain a precursor product.

[0094] Step 3: Grind the precursor product evenly, add the powder obtained after grinding into a tubular furnace filled with nitrogen, heat it at a heating rate of 10°C / min, and calcine it at 550°C for 5h to obtain the sodium iron pyrophosphate positive electrode material.

[0095] Comparative Example 8 (calcination temperature changed compared with Example 1)

[0096] Preparation of cobalt-doped carbon-coated Na4Fe 2.4 Co 0.6 (PO4)2(P2O7) cathode material

[0097] Step 1: Dissolve ferric nitrate, sodium dihydrogen phosphate, citric acid and cobalt nitrate in 50 mL of deionized water at a molar ratio of 2.4:4:1.5:0.6 to obtain a clear solution.

[0098] Step 2: spray drying the solution obtained in step 1 at an inlet air rate of 100%, an inlet air temperature of 220° C., an outlet air temperature of 160° C., and a feed rate of 10% to obtain a precursor product.

[0099] Step 3: Grind the precursor product evenly, add the powder obtained after grinding into a tube furnace filled with nitrogen, heat it at a heating rate of 10°C / min, and calcine it at 450°C for 5h to obtain the sodium iron pyrophosphate positive electrode material.

[0100] Performance Testing

[0101] The final products of Examples 1, 2 and Comparative Example 1 were mixed with the conductive agent Super P, the binder polyvinylidene fluoride and an appropriate amount of solvent N-methylpyrrolidone (NMP) in a mass ratio of 7:2:1 to form a homogeneous slurry, and then evenly coated on the aluminum current collector. After being dried at a constant temperature of 60°C for 12 hours, they were punched into discs with a diameter of 1.2 cm as sodium ion battery electrodes. A glass microfiber diaphragm was used as a diaphragm, and a half-cell was assembled using an electrolyte composed of ethylene carbonate / diethyl carbonate with a volume ratio of 1:1 as a solvent, NaClO4 as a solute, and a sodium sheet as a counter electrode. The assembled sodium ion half-cell was left to stand for 24 hours in a constant temperature environment of 35°C, and then an electrochemical test was performed.

[0102] Figure 1 The XRD diagrams of the products of Example 1 and Comparative Example 1, the peak positions of which all match the standard card of sodium ferric pyrophosphate, indicating that a pure phase of sodium ferric pyrophosphate was prepared.

[0103] Figure 2 It is the SEM picture of the product of Example 1. Figure 2 It can be seen that the spray-dried particles are in the form of small round particles, which look very fine and uniform as a whole, and are an ideal powder form.

[0104] Figure 3 This is the battery rate performance diagram of Example 1. As can be seen from the figure, the reversible capacity of the button half-cell at 0.1C reaches 100 mAh g -1 , the reversible capacity reaches 80 mAh g at 10C -1 , compared with the comparison example 1 without metal Co 2+ The rate performance of the doped sodium iron pyrophosphate positive electrode material has been significantly improved.

[0105] Figure 4This is the cycle performance diagram of Example 1 at 5C. At a current density of 5C, the capacity retention rate reaches 84% ​​after 250 cycles, and the cycle stability is good;

[0106] Table 1

[0107]

[0108] According to the data in Table 1, the following conclusions can be drawn:

[0109] By comparing Example 1 with Examples 2 and 3, it can be concluded that the reduction of the iron source (Example 2) will cause the product to tend to form NaFePO4 and Na2FeP2O7, affecting the formation of the target product, and the capacity decay is significant at high rates. However, the increase of the iron source (Example 3) will cause incomplete conversion of iron, resulting in some iron still not participating in the reaction, generating non-stoichiometric Fe impurities and NaFePO4 during the calcination process, thereby reducing the performance of the final product.

[0110] By comparing Example 1 with Examples 4 and 5, it can be concluded that excessive cobalt doping (Example 4) will lead to changes in the material structure, thereby reducing the material performance, while insufficient cobalt doping (Example 5) fails to effectively improve the material performance. Therefore, accurately grasping the optimal cobalt doping amount is crucial to improving the rate and cycle performance of the material.

[0111] Table 2

[0112]

[0113] According to the data in Table 2, the following conclusions can be drawn:

[0114] By comparing Example 1 and Comparative Example 1, it can be seen that the doping of cobalt element in Example 1 significantly improves the electrochemical performance of sodium iron pyrophosphate, especially at a rate of 10C.

[0115] It can be clearly observed that Example 1 has a significant improvement in charge and discharge specific capacity compared to Comparative Example 2, which indicates that citric acid as the carbon source (Example 1) can better chelate Fe 3+ , while the strong reducing property of ascorbic acid as a carbon source (Comparative Example 2) would reduce Fe in advance before calcination 3+ , resulting in a mixed phase that reduces the capacity. In Comparative Example 3, glucose and citric acid are used as mixed carbon sources. The different decomposition temperatures of the two carbon sources result in poor carbon distribution uniformity in the material, and carbon agglomeration or uneven carbon layers are easily formed. This will affect the charge transfer performance and ion diffusion path of the material, thereby reducing the electrochemical performance of the material. .

[0116] Comparing Example 1 and Comparative Example 4, the Co2+ The introduction of Fe 3 + The metal complex formed with oxalic acid turns the solution into a suspension. Precipitated particles with larger particle sizes tend to agglomerate during the spraying process, and uneven heating and incomplete oxidation and reduction occur during the calcination process, thus generating impurities that reduce the capacity.

[0117] Comparing Example 1 and Comparative Example 6, the higher inlet air temperature during spray drying in Example 1 can promote the grain growth and morphology control of the material, and help to form more microporous structures and nano-scale particles, thereby increasing the specific surface area of ​​the electrode material. However, too high an inlet air temperature may cause excessive heat treatment or sintering of the electrode material, causing changes in the material structure, and even causing sintering or melting of the material, thereby affecting the battery performance. Conversely, too low an inlet air temperature in Comparative Example 5 will also lead to incomplete or uneven drying of the material, which in turn causes the material to agglomerate and reduce its specific surface area. Therefore, setting the inlet air temperature to around 220°C is more conducive to improving material performance.

[0118] Comparative Example 1 and Comparative Example 7 show that a too fast feed rate during spray drying in Comparative Example 7 may result in the solution not being fully dispersed when entering the spray dryer, forming larger particles, and increasing the tendency of the particles to agglomerate during formation, causing the particles to combine into a larger block structure.

[0119] Comparative Example 1 and Comparative Example 8 show that during the calcination process, the furnace heat provided by the too low temperature of Comparative Example 8 makes the binding energy too low to reach the thermal energy for generating the final product, thereby generating inert intermediate phases NaFePO4 and Na2FeP2O7, resulting in a sudden drop in capacity.

[0120] In summary, the carbon-coated cobalt-doped sodium iron phosphate pyrophosphate cathode material prepared by spray drying has good electrochemical performance. The invention has high production efficiency and low synthesis cost, making it an effective method worthy of consideration in the preparation of sodium ion battery materials.

Claims

1. A method for preparing a carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material, characterized in that The following steps are involved: Step 1: dissolving an iron source, a sodium source, a phosphorus source, a carbon source and a cobalt source in water to obtain a uniform clear solution; the iron source is ferric nitrate and / or ferric phosphate; the carbon source is citric acid; the molar ratio of the iron element, the sodium element, the phosphorus element, the carbon element and the cobalt element in the clear solution is (2-3): (3.5-4.5): (3.5-4.5): (1-4): (0.01-1.0); Step 2: preparing the precursor powder from the clarified solution by spray drying; Step 3: calcining the precursor powder under an inert atmosphere; Step 4: Crush and grind the calcined product obtained in step 3 to obtain a carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material.

2. The preparation method according to claim 1, characterized in that: In step 1, the sodium source is one or more of sodium carbonate, sodium dihydrogen phosphate, sodium bicarbonate, sodium pyrophosphate, disodium dihydrogen pyrophosphate, sodium citrate, and sodium phosphate; The phosphorus source is one or more of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, pyrophosphoric acid, and sodium pyrophosphate; The cobalt source is one or more of cobalt nitrate, cobalt acetate, potassium cobalt cyanide, cobalt sulfate and cobalt chloride.

3. The preparation method according to claim 2, characterized in that: In step 1, the iron source is ferric nitrate; the carbon source is citric acid; the sodium source and phosphorus source are sodium dihydrogen phosphate; and the cobalt source is cobalt nitrate.

4. The preparation method according to claim 1, characterized in that: In step 1, the molar ratio of iron, sodium, phosphorus, carbon and cobalt in the clarified solution is (2.35-2.45):(3.5-4.5):(3.5-4.5):(1-4):(0.4-0.8).

5. The preparation method according to claim 1, characterized in that: In step 3, the inlet temperature of the spray drying is 100-250°C, the outlet temperature is 100-180°C, and the feed rate is 0.5-20%.

6. The preparation method according to claim 5, characterized in that: In step 3, the inlet temperature of the spray drying is 200-230°C, the outlet temperature is 100-180°C, and the feed rate is 0.5-15%.

7. The preparation method according to claim 1, characterized in that: In step 4, the inert gas is one of nitrogen, argon and argon-hydrogen mixed gas.

8. The preparation method according to claim 1, characterized in that: In step 4, the calcination temperature is 500-800° C. and the calcination time is 5-15 h.

9. Use of the carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material prepared by the preparation method according to any one of claims 1 to 8 in sodium ion batteries.

10. The use according to claim 9, characterized in that: The sodium ion battery is assembled by first mixing a carbon-coated cobalt-doped sodium iron phosphate pyrophosphate positive electrode material with a conductive agent and a binder, then adding an organic solvent and mixing evenly to obtain a positive electrode slurry of the sodium ion battery; evenly coating the positive electrode slurry on a positive electrode current collector, rolling and drying to obtain a positive electrode, and assembling the battery using a sodium metal sheet as a negative electrode, a diethyl carbonate / ethylene carbonate solution of sodium perchlorate as an electrolyte, and glass fiber as a battery separator.

Images