A sodium-ion battery cathode material with core-shell structure and a preparation method thereof

Abstract

This invention discloses a core-shell structured sodium-ion battery cathode material and its preparation method. The sodium-ion battery cathode material is a polyanionic phosphate material, comprising a core material and a shell material; the core material is lithium iron phosphate (LiFePO4); the shell material is selected from Na2FeP2O7, Na... 3.12 Fe 2.44 (P2O7)2, Na4Fe3(PO4)2P2O7, Na7Fe 4.5 One or more of (P2O7)4 and Na3Fe2(PO4)3. The material provided by this invention has the advantages of high specific capacity, high compaction density, low cost, and environmental friendliness as a cathode material for sodium-ion batteries, and can be mass-produced and widely used.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a core-shell structured sodium-ion battery cathode material and its preparation method, and more specifically, to a core-shell structured polyanionic phosphate cathode material for sodium-ion batteries and its preparation method. Background Technology

[0002] In recent years, with the increasing depletion of fossil fuels and the growing severity of environmental problems such as global warming, research related to new energy sources has received widespread attention. With advancements in battery technology, the demand for batteries is constantly increasing, especially with the widespread use of lithium-ion batteries in electric vehicles and energy storage devices. However, due to the scarcity of active materials for lithium-ion batteries, battery costs remain high, and the industry faces serious challenges such as resource depletion. The development of lithium resources and precious metals such as nickel and cobalt further increases the production cost of lithium-ion batteries, severely restricting the promotion and application of new energy vehicles and battery energy storage. Finding resource-rich, low-cost, environmentally friendly, and safe battery technology solutions remains a primary challenge.

[0003] Sodium-ion batteries and lithium-ion batteries operate on essentially the same principle. Sodium is one of the most abundant metallic elements in the Earth's crust, and the materials used in sodium-ion batteries are readily available and inexpensive. Therefore, sodium-ion batteries are one of the most promising technologies for development. However, the specific energy of sodium-ion batteries is lower than that of lithium-ion batteries. Therefore, improving the specific energy of sodium-ion batteries is a key technical problem that needs to be solved, and it is also a problem that needs to be addressed first in order to overcome the difficulties in the promotion and application of new energy vehicles and battery energy storage.

[0004] The mainstream cathode materials for sodium-ion secondary batteries are mainly divided into three categories: layered transition metal oxides, Prussian blue, and polyanionic materials. Among them, layered transition metal oxides have low reversible capacity, contain precious metals such as copper and nickel, have high cost, and are sensitive to moisture, making them difficult to use on a large scale. Prussian blue compounds have disadvantages such as low compaction density and poor thermal stability, and the material contains toxic CN- groups.

[0005] Polyanionic compounds, especially iron-based phosphates, have attracted widespread attention as a structurally stable and low-cost cathode material. Patent publication number "CN105845974A" proposes a carbothermal reduction solid-state method for synthesizing carbon-coated NaFePO4 sodium-ion battery cathode material, but this material only has a reversible capacity of 75 mAh / g as a sodium-ion battery cathode. Patent publication number "CN105152154A" provides a method for preparing NaFePO4 sodium-ion battery cathode material via electrochemical means, with a reversible capacity of 147.9 mAh / g; however, this process is too complex, requiring electrochemical sodium intercalation, making it difficult to apply in practical production. Patent publication number "CN108123129A" ​​describes a carbon-coated sodium iron pyrophosphate nanosheet (Na2FeP2O7) obtained by sequentially ball-milling and mixing an organic polymer surfactant, a phosphorus source, a hydrocarbon mixture, an iron source, and a sodium source, resulting in a reversible specific capacity of 90 mAh / g as a sodium-ion battery cathode material. Patent publication number "CN112768673A" reports a Na4Fe with iron defects. 3-x (PO4)2P2O7 / C sodium-ion battery cathode material, which has a specific capacity of 108 mAh / g.

[0006] The aforementioned existing technologies have many drawbacks. These mainly include:

[0007] 1. The reversible capacity of patent publication numbers “CN105845974A”, “CN105152154A”, and “CN112768673A” is relatively low, and the specific capacity is difficult to reach 140mAh / g.

[0008] 2. Patent publication number “CN105152154A” has a high reversible capacity, but the preparation method uses an electrochemical method, which is complicated and difficult to industrialize. Summary of the Invention

[0009] To address the aforementioned shortcomings in the prior art, this invention provides a high-capacity, long-cycle, low-cost, and environmentally friendly core-shell structured polyanionic phosphate cathode material for sodium-ion batteries, along with its preparation method.

[0010] To achieve the above objectives, the embodiments of the present invention provide the following technical solutions:

[0011] According to a first aspect of the present invention, a core-shell structured sodium-ion battery cathode material is provided. The sodium-ion battery cathode material is a polyanionic phosphate material, comprising a core material and a shell material; the core material is lithium iron phosphate (LiFePO4); and the shell material is selected from Na2FeP2O7, Na... 3.12 Fe 2.44 (P2O7)2, Na4Fe3(PO4)2P2O7, Na7Fe4.5 One or more of (P2O7)4 and Na3Fe2(PO4)3.

[0012] Furthermore, the mass ratio of the core material to the outer shell material is 5–95:95–5. This material, as a positive electrode material for sodium-ion batteries, has advantages such as high specific capacity, high compaction density, low cost, and environmental friendliness.

[0013] According to a second aspect of the present invention, a method for preparing a core-shell structured sodium-ion battery cathode material as described above is provided, comprising the following steps:

[0014] (1) Weigh a certain mass of commercial lithium iron phosphate material and add it to a reaction vessel along with a certain mass of deionized water, dispersant, and gelling agent. Under an inert atmosphere, disperse the materials at high speed.

[0015] (2) Weigh out the iron source and phosphorus source according to a certain molar ratio of elements, add the iron source and phosphorus source to the reaction vessel under an inert atmosphere, and mix and react under the protection of an inert atmosphere;

[0016] (3) Prepare a sodium source solution, add it to the reaction vessel, and mix and react under an inert atmosphere;

[0017] (4) The product obtained in step (3) is spray-dried to obtain a precursor. The precursor is then heat-treated under an inert atmosphere to obtain the sodium-ion battery cathode material.

[0018] Further, in steps (2) to (3), the molar ratio of the sodium source, iron source and phosphorus source is the molar ratio of sodium, iron and phosphorus elements in the shell material.

[0019] Further, the sodium source is selected from one or more of sodium hydroxide, sodium carbonate, and sodium bicarbonate; the iron source is selected from at least one of iron powder and ferrous carbonate; the phosphorus source is selected from at least one of phosphoric acid and ammonium dihydrogen phosphate; the dispersant is selected from at least one of polyvinylpyrrolidone, polyacrylamide, and polyvinyl alcohol; and the gelling agent is selected as citric acid.

[0020] Furthermore, the mass ratio of the dispersant to lithium iron phosphate is 1 to 5:100.

[0021] Furthermore, the molar ratio of the gelling agent to the iron source is 100-110:100.

[0022] Furthermore, in step (2), the feeding time is controlled to be 2-3 hours, the reaction time is controlled to be 24-48 hours, the reaction temperature is controlled to be 60-80℃, and the reaction vessel rotation speed is 200-2000 rpm.

[0023] Furthermore, in step (3), the molar concentration of sodium source is controlled to be 1-20 mol / L, the feeding time is controlled to be 30-60 minutes, the reaction temperature is controlled to be 60-80℃, and the reaction vessel rotation speed is 200-2000 rpm.

[0024] Furthermore, in step (4), the heat treatment temperature is 500–700°C and the time is 5–20 h;

[0025] And / or, after performing the heat treatment step, the method further includes: pulverizing and passing the material through a 300-500 mesh sieve.

[0026] Through the above heat treatment, the gelling agent decomposes under a high-temperature inert atmosphere, thereby generating a carbon coating layer in situ on the material surface, which can further improve the electronic conductivity of the material of the present invention. In the present invention, the inert atmosphere is an argon atmosphere or a nitrogen atmosphere.

[0027] The embodiments of the present invention have the following advantages:

[0028] First: This invention provides a positive electrode material for non-aqueous sodium-ion secondary batteries, which is a polyanionic phosphate compound containing sodium, lithium, and iron. The raw materials used in the synthesis of this invention are all commercially available, inexpensive, and readily available materials, and do not contain any precious or heavy metals, so they can be widely used.

[0029] Second: The material of this invention has a specific capacity of ≥130mAh / g, which is much greater than the specific capacity of sodium metal oxide (~110mAh / g) and sodium Prussian blue compound (~125mAh / g), thus greatly improving the specific energy of sodium-ion batteries.

[0030] Third: The preparation method of the material of this invention adopts the mature gelation method and uses citric acid as a complexing agent to generate a homogeneous precursor. At the same time, no pollutants are emitted during the production process, the production cost is low, and it is green and environmentally friendly. Therefore, it can be produced and promoted on a large scale.

[0031] Fourth: The compaction density of this invention can reach 2.2–2.5 g / cm³. 3 It has reached the same compaction density level as commercial lithium iron phosphate materials, and is higher than Prussian blue sodium-ion cathode materials, thus improving the volumetric energy density of sodium-ion batteries.

[0032] Fifth: The cycle life of this invention can reach more than 5,000 times, which is far higher than that of sodium metal oxide, and can be applied on a large scale in the field of energy storage. Attached Figure Description

[0033] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0034] Figure 1 This is a cycle life curve of an 18650 battery provided in Embodiment 3 of the present invention;

[0035] Figure 2 This is a rate characteristic diagram of an 18650 battery provided in Embodiment 3 of the present invention;

[0036] Figure 3 The discharge curve of the 18650 battery provided in Embodiment 3 of the present invention. Detailed Implementation

[0037] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] Example 1

[0039] This embodiment uses the gelation method to prepare sodium-ion battery cathode materials. The method is as follows:

[0040] Step 1: Weigh 1656 grams of lithium iron phosphate material, 15000 grams of deionized water, 50 grams of polyvinylpyrrolidone, and 315 grams of citric acid and add them to the reaction vessel.

[0041] Step 2: Weigh 84 grams of iron powder and 346 grams of phosphoric acid, add them to the reaction vessel, control the feeding time to 3 hours, control the reaction time to 24 hours, control the reaction temperature to 80℃, and control the reaction vessel rotation speed to 1000 rpm.

[0042] Step 3: Weigh 120 grams of sodium hydroxide, prepare a 5 mol / L aqueous solution of sodium hydroxide, add it to the reaction vessel, control the feeding time to be 45 minutes, control the reaction temperature to be 80℃, and control the reaction vessel rotation speed to be 1000 rpm.

[0043] Step 4: Spray dry the product from step 3, and then heat treat it under an argon atmosphere at a temperature of 650°C for 8 hours. After heat treatment, pulverize the material and sieve it through a 350-mesh sieve to obtain the sodium-ion battery cathode material of this invention.

[0044] According to the molecular formula molar ratio of the elements in this embodiment, the outer shell material of the synthesized material in this embodiment has the molecular formula Na2FeP2O7, and the mass ratio of the core material to the outer shell material is 4:1. Citric acid, as a gelling agent, has a chelating effect on metal particles and can generate a uniform and stable precursor. At the same time, carbonization is carried out under the high temperature treatment in the fourth step to obtain carbon-coated Na2FeP2O7, which improves the electronic conductivity of the material of the present invention.

[0045] The material obtained in Example 1 was ball-milled and used to fabricate the positive electrode sheet for a sodium-ion secondary battery. The specific steps included: mixing the material from Example 1, acetylene black, and polyvinylidene fluoride (PVDF) binder in a mass ratio of 85:10:5 using NMP solvent; coating the resulting slurry onto aluminum foil; and drying it under vacuum at 120°C for 10 hours. Simulated coin cells were tested in an Ar-filled glove box. Metallic sodium was used as the counter electrode, and commercially available sodium-ion battery electrolyte was used. A CR2032 battery was assembled, and the tested voltage range was between 1.5 and 4.2 V. At a C / 10 rate, the first-cycle charge specific capacity was 158 mAh / g, the discharge specific capacity was 141 mAh / g, and the initial efficiency was 89%, as shown in Table 1. The material obtained in Example 1 was measured using a powder compaction density tester under a test condition of 100 MPa, and the measured compaction density was 2.34 g / cm³. 3 The results are shown in Table 2.

[0046] Example 2

[0047] This embodiment uses the gelation method to prepare sodium-ion battery cathode materials. The method is as follows:

[0048] Step 1: Weigh 556 grams of lithium iron phosphate material, 3000 grams of deionized water, 15 grams of polyvinylpyrrolidone, and 256 grams of citric acid and add them to the reaction vessel.

[0049] Step 2: Weigh 68.3 grams of iron powder and 230.6 grams of phosphoric acid, add them to the reactor, control the feeding time to 3 hours, control the reaction time to 26 hours, control the reaction temperature to 65℃, and control the reactor rotation speed to 800 rpm.

[0050] Step 3: Weigh 62.4 grams of sodium hydroxide, prepare a 10 mol / L aqueous solution of sodium hydroxide, add it to the reaction vessel, control the feeding time to be 40 minutes, control the reaction temperature to be 65℃, and control the reaction vessel rotation speed to be 8000 rpm.

[0051] Step 4: Spray dry the product from step 3, and then heat treat it under an argon atmosphere at a temperature of 600°C for 9 hours. After heat treatment, pulverize the material and sieve it through a 500-mesh sieve to obtain the sodium-ion battery cathode material of this invention.

[0052] According to the elemental molar ratio in this embodiment, the outer shell material of the synthesized material in this embodiment has the molecular formula Na. 3.12 Fe 2.44 (P2O7)2, the mass ratio of core material to shell material is 2:1; citric acid, as a gelling agent, has a chelating effect on metal particles, generating a uniform and stable precursor, which is then carbonized in the fourth step at high temperature to obtain carbon-coated Na. 3.12 Fe 2.44 (P2O7)2 improves the electronic conductivity of the material of the present invention.

[0053] The material obtained in Example 2 was ball-milled and used to manufacture the positive electrode sheet for a sodium-ion secondary battery. The specific steps included: mixing the material from Example 2, acetylene black, and polyvinylidene fluoride (PVDF) binder in a mass ratio of 85:10:5 using NMP solvent; coating the resulting slurry onto aluminum foil; and drying it under vacuum at 120°C for 10 hours. Simulated coin cells were tested in an Ar-filled glove box. Metallic sodium was used as the counter electrode, and commercially available sodium-ion battery electrolyte was used. A CR2032 battery was assembled, and the tested voltage range was between 1.5 and 4.2 V. At a C / 10 rate, the first-cycle charge specific capacity was 150 mAh / g, the discharge specific capacity was 136 mAh / g, and the initial efficiency was 90.6%, as shown in Table 1. The material obtained in Example 2 was measured using a powder compaction density tester under a test condition of 100 MPa, and the measured compaction density was 2.35 g / cm³. 3 The results are shown in Table 2.

[0054] Example 3

[0055] This embodiment uses the gelation method to prepare sodium-ion battery cathode materials. The method is as follows:

[0056] Step 1: Weigh 936 grams of lithium iron phosphate material, 9000 grams of deionized water, 18.7 grams of polyvinylpyrrolidone, and 315 grams of citric acid and add them to the reaction vessel.

[0057] Step 2: Weigh 84 grams of iron powder and 230.6 grams of phosphoric acid, add them to the reactor, control the feeding time to 3 hours, control the reaction time to 30 hours, control the reaction temperature to 70℃, and control the reactor rotation speed to 1200 rpm.

[0058] Step 3: Weigh 80 grams of sodium hydroxide, prepare an 8 mol / L aqueous solution of sodium hydroxide, add it to the reaction vessel, control the feeding time to be 30 minutes, control the reaction temperature to be 70℃, and control the reaction vessel rotation speed to be 1200 rpm.

[0059] Step 4: Spray dry the product from step 3, and then heat treat it under an argon atmosphere at a temperature of 580°C for 10 hours. After heat treatment, pulverize the material and sieve it through a 500-mesh sieve to obtain the sodium-ion battery cathode material of this invention.

[0060] According to the molecular formula molar ratio of the elements in this embodiment, the outer shell material of the synthesized material in this embodiment has the molecular formula Na4Fe3(PO4)2P2O7, and the mass ratio of the core material to the outer shell material is 3:1. Citric acid, as a gelling agent, has a chelating effect on metal particles and can generate a uniform and stable precursor. At the same time, carbonization is carried out under the high temperature treatment in the fourth step to obtain carbon-coated Na4Fe3(PO4)2P2O7, which improves the electronic conductivity of the material of the present invention.

[0061] The material obtained in Example 3 was ball-milled and used to manufacture the positive electrode sheet for a sodium-ion secondary battery. The specific steps included: mixing the material from Example 3, acetylene black, and polyvinylidene fluoride (PVDF) binder in a mass ratio of 85:10:5 using NMP solvent; coating the resulting slurry onto aluminum foil; and drying it under vacuum at 120°C for 10 hours. Simulated coin cells were tested in an Ar-filled glove box. Metallic sodium was used as the counter electrode, and commercially available sodium-ion battery electrolyte was used. A CR2032 battery was assembled, and the tested voltage range was between 1.5 and 4.2 V. At a C / 10 rate, the first-cycle charge specific capacity was 159 mAh / g, the discharge specific capacity was 145 mAh / g, and the initial efficiency was 91%, as shown in Table 1. The material obtained in Example 3 was measured using a powder compaction density tester under a test condition of 100 MPa, and the measured compaction density was 2.36 g / cm³. 3 The results are shown in Table 2.

[0062] The material obtained in Example 3 was combined with PVDF, carbon nanotubes, and NMP to prepare an 18650 cylindrical battery positive electrode sheet using the 18650 battery positive electrode sheet manufacturing process.

[0063] Hard carbon material, carbon nanotubes, styrene-butadiene rubber, and CMC are used to prepare 18650 cylindrical battery negative electrode sheets using the manufacturing process of 18650 battery negative electrode sheets.

[0064] The diaphragm is a commercially available polyethylene diaphragm.

[0065] The electrolyte is a mixture of commercial lithium-ion electrolyte and commercial sodium-ion electrolyte, and the mass ratio of lithium-ion electrolyte to sodium-ion electrolyte is controlled at 3:1.

[0066] The positive electrode, negative electrode, electrolyte, and separator prepared above were assembled into an 18650 battery according to the assembly process of an 18650 battery and tested. The tested voltage range was between 1.5 and 4.2V.

[0067] The prepared batteries were discharged at rates of 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, and 10C, respectively. The rate characteristic graphs are shown below. Figure 2 As shown, by Figure 2 It can be seen that the battery's 10C discharge capacity is 77% of its 0.1C discharge capacity, indicating good rate capability of the material. The battery's 1C discharge curve is shown in the figure below. Figure 3 As shown.

[0068] The 18650 battery was subjected to a battery cycle performance test. The test method was as follows: under a temperature of 25℃, constant current and constant voltage at 1C to the upper limit voltage, and then left to stand for 10 minutes.

[0069] Discharge at a constant current of 1C to the lower limit voltage and let stand for 10 minutes;

[0070] Repeat the process 5000 times;

[0071] The results are as follows Figure 1 As shown.

[0072] Example 4

[0073] This embodiment uses the gelation method to prepare sodium-ion battery cathode materials. The method is as follows:

[0074] Step 1: Weigh 1109g of lithium iron phosphate material, 10000g of deionized water, 33g of polyvinylpyrrolidone, and 189g of citric acid and add them to the reaction vessel.

[0075] Step 2: Weigh 50.4 g of iron powder and 184.5 g of phosphoric acid, add them to the reactor, control the feeding time to 3 h, control the reaction time to 28 h, control the reaction temperature to 75 ℃, and control the reactor rotation speed to 1100 rpm.

[0076] Step 3: Weigh 56 grams of sodium hydroxide, prepare a 10 mol / L aqueous solution of sodium hydroxide, add it to the reaction vessel, control the feeding time to be 30 minutes, control the reaction temperature to be 75℃, and control the reaction vessel rotation speed to be 1100 rpm.

[0077] Step 4: Spray dry the product from step 3, and then heat treat it under an argon atmosphere at a temperature of 570°C for 12 hours. After heat treatment, pulverize the material and sieve it through a 400-mesh sieve to obtain the sodium-ion battery cathode material of this invention.

[0078] According to the elemental molar ratio in this embodiment, the outer shell material of the synthesized material in this embodiment has the molecular formula Na7Fe. 4.5 (P2O7)4, with a core material to shell material mass ratio of 5:1; citric acid, acting as a gelling agent, chelates the metal particles, generating a uniform and stable precursor. Simultaneously, carbonization is performed in the fourth step at high temperature to obtain carbon-coated Na7Fe.4.5 (P2O7)4 improves the electronic conductivity of the material of the present invention.

[0079] The material obtained in Example 4 was ball-milled and used to fabricate the positive electrode sheet for a sodium-ion secondary battery. The specific steps included: mixing the material from Example 4, acetylene black, and polyvinylidene fluoride (PVDF) binder in a mass ratio of 85:10:5 using NMP solvent; coating the resulting slurry onto aluminum foil; and drying it under vacuum at 120°C for 10 hours. Simulated coin cells were tested in an Ar-filled glove box. Metallic sodium was used as the counter electrode, and commercially available sodium-ion battery electrolyte was used. A CR2032 battery was assembled, and the tested voltage range was between 1.5 and 4.2 V. At a C / 10 rate, the first-cycle charge specific capacity was 162 mAh / g, the discharge specific capacity was 143 mAh / g, and the initial efficiency was 88%. The results are shown in Table 1. The material obtained in Example 4 was measured using a powder compaction density tester under a test condition of 100 MPa, and the measured compaction density was 2.38 g / cm³. 3 The results are shown in Table 2.

[0080] Example 5

[0081] This embodiment uses the gelation method to prepare sodium-ion battery cathode materials. The method is as follows:

[0082] Step 1: Weigh 838.8 grams of lithium iron phosphate material, 8000 grams of deionized water, 16.8 grams of polyvinylpyrrolidone, and 210 grams of citric acid and add them to the reaction vessel.

[0083] Step 2: Weigh 56 grams of iron powder and 179.3 grams of phosphoric acid, add them to the reactor, control the feeding time to 3 hours, control the reaction time to 24 hours, control the reaction temperature to 60℃, and control the reactor rotation speed to 800 rpm.

[0084] Step 3: Weigh 60 grams of sodium hydroxide, prepare a 12 mol / L aqueous solution of sodium hydroxide, add it to the reaction vessel, control the feeding time to be 45 minutes, control the reaction temperature to be 60℃, and control the reaction vessel rotation speed to be 800 rpm.

[0085] Step 4: Spray dry the product from step 3, and then heat treat it under an argon atmosphere at a temperature of 650°C for 7 hours. After heat treatment, pulverize the material and sieve it through a 500-mesh sieve to obtain the sodium-ion battery cathode material of this invention.

[0086] According to the molecular formula element molar ratio of this embodiment, the outer shell material of the synthesized material in this embodiment has the molecular formula Na3Fe2(PO4)3, and the mass ratio of the core material to the outer shell material is 9:1. Citric acid, as a gelling agent, has a chelating effect on metal particles and can generate a uniform and stable precursor. At the same time, carbonization is carried out under the high temperature treatment in the fourth step to obtain carbon-coated Na3Fe2(PO4)3, which improves the electronic conductivity of the material of the present invention.

[0087] The material obtained in Example 5 was ball-milled and used to fabricate the positive electrode sheet for a sodium-ion secondary battery. The specific steps included: mixing the material from Example 5, acetylene black, and polyvinylidene fluoride (PVDF) binder in a mass ratio of 85:10:5 using NMP solvent; coating the resulting slurry onto aluminum foil; and drying it under vacuum at 120°C for 10 hours. Simulated coin cells were tested in an Ar-filled glove box. Metallic sodium was used as the counter electrode, and commercially available sodium-ion battery electrolyte was used. A CR2032 battery was assembled, and the tested voltage range was between 1.5 and 4.2 V. At a C / 10 rate, the first-cycle charge specific capacity was 165 mAh / g, the discharge specific capacity was 148 mAh / g, and the initial efficiency was 89.7%, as shown in Table 1. The material obtained in Example 5 was measured using a powder compaction density tester under a test condition of 100 MPa, and the compaction density was found to be 2.4 g / cm³. 3 The results are shown in Table 2.

[0088] Comparative Example 1

[0089] Commercially available sodium ferrous ferrocyanide (Na2Fe[Fe(CN)6]) was used to fabricate the positive electrode for sodium-ion secondary batteries. The specific steps included: mixing Na2Fe[Fe(CN)6] powder with acetylene black and polyvinylidene fluoride (PVDF) binder at a mass ratio of 85:10:5 using NMP solvent; coating the resulting slurry onto aluminum foil; and drying it under vacuum at 120°C for 24 hours. Simulated coin cells were tested in an Ar-filled glove box. Metallic sodium was used as the counter electrode, and commercially available sodium-ion battery electrolyte was used. The cells were assembled into CR2032 batteries. The tested voltage range was 2.0-4.0V. At a C / 10 rate, the first-cycle charge specific capacity was 150 mAh / g, the discharge specific capacity was 128 mAh / g, and the initial efficiency was 85%. The results are shown in Table 1. The material prepared in Comparative Example 1 was measured using a powder compaction density tester under a test condition of 100 MPa, and the compaction density was found to be 1.5 g / cm³. 3 The results are shown in Table 2.

[0090] Therefore, the cathode material provided in this embodiment of the invention, when used as a cathode material for sodium-ion secondary batteries, exhibits a superior specific capacity compared to the existing Na2Fe[Fe(CN)6] cathode material. Furthermore, Na2Fe[Fe(CN)6] contains toxic (CN)- groups, which are environmentally unfriendly. The iron element in the material of this invention is inexpensive and readily available, making it green and environmentally friendly, and thus suitable for large-scale application.

[0091] Comparative Example 2

[0092] Comparative Example 2 uses a high-temperature solid-state synthesis method to prepare sodium-ion battery cathode materials. The preparation method of Comparative Example 2 is as follows:

[0093] Step 1: Weigh out 160g of sodium hydroxide, 348g of ferrous carbonate, 460g of ammonium dihydrogen phosphate, 5000g of deionized water, and 30g of sucrose. Place the weighed raw materials into the reaction vessel and react at room temperature (25℃) for 9 hours. Control the rotation speed of the reaction vessel to 200-300 rpm. To protect the materials from oxidation by air during the reaction, argon gas is introduced for protection.

[0094] Step 2: The material after the reaction is completed is dispersed by sand milling. The material dispersed by the sand mill has a D50 particle size of 500nm.

[0095] Step 3: Spray dry the milled material. The dried powder is the precursor. Heat treat the precursor powder at 610°C in an argon atmosphere for 14 hours to obtain the sodium-ion battery cathode material.

[0096] According to the element molar ratio in Comparative Example 2, it is equivalent to the general chemical formula: Na4Fe3(PO4)2P2O7, that is, the molecular formula of the synthesized material is Na4Fe3(PO4)2P2O7.

[0097] The material obtained in Comparative Example 2 was ground and used to fabricate the positive electrode sheet for a sodium-ion secondary battery. The specific steps included: mixing Na4Fe3(PO4)2P2O7 powder, acetylene black, and polyvinylidene fluoride (PVDF) binder in a mass ratio of 85:10:5 using NMP solvent; coating the resulting slurry onto aluminum foil; and drying it under vacuum at 120°C for 24 hours. Simulated coin cells were tested in an Ar-filled glove box. Metallic sodium was used as the counter electrode, and commercially available sodium-ion battery electrolyte was used. A CR2032 battery was assembled, and the tested voltage range was between 2.5-4.0V. At a C / 10 rate, the first-week charge specific capacity was 130 mAh / g, the discharge specific capacity was 102 mAh / g, and the initial efficiency was 78.5%, as shown in Table 1. The material obtained in Comparative Example 2 was measured using a powder compaction density tester under a test condition of 100 MPa, and the measured compaction density was 2 g / cm³. 3 The results are shown in Table 2.

[0098] Comparative Example 3

[0099] Commercially available sodium vanadium phosphate (Na3V2(PO4)3) was used to fabricate the positive electrode for sodium-ion secondary batteries. The specific steps included: mixing Na3V2(PO4)3 powder with acetylene black and polyvinylidene fluoride (PVDF) binder at a mass ratio of 85:10:5 using NMP solvent; coating the resulting slurry onto aluminum foil; and drying under vacuum at 120°C for 24 hours. Simulated coin cells were tested in an Ar-filled glove box. Metallic sodium was used as the counter electrode, and commercially available sodium-ion battery electrolyte was used. A CR2032 battery was assembled, and the tested voltage range was 2.0-4.0V. At a C / 10 rate, the first-week charge specific capacity was 110 mAh / g, the discharge specific capacity was 92 mAh / g, and the initial efficiency was 83.6%, as shown in Table 1. The material prepared in Comparative Example 3 was measured using a powder compaction density tester under a test condition of 100 MPa, and the compaction density was found to be 2.2 g / cm³. 3 The results are shown in Table 2.

[0100] Therefore, the cathode material provided in this invention, when used as a cathode material in sodium-ion secondary batteries, exhibits a superior specific capacity compared to the existing Na3V2(PO4)3 cathode material. Furthermore, the iron in this material is inexpensive and readily available, resulting in a lower cost than vanadium materials, thus enabling its large-scale application.

[0101] Table 1. Comparison of battery electrical performance between the examples and comparative examples.

[0102]

[0103] Table 2 Comparison of compaction density of cathode materials in the examples and comparative examples

[0104]

[0105]

[0106] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A core-shell structured sodium-ion battery cathode material, characterized in that, The sodium ion battery cathode material is a polyanion phosphate material, comprising a core material and a shell material; The core material is lithium iron phosphate LiFePO4; The shell material is selected from one or more of Na2FeP207, Na 3.12 Fe 2.44 (P207)2, Na4Fe3(PO4)2P207, Na7Fe 4.5 (P207)4, Na3Fe2(PO4)3.

2. The core-shell structured sodium-ion battery cathode material of claim 1, wherein, The mass ratio of the core material to the shell material is 5-95:95-5.

3. The method of producing a core-shell structured sodium-ion battery cathode material according to claim 1, characterized in that, The method comprises the following steps: (1) A certain mass of commercial lithium iron phosphate material is weighed, and a certain mass of deionized water, a dispersing agent and a gelling agent are added into a reaction kettle, and high-speed dispersion is carried out under the protection of an inert atmosphere; (2) Iron source and phosphorus source are weighed according to a certain element molar ratio, and the iron source and the phosphorus source are added into the reaction kettle under an inert atmosphere, and mixed reaction is carried out under the protection of an inert atmosphere; (3) A sodium source is configured into a solution and added into the reaction kettle, and mixed reaction is carried out under the protection of an inert atmosphere; (4) The product obtained in step (3) is subjected to spray drying to obtain a precursor, and the precursor is subjected to heat treatment under the protection of an inert atmosphere to obtain the sodium ion battery cathode material; The gelling agent is citric acid.

4. The method of claim 3, wherein the method further comprises a step of calcining the mixture at a temperature of 600-800 °C for 5-10 hours. In steps (2)-(3), the element molar ratio of the sodium source, the iron source and the phosphorus source is the molar ratio of sodium elements, iron elements and phosphorus elements of the shell material.

5. The method of producing a core-shell structured sodium-ion battery cathode material according to claim 3, wherein, The sodium source is selected from one or more of sodium hydroxide, sodium carbonate and sodium bicarbonate; the iron source is selected from at least one of iron powder and ferrous carbonate; the phosphorus source is selected from at least one of phosphoric acid and ammonium dihydrogen phosphate; and the dispersing agent is selected from at least one of polyvinylpyrrolidone, polyacrylamide and polyvinyl alcohol.

6. The method of producing a core-shell structured sodium-ion battery cathode material according to claim 3, wherein, The mass ratio of the dispersing agent to the lithium iron phosphate is 1-5:

100.

7. The method of producing a core-shell structured sodium-ion battery cathode material according to claim 3, characterized in that, The molar ratio of the gelling agent to the iron source is 100-110:

100.

8. The method of producing a core-shell structured sodium-ion battery cathode material according to claim 3, characterized in that, In step (2), the feeding time is controlled to be 2-3h, the reaction time is controlled to be 24-48h, the reaction temperature is controlled to be 60-80℃, and the rotation speed of the reaction kettle is controlled to be 200-2000rpm.

9. The method of producing a core-shell structured sodium-ion battery cathode material according to claim 3, wherein, In step (3), the molar concentration of the sodium source is controlled to be 1-20mol / L, the feeding time is controlled to be 30-60min, the reaction temperature is controlled to be 60-80℃, and the rotation speed of the reaction kettle is controlled to be 200-2000rpm.

10. The method of producing a core-shell structured sodium-ion battery cathode material according to claim 3, wherein, In step (4), the heat treatment temperature is 500-700℃, and the time is 5-20h; And / or, after the step of heat treatment, the method further comprises: crushing and then passing through a 300-500 mesh sieve.

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