A composite polyanion sodium cathode material, preparation method and application thereof, and sodium ion battery

Abstract

The present invention provides a composite polyanion sodium cathode material, its preparation method and application, and sodium ion battery. The preparation method of the composite polyanion sodium cathode material of the present invention innovatively integrates the finished sodium ferric phosphate pyrophosphate material into the synthesis process of sodium ferric sulfate, avoiding the conflict between the synthesis temperatures of sodium ferric sulfate when the temperature is higher than 400°C, sulfur dioxide precipitation, and the sintering temperature of sodium ferric phosphate pyrophosphate, which needs to be above 450°C. At the same time, the properties of the sodium ferric phosphate pyrophosphate material do not change during the low-temperature sintering process, and the carbon source introduced during the synthesis process can also improve the conductivity of the composite material. The present invention uses a secondary sintering method to jointly sinter the two materials to form a uniform and stable composite polyanion sodium cathode material without generating any by-products. The synthetic composite polyanion sodium cathode material has the excellent properties of low cost, long cycle, and high capacity.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion batteries, and in particular to a composite polyanion sodium cathode material, a preparation method and application thereof, and a sodium ion battery. Background Art

[0002] Sodium-ion batteries have the advantages of abundant resources, low cost, and high cost-effectiveness. Their working principle is similar to that of lithium-ion batteries and they are highly compatible with lithium-ion batteries. They have become an ideal alternative to lithium-ion batteries and are expected to be widely used in the energy storage field in the future. The current mainstream sodium cathode material routes each have their own advantages. Among them, polyanion sodium cathode materials currently have comprehensive performance advantages such as low material cost and good cycle performance. Sodium ferric sulfate and sodium ferric pyrophosphate perform very well in the fields of low cost and long cycle respectively. At the same time, the two materials have similar voltage platforms. If they are synthesized together into a composite polyanion sodium cathode material, they will have the common advantages of low cost and long cycle, which can promote the faster entry of sodium cathode materials into the market and expand their application areas.

[0003] Sodium ferric sulfate is a positive electrode material with extremely low production cost and stable electrochemical performance, but its poor conductivity and low theoretical capacity (80mah / g) limit its commercial development. In the conventional synthesis method, the sintering temperature needs to reach 300-400°C during the sintering process; at the same time, too high a temperature during the sintering process will cause sulfate ions to volatilize into sulfur dioxide, resulting in the material being unable to form a high-purity sodium ferric sulfate positive electrode material. The theoretical capacity and conductivity of sodium ferric sulfate positive electrode materials can be improved by compounding materials. In the conventional sintering process, sodium ferric pyrophosphate needs to be heated to 500-600°C to form a uniform phase. Therefore, the two processes cannot be sintered into a phase together during the sintering process, which is also the current obstacle to the preparation of composite polyanion sodium positive electrode materials.

[0004] Based on the defects in the current preparation of composite polyanion sodium cathode materials, it is necessary to improve them. Summary of the Invention

[0005] In view of this, the present invention provides a composite polyanion sodium cathode material, a preparation method and application thereof, and a sodium ion battery to solve or at least partially solve the defects in the prior art.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] The present invention provides a method for preparing a composite polyanion sodium cathode material, comprising the following steps:

[0008] An iron source, a sodium source, a sulfur source, a carbon source, an antioxidant, and sodium ferric pyrophosphate are mixed under an inert atmosphere and ball-milled to obtain a precursor powder;

[0009] After compacting the precursor powder, calcining it in an inert atmosphere to obtain a composite polyanion sodium cathode material;

[0010] Among them, the calcination is specifically as follows: the compacted precursor powder is heated from room temperature to 80-120°C at a rate of 1-10°C / min, and kept warm for 10-15 hours, then heated to 320-380°C at a rate of 1-10°C / min, and kept warm for 20-25 hours, and then cooled to room temperature with the furnace to obtain a composite polyanion sodium cathode material.

[0011] Preferably, an iron source, a sodium source, a sulfur source, a carbon source, an antioxidant, and sodium ferric pyrophosphate are mixed under an inert atmosphere to obtain a mixed powder, which is then ball-milled to obtain a precursor powder;

[0012] Among them, the ball milling method is dry ball milling;

[0013] The ball milling speed is 300-600 r / min, the ball milling time is 1-5 h, the mass ratio of the ball milling beads to the mixed powder is (4-8): (2-6), the diameter of the ball milling beads is 1-5 mm, and the D of the precursor powder obtained after ball milling is 50 The particle size is 4 to 8 μm.

[0014] Preferably, the iron source includes at least one of a divalent iron salt or a trivalent iron salt;

[0015] The sodium source includes at least one of sodium carbonate and its hydrate, sodium bicarbonate and its hydrate, sodium acetate and its hydrate, sodium oxalate and its hydrate, sodium citrate and its hydrate, sodium nitrate and its hydrate, sodium sulfate and its hydrate, and sodium bisulfate and its hydrate;

[0016] The sulfur source includes at least one of sodium sulfate and its hydrate, sodium bisulfate and its hydrate, ammonium bisulfate and its hydrate, ammonium sulfate and its hydrate, ferric sulfate and its hydrate, ferrous sulfate and its hydrate, ferrous ammonium sulfate and its hydrate, and ferric ammonium sulfate and its hydrate;

[0017] The carbon source includes at least one of organic carbon and inorganic carbon;

[0018] The antioxidant includes at least one of ascorbic acid, citric acid, oxalic acid, sodium sulfite, sodium D-isoascorbate, sodium bisulfite, hydrazine and paraformaldehyde.

[0019] Preferably, the iron source includes ferrous sulfate and its hydrate, ferric sulfate and its hydrate, ammonium ferrous sulfate and its hydrate, ammonium ferric sulfate and its hydrate, ferrous nitrate and its hydrate, ferric nitrate and its hydrate, ferrous chloride and its hydrate, ferric chloride and its hydrate, ammonium ferric citrate and its hydrate;

[0020] The inorganic carbon includes at least one of sucrose, graphene, carbon nanotubes, and carbon black.

[0021] Preferably, the molar ratio of the sodium source, the iron source, and the sulfur source is (0.7-1.2):1:(1-2);

[0022] The molar ratio of the iron source to sodium ferric pyrophosphate is (0.5-0.95):(0.05-0.5);

[0023] The mass fraction of the carbon source in the mixed powder is 0.5 to 5%;

[0024] The molar ratio of the antioxidant to the iron source is (0.01-0.1):1.

[0025] Preferably, the chemical formula of the sodium ferric phosphate is Na 4+x Fe 3+x (PO4) 2+x (P2O7), 0≤x≤1;

[0026] The inert atmosphere includes at least one of nitrogen, helium, neon and argon.

[0027] Preferably, the precursor powder is compacted under a pressure of 0.05 to 5 GPa, calcined under an inert atmosphere, and crushed to D 50 The particle size is 3 to 10 μm, and a composite polyanion sodium positive electrode material is obtained.

[0028] In a second aspect, the present invention further provides a composite polyanion sodium cathode material, which is prepared using the above-mentioned preparation method.

[0029] In a third aspect, the present invention also provides a composite polyanion sodium cathode material prepared by the preparation method or the use of the composite polyanion sodium cathode material in the preparation of a sodium ion battery.

[0030] In a fourth aspect, the present invention further provides a sodium ion battery comprising the composite polyanion sodium cathode material.

[0031] The method for preparing a composite polyanion sodium cathode material of the present invention has the following beneficial effects compared with the prior art:

[0032] The preparation method of the composite polyanion sodium cathode material of the present invention innovatively integrates the finished sodium ferric phosphate pyrophosphate material into the synthesis process of sodium ferric sulfate, avoiding the problem of sulfur dioxide precipitation at a sodium ferric sulfate temperature higher than 400°C and the conflict between the synthesis temperatures of the two, which is caused by the sintering temperature of sodium ferric phosphate pyrophosphate being required to reach 450°C or above; at the same time, the properties of the sodium ferric phosphate pyrophosphate material do not change during the low-temperature sintering process, and the carbon source introduced during the synthesis process can also improve the conductivity of the composite material; the present invention jointly fires the two materials through a secondary sintering method to form a uniform and stable composite polyanion sodium cathode material, and no by-products are produced. Taking into account the problem that the sodium ferric sulfate material has extremely low cost and excellent cycle performance but low theoretical capacity, a certain proportion of sodium ferric phosphate pyrophosphate cathode material with a similar voltage platform is introduced to synthesize a composite polyanion sodium cathode material, which has the excellent properties of low cost, long cycle and high capacity. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive effort.

[0034] Figure 1 This is a SEM image of a composite polyanion sodium cathode material prepared according to the method of Example 1;

[0035] Figure 2 The XRD pattern of the composite polyanion sodium cathode material prepared according to the method in Example 1;

[0036] Figure 3 This is the charge and discharge curve of the sodium ion battery in Example 2. DETAILED DESCRIPTION

[0037] To make the purpose, technical solutions and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments.

[0038] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0039] It should be noted that the order of description of the following embodiments is not intended to limit the preferred order of the embodiments. In addition, in the description of this application, the term "including" means "including but not limited to". Various embodiments of the present invention may be presented in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be understood as a rigid limitation on the scope of the invention; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0040] The present invention provides a method for preparing a composite polyanion sodium cathode material, comprising the following steps:

[0041] S1. Mixing an iron source, a sodium source, a sulfur source, a carbon source, an antioxidant, and sodium ferric pyrophosphate under an inert atmosphere, and ball milling to obtain a precursor powder;

[0042] S2. After compacting the precursor powder, calcining it under an inert atmosphere to obtain a composite polyanion sodium cathode material;

[0043] Among them, the calcination is specifically as follows: the compacted precursor powder is heated from room temperature to 80-120°C at a rate of 1-10°C / min, and kept warm for 10-15 hours, then heated to 320-380°C at a rate of 1-10°C / min, and kept warm for 20-25 hours, and then cooled to room temperature with the furnace to obtain a composite polyanion sodium cathode material.

[0044] The preparation method of the composite polyanion sodium cathode material of the present invention innovatively integrates the finished sodium ferric phosphate pyrophosphate material into the synthesis process of sodium ferric sulfate, avoiding the problem of sulfur dioxide precipitation at a sodium ferric sulfate temperature higher than 400°C and the conflict between the synthesis temperatures of the two, which is caused by the sintering temperature of sodium ferric phosphate pyrophosphate being required to reach 450°C or above; at the same time, the properties of the sodium ferric phosphate pyrophosphate material do not change during the low-temperature sintering process, and the carbon source introduced during the synthesis process can also improve the conductivity of the composite material; the present invention jointly fires the two materials through a secondary sintering method to form a uniform and stable composite polyanion sodium cathode material, and no by-products are produced. Taking into account the problem that the sodium ferric sulfate material has extremely low cost and excellent cycle performance but low theoretical capacity, a certain proportion of sodium ferric phosphate pyrophosphate cathode material with a similar voltage platform is introduced to synthesize a composite polyanion sodium cathode material, which has the excellent properties of low cost, long cycle and high capacity.

[0045] Specifically, the iron source, sodium source, and sulfur source are synthesized sodium ferric sulfate (Na 2+2x Fe 2-x (SO4)3, 0≤x≤1) raw materials. During the synthesis process, the addition of antioxidants can effectively prevent divalent iron from being oxidized during high-energy ball milling. At the same time, as a source of carbon source, it can increase the carbon coating content of the material and improve conductivity and stability.

[0046] Specifically, the calcination is heated in a step-by-step manner. The first stage is: the compacted precursor powder is heated from room temperature to 80-120°C at a rate of 1-10°C / min, and kept warm for 10-15 hours. This can remove the bound water and adsorbed water in the precursor, and at the same time, the carbon source is carbonized and coated on the surface of the material to improve the conductivity of the material; the second stage: the temperature is raised to 320-380°C at a rate of 1-10°C / min, and kept warm for 20-25 hours, and cooled to room temperature with the furnace to obtain a composite polyanion sodium cathode material; the step-by-step heating method can convert the carbon source into carbon coated on the surface of the material in its carbonization temperature range, avoiding the phenomenon of carbon source sublimation caused by too rapid heating, reducing material loss while increasing the carbon coating ratio of the material, and at the same time, the bound water and adsorbed water in the precursor or raw material can be volatilized in a lower temperature range, avoiding the influence of moisture on the sodium ferric sulfate material.

[0047] In some embodiments, an iron source, a sodium source, a sulfur source, a carbon source, an antioxidant, and sodium ferric pyrophosphate are mixed under an inert atmosphere to obtain a mixed powder, which is then ball-milled to obtain a precursor powder;

[0048] Among them, the ball milling method is dry ball milling;

[0049] The ball milling speed is 300-600 r / min, the ball milling time is 1-5 h, the mass ratio of the ball milling beads to the mixed powder is (4-8): (2-6), the diameter of the ball milling beads is 1-5 mm, and the D of the precursor powder obtained after ball milling is 50 The particle size is 4 to 8 μm.

[0050] In the above embodiment, the ball milling method is dry ball milling, using inert wear-resistant ball milling beads such as mixed zirconia and stainless steel with a size of 1 to 5 mm. Ball milling is performed under an inert atmosphere to protect the material from oxidation.

[0051] In some embodiments, the precursor powder is compacted under a pressure of 0.05 to 5 GPa, calcined under an inert atmosphere, and crushed to D 50The particle size is 3 to 10 μm, and a composite polyanion sodium cathode material is obtained; the compression molding method of the precursor can avoid the situation where the distance between particles is generally at the micron level after high-energy ball milling, and the heat transfer effect between the precursor interfaces is poor. During the sintering process, the heat transfer of the solid phase interface is better utilized to reduce the difference between the internal temperature and the external temperature of the powder and the heat transfer delay, which can better enable the precursor to absorb energy and undergo phase change while reducing the generation of miscellaneous products, thereby improving the consistency of the material and improving the sodium storage performance of the composite polyanion sodium cathode material.

[0052] In some embodiments, an iron source, a sodium source, a sulfur source, a carbon source, an antioxidant, and sodium ferric pyrophosphate are placed in a mixing tank, the air in the mixing tank is discharged, and an inert gas such as argon is introduced to prevent the divalent iron from being oxidized during the ball milling process. After mixing evenly, the mixture is ball-milled under an inert atmosphere to obtain a highly dispersed precursor powder; the air is discharged by vacuuming, and the vacuum degree is 0.1-0.5 MPa, and the inert gas introduced is at least one of argon, nitrogen, hydrogen, etc.

[0053] In some embodiments, the iron source includes at least one of a soluble ferrous salt or a soluble ferric salt.

[0054] In some embodiments, the sodium source includes at least one of sodium carbonate and its hydrates, sodium bicarbonate and its hydrates, sodium acetate and its hydrates, sodium oxalate and its hydrates, sodium citrate and its hydrates, sodium nitrate and its hydrates, sodium sulfate and its hydrates, and sodium bisulfate and its hydrates.

[0055] In some embodiments, the sulfur source includes at least one of sodium sulfate and its hydrates, sodium bisulfate and its hydrates, ammonium bisulfate and its hydrates, ammonium sulfate and its hydrates, ferric sulfate and its hydrates, ferrous sulfate and its hydrates, ferrous ammonium sulfate and its hydrates, and ferric ammonium sulfate and its hydrates.

[0056] In some embodiments, the carbon source includes at least one of organic carbon and inorganic carbon.

[0057] In some embodiments, the antioxidant includes at least one of ascorbic acid, citric acid, oxalic acid, sodium sulfite, sodium D-isoascorbate, sodium bisulfite, hydrazine, and paraformaldehyde.

[0058] In some embodiments, the iron source includes ferrous sulfate and its hydrates, ferric sulfate and its hydrates, ferrous ammonium sulfate and its hydrates, ferrous ammonium sulfate and its hydrates, ferrous nitrate and its hydrates, ferric nitrate and its hydrates, ferrous chloride and its hydrates, ferric chloride and its hydrates, and ferric ammonium citrate and its hydrates;

[0059] In some embodiments, the inorganic carbon includes at least one of sucrose, graphene, carbon nanotubes, and carbon black.

[0060] In some embodiments, the molar ratio of the sodium source, the iron source, and the sulfur source is (0.7-1.2):1:(1-2).

[0061] In some embodiments, the molar ratio of the iron source to the sodium ferric pyrophosphate is (0.5-0.95):(0.05-0.5).

[0062] In some embodiments, the mass fraction of the carbon source in the mixed powder is 0.5-5%.

[0063] In some embodiments, the molar ratio of the antioxidant to the iron source is (0.01-0.1):1.

[0064] In some embodiments, the chemical formula of sodium ferric pyrophosphate is Na 4+x Fe 3+x (PO4) 2+x (P2O7), 0≤x≤1.

[0065] In some embodiments, the preparation method of sodium ferric pyrophosphate is a conventional method, specifically comprising:

[0066] S1. Weigh a sodium source, an organic phosphonic acid, and an iron source, add deionized water and stir, heat and stir to evaporate to dryness, and dry the resulting solid;

[0067] S2. Grind the solid obtained in step S1, pre-sinter under nitrogen protection, cool naturally, take out and grind again, sinter under nitrogen protection, and cool naturally to obtain the product sodium ferric pyrophosphate.

[0068] In some embodiments, in step S1, the ratio of the raw materials must satisfy the sodium:iron:phosphonic acid molar ratio of 4:3:4 or 3:2:3.

[0069] In some embodiments, in step S1, the sodium source is sodium hydroxide, sodium carbonate, sodium acetate or sodium oxalate.

[0070] In some embodiments, in step S1, the organic phosphonic acid is aminotrimethylenephosphonic acid, ethylenediaminetetramethylenephosphonic acid, hexamethylenediaminetetramethylenephosphonic acid or hydroxyethylidene diphosphonic acid.

[0071] In some embodiments, the iron source in step S1 is battery-grade ferrous oxalate or ferrous carbonate.

[0072] In some embodiments, in step S1, the temperature of heating and stirring is 80°C to 100°C; the conditions of the drying treatment are: drying in an oven at 80°C to 100°C for 5 to 12 hours.

[0073] In some embodiments, in step S2, the pre-sintering conditions are: pre-sintering at 300-400°C under nitrogen protection for 5-10 hours; and the sintering conditions are: sintering at 550-650°C under nitrogen protection for 8-20 hours.

[0074] In some embodiments, the inert atmosphere includes at least one of nitrogen, helium, neon, and argon.

[0075] In some embodiments, the precursor powder is compacted under a pressure of 0.05 to 5 GPa, calcined under an inert atmosphere, and crushed to D 50 The particle size is 3 to 10 μm, and a composite polyanion sodium positive electrode material is obtained.

[0076] Specifically, the sintered material is crushed and pulverized in an inert gas flow to form a 50 The size range is adjusted to 3 to 10 μm to obtain a composite polyanion sodium cathode material.

[0077] Based on the same inventive concept, the present invention also provides a composite polyanion sodium cathode material, which is prepared using the above-mentioned preparation method.

[0078] Based on the same inventive concept, the present invention also provides a composite polyanion sodium cathode material prepared by the above-mentioned preparation method or the use of the above-mentioned composite polyanion sodium cathode material in the preparation of a sodium ion battery.

[0079] Based on the same inventive concept, the present invention also provides a sodium ion battery, comprising the above-mentioned composite polyanion sodium cathode material.

[0080] The above-mentioned sodium ion battery, in addition to the above-mentioned composite polyanion sodium cathode material, also includes conventional negative electrode materials, diaphragms, electrolytes, etc.; for example, the negative electrode is hard carbon, sodium sheet, etc., the diaphragm is British Waterman GF / D glass fiber filter paper membrane, the electrolyte is sodium perchlorate or sodium hexafluorophosphate solution containing 0.5-1 mol / L, and the solvent in the electrolyte includes a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 3:5:2.

[0081] The following further illustrates the preparation method of the composite polyanion sodium cathode material of the present application with specific examples. This section further illustrates the content of the present invention in conjunction with specific examples, but should not be construed as limiting the present invention. Unless otherwise specified, the technical means adopted in the examples are conventional means well known to those skilled in the art. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the art.

[0082] Example 1

[0083] The present invention provides a method for preparing a composite polyanion sodium cathode material, comprising the following steps:

[0084] S1. Mixing an iron source, a sodium source, a sulfur source, a carbon source, an antioxidant, and sodium ferric pyrophosphate under an argon atmosphere to obtain a mixture, and then ball-milling the mixture to obtain a precursor powder;

[0085] S2. The precursor powder was compacted under a pressure of 1 GPa, and then placed in a muffle furnace. In an argon atmosphere, the temperature was raised from room temperature (25 ° C) to 100 ° C at a rate of 5 ° C / min and kept at this temperature for 10 h. Then the temperature was raised to 350 ° C at a rate of 5 ° C / min and kept at this temperature for 22 h. The precursor powder was cooled to room temperature with the furnace and crushed to D 50 The particle size is 8 μm, and a composite polyanion sodium cathode material is obtained;

[0086] Among them, the ball milling method is dry ball milling;

[0087] The ball milling speed was 500 r / min, the ball milling time was 3 h, the mass ratio of the ball milling beads to the mixed powder was 6:4, the diameter of the ball milling beads was 3 mm, and the D of the precursor powder obtained after ball milling was 0. 50 The particle size is 6 μm, and the ball milling beads are zirconia ball milling beads;

[0088] The iron source is ferrous sulfate;

[0089] The sodium source is sodium sulfate; the sulfur source is sodium bisulfate;

[0090] The antioxidant is ascorbic acid;

[0091] The carbon source is carbon black;

[0092] The molar ratio of sodium source, iron source and sulfur source is 1:1:1.5;

[0093] The molar ratio of the iron source to sodium ferric pyrophosphate is 0.8:0.2;

[0094] The mass fraction of carbon source in the mixed powder is 1%;

[0095] The molar ratio of antioxidant to iron source is 0.05:1;

[0096] The preparation of sodium ferric phosphate pyrophosphate comprises the following steps:

[0097] Sodium hydroxide, battery-grade ferrous oxalate, and aminotrimethylenephosphonic acid were weighed in a sodium:iron:phosphonate molar ratio of 4:3:4. First, dissolve the sodium hydroxide and aminotrimethylenephosphonic acid in deionized water, then add the battery-grade ferrous oxalate. Heat the solution at 80°C with stirring. Once the water evaporates and the sample becomes solid, place it in an oven and dry it at 80°C for 10 hours. The resulting solid was ground and pre-sintered at 350°C under nitrogen for 8 hours. Cool naturally to room temperature, remove it, grind it again, and sinter it at 600°C under nitrogen for 8 hours. Cool naturally to obtain the product, sodium ferric pyrophosphate.

[0098] Example 2

[0099] This embodiment provides a sodium ion battery, using the composite polyanion sodium cathode material in Example 1 as a positive electrode active material; specifically, the composite polyanion sodium cathode material in Example 1, super-p (conductive carbon black), and cmc (sodium carboxymethyl cellulose) are dissolved in a 90:5:5 mass ratio using nmp (N-methylpyrrolidone) solvent to prepare a slurry with a solid content of 50% (i.e., the sum of the mass fractions of the composite polyanion sodium cathode material, super-p, and cmc in the slurry is 50%); the prepared slurry is coated on a 20 μm aluminum foil current collector with a coating thickness of 40 μm, and the coated current collector is placed in an oven for drying, and a complete positive electrode sheet of a 2032 button battery is obtained by cutting and pressing.

[0100] Assemble the positive electrode sheet, negative electrode sheet, electrolyte, and separator into a button battery and then conduct relevant tests;

[0101] Among them, the negative electrode plate is a purchased sodium plate; the electrolyte includes a solvent and a sodium salt, the sodium salt is sodium hexafluorophosphate, the solvent in the electrolyte includes a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 3:5:2, and the concentration of sodium hexafluorophosphate in the electrolyte is 1 mol / L; the diaphragm is the British Waterman GF / D glass fiber filter paper membrane

[0102] Comparative Example 1

[0103] This comparative example provides a sodium ion battery, which is the same as Example 2, except that the positive electrode active material is sodium iron pyrophosphate prepared according to the method in Example 1, and the rest is the same as Example 2.

[0104] Comparative Example 2

[0105] This comparative example provides a sodium ion battery, which is the same as Example 2, except that the positive electrode active material is sodium ferric sulfate;

[0106] The preparation method of sodium ferric sulfate comprises the following steps:

[0107] S1. Mixing an iron source, a sodium source, a sulfur source, a carbon source, and an antioxidant under an argon atmosphere to obtain a mixture, and then ball-milling the mixture to obtain a precursor powder;

[0108] S2. The precursor powder was compacted under a pressure of 1 GPa, and then placed in a muffle furnace. In an argon atmosphere, the temperature was raised from room temperature (25 ° C) to 100 ° C at a rate of 5 ° C / min and kept at this temperature for 10 h. Then the temperature was raised to 350 ° C at a rate of 5 ° C / min and kept at this temperature for 22 h. The precursor powder was cooled to room temperature with the furnace and crushed to D 50 The particle size is 8 μm, and sodium iron sulfate is obtained;

[0109] Among them, the ball milling method is dry ball milling;

[0110] The ball milling speed was 500 r / min, the ball milling time was 3 h, the mass ratio of the ball milling beads to the mixed powder was 6:4, the diameter of the ball milling beads was 3 mm, and the D of the precursor powder obtained after ball milling was 0. 50 The particle size is 6 μm, and the ball milling beads are zirconia ball milling beads;

[0111] The iron source is ferrous sulfate;

[0112] The sodium source is sodium sulfate; the sulfur source is sodium bisulfate;

[0113] The antioxidant is ascorbic acid;

[0114] The carbon source is carbon black;

[0115] The molar ratio of sodium source, iron source and sulfur source is 1:1:1.5;

[0116] The mass fraction of carbon source in the mixed powder is 1%;

[0117] The molar ratio of antioxidant to iron source is 0.05:1.

[0118] Figure 1 This is an SEM image of the composite polyanion sodium cathode material prepared according to the method in Example 1.

[0119] from Figure 1 It can be seen that the composite polyanion sodium cathode material prepared in Example 1 is a uniform spherical structure and has two different morphologies, which can effectively achieve a denser distribution.

[0120] Figure 2 The XRD pattern of the composite polyanion sodium cathode material prepared according to the method in Example 1 is shown.

[0121] from Figure 2 It can be seen that the composite polyanion sodium cathode material prepared in Example 1 has obvious characteristic peaks of sodium ferric pyrophosphate and sodium ferric sulfate.

[0122] Figure 3 This is the 1C charge and discharge curve of the sodium ion battery in Example 2.

[0123] from Figure 3 It can be seen that the composite polyanion sodium cathode material prepared in Example 1 has a higher specific capacity than sodium ferric sulfate, and has a higher voltage platform, which makes the composite material have more abundant application scenarios.

[0124] The performance comparison of the sodium ion batteries in Comparative Examples 1-2 and Example 2 is shown in Table 1 below.

[0125] Table 1 - Performance of sodium ion batteries in different embodiments

[0126]

[0127] As can be seen from Table 1, the sodium ion battery prepared in Example 2 of the present invention has low cost, and its discharge specific capacity is close to that of the sodium ion battery prepared from sodium iron pyrophosphate, and its reversible cycle number is high.

[0128] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a composite polyanion sodium cathode material, characterized in that: The following steps are involved: An iron source, a sodium source, a sulfur source, a carbon source, an antioxidant, and sodium ferric pyrophosphate are mixed under an inert atmosphere and ball-milled to obtain a precursor powder; After compacting the precursor powder, calcining it in an inert atmosphere to obtain a composite polyanion sodium cathode material; Among them, the calcination is specifically as follows: the compacted precursor powder is heated from room temperature to 80-120°C at a rate of 1-10°C / min, and kept warm for 10-15 hours, then heated to 320-380°C at a rate of 1-10°C / min, and kept warm for 20-25 hours, and then cooled to room temperature with the furnace to obtain a composite polyanion sodium cathode material.

2. The method for preparing the composite polyanion sodium cathode material according to claim 1, wherein: An iron source, a sodium source, a sulfur source, a carbon source, an antioxidant, and sodium ferric pyrophosphate are mixed under an inert atmosphere to obtain a mixed powder, which is then ball-milled to obtain a precursor powder; Among them, the ball milling method is dry ball milling; The ball milling speed is 300-600 r / min, the ball milling time is 1-5 h, the mass ratio of the ball milling beads to the mixed powder is (4-8): (2-6), the diameter of the ball milling beads is 1-5 mm, and the D of the precursor powder obtained after ball milling is 50 The particle size is 4 to 8 μm.

3. The method for preparing the composite polyanion sodium cathode material according to claim 1, wherein: The iron source includes at least one of a divalent iron salt or a trivalent iron salt; The sodium source includes at least one of sodium carbonate and its hydrate, sodium bicarbonate and its hydrate, sodium acetate and its hydrate, sodium oxalate and its hydrate, sodium citrate and its hydrate, sodium nitrate and its hydrate, sodium sulfate and its hydrate, and sodium bisulfate and its hydrate; The sulfur source includes at least one of sodium sulfate and its hydrate, sodium bisulfate and its hydrate, ammonium bisulfate and its hydrate, ammonium sulfate and its hydrate, ferric sulfate and its hydrate, ferrous sulfate and its hydrate, ferrous ammonium sulfate and its hydrate, and ferric ammonium sulfate and its hydrate; The carbon source includes at least one of organic carbon and inorganic carbon; The antioxidant includes at least one of ascorbic acid, citric acid, oxalic acid, sodium sulfite, sodium D-isoascorbate, sodium bisulfite, hydrazine and paraformaldehyde.

4. The method for preparing the composite polyanion sodium cathode material according to claim 3, wherein: The iron source includes ferrous sulfate and its hydrate, ferric sulfate and its hydrate, ammonium ferrous sulfate and its hydrate, ammonium ferric sulfate and its hydrate, ferrous nitrate and its hydrate, ferric nitrate and its hydrate, ferrous chloride and its hydrate, ferric chloride and its hydrate, and ammonium ferric citrate and its hydrate; The inorganic carbon includes at least one of graphene, carbon nanotubes, and carbon black; The organic carbon is sucrose.

5. The method for preparing the composite polyanion sodium cathode material according to claim 2, wherein: The molar ratio of the sodium source, the iron source, and the sulfur source is (0.7-1.2):1:(1-2); The molar ratio of the iron source to sodium ferric pyrophosphate is (0.5-0.95):(0.05-0.5); The mass fraction of the carbon source in the mixed powder is 0.5 to 5%; The molar ratio of the antioxidant to the iron source is (0.01-0.1):

1.

6. The method for preparing the composite polyanion sodium cathode material according to any one of claims 1 to 5, characterized in that: The chemical formula of the sodium ferric phosphate pyrophosphate is Na 4+x Fe 3+x (PO4) 2+x (P2O7), 0≤x≤1; The inert atmosphere includes at least one of nitrogen, helium, neon and argon.

7. The method for preparing the composite polyanion sodium cathode material according to any one of claims 1 to 5, characterized in that: The precursor powder is compacted under a pressure of 0.05-5 GPa, calcined under an inert atmosphere, and crushed to D 50 The particle size is 3 to 10 μm, and a composite polyanion sodium positive electrode material is obtained.

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