Preparation method of iron-based pyrophosphate-based secondary battery positive electrode material with good air stability

Abstract

The application discloses a preparation method of an iron-based pyrophosphate secondary battery positive electrode material with good air stability, and comprises the following steps: S1, preparation of a precursor slurry; S2, preparation of unmodified precursor powder; S3, preparation of unmodified iron-based pyrophosphate material; S4, preparation of a milk emulsion; S5, preparation of modified precursor powder; and S6, high-temperature sintering of the modified iron-based pyrophosphate secondary battery positive electrode material. The preparation method of the iron-based pyrophosphate secondary battery positive electrode material with good air stability has the characteristics of high compaction density, good air stability and excellent electrochemical performance.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of secondary battery positive electrode materials, in particular to a preparation method of an iron-based pyrophosphate secondary battery positive electrode material with good air stability. BACKGROUND

[0002] In sodium ion batteries, transition metal oxides, polyanionic materials and Prussian blue and its analogues have been rapidly developed as positive electrode materials for sodium ion batteries in the past two years. Among them, Prussian blue and its analogues have high reversible capacity and discharge voltage, and correspond to a high energy density of the sodium ion battery system, but the lattice water introduced in the structure during the synthesis process can easily lead to gas production failure of the battery during charging and discharging; transition metal oxides have high tap density and discharge capacity, but the complex phase change during sodium extraction can easily lead to material structure collapse and rapid performance decay; polyanionic materials have excellent cycle stability and rate performance, but their capacity is low, and the tap density is small, which is only suitable for energy storage fields with low energy density requirements.

[0003] Polyanionic materials are diverse, and can be divided into phosphate, pyrophosphate, sulfate, borate, silicate and fluorinated mixed polyanion types according to the anion groups in the materials. All the above-mentioned materials are composed of alkali metal ions, transition metal ions and anion groups connected in a framework structure. Among them, iron-based pyrophosphate materials, such as sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7), are favored by the market due to their low cost and excellent electrochemical performance. The transition metal element in this type of material is divalent iron, and the anion group is phosphate (PO4 3- ) and pyrophosphate (P2O7 4- ). Due to the weak induction of this type of anion group, the redox potential of divalent iron in this type of material is generally low (≤3.3V). Therefore, when this type of material is exposed to humid air, the divalent iron with low redox potential begins to oxidize, accompanied by the extraction of sodium ions at the material interface, and at the same time, P2O7 4- is triggered to absorb water and decompose to generate Na x H y PO4 and other impurities to maintain charge balance, ultimately leading to a decrease in sodium extraction capacity and loss of electrochemical activity. SUMMARY

[0004] The purpose of the present application is to provide a preparation method of an iron-based pyrophosphate secondary battery positive electrode material with good air stability, which has the characteristics of high tap density, good air stability and excellent electrochemical performance.

[0005] The present application can be realized by the following technical solutions:

[0006] The application discloses a preparation method of an iron-based pyrophosphate secondary battery positive electrode material with good air stability, and comprises the following steps:

[0007] S1, preparation of a precursor slurry: a first alkali metal source, an iron-containing compound, and a first phosphorus source are mixed and dispersed in water to form a uniform precursor slurry;

[0008] S2, preparation of an unmodified precursor powder: the precursor slurry is dried to remove water, and a uniform unmodified precursor powder is obtained;

[0009] S3, preparation of an unmodified iron-based pyrophosphate material: the unmodified precursor powder is calcined in a protective atmosphere to obtain an unmodified iron-based pyrophosphate material;

[0010] S4, preparation of a milk emulsion: the unmodified iron-based pyrophosphate material, a second alkali metal source, a second phosphorus source, a water-soluble redox transition metal source, and a carbon source are dispersed in water to form a uniform milk emulsion;

[0011] S5, preparation of a modified precursor powder: the milk emulsion is dried to obtain a uniform modified precursor powder;

[0012] S6, high-temperature sintering of a modified iron-based pyrophosphate secondary battery positive electrode material: the modified precursor powder is calcined in a protective atmosphere to obtain an iron-based pyrophosphate secondary battery positive electrode material with a hetero-element interface layer doping.

[0013] The application adopts a hetero-element interface layer doping process to improve the air stability of the iron-based pyrophosphate material. The process comprises two key steps: first, synthesizing an iron-based pyrophosphate material body structure; and second, doping high-oxidation-state (such as Mn, Ni, and Co) elements at the interface of the body structure, based on the high oxidation potential of these elements to inhibit the oxidation of divalent iron at the interface of the iron-based pyrophosphate material in humid air, thereby avoiding the release of sodium ions at the interface, and finally obtaining an iron-based pyrophosphate material with high hydrophobicity and strong stability. 4- hydrolyzed into Na x H y PO4 and other impurities.

[0014] In the application, the thickness of the externally coated hetero-element interface layer is 5-20 nm. When the thickness of the hetero-element interface layer is less than 5 nm, water and oxygen in the air will slowly penetrate into the inner core and cause oxidation of the inner core material. When the thickness of the hetero-element interface layer is greater than 20 nm, the release speed of sodium ions is slow due to the high oxidation potential of the transition metals in the layer, and the long release distance will reduce the rate performance of the material. At the same time, the carbon distribution range in the over-thick coating layer is deep, and the specific surface area is large, resulting in a decrease in the sintered material compaction density.

[0015] Further, in step S4, the water-soluble redox transition metal source contains a manganese-containing compound, a nickel-containing compound and / or a cobalt-containing compound; the manganese-containing compound is one or more of manganese nitrate, manganese acetate and / or manganese chloride; the nickel-containing compound is one or more of nickel nitrate, nickel sulfate and / or nickel chloride; and the cobalt-containing compound is cobalt sulfate and / or cobalt nitrate.

[0016] Further, in step S3, the protective atmosphere is a gas having reducing properties at high temperatures, such as hydrogen and / or carbon monoxide; and the calcination temperature is 400-650°C. If the temperature is lower than 400°C, the phosphate groups cannot be deoxidized and condensed into pyrophosphate groups, and the material cannot grow into a complete crystal structure. If the temperature is higher than 650°C, the material structure is easily decomposed into phosphides and transition metal oxides.

[0017] Further, in step S6, the protective inert atmosphere is argon, nitrogen, a mixture of nitrogen and hydrogen, or a mixture of argon and hydrogen; and the calcination temperature is 550-650°C. At this temperature range, the alkali metal, the phosphorus source and the transition metal source in step S4 can be completely decomposed, and the free alkali metal ions, phosphorus and transition metal elements can undergo displacement reactions with the surface elements of the unmodified iron-based pyrophosphate material, thereby generating a pyrophosphate material coating layer with a higher redox potential, effectively inhibiting the oxidation of the transition metal in the structure in air, thereby avoiding the migration of sodium ions in the structure to the surface, and indirectly improving the air stability of the material.

[0018] Further, in steps S1 and S4, the first alkali metal source and the second alkali metal source are a lithium-containing compound, a sodium-containing compound and / or a potassium-containing compound; the lithium-containing compound is one or more of lithium carbonate, lithium hydroxide, lithium nitrate, lithium dihydrogen phosphate and / or lithium acetate; the sodium-containing compound is one or more of sodium carbonate, sodium bicarbonate, sodium nitrate, sodium formate, sodium acetate and / or sodium phosphate; and the potassium-containing compound is one or more of potassium nitrate, potassium dihydrogen phosphate, potassium hydroxide and / or potassium carbonate.

[0019] Further, in step S1, the iron-containing compound is one or more of iron, iron oxide, ferrous oxide, ferric oxide, iron nitrate, iron acetate, iron phosphate, ferrous oxalate, iron sulfate and / or iron chloride.

[0020] Further, in steps S1 and S4, the first phosphorus source and the second phosphorus source are one or more of pyrophosphoric acid, metaphosphoric acid, sodium pyrophosphate, sodium hexametaphosphate, sodium polyphosphate, phosphoric acid, sodium dihydrogen phosphate, sodium monohydrogen phosphate, sodium phosphate, ammonium phosphate and / or ammonium dihydrogen phosphate.

[0021] Further, in steps S1 and S4, the dispersion method is ball milling, sand milling and / or high-speed dispersion, which realizes uniform dispersion of the materials in the form of mechanical force and shear force.

[0022] Further, in steps S2 and S5, the drying method is natural evaporation, freeze drying, flash drying, spray drying or vacuum drying, etc.

[0023] Further, in step S4, the carbon source is one or more of carbon black, graphene, carbon nanotubes, graphite, hard carbon, etc.

[0024] The application has the following advantages:

[0025] First, the compaction density is high. In the synthesis process of the unmodified iron-based pyrophosphate material, the introduction of the carbon source is avoided. The protective gas introduced during sintering is used to reduce the transition metal elements. The advantage of this process is that after the volatile components in the raw materials are volatilized, the remaining alkali metal, transition metal elements and phosphorus elements can realize dense contact, melt crystallization and easy growth into single crystal particles with high crystallinity without the isolation of the carbon source, which greatly helps to improve the compaction density of the material.

[0026] Second, the air stability is good. The unmodified iron-based pyrophosphate material is mixed and dispersed with a water-soluble transition metal source, an additional small amount of an alkali metal source and a phosphorus source. After the above-mentioned transition metal source, alkali metal source and phosphorus source are dissolved in water, they are uniformly dispersed on the surface of the unmodified iron-based pyrophosphate material. In the subsequent sintering process, the transition metal elements, alkali metal elements and phosphorus elements released by the decomposition of the transition metal source, alkali metal source and phosphorus source are mutually molten with the interface of the unmodified iron-based pyrophosphate material, and then an iron-based pyrophosphate material doped with interface impurities is generated. Since the transition metal elements in the interface layer are mainly Mn, Co, Ni and other elements with high oxidation potential, it is difficult for them to undergo oxidation in air, effectively inhibiting the dissolution of sodium ions and the hydrolysis of pyrophosphate, thereby greatly improving the air stability of the material.

[0027] Third, the electrochemical performance is excellent. In the application, the carbon coating layer is located in the outermost layer, and the iron-based pyrophosphate material core contains almost no carbon. Therefore, the overall porosity of the material is low, and the specific surface area is small, which can isolate the oxygen and water in the air from eroding the material core. In addition, the low specific surface area can effectively reduce the dissolution of the material core in the electrolyte, thereby greatly improving the cycle stability of the material. DETAILED DESCRIPTION

[0028] In order to make the technical personnel in the art 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 embodiments.

[0029] The application discloses a preparation method of an iron-based pyrophosphate secondary battery positive electrode material with good air stability.

[0030] S1, preparation of a precursor slurry: a first alkali metal source, an iron-containing compound, and a first phosphorus source are mixed and dispersed with water to form a uniform precursor slurry;

[0031] S2, preparation of an unmodified precursor powder: the precursor slurry is dried to remove water to obtain a uniform unmodified precursor powder;

[0032] S3, preparation of an unmodified iron-based pyrophosphate material: the unmodified precursor powder is calcined in a protective atmosphere to obtain an unmodified iron-based pyrophosphate material;

[0033] S4, preparation of a milk emulsion: the unmodified iron-based pyrophosphate material, a second alkali metal source, a second phosphorus source, a water-soluble redox transition metal source, and a carbon source are dispersed with water to form a uniform milk emulsion;

[0034] S5, preparation of a modified precursor powder: the milk emulsion is dried to obtain a uniform modified precursor powder;

[0035] S6, high-temperature sintering of a modified iron-based pyrophosphate secondary battery positive electrode material: the modified precursor powder is calcined in a protective atmosphere to obtain an iron-based pyrophosphate secondary battery positive electrode material with a hetero-element interface layer doped.

[0036] In the present application, the thickness of the hetero-element interface layer is 5-20 nm, when the thickness of the hetero-element interface layer is less than 5 nm, water and oxygen in the air will slowly penetrate into the inner core and cause oxidation of the inner core material, and when the thickness of the hetero-element interface layer is greater than 20 nm, the sodium ion deintercalation speed is slow due to the high transition metal oxidation potential in the layer, and the long deintercalation distance will reduce the rate performance of the material. At the same time, the carbon distribution range in the over-thick coating layer is deep, and the specific surface is large, which causes the compaction density of the material after sintering to decrease.

[0037] Further, in step S4, the water-soluble redox transition metal source contains a manganese-containing compound, a nickel-containing compound, and / or a cobalt-containing compound; the manganese-containing compound is one or two or more of manganese nitrate, manganese acetate, and / or manganese chloride; the nickel-containing compound is one or two or more of nickel nitrate, nickel sulfate, and / or nickel chloride; and the cobalt-containing compound is cobalt sulfate and / or cobalt nitrate.

[0038] Further, in step S3, the protective atmosphere is hydrogen and / or carbon monoxide; and the calcination temperature is 400-650°C.

[0039] Further, in step S6, the protective inert atmosphere is argon, nitrogen, nitrogen-hydrogen mixed gas or argon-hydrogen mixed gas; the calcination temperature is 550-650°C.

[0040] Further, in steps S1 and S4, the first alkali metal source and the second alkali metal source are lithium-containing compounds, sodium-containing compounds and / or potassium-containing compounds; the lithium-containing compounds are one or two or more of lithium carbonate, lithium hydroxide, lithium nitrate, lithium dihydrogen phosphate and / or lithium acetate; the sodium-containing compounds are one or two or more of sodium carbonate, sodium bicarbonate, sodium nitrate, sodium formate, sodium acetate and / or sodium phosphate; the potassium-containing compounds are one or two or more of potassium nitrate, potassium dihydrogen phosphate, potassium hydroxide and / or potassium carbonate.

[0041] Further, in step S1, the iron-containing compound is one or two or more of iron, iron oxide, ferrous oxide, ferric oxide, iron nitrate, iron acetate, iron phosphate, ferrous oxalate, iron sulfate and / or iron chloride.

[0042] Further, in steps S1 and S4, the first phosphorus source and the second phosphorus source are one or two or more of pyrophosphoric acid, metaphosphoric acid, sodium pyrophosphate, sodium hexametaphosphate, sodium polyphosphate, phosphoric acid, sodium dihydrogen phosphate, sodium monohydrogen phosphate, sodium phosphate, ammonium phosphate and / or ammonium dihydrogen phosphate.

[0043] Further, in steps S1 and S4, the dispersion method is ball milling, sand milling and / or high-speed dispersion.

[0044] Further, in steps S2 and S5, the drying method is natural evaporation, freeze drying, flash drying, spray drying or vacuum drying.

[0045] Further, in step S4, the carbon source is one or two or more of inorganic carbon-containing compounds such as carbon black, graphene, carbon nanotubes, graphite and hard carbon; and one or two or more of sucrose, starch, citric acid, glucose, polyvinyl alcohol, polyethylene glycol, maltose, cyclodextrin and / or polyacrylic acid.

[0046] In the present application, the unmodified iron-based pyrophosphate material in step S3 is sodium / lithium / potassium pyrophosphate iron phosphate, a compound material containing pyrophosphate. The high-air-stability iron-based pyrophosphate material with a hetero-element interface layer in step S6 includes a triple structure of an iron-based pyrophosphate material core, a hetero-element-doped pyrophosphate material interface modification layer and a carbon-coated outer layer.

[0047] Application Example 1 Synthesis of Na2FeP2O7 / Na2MnP2O7 / C and its electrochemical performance

[0048] In the present embodiment, the synthesis method of Na2FeP2O7 / Na2MnP2O7 / C includes the following steps:

[0049] Step 1: sodium carbonate, iron oxide, and phosphoric acid are mixed in a molar ratio of 1:1:2 and grinded with water to obtain a uniform reddish-brown precursor slurry;

[0050] Step 2: the above precursor slurry is spray dried at an inlet temperature of 260°C and an outlet temperature of 100°C to ensure complete evaporation of moisture, to obtain dry precursor powder;

[0051] Step 3: the precursor powder is calcined in a reducing carbon monoxide atmosphere at a sintering temperature of 500°C for 10H to obtain unmodified Na2FeP2O7 material;

[0052] Step 4: the unmodified Na2FeP2O7 material and sodium acetate, phosphoric acid, and manganese acetate with a total solid content of about 5% are dispersed and dissolved in water in a molar ratio of 2:1:2, and additional glucose with a total solid content of 12% is added as a carbon source to the above slurry to form a uniform emulsion;

[0053] Step 5: the above emulsion is flash dried at a temperature of about 300°C to obtain a uniform modified precursor powder;

[0054] Step 6: the modified precursor powder is calcined in a nitrogen atmosphere at a sintering temperature of 600°C for 12H, and after natural cooling, a high air stability Na2FeP2O7 / Na2MnP2O7 / C material with a hetero-element interface layer doping is obtained.

[0055] After uniformly mixing Na2FeP2O7 / Na2MnP2O7 / C, AB, and PVDF in a mass ratio of 8:1:1, the black slurry is coated on an aluminum foil using a 150um four-side coater, and then the film is dried in a 100°C vacuum drying oven for 2 hours. The electrode film is punched into a circular sheet with a radius of 0.6mm using a sheet puncher, and a CR2016 type button cell is assembled in a glove box using metallic sodium as the counter electrode, 1mol / L NaClO4EC+DEC(1:1vol%)+5%FEC as the electrolyte, and PP / PE / PP three-layer separator as the separator.

[0056] Table 1 shows that the Na2FeP2O7 / Na2MnP2O7 / C material has a compaction density of 2.20g / cm 3 , compared to Comparative Example 1 (1.80g / cm 3) high about 22.2%, the reason is, Na2FeP2O7 core material synthesis process without adding carbon source, so the sintering process of the elements of the molten combination more closely, it is easy to generate a higher degree of compaction of single crystal particles, the final reaction to the finished product is the improvement of the tap density. At the same time, the results of table 1 show that the specific surface area of the material is only 6.3m 2 / g, the smaller specific surface area and the porosity of the material, pore distribution, etc. there is a direct link, said in the premise of not adding carbon source, the material melting growth of the density of the material between the first particle porosity, and thus reduce the specific surface area. In addition, the material at 0.1C rate of 97mAh / g, the theoretical capacity (98mAh / g) of 98.98%, far higher than the 76mAh / g in comparative example 1, the process prepared by the material has higher crystallinity and phase purity, in the process of charging and discharging, sodium ion deintercalation path usually, the higher degree of deintercalation, and thus show higher capacity. Finally, we tested the air stability of Na2FeP2O7 / Na2MnP2O7 / C, by exposing the material in a saturated humidity environment for a week, found that the prepared electrode reversible capacity is 95.8mAh / g, almost no obvious capacity attenuation, the Na2MnP2O7 interface layer of divalent manganese higher oxidation state effectively avoid the oxidation, desodium and pyrophosphate hydrolysis reaction in the air, at the same time, under the protection of Na2MnP2O7 interface layer, Na2FeP2O7 core layer cannot contact the oxygen and water in the air, and thus greatly improve its stability.

[0057] Application Example 2 Synthesis and electrochemical performance of Na4Fe3(PO4)2P2O7 / Na4Mn3(PO4)2P2O7 / C

[0058] In this example, the synthesis method of Na4Fe3(PO4)2P2O7 / Na4Mn3(PO4)2P2O7 / C includes the following steps:

[0059] Step 1: sodium dihydrogen phosphate, iron nitrate nine water according to the molar ratio of 4:3 mixed with water grinding, get uniform light red precursor solution;

[0060] Step 2: the precursor solution is spray dried, the inlet temperature is 300°C, and the outlet temperature is 101°C, to ensure that the water is completely volatilized, and the dry precursor powder is obtained;

[0061] Step 3: the precursor powder is calcined in a reducing hydrogen atmosphere, the sintering temperature is 550°C, and the time is 10H, to obtain unmodified Na4Fe3(PO4)2P2O7 material;

[0062] Step 4: The unmodified Na4Fe3(PO4)2P2O7 material and sodium dihydrogen phosphate and manganese acetate with a total solid content of about 7% were dispersed and dissolved in water according to a molar ratio of 4:3, and citric acid with a total solid content of 15% was additionally added as a carbon source to the slurry to form a uniform emulsion;

[0063] Step 5: The emulsion was flash dried at a temperature of about 300°C to obtain a uniform modified precursor powder;

[0064] Step 6: The modified precursor powder was calcined in a nitrogen atmosphere at a sintering temperature of 580°C for 8H, and a high-air-stability Na4Fe3(PO4)2P2O7 / Na4Mn3(PO4)2P2O7 / C material with a hetero-element interface layer was obtained after natural cooling.

[0065] After uniformly mixing the Na4Fe3(PO4)2P2O7 / Na4Mn3(PO4)2P2O7 / C, AB, and PVDF according to a mass ratio of 8:1:1, the black slurry was coated on an aluminum foil using a 150um four-side coater, and then the film was dried in a vacuum drying oven at 100°C for 2 hours. The electrode film was punched into a circular sheet with a radius of 0.6mm using a sheet puncher, and a CR2016 type button cell was assembled in a glove box using metallic sodium as the counter electrode, 1mol / L NaClO4 EC+DEC (1:1 vol%)+5% FEC as the electrolyte, and a PP / PE / PP three-layer separator.

[0066] The results in Table 1 show that the Na4Fe3(PO4)2P2O7 / Na4Mn3(PO4)2P2O7 / C material has a tap density of 2.25g / cm 3 , which is about 19.7% higher than that of Comparative Example 2 (1.88g / cm 3 ). The reason is that no carbon source was added during the preparation of the Na4Fe3(PO4)2P2O7 core material precursor, which can effectively increase the degree of element fusion during the high-temperature process, thereby promoting the growth of large single-crystal granular materials and ultimately improving the tap density of the product. At the same time, the results in Table 1 show that the specific surface area of the material is only 7.5m 2 / g, the smaller specific surface area indicates less pores between primary particles of the material, which is consistent with the higher tap density. In addition, the reversible capacity of the material at 0.1C rate is 123 mAh / g, which is almost close to the theoretical specific capacity, much higher than 114 mAh / g in Comparative Example 2, indicating that the material prepared by the process has higher crystallinity and phase purity, and the regular and ordered material is beneficial to the extraction of sodium ions, thereby improving the reversible capacity. Finally, we tested the air stability of Na4Fe3(PO4)2P2O7 / Na4Mn3(PO4)2P2O7 / C material. After exposing the material to a saturated humidity environment for one week, it was found that the reversible capacity of the prepared electrode was 121.4 mAh / g, with almost no obvious capacity attenuation, indicating that the existence of the Na4Mn3(PO4)2P2O7 interface layer avoids the oxidation of sodium and the hydrolysis of pyrophosphoric acid in the core material. The stability of the interface layer is related to the high oxidation state of the divalent manganese contained therein. The high oxidation state can effectively avoid the oxidation reaction when it contacts oxygen, thereby inhibiting a series of side reactions such as sodium ion extraction, pyrophosphoric acid hydrolysis, and thus greatly improving the stability of the material.

[0067] Synthesis of Na2FeP2O7 / C and its electrochemical performance

[0068] In this example, the synthesis method of Na2FeP2O7 / C includes the following steps:

[0069] Step 1: Mix sodium carbonate, iron oxide, and phosphoric acid in a molar ratio of 1:1:2 with water, and additionally add 12% of the total solid content of glucose as a carbon source. After grinding, a uniform reddish-brown precursor slurry is obtained;

[0070] Step 2: Spray dry the above precursor slurry with an inlet temperature of 260°C and an outlet temperature of 100°C to ensure complete evaporation of water, to obtain a dry precursor powder;

[0071] Step 3: Calcine the precursor powder in a reducing carbon monoxide atmosphere at a sintering temperature of 500°C for 10H to obtain a Na2FeP2O7 / C material.

[0072] After uniformly mixing Na2FeP2O7 / C, AB, and PVDF in a mass ratio of 8:1:1, the black slurry was coated on an aluminum foil using a 150um four-side coater. Then the film was dried in a 100°C vacuum drying oven for 2 hours. The electrode film was punched into a circular sheet with a radius of 0.6mm using a sheet puncher. A CR2016 type button cell was assembled in a glove box using metallic sodium as the counter electrode, 1 mol / L NaClO4 EC+DEC (1:1 vol%) + 5% FEC as the electrolyte, and PP / PE / PP three-layer separator.

[0073] Table 1 shows that the compacted density of Na2FeP2O7C material is only 1.80 g / cm³. 3 This is far lower than the 2.20 g / cm³ observed in Application Example 1. 3 The reason for this is that a carbon source was introduced during the precursor preparation of the Na2FeP2O7 material. During sintering, the carbon source decomposed to produce an isolation layer, hindering inter-element melt growth and thus generating a large number of nanoparticles. These nanoparticles are interwoven, leaving numerous pores. Simultaneously, the carbon layer formed by the decomposition of the carbon source contains a large number of micropores, and the presence of these pores significantly reduces the material's compaction density. Furthermore, Table 1 shows that the specific surface area of ​​this material is 12.2 m². 2 The relatively large specific surface area is directly related to the particle size, porosity, and distribution of the material, indicating that adding a carbon source in the precursor stage reduces the material size, increases porosity, and thus increases the specific surface area. Furthermore, the reversible capacity of this material at 0.1C rate is 83 mAh / g, slightly lower than the 97 mAh / g in Application Example 1. This indicates that the material prepared by this process has low crystallinity and phase separation, which hinders the sodium ion insertion / extraction process during charging and discharging, reducing the number of insertions and extractions and severely affecting capacity. Finally, we tested the air stability of the Na2FeP2O7 / C material. After exposing the material to a saturated humidity environment for one week, we found that the reversible capacity of the electrode prepared from it decreased to 57 mAh / g. This significant capacity decay indicates that the divalent iron in the Na2FeP2O7 material oxidized in the air, causing sodium ions to be released from the structure. Simultaneously, pyrophosphate absorbs water from the air, undergoes hydrolysis, and combines with the released sodium ions to form Na+. x H y Substances such as PO4 cause a significant decrease in its electrochemical performance.

[0074] Comparative Example 2: Synthesis and Electrochemical Performance of Na4Fe3(PO4)2P2O7 / C

[0075] In this embodiment, the synthesis method of Na4Fe3(PO4)2P2O7 / C includes the following steps:

[0076] Step 1: Mix sodium dihydrogen phosphate and ferric nitrate nonahydrate in a molar ratio of 4:3 and disperse with water. Add citric acid with a total solid content of 15% as a carbon source to the above slurry to obtain a uniform light red precursor solution.

[0077] Step 2: Spray dry the above precursor solution with an inlet air temperature of 300°C and an outlet air temperature of 101°C to ensure complete evaporation of moisture and obtain dry precursor powder.

[0078] Step 3: Calcining the precursor powder in a reducing hydrogen atmosphere, sintering temperature 550°C, time 10H, to obtain Na4Fe3(PO4)2P2O7 / C material.

[0079] After mixing Na4Fe3(PO4)2P2O7 / C, AB, PVDF in a ratio of 8:1:1, the black slurry was coated on aluminum foil using a 150um four-side coater, and then the film was dried in a vacuum drying oven at 100°C for 2 hours. The electrode film was punched into a circular sheet with a radius of 0.6mm using a sheet puncher, and a CR2016 type button cell was assembled in a glove box using metallic sodium as the counter electrode, 1mol / L NaClO4EC+DEC(1:1vol%)+5%FEC as the electrolyte, and a PP / PE / PP three-layer separator.

[0080] The results in Table 1 show that the Na4Fe3(PO4)2P2O7 / C material has a tap density of 1.88g / cm 3 , which is much lower than that of Example 2, which is related to the addition of carbon source during the preparation of the Na4Fe3(PO4)2P2O7 precursor, resulting in a small grain size and high porosity of the material, which ultimately affects the tap density of the product. At the same time, the results in Table 1 show that the specific surface area of the material is only about 15.8m 2 / g, which indicates that the material has a large specific surface area and small pore size, which is consistent with the small tap density of the material. In addition, the reversible capacity of the material at 0.1C rate is 114mAh / g, which is slightly lower than that of Example 2, which may be related to the uneven distribution of elements and the phase separation caused by the introduction of carbon source during the preparation of the material, which seriously affects the capacity of the material. Finally, we tested the air stability of the Na4Fe3(PO4)2P2O7 / C material, and found that the reversible capacity of the electrode prepared from the material was 79mAh / g after exposure to a saturated humidity environment for one week, which indicates that the divalent iron in the Na4Fe3(PO4)2P2O7 material was oxidized in the air, and in order to maintain charge balance, sodium ions were removed from the structure, accompanied by the hydrolysis of pyrophosphate to form Na x H y PO4 and other impurities, which ultimately leads to a significant decline in the electrochemical performance of the material.

[0081] Table 1 Performance test results

[0082]

[0083] The above embodiments are only specific embodiments of the present application, and the description is more specific and detailed, but it cannot be understood as a limitation on the scope of the patent of the present application. It should be noted that for ordinary skilled in the art, without departing from the concept of the present application, several modifications and improvements can be made, and these obvious replacement forms belong to the protection scope of the present application.

Claims

1. A method for preparing an iron-based pyrophosphate secondary battery cathode material with good air stability, characterized in that... Includes the following steps: S1. Preparation of precursor slurry: The first alkali metal source, iron-containing compound and first phosphorus source are mixed and dispersed with water to form a uniform precursor slurry; S2. Preparation of unmodified precursor powder: The above precursor slurry is dried to remove water, and uniform unmodified precursor powder is obtained. S3. Preparation of unmodified iron-based pyrophosphate materials: Unmodified precursor powder is calcined in a protective atmosphere to obtain unmodified iron-based pyrophosphate materials. S4. Preparation of emulsion: Unmodified iron-based pyrophosphate material, second alkali metal source, second phosphorus source, water-soluble redox transition metal source, and carbon source are dispersed in water to form a uniform emulsion. S5. Preparation of modified precursor powder: The above emulsion is dried to obtain a uniform modified precursor powder. S6. High-temperature sintering of modified iron-based pyrophosphate secondary battery cathode material: The modified precursor powder is calcined in a protective atmosphere to obtain a high-air-stability iron-based pyrophosphate secondary battery cathode material with heterogeneous element interface layer doping. In step S4, the water-soluble redox transition metal source contains manganese compounds, nickel compounds, and / or cobalt compounds; the manganese compounds are one or more of manganese nitrate, manganese acetate, and / or manganese chloride; the nickel compounds are one or more of nickel nitrate, nickel sulfate, and / or nickel chloride; and the cobalt compounds are cobalt sulfate and / or cobalt nitrate. In step S3, the protective atmosphere is hydrogen and / or carbon monoxide; the calcination temperature is 400-650°C; In step S6, the protective inert atmosphere is argon, nitrogen, a nitrogen-hydrogen mixture, or an argon-hydrogen mixture; the calcination temperature is 550-650°C. In steps S1 and S4, the first alkali metal source and the second alkali metal source are lithium-containing compounds, sodium-containing compounds and / or potassium-containing compounds; the lithium-containing compound is one or more of lithium carbonate, lithium hydroxide, lithium nitrate, lithium dihydrogen phosphate and / or lithium acetate; the sodium-containing compound is one or more of sodium carbonate, sodium bicarbonate, sodium nitrate, sodium formate, sodium acetate and / or sodium phosphate; and the potassium-containing compound is one or more of potassium nitrate, potassium dihydrogen phosphate, potassium hydroxide and / or potassium carbonate.

2. The method for preparing the iron-based pyrophosphate secondary battery cathode material with good air stability according to claim 1, characterized in that: In step S1, the iron-containing compound is one or more of the following: iron, ferric oxide, iron(II) oxide, ferric nitrate, ferric acetate, ferric phosphate, ferrous oxalate, ferric sulfate, and / or ferric chloride.

3. The method for preparing the iron-based pyrophosphate secondary battery cathode material with good air stability according to claim 1, characterized in that: In steps S1 and S4, the first phosphorus source and the second phosphorus source are one or more of the following: pyrophosphate, metaphosphoric acid, sodium pyrophosphate, sodium hexametaphosphate, sodium polyphosphate, phosphoric acid, sodium dihydrogen phosphate, sodium monohydrogen phosphate, sodium phosphate, ammonium phosphate and / or ammonium dihydrogen phosphate.

4. The method for preparing the iron-based pyrophosphate secondary battery cathode material with good air stability according to claim 1, characterized in that: In steps S1 and S4, the dispersion method is ball milling, sand milling, and / or high-speed dispersion.

5. The method for preparing the iron-based pyrophosphate secondary battery cathode material with good air stability according to claim 1, characterized in that: In steps S2 and S5, the drying method is natural evaporation, freeze drying, flash drying, spray drying, or vacuum drying.

6. The method for preparing the iron-based pyrophosphate secondary battery cathode material with good air stability according to claim 1, characterized in that: In step S4, the carbon source is one or more of graphene, carbon nanotubes, graphite, hard carbon, sucrose, starch, citric acid, glucose, polyvinyl alcohol, polyethylene glycol, maltose, cyclodextrin and / or polyacrylic acid.

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