Method for synthesizing composite polyanion positive electrode material in alkaline environment

Abstract

The application discloses a method for synthesizing a composite polyanion positive electrode material in an alkaline environment, and comprises the following steps: S1, preparation of a raw material solution: an anion source is added into a solvent to be dissolved, and an alkaline additive is added at the same time, so that the pH range is adjusted to 10-12, and the raw material solution is obtained; S2, preparation of a precursor solution: a transition metal ion source, a sodium source and a carbon source are added into the raw material solution to form a precipitate, and liquid phase sand milling is simultaneously performed to obtain a uniformly dispersed precursor solution; S3, preparation of a homogeneous precursor powder: the precursor solution is heated and dried to obtain a homogeneous precursor powder composed of sodium elements, transition metal ion elements, an anion source and a carbon source; and S4, high-temperature sintering: the precursor powder is calcined at high temperature in a protective atmosphere, and the composite polyanion positive electrode material is obtained. The application has the characteristics of good system stability, high compaction density, strong process controllability and excellent electrochemical performance.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of sodium ion batteries, in particular to a method for synthesizing a composite polyanion positive electrode material in an alkaline environment. BACKGROUND

[0002] With the continuous development of modern science and technology, the living standards of people have been greatly improved, but the problems of environmental pollution and depletion of non-renewable energy sources such as oil are becoming more and more serious. Therefore, clean secondary energy such as wind energy, tidal energy and solar energy has entered the field of vision of people and has developed rapidly. However, the discontinuity, uncertainty and instability of wind energy, solar energy and the like have caused great resistance to the use of these clean energy. A relatively effective solution is to store the obtained electric energy, and in view of the important role of secondary batteries in large-scale energy storage, the stability, use cost and environmental protection of secondary batteries have great advantages, and therefore become the first choice. Among all the secondary batteries, lithium / sodium ion batteries are undoubtedly the leaders, and the research on lithium ion batteries has been quite mature. However, uneven distribution of lithium resources, insufficient reserves and other factors make it unable to meet the requirements of large-scale energy storage. Correspondingly, sodium ion batteries have similar working principles as lithium ion batteries, and the reserves of sodium resources are abundant, the cost is low, and the environment is pollution-free, which is the most promising technology for large-scale energy storage at present.

[0003] At present, the mainstream sodium ion battery positive electrode materials include transition metal oxides, polyanion type, prussian blue compounds and organic type, each of which has its own advantages and disadvantages. The transition metal oxides are easy to absorb water and react with air, and often accompanied by multiple phase changes in the process of deintercalating sodium ions, and have poor cycle stability. The transition metal ions of the prussian blue compound are dissolved, and the crystal water is difficult to remove, and gas is easily generated at high potential. The electronic conductivity of the organic positive electrode material is generally poor, and it is easy to dissolve in the organic electrolyte. The polyanion type material often has an open three-dimensional framework structure, and thus has good cycle stability and excellent rate performance, and is the best choice for commercial sodium ion batteries.

[0004] The composite polyanion type material is generally a framework structure formed by mutual connection of alkali metal ions, transition metal ions and two anion groups in the form of point / line / surface. Taking sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) as an example, the material contains [PO4] and [P2O7] two anion groups. In the prior art, the synthesis of the material mainly adopts a spray drying-calcination method in a liquid phase, but in an acidic environment, P2O7 4- acidic hydrolysis reaction to generate PO4 3- , so that PO4 3- is excessive in the synthesis process, PO43- with P2O7 4- The generated Na4Fe3(PO4)2P2O7 product is not only poor in crystallinity, but also contains a large amount of NaFePO4 impurities, which limits the exertion of its electrochemical performance. SUMMARY

[0005] The purpose of the present application is to provide a method for synthesizing a composite polyanion positive electrode material in an alkaline environment, which has the characteristics of good system stability, high compaction density, strong controllability of process and excellent electrochemical performance.

[0006] The present application can be realized by the following technical solutions:

[0007] The present application discloses a method for synthesizing a composite polyanion positive electrode material in an alkaline environment, comprising the following steps:

[0008] S1, preparation of a raw material solution: add an anion source to a solvent to dissolve it, and at the same time, add an alkaline additive to adjust the pH range to 10-12, to obtain a raw material solution;

[0009] S2, preparation of a precursor solution: add a transition metal ion source, a sodium source and a carbon source to the raw material solution to form a precipitate, and at the same time, perform liquid phase sanding to obtain a uniformly dispersed precursor solution;

[0010] S3, preparation of a homogeneous precursor powder: heat and dry the precursor solution to obtain a homogeneous precursor powder composed of sodium elements, transition metal ion elements, anion sources and carbon sources;

[0011] S4, high-temperature sintering: after high-temperature calcination of the precursor powder in a protective atmosphere, a composite polyanion positive electrode material is obtained.

[0012] The present application discloses a method for synthesizing a composite polyanion positive electrode material in an alkaline environment. In a liquid phase alkaline environment (10<pH<12), transition metal ions will react with pyrophosphate and phosphate to rapidly generate pyrophosphate iron and iron phosphate precipitates, thereby ensuring the integrity of pyrophosphate and the ratio of PO4 3- to P2O7 4- remains unchanged. At the same time, the precipitate can ensure close contact of iron, sodium and anion groups, and liquid phase sanding can completely mix and evenly disperse pyrophosphate iron, iron phosphate, sodium sources and carbon sources, thereby ensuring the full reaction during calcination. The method is simple to operate, the raw materials used are low in price, the synthesized Na4Fe3(PO4)2P2O7 is good in crystallinity, contains less NaFePO4 impurities, and can exhibit excellent electrochemical performance.

[0013] Further, the anion source is F - , P2O7 4- , CO32- BO3 3- SiO4 4- SO3 2- and / or two or more of the above.

[0014] Further, the solvent is one or two or more of the above of inorganic or organic solvents such as water, ethanol, chloroform, acetone, benzene, diethyl ether, ethyl acetate, carbon tetrachloride, etc.

[0015] Further, the basic additive is one or two or more of the above of sodium hydroxide solution, ammonium bicarbonate solution, ammonium acetate solution, ester-based quaternary ammonium salt, ammonium citrate, tetraethylammonium sulfate, ammonium oxalate monohydrate, cetyltrimethylammonium fluoride, dimethylalkyl-C12-18-ethanol ammonium chloride salt.

[0016] Further, the transition metal source is one or two or more of the above of iron-containing compounds, manganese-containing compounds, vanadium-containing compounds, cobalt-containing compounds, nickel-containing compounds, and / or titanium-containing compounds; the iron-containing compounds are one or two or more of the above of ferric nitrate, ferrous nitrate, ferric sulfate, ferrous sulfate, and / or ferrous acetate; the manganese-containing compounds are one or two or more of the above of manganese nitrate, manganese acetate, and / or manganese sulfate; the vanadium-containing compounds are one or two or more of the above of vanadium nitrate, vanadium sulfate, and / or vanadium acetate; the cobalt-containing compounds are one or two or more of the above of cobalt nitrate, cobalt sulfate, and / or cobalt acetate; the nickel-containing compounds are one or two or more of the above of nickel nitrate, nickel sulfate, and / or nickel acetate; the titanium-containing compounds are one or two or more of the above of titanium nitrate, titanium sulfate, and / or titanium acetate.

[0017] Further, the sodium source is one or two or more of the above of sodium bicarbonate, sodium formate, sodium acetate, sodium nitrate, sodium sulfate, sodium citrate, sodium propionate, sodium acrylate, sodium benzoate, and / or sodium bisulfate.

[0018] Further, the carbon source is one or two or more of the above of glucose, sucrose, starch, maltose, lactose, polyvinyl alcohol, cyclodextrin, polyacrylic acid, polyacrylonitrile, cellulose, and / or polyethylene.

[0019] Further, in step S2, the liquid phase sanding rotation speed is 1000-1500 r / min, and the sanding time is 4-8 hours.

[0020] Further, in step S3, the heating drying method includes spray drying, oil bath / water bath drying, and flash drying, and the heating temperature is 105-115℃, which can ensure the rapid evaporation of the solvent while the raw materials are not deteriorated due to high temperature.

[0021] Further, in step S4, the protective atmosphere is nitrogen, hydrogen, carbon monoxide, argon, nitrogen-hydrogen mixed gas, or argon-hydrogen mixed gas.

[0022] Further, in step S4, the sintering temperature is 500-800°C, and the holding time is 3-15H.

[0023] The method for synthesizing the composite polyanion positive electrode material in an alkaline environment has the following beneficial effects:

[0024] First, the system has good stability. In the process of the present application, the anion groups can be kept intact in the form of precipitates by providing an alkaline environment, thereby ensuring that the ratio between the anion groups remains stable.

[0025] Second, the compaction density is high. In the process of the present application, the pH value of the solution has a great influence on the characteristics of the material. By controlling the pH value, the particle size of the generated precipitate can be adjusted. Small particles are beneficial for preparing high-rate nanoscale materials, and large particles are beneficial for preparing dense high-compaction materials.

[0026] Third, the process is highly controllable. In the process of the present application, the liquid-phase sanding mixing method is used to achieve uniform mixing between sodium ions, transition metal ions, and anion groups, thereby improving the controllability of the process synthesis. Uniform material contact is beneficial for the growth of crystals at high temperatures.

[0027] Fourth, the electrochemical performance is excellent. In the process of the present application, the carbon source is in-situ cracked into carbon at high temperature, which can ensure that the carbon source is uniformly dispersed and filled in the particle surface and its gaps, thereby constructing a three-dimensional conductive network and greatly increasing the electronic conductivity. DETAILED DESCRIPTION

[0028] In order to enable those skilled in the art to better understand the technical solutions of the present application, the product of the present application will be further described in detail below in conjunction with examples.

[0029] The present application discloses a method for synthesizing a composite polyanion positive electrode material in an alkaline environment, comprising the following steps:

[0030] S1, preparation of a raw material solution: add an anion source to a solvent to dissolve it, and add an alkaline additive to adjust the pH range to 10-12 to obtain a raw material solution;

[0031] S2, preparation of a precursor solution: add a transition metal ion source, a sodium source, and a carbon source to the raw material solution to form a precipitate, and perform liquid-phase sanding to obtain a uniformly dispersed precursor solution;

[0032] S3, preparation of a homogeneous precursor powder: heat and dry the precursor solution to obtain a homogeneous precursor powder composed of sodium elements, transition metal ion elements, anion sources, and carbon sources;

[0033] S4, high-temperature sintering: the precursor powder is calcined at high temperature under a protective atmosphere to obtain the composite polyanion positive electrode material.

[0034] Further, the anion source is one or two of F - , P2O7 4- , CO3 2- , BO3 3- , SiO4 4- , SO3 2- , and the like.

[0035] Further, the solvent is one or a mixture of two or more of water, ethanol, chloroform, acetone, benzene, diethyl ether, ethyl acetate, carbon tetrachloride, and the like inorganic or organic solvents.

[0036] Further, the alkaline additive is one or a mixture of two or more of sodium hydroxide solution, ammonium bicarbonate solution, ammonium acetate solution, ester-based quaternary ammonium salt, ammonium citrate, tetraethylammonium sulfate, ammonium oxalate monohydrate, cetyltrimethylammonium fluoride, dimethylalkyl-C12-18-ethanol ammonium chloride salt.

[0037] Further, the transition metal source is a compound containing iron, a compound containing manganese, a compound containing vanadium, a compound containing cobalt, a compound containing nickel, and / or a compound containing titanium; the compound containing iron is one or two or more of ferric nitrate, ferrous nitrate, ferric sulfate, ferrous sulfate, and / or ferrous acetate; the compound containing manganese is one or two or more of manganese nitrate, manganese acetate, and / or manganese sulfate; the compound containing vanadium is one or two or more of vanadium nitrate, vanadium sulfate, and / or vanadium acetate; the compound containing cobalt is one or two or more of cobalt nitrate and / or cobalt sulfate, cobalt acetate; the compound containing nickel is one or two or more of nickel nitrate, nickel sulfate, and / or nickel acetate; the compound containing titanium is one or two or more of titanium nitrate, titanium sulfate, and / or titanium acetate.

[0038] Further, the sodium source is one or a mixture of two or more of sodium bicarbonate, sodium formate, sodium acetate, sodium nitrate, sodium sulfate, sodium citrate, sodium propionate, sodium acrylate, sodium benzoate, and / or sodium bisulfate.

[0039] Further, the carbon source is one or a mixture of two or more of glucose, sucrose, starch, maltose, lactose, polyvinyl alcohol, cyclodextrin, polyacrylic acid, polyacrylonitrile, cellulose, and / or polyethylene.

[0040] Further, in step S2, the liquid-phase sanding rotation speed is 1000-1500 r / min, and the sanding time is 4-8 hours.

[0041] Further, in step S3, the heating and drying method includes spray drying, oil bath / water bath drying, and flash drying, and the heating temperature is 105-115°C.

[0042] Further, in step S4, the protective atmosphere is nitrogen, hydrogen, carbon monoxide, argon, nitrogen-hydrogen mixed gas, or argon-hydrogen mixed gas.

[0043] Further, in step S4, the sintering temperature is 500-800°C, and the holding time is 3-15H.

[0044] Application Example 1 Synthesis of Na4Fe3(PO4)2P2O7 / C and its electrochemical performance

[0045] The synthesis of Na4Fe3(PO4)2P2O7 / C in this example includes the following steps:

[0046] S1, Preparation of raw material solution: ammonium dihydrogen phosphate and sodium pyrophosphate are added in a molar ratio of 2:1 with water to completely dissolve them, and an appropriate amount of ammonia solution is added to adjust the pH value to 10, to obtain a clear raw material solution (solution A);

[0047] S2, Preparation of precursor solution: iron nitrate is added to solution A in an amount of 3:1 with sodium pyrophosphate, and glucose is added, and the mixture is fully stirred to form a soft small particle precipitate, and liquid sanding is performed, with a sanding machine speed of 1200 r / min and a sanding time of 6 hours, to obtain a uniformly dispersed suspension. After sufficient sanding in this step, a completely mixed and uniform suspension, i.e., the precursor solution, is obtained;

[0048] S3, Preparation of precursor powder: the precursor solution is spray dried at 105°C, and the ammonia gas volatilized is collected and recycled, to obtain a precursor powder in which iron, sodium, and anion groups are uniformly mixed;

[0049] S4, High-temperature sintering: the precursor powder is sintered at 600°C for 10 hours in a nitrogen atmosphere, and then naturally cooled to obtain the Na4Fe3(PO4)2P2O7 / C material.

[0050] The Na4Fe3(PO4)2P2O7 / C, AB, and PVDF are mixed at a mass ratio of 8:1:1, and homogenized for 15 minutes to obtain a uniform slurry. The slurry is coated on an aluminum foil using a 150 μm four-side coater, and then the film is dried in a vacuum drying oven at 100°C for 12 hours. The electrode film is punched into a circular sheet with a diameter of 12 mm using a sheet puncher, and a metal sodium is used as the counter electrode, 1 mol / L NaClO4 EC+DEC (1:1 vol%)+5% FEC is used as the electrolyte, and a PP / PE / PP three-layer separator is used as the separator, to assemble a CR2016 type button cell in a glove box.

[0051] Table 1 shows that the Na4Fe3(PO4)2P2O7 / C material has a tap density of 2.02 g / cm 3, which is about 17.4% higher than that of Comparative Example 1 (1.72 g / cm 3 ), the main reason being that in Step 1, ammonia water is added to adjust the pH to 10, thereby generating soft small-particle precipitates, so that the iron, sodium and anion groups in the raw materials are in close contact, and in the subsequent sintering process, the material is prone to nucleation and crystallization to generate larger single-crystal particles, thereby forming a material with a larger tap density.

[0052] At the same time, the results in Table 1 show that the specific surface area of the material is only 10.2 m 2 / g, which is about 22.7% lower than that of Comparative Example 1 (13.2 m 2 / g), indicating that the porosity of the material is lower, which corresponds to its larger tap density.

[0053] In addition, the reversible capacity of the material at 0.1C rate is 110 mAh / g, which is much higher than 98 mAh / g in Comparative Example 1, and the capacity retention rate at 5C rate is 98.2%, which is much higher than 89.2% in Comparative Example 1. This indicates that inhibiting the hydrolysis of the anion group, maintaining the proportion of the anion group, and improving the nucleation and crystallization of the material have a great improvement on the performance of the material.

[0054] Application Example 2 Synthesis of Na4Fe3(PO4)2P2O7 / C and its electrochemical performance

[0055] The synthesis of Na4Fe3(PO4)2P2O7 / C in this example includes the following steps:

[0056] S1, Preparation of raw material solution: ammonium dihydrogen phosphate and sodium pyrophosphate are added to water in a molar ratio of 2:1 to completely dissolve them, and an appropriate amount of ammonia solution is added to adjust the pH value to 11 to obtain a clear raw material solution (A solution);

[0057] S2, Preparation of precursor solution: iron nitrate is added to the A solution in an amount of 3:1 with sodium pyrophosphate, and glucose is added, and the mixture is stirred to form a dense precipitate, and liquid sanding is performed at a sanding machine speed of 1200 r / min for 6 hours, at which time a uniformly dispersed suspension is obtained. After sufficient sanding in this step, a completely mixed and uniform suspension, i.e. a precursor solution, is obtained;

[0058] S3, Preparation of precursor powder: the precursor solution is spray dried at 105°C, and the ammonia gas volatilized is collected and recycled to obtain a precursor powder in which iron, sodium and anion groups are uniformly mixed;

[0059] S4, High-temperature sintering: the precursor powder is heated at 600°C for 10 hours in a nitrogen atmosphere, and then naturally cooled to obtain a Na4Fe3(PO4)2P2O7 / C material.

[0060] The Na4Fe3(PO4)2P2O7 / C, AB, PVDF were mixed in a ratio of 8:1:1 by mass and homogenized for 15 minutes to obtain a uniform slurry. The slurry was coated on an aluminum foil using a 150 μm four-side coater, and then the film was dried in a vacuum drying oven at 100°C for 12 hours. The electrode film was punched into a 12 mm diameter circle using a puncher, and a CR2016 type button cell was assembled in a glove box using a metal sodium as a counter electrode, 1 mol / L NaClO4 EC+DEC (1:1 vol%)+5% FEC as an electrolyte, and a PP / PE / PP three-layer separator.

[0061] The results in Table 1 show that the Na4Fe3(PO4)2P2O7 / C material has a tap density of 2.13 g / cm3 3 , which is about 23.8% higher than that of Comparative Example 1 (1.72 g / cm3 3 ). The main reason is that in Step 1, ammonia water is added to adjust the pH to 11, thereby generating a dense precipitate, so that the iron, sodium and anion groups in the raw materials are in close contact, and in the subsequent sintering process, the material is easy to nucleate and crystallize, generating large single crystal particles, and thus a material with high tap density is formed.

[0062] At the same time, the results in Table 1 show that the specific surface area of the material is only 9.0 m 2 / g, which is about 31.8% lower than that of Comparative Example 1 (13.2 m 2 / g), indicating that the porosity of the material is low, which corresponds to its high tap density.

[0063] In addition, the reversible capacity of the material at 0.1C rate is 117 mAh / g, which is much higher than 98 mAh / g in Comparative Example 1, and the capacity retention rate at 5C rate is 96.3%, which is much higher than 89.2% in Comparative Example 1. This shows that inhibiting the hydrolysis of the anion group, maintaining the proportion of the anion group, and improving the nucleation and crystallization of the material can greatly improve the performance of the material.

[0064] In addition, the process of Application Example 2 is changed compared to Application Example 1, i.e., more ammonia water is added to obtain a more alkaline liquid environment, and the generated precipitate is more dense, which is beneficial to the formation of large single crystal particles in the later sintering process, thereby obtaining a material with better crystallinity, and thus the specific capacity is higher. However, in Application Example 1, the smaller single crystal particles can shorten the sodium ion diffusion distance to a certain extent, and thus the rate performance (98.2%) is more excellent than that of Application Example 2 (96.3%). Therefore, this process can adjust the amount of ammonia water solution added in the early stage to control the pH of the solution, thereby realizing the switching between energy-type and rate-type materials.

[0065] Synthesis and electrochemical performance of Na4Fe3(PO4)2P2O7 / C

[0066] The synthesis of Na4Fe3(PO4)2P2O7 / C in this example includes the following steps:

[0067] S1, Preparation of raw material solution: ammonium dihydrogen phosphate and sodium pyrophosphate were added according to a molar ratio of 2:1, and water was added to completely dissolve them. At the same time, an appropriate amount of ammonia solution was added to adjust the pH value to 12, and a clear raw material solution (solution A) was obtained.

[0068] S2, Preparation of precursor solution: iron nitrate was added to solution A according to a ratio of 3:1 to sodium pyrophosphate, and glucose was added. After stirring, a blocky and large particle precipitate was formed, and liquid sanding was performed. The sanding machine was operated at a speed of 1200 r / min, and the sanding time was 6 hours. At this time, a uniformly dispersed suspension was obtained. After sufficient sanding in this step, a completely mixed and uniform suspension, i.e., the precursor solution, was obtained.

[0069] S3, Preparation of precursor powder: the precursor solution was spray dried at 105°C, and the ammonia gas volatilized was collected and recycled. A precursor powder uniformly mixed with iron, sodium, and anion groups was obtained.

[0070] S4, High-temperature sintering: the precursor powder was sintered at 600°C for 10 hours in a nitrogen atmosphere, and then naturally cooled to obtain the Na4Fe3(PO4)2P2O7 / C material.

[0071] Na4Fe3(PO4)2P2O7 / C, AB, and PVDF were mixed at a mass ratio of 8:1:1, and homogenized for 15 minutes to obtain a uniform slurry. The slurry was coated on an aluminum foil using a 150 μm four-sided coater, and then the film was dried in a vacuum drying oven at 100°C for 12 hours. The electrode film was punched into a circular sheet with a diameter of 12 mm using a sheet puncher. A CR2016 type button cell was assembled in a glove box using a metal sodium as the counter electrode, 1 mol / L NaClO4 EC+DEC (1:1 vol%)+5% FEC as the electrolyte, and a PP / PE / PP three-layer separator.

[0072] Table 1 shows that the Na4Fe3(PO4)2P2O7 / C material has a tap density of 2.24 g / cm 3 , which is about 30.2% higher than that of Comparative Example 1 (1.72 g / cm 3 ). The main reason is that in step 1, ammonia water is added to adjust the pH to 12, thereby generating a blocky and large particle precipitate. This allows the iron, sodium, and anion groups in the raw material to be in close contact, and the material is easy to nucleate and crystallize during the subsequent sintering process, generating large single crystal particles and thus a material with high tap density.

[0073] Meanwhile, the results of Table 1 show that the specific surface area of the material is only 8.3 m 2 / g, which is 37.2% lower than that of Comparative Example 1 (13.2 m 2 / g), indicating that the porosity of the material is low, which corresponds to its large compaction density

[0074] In addition, the reversible capacity of the material at 0.1C rate is 119 mAh / g, which is much higher than 98 mAh / g in Comparative Example 1, and the capacity retention rate at 5C rate is 93.4%, which is much higher than 89.2% in Comparative Example 1. It is shown that inhibiting the hydrolysis of anion groups, maintaining the proportion of anion groups, and improving the nucleation crystallinity of the material have great improvement on the performance of the material.

[0075] In addition, the process of Application Example 3 is changed compared to Application Examples 1 and 2, and the amount of ammonia water added in the former is the largest, so that the most alkaline liquid environment is obtained, and blocky large particle precipitates are generated, which is beneficial to the formation of large single crystal particles in the later sintering process, so that a material with better crystallinity is obtained, and thus the specific capacity is the highest. However, in Application Example 3, the excessively large single crystal particles prolong the diffusion distance of sodium ions to a certain extent, so that the rate performance (93.4%) is poorer than that of Application Examples 1 and 2. It can be seen that, continuing to increase the pH value has a smaller improvement in specific capacity, but a larger loss in rate performance. Therefore, the process can adjust the pH of the solution by changing the amount of ammonia water solution added in the early stage, so as to switch between energy type and rate type materials, and the recommended optimal pH value is 11.

[0076] Synthesis of Comparative Example 1 Na4Fe3(PO4)2P2O7 / C and its electrochemical performance

[0077] The synthesis of Na4Fe3(PO4)2P2O7 / C in this example includes the following steps:

[0078] Step 1: ammonium dihydrogen phosphate, sodium pyrophosphate were added according to the molar ratio of 2:1 with water to completely dissolve them, and an appropriate amount of oxalic acid solution was added to adjust the pH value to 5, to obtain a clear solution (A solution);

[0079] Step 2: iron nitrate was added to the A solution according to the amount of 3:1 with sodium pyrophosphate, and glucose was added at this time, to obtain a light yellow clear solution;

[0080] Step 3: then the precursor solution was spray dried at 105°C to obtain a precursor powder in which iron, sodium and anion groups were uniformly mixed;

[0081] Step 4: After the precursor powder is kept at 600℃ for 10 hours in a nitrogen atmosphere, the Na4Fe3(PO4)2P2O7 / C material is obtained by natural cooling.

[0082] The Na4Fe3(PO4)2P2O7 / C, AB and PVDF are mixed at a mass ratio of 8:1:1 and homogenized for 15 minutes to obtain a uniform slurry. The slurry is coated on an aluminum foil using a 150μm four-side coater, and then the film is dried in a vacuum drying oven at 100℃ for 12 hours. The electrode film is punched into a 12mm diameter disc using a puncher, and a CR2016 button cell is assembled in a glove box using a metal sodium as a counter electrode, 1mol / L NaClO4 EC+DEC (1:1 vol%)+5% FEC as an electrolyte, and a PP / PE / PP three-layer separator.

[0083] The results in Table 1 show that the Na4Fe3(PO4)2P2O7 / C material has a tap density of 1.72g / cm 3 , which is lower than that of Application Example 1 (2.02g / cm 3 ), and a specific surface area of 13.2 m 2 / g, which is higher than that of Application Example 1 (10.2 m 2 / g). The lower tap density corresponds to the presence of more pores between the materials, which is mainly due to the fact that in an acidic environment, iron, sodium and anion groups are dissolved in water, the precursor powder obtained by spraying is loose, and there is a lack of crystal nucleation core in the subsequent sintering process, resulting in poor crystallinity and low tap density.

[0084] In addition, the reversible capacity of the material at 0.1C rate is 98 mAh / g, which is much lower than 110mAh / g in Application Example 1, and the capacity retention rate at 5C rate is 89.2%, which is much lower than 98.2% in Application Example 1. This is because on the one hand, the poor crystallinity makes it difficult for sodium ions to transport, and on the other hand, the hydrolysis of anion groups in an acidic environment causes an imbalance in the proportion of anion groups during the reaction, resulting in a large amount of impurity substances, which greatly affects the release of the electrochemical performance.

[0085] Table 1: Performance test results

[0086]

[0087] The above examples are only specific embodiments of the present application, which are described in detail, but should not be construed as limiting the scope of the present patent. It should be noted that for those skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, and these obvious alternative forms all belong to the protection scope of the present application.

Claims

1. A method for synthesizing composite polyanionic cathode materials in an alkaline environment, characterized in that... Includes the following steps: S1. Preparation of raw material solution: Add the anion source to the solvent to dissolve it, and add an alkaline additive to adjust the pH range to 10-12 to obtain the raw material solution. S2. Preparation of precursor solution: A transition metal ion source, sodium source and carbon source are added to the raw material solution to form a precipitate, and liquid phase milling is performed to obtain a uniformly dispersed precursor solution. S3. Preparation of homogeneous precursor powder: The precursor solution is heated and dried to obtain homogeneous precursor powder composed of sodium, transition metal ions, anion source and carbon source. S4. High-temperature sintering: The composite polyanion cathode material is obtained by calcining the precursor powder at high temperature under a protective atmosphere.

2. The method for synthesizing composite polyanionic cathode materials in an alkaline environment according to claim 1, characterized in that: The anion source is F - P2O7 4- CO3 2- BO3 3- SiO4 4- SO3 2- One or two of them.

3. The method for synthesizing composite polyanionic cathode materials in an alkaline environment according to claim 1, characterized in that: The solvent is one or a mixture of two or more inorganic or organic solvents such as water, ethanol, chloroform, acetone, benzene, diethyl ether, ethyl acetate, and carbon tetrachloride.

4. The method for synthesizing composite polyanionic cathode materials in an alkaline environment according to claim 1, characterized in that: The alkaline additive is one or a mixture of two or more of the following: sodium hydroxide solution, ammonium bicarbonate solution, ammonium acetate solution, ester-based quaternary ammonium salt, ammonium citrate, tetraethylammonium sulfate, ammonium oxalate monohydrate, hexadecyltrimethylammonium fluoride, and dimethylalkyl-C12-18-ethanol ammonium salt.

5. The method for synthesizing composite polyanionic cathode materials in an alkaline environment according to claim 1, characterized in that: The transition metal source is an iron-containing compound, a manganese-containing compound, a vanadium-containing compound, a cobalt-containing compound, a nickel-containing compound, and / or a titanium-containing compound; The iron-containing compound is one or more of ferric nitrate, ferrous nitrate, ferrous sulfate, ferrous sulfate and / or ferrous acetate; The manganese-containing compound is one or more of manganese nitrate, manganese acetate, and / or manganese sulfate; The vanadium-containing compound is one or more of vanadium nitrate, alum sulfate and / or vanadium acetate; The cobalt-containing compounds are cobalt nitrate and / or cobalt sulfate and cobalt acetate; The nickel-containing compound is one or more of nickel nitrate, nickel sulfate, and / or nickel acetate; The titanium-containing compound is one or more of titanium nitrate, titanium sulfate, and / or titanium acetate.

6. The method for synthesizing composite polyanionic cathode materials in an alkaline environment according to claim 1, characterized in that: The sodium source is one or a mixture of two or more of sodium bicarbonate, sodium formate, sodium acetate, sodium nitrate, sodium sulfate, sodium citrate, sodium propionate, sodium acrylate, sodium benzoate and / or sodium bisulfate.

7. The method for synthesizing composite polyanionic cathode materials in an alkaline environment according to claim 1, characterized in that: The carbon source is one or a mixture of two or more of glucose, sucrose, starch, maltose, lactose, polyvinyl alcohol, cyclodextrin, polyacrylic acid, polyacrylonitrile, cellulose and / or polyethylene.

8. The method for synthesizing composite polyanionic cathode materials in an alkaline environment according to claim 1, characterized in that: In step S2, the liquid phase milling speed is 1000-1500 r / min, and the milling time is 4-8 hours.

9. The method for synthesizing composite polyanionic cathode materials in an alkaline environment according to claim 1, characterized in that: In step S3, the heating and drying methods include spray drying, oil bath / water bath drying, and flash drying, with a heating temperature of 105-115℃.

10. The method for synthesizing composite polyanionic cathode materials in an alkaline environment according to claim 1, characterized in that: In step S4, the protective atmosphere is nitrogen, hydrogen, carbon monoxide, argon, a nitrogen-hydrogen mixture, or an argon-hydrogen mixture; in step S4, the sintering temperature is 500~800°C, and the holding time is 3~15H.

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