Polyanion composite cathode material for sodium ion battery and preparation method and application thereof

Abstract

This invention provides a polyanionic composite cathode material, its preparation method, and its applications. The composite cathode material includes a core and a coating structure encapsulating the core. The core comprises sodium ferric sulfate particles, and the coating structure comprises sodium ferric phosphate pyrophosphate and a nano-conductive compound. The particle size of the sodium ferric sulfate particles is larger than that of the sodium ferric phosphate pyrophosphate, and the particle size of the sodium ferric phosphate pyrophosphate is larger than that of the nano-conductive compound. The coating structure includes a first coating layer and a second coating layer sequentially arranged in a direction away from the core. The first coating layer comprises sodium ferric phosphate pyrophosphate and a first portion of the nano-conductive compound, which are bonded to the surface of the sodium ferric sulfate particles. The second coating layer comprises a second portion of the nano-conductive compound, which is coated on the surface of the sodium ferric phosphate pyrophosphate in the first coating layer. The composite cathode material provided by this invention exhibits good environmental stability and air stability, as well as high conductivity, good specific capacity, and good rate performance.

Description

Technical Field

[0001] This invention belongs to the field of new energy technology, specifically relating to a polyanion composite cathode material for sodium-ion batteries, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries play a crucial role in today's vast energy storage landscape due to their low cost, abundant resources, and environmentally friendly characteristics. As a strong competitor to lithium-ion batteries, sodium-ion batteries are gradually demonstrating their immeasurable potential in emerging fields such as large-scale energy storage networks and electric vehicles. In the global journey towards a new era of sustainable development and green energy, in-depth research and development of high-performance sodium-ion batteries is undoubtedly a key link in meeting future energy demands.

[0003] Among the discovered cathode materials, polyanionic compounds have attracted widespread attention due to their stable crystal structure. Based on the type of anionic functional groups, polyanionic compounds can be classified into phosphates, silicates, sulfates, pyrophosphates, and mixed pyrophosphates. Ferric sulfate-based materials, with their advantages of low cost and high operating voltage, are considered one of the more promising candidate materials for sodium-ion batteries. Sodium ferric sulfate possesses a high operating voltage platform, enabling higher energy output, while its raw materials are abundant and its preparation cost is low, resulting in high cost-effectiveness. Furthermore, this material exhibits good cycle stability during multiple charge-discharge cycles, ensuring battery lifespan. Simultaneously, sodium ferric sulfate is highly safe and environmentally friendly, aligning with the principles of green chemistry and sustainable development.

[0004] However, sodium ferric sulfate cathode material has an unstable crystal structure and is sensitive to water, making it prone to deactivation in high humidity environments, thus affecting battery performance. Furthermore, sodium ferric sulfate has relatively low conductivity, which impacts the battery's charge / discharge efficiency and power performance. Summary of the Invention

[0005] To solve all or part of the above-mentioned technical problems, the present invention provides the following technical solutions: One objective of this invention is to provide a polyanion composite cathode material for sodium-ion batteries. The polyanion composite cathode material includes a core and a coating structure covering the core. The core includes sodium iron sulfate particles, and the coating structure includes sodium iron phosphate pyrophosphate and a nano-conductive compound. The particle size of the sodium ferric sulfate particles is larger than that of the sodium ferric pyrophosphate, and the particle size of the sodium ferric pyrophosphate is larger than that of the nano-conductive compound. The coating structure includes a first coating layer and a second coating layer arranged sequentially in a direction away from the core. The first coating layer includes sodium iron phosphate pyrophosphate and a first portion of a nano-conductive compound, the first portion of which is bonded to the surface of the sodium iron sulfate particles. The second coating layer includes a second portion of a nano-conductive compound, the second portion of which is coated on the surface of the sodium iron phosphate pyrophosphate in the first coating layer.

[0006] This invention utilizes sodium ferric phosphate pyrophosphate and nano-conductive compounds to form a gradient coating layer on the surface of sodium ferric sulfate particles, which can suppress the water absorption of sodium ferric sulfate. The resulting composite material exhibits good environmental and air stability, solving the problem of water sensitivity and easy deactivation of sodium ferric sulfate cathode materials in existing technologies. Furthermore, the polyanionic composite cathode material also possesses high conductivity, good specific capacity, and rate performance, demonstrating excellent electrical properties.

[0007] In some embodiments, the median particle size of the sodium ferric sulfate particles is 15~30 μm.

[0008] In some embodiments, the median particle size of the sodium iron pyrophosphate is less than 10 μm, preferably less than 2 μm, and more preferably 1~2 μm.

[0009] In some embodiments, the particle size of the nano-conductive compound is 1~100nm, preferably 10~50nm.

[0010] The particle size matching of sodium ferric sulfate particles, sodium ferric pyrophosphate, and the nano-conductive chemical has a significant impact on the formation of the coating structure described in this invention. If the particle size of sodium ferric pyrophosphate is large, it will not be able to effectively coat the surface of sodium ferric sulfate particles. Ultimately, the sodium ferric pyrophosphate and sodium ferric sulfate particles tend to be presented as a uniform mixture of the two materials, and the coating structure is only a single coating structure formed by the nano-conductive compound. If the particle size of sodium ferric pyrophosphate is small (e.g., nanoscale), or the particle size of the conductive compound is large, the sodium ferric pyrophosphate and the nano-conductive compound will concentrate on coating the surface of sodium ferric sulfate particles. That is, the formed coating structure is a single coating structure formed by the mixture of sodium ferric pyrophosphate and the nano-conductive compound, and a gradient coating structure cannot be formed.

[0011] In some embodiments, the thickness ratio of the first coating layer to the second coating layer is 200:1 to 20:1. The thickness of the first coating layer is preferably 1 to 2 micrometers, and the thickness of the second coating layer is preferably 10 to 50 nm. If the first coating layer is too thick, the nano-conductive compound tends to concentrate mainly on the surface of the sodium iron phosphate pyrophosphate particles. If the second coating layer is too thick, the proportion of the nano-conductive compound on the surface of the sodium iron sulfate particles tends to be high, leading to a decrease in the specific capacity of the cathode material.

[0012] In some embodiments, the second part of the nano-conductive compound accounts for 2-95% of the total mass of the nano-conductive compound. At this time, the proportion of the nano-conductive compound on the surface of sodium ferric sulfate particles and the proportion coated on the surface of sodium ferric phosphate pyrophosphate particles are more suitable, so that the composite material has both good environmental stability and good conductivity, specific capacity and rate performance.

[0013] In some embodiments, the polyanion composite cathode material is spherical particles with a median particle size of 15~40μm.

[0014] In some embodiments, the chemical formula of the polyanion composite cathode material is: Na x Fe(SO4) y @Na a Fe b (PO4) c P2O7 / M d O e , where 1≤x≤3, y=(x+2) / 2, 3≤a≤4.5, 2≤b≤3, 1≤c≤2, 1≤d≤2, and 1≤e≤3.

[0015] In some embodiments, the mass of the sodium ferric pyrophosphate is 0.1-10 wt% of the sodium ferric sulfate particles, and the mass of the nano-conductive compound is 0.1%-10 wt% of the sodium ferric sulfate particles.

[0016] In some embodiments, the sodium ferric sulfate particles contain 0.1-10 wt% conductive material, preferably 2-5 wt% conductive material. The conductive material is distributed within the sodium ferric sulfate, at its interface, or uniformly dispersed between the sodium ferric sulfate particles.

[0017] In some embodiments, the conductive material includes a carbon material. The carbon material can be any conductive carbon material known in the art, such as one or a combination of carbon nanotubes, carbon fibers, reduced graphene oxide, graphene, and conductive carbon black, but is not limited thereto.

[0018] In some embodiments, the nano-conductive compound includes one or more of aluminum oxide, aluminum fluoride, copper oxide, titanium dioxide, or silver oxide, but is not limited thereto.

[0019] The second objective of this invention is to provide a method for preparing a polyanion composite cathode material for sodium-ion batteries, comprising: The sodium ferric sulfate precursor material, powdered sodium ferric pyrophosphate, and nano-conductive compound were mechanically fused to obtain a mixture. Under a protective atmosphere, the mixture is sintered at high temperature to obtain a polyanion composite cathode material.

[0020] Compared to simple mixing, mechanical fusion can effectively coat sodium ferric pyrophosphate and nano-conductive compounds onto the surface of sodium ferric sulfate, forming the coating structure described in this invention, thereby improving the air stability and conductivity of sodium ferric sulfate.

[0021] In some embodiments, the mechanical fusion is performed in a high-speed mixer with a linear velocity of 10-40 m / s, and the mechanical fusion time is 10-30 min. Under these process conditions, the polyanion composite cathode material coating structure described in this invention can be formed.

[0022] In some embodiments, the mixture is sintered at 300-400°C for 5-24 hours to obtain the polyanion composite cathode material.

[0023] In some embodiments, the protective atmosphere includes one or a combination of argon and nitrogen, or the protective atmosphere includes a hydrogen-argon mixture or a nitrogen-hydrogen mixture. Compared to a simple inert gas protective atmosphere, using an argon-hydrogen mixture or a nitrogen-hydrogen mixture can provide a reducing atmosphere during sintering, suppressing the oxidation of ferrous iron.

[0024] In some embodiments, the volume ratio of hydrogen in the argon-hydrogen mixture is less than 5%; the volume ratio of hydrogen in the nitrogen-hydrogen mixture is less than 5%.

[0025] In some embodiments, the mass of sodium ferric pyrophosphate in the mixture is 0.1-10 wt% of the sodium ferric sulfate precursor material, and the mass of the nano-conductive compound is 0.1%-10 wt% of the sodium ferric sulfate precursor material.

[0026] In some embodiments, the median particle size of the sodium iron pyrophosphate is less than 10 μm, preferably less than 2 μm, and more preferably 1~2 μm.

[0027] In some embodiments, the particle size of the nano-conductive compound is 1~100 nm.

[0028] The sodium ferric sulfate precursor material can be prepared using any method known in the art, and the present invention does not impose any particular limitation on it.

[0029] In some embodiments, the preparation method of the sodium ferric sulfate precursor material includes: dissolving sodium sulfate and ferrous sulfate in a solvent, and uniformly dispersing an antioxidant and a conductive material in the solvent to obtain a mixture; spray drying the mixture to obtain the sodium ferric sulfate precursor material.

[0030] In some embodiments, the molar ratio of sodium sulfate to ferrous sulfate is 1:1 to 3:1.

[0031] In some embodiments, the antioxidant includes one or more of ascorbic acid, citric acid, oxalic acid, or sodium sulfite, but is not limited thereto.

[0032] In some embodiments, the molar ratio of the antioxidant to ferrous sulfate is 2:10~100.

[0033] In some embodiments, the conductive material includes one or more of carbon nanotubes, carbon fibers, reduced graphene oxide, graphene, and conductive carbon black, but is not limited thereto.

[0034] In some embodiments, the amount of the conductive material is 0.1 to 10 wt% of the mixture, and more preferably 2 to 5 wt%.

[0035] In some embodiments, the inlet air temperature of the spray dryer is 150~250℃, and the outlet air temperature is 80~120℃.

[0036] The third objective of this invention is to provide a polyanion composite cathode material for sodium-ion batteries, which is prepared by the method described above.

[0037] The fourth objective of this invention is to provide a positive electrode for a sodium-ion battery, the positive electrode comprising a current collector and a positive electrode active material layer formed on the current collector, the positive electrode active material layer comprising the aforementioned polyanion composite positive electrode material for sodium-ion batteries.

[0038] The fifth objective of this invention is to provide a sodium-ion battery, comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode is the positive electrode for use in a sodium-ion battery.

[0039] Compared with existing technologies, the present invention has at least the following beneficial effects: The present invention uses sodium ferric phosphate pyrophosphate and nano-conductive compounds to form a gradient coating layer on the surface of sodium ferric sulfate particles, which can suppress the water absorption of sodium ferric sulfate. The resulting composite material has good environmental stability and air stability, solving the problem of water sensitivity and easy deactivation of sodium ferric sulfate cathode materials in existing technologies. Furthermore, the polyanion composite cathode material also exhibits high conductivity, good specific capacity and rate performance, demonstrating excellent electrical performance. Attached Figure Description

[0040] To more clearly illustrate the technical solutions in the embodiments of this application or 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 only some embodiments recorded in this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0041] Figure 1 This is a schematic diagram of the structure of the polyanion composite cathode material prepared in Example 1; Figure 2 This is the XRD pattern of the polyanion composite cathode material prepared in Example 1. Detailed Implementation

[0042] The technical solutions of the present invention will be described in detail below with reference to specific embodiments, so that those skilled in the art can better understand and implement the technical solutions of the present invention. The specific functional details disclosed herein should not be construed as limiting, but are merely intended to form the basis of the claims and to teach those skilled in the art to employ the representative basis of the invention in different ways in any suitable detailed embodiment.

[0043] In addition, unless otherwise specified, all raw materials used in the following embodiments can be purchased from the market or other sources, and all production and testing equipment used are known in the art, and the testing methods used are conventional methods in the art.

[0044] Example 1 This embodiment provides a polyanion composite cathode material and its preparation method, specifically including the following steps: Sodium sulfate and ferrous sulfate heptahydrate in a molar ratio of 1.5:1 were weighed and dissolved in deionized water, resulting in a total concentration of 2.5 mol / L. Citric acid in a molar ratio of 1:10 to ferrous sulfate was then added and stirred. CNTs were then added and uniformly dispersed to obtain a mixture containing 2% CNTs. The mixture was then spray-dried at an inlet temperature of 250°C and an outlet temperature of 120°C to obtain a sodium ferric sulfate precursor.

[0045] A mixture of sodium ferric sulfate precursor, Na4Fe3(PO4)2P2O7, and Al2O3 was mechanically fused in a high-speed mixer to form a composite material. The mass of Na4Fe3(PO4)2P2O7 was 1 wt% of the sodium ferric sulfate precursor, with a median particle size (D50) of 1 μm. The mass of Al2O3 was 5 wt% of the sodium ferric sulfate precursor, with a particle size of 20 nm. The mechanical fusion process parameters were: a high-speed mixer linear velocity of 25 m / s and a fusion time of 15 min. The mechanically fused composite material was sintered at 350 °C for 16 h in a nitrogen atmosphere to obtain a polyanion composite cathode material.

[0046] Figure 1 This is a schematic diagram of the structure of the polyanion composite cathode material prepared in this embodiment. The polyanion composite cathode material consists of spherical particles with a median particle size of approximately 21 μm. Its core is composed of sodium ferric sulfate particles with a median particle size of approximately 20 μm. A first coating layer and a second coating layer are sequentially formed on the surface of the sodium ferric sulfate particles. The first coating layer comprises Na4Fe3(PO4)2P2O7 and a portion of Al2O3, with the Al2O3 bound to the surface of the sodium ferric sulfate particles; the thickness of the first coating layer is approximately 1 μm. The second coating layer comprises Al2O3, with the Al2O3 coating Na4Fe3(PO4)2P2O7 to form the second coating layer; the thickness of the second coating layer is approximately 20 nm. The mass of the Al2O3 forming the second coating layer is approximately 5% of the total mass of Al2O3.

[0047] Figure 2 This is the XRD pattern of the polyanion composite cathode material prepared in this embodiment.

[0048] Example 2 This embodiment provides a polyanion composite cathode material and its preparation method, specifically including the following steps: Sodium sulfate and ferrous sulfate heptahydrate in a molar ratio of 1.5:1 were weighed and dissolved in deionized water to a total concentration of 3 mol / L. Oxalic acid in a molar ratio of 1:5 to ferrous sulfate was then added and stirred. Graphene was then added and uniformly dispersed to obtain a mixture containing 5% graphene. The mixture was then spray-dried at an inlet temperature of 200°C and an outlet temperature of 100°C to obtain a sodium ferric sulfate precursor.

[0049] The sodium ferric sulfate precursor was reacted with Na4Fe 2.9 (PO4)2P2O7 and Al2O3 are mechanically fused in a high-speed mixer to form a mixture, in which Na4Fe 2.9 The mass of (PO4)2P2O7 is 10 wt% of the sodium ferric sulfate precursor, and Na4Fe 2.9The median particle size (D50) of (PO4)2P2O7 is 2 μm, and the mass of Al2O3 is 0.1 wt% of the sodium ferric sulfate precursor, with a particle size of 20 nm. The mechanical fusion process parameters are: linear velocity 40 m / s, time 10 min. The mechanically fused mixture is sintered at 400 °C for 12 h in an Ar atmosphere to obtain the polyanion composite cathode material.

[0050] The prepared polyanion composite cathode material consists of spherical particles with an overall particle size of 25 μm. Its core comprises sodium ferric sulfate particles with a particle size of approximately 23 μm. A first coating layer and a second coating layer are sequentially formed on the surface of the sodium ferric sulfate particles. The first coating layer includes Na₄Fe₂O₃. 2.9 The first coating layer consists of (PO4)2P2O7 and a portion of Al2O3, which is bound to the surface of the sodium ferric sulfate particles; the thickness of the first coating layer is approximately 2 μm. The second coating layer comprises Al2O3, which is coated onto Na4Fe. 2.9 A second coating layer is formed by (PO4)2P2O7, with a thickness of approximately 20 nm. The mass of Al2O3 forming the second coating layer is approximately 95% of the total mass of Al2O3.

[0051] Example 3 This embodiment provides a polyanion composite cathode material and its preparation method, specifically including the following steps: Sodium sulfate and ferrous sulfate heptahydrate in a molar ratio of 1.5:1 were weighed and dissolved in deionized water to a total concentration of 3 mol / L. Oxalic acid in a molar ratio of 1:10 to ferrous sulfate was then added and stirred. Conductive carbon black was then added and uniformly dispersed to obtain a mixture containing 5% conductive carbon black. The mixture was then spray-dried at an inlet temperature of 250°C and an outlet temperature of 110°C to obtain the sodium ferric sulfate precursor.

[0052] The sodium ferric sulfate precursor was reacted with Na4Fe 2.9 (PO4)2P2O7 and AlF3 are mechanically fused in a high-speed mixer to form a mixture, in which Na4Fe 2.9 The mass of (PO4)2P2O7 is 5 wt% of the sodium ferric sulfate precursor, and Na4Fe 2.9 The median particle size (D50) of (PO4)2P2O7 was 1.5 μm, and the mass of AlF3 was 5 wt% of the sodium ferric sulfate precursor, with a particle size of 50 nm. The mechanical fusion process parameters were a linear velocity of 30 m / s and a time of 20 min. The mechanically fused mixture was sintered at 400 °C for 10 h in an Ar atmosphere to obtain a polyanion composite cathode material.

[0053] The prepared polyanion composite cathode material consists of spherical particles with an overall particle size of approximately 15 μm. Its core comprises sodium ferric sulfate particles with a particle size of approximately 13.5 μm. A first coating layer and a second coating layer are sequentially formed on the surface of the sodium ferric sulfate particles. The first coating layer includes Na₄Fe₂O₃. 2.9 (PO4)2P2O7 and AlF3 are present, with the AlF3 portion bound to the surface of the sodium ferric sulfate particles. The thickness of the first coating layer is approximately 1.5 μm. The second coating layer consists of AlF3, which is coated onto Na4Fe. 2.9 The second coating layer is formed by (PO4)2P2O7, and the thickness of the second coating layer is approximately 50 nm. The mass of AlF3 forming the second coating layer is approximately 40% of the total mass of AlF3.

[0054] Example 4 This embodiment provides a polyanion composite cathode material and its preparation method, specifically including the following steps: Sodium sulfate and ferrous sulfate heptahydrate in a molar ratio of 1.5:1 were weighed and dissolved in deionized water, resulting in a total concentration of 2.5 mol / L. Citric acid in a molar ratio of 1:10 to ferrous sulfate was then added and stirred. CNTs were then added and uniformly dispersed to obtain a mixture containing 3% CNTs. The mixture was then spray-dried at an inlet air temperature of 150°C and an outlet air temperature of 80°C to obtain a sodium ferric sulfate precursor.

[0055] A mixture of sodium ferric sulfate precursor, Na4Fe3(PO4)2P2O7, and copper oxide was mechanically fused in a high-speed mixer to form a composite material. The mass of Na4Fe3(PO4)2P2O7 was 0.5 wt% of the sodium ferric sulfate precursor, with a median particle size (D50) of 1 μm. The mass of copper oxide was 10 wt% of the sodium ferric sulfate precursor, with a particle size of 50 nm. The mechanical fusion process parameters were 25 m / s and 20 min. The mechanically fused mixture was sintered at 300 °C for 24 h in a nitrogen atmosphere to obtain a polyanion composite cathode material.

[0056] Example 5 Example 5 is basically the same as Example 1, except that in Example 6, sintering is carried out in a hydrogen-argon mixed gas with a volume ratio of hydrogen to argon of 5:95. The rest is the same as in Example 1, and will not be repeated here.

[0057] Comparative Example 1 Sodium sulfate and ferrous sulfate heptahydrate in a molar ratio of 1.5:1 were weighed and dissolved in deionized water, resulting in a total concentration of 2.5 mol / L. Citric acid in a molar ratio of 1:10 to ferrous sulfate was then added and stirred. CNTs were then added and uniformly dispersed to obtain a mixture containing 2% CNTs. The mixture was then spray-dried at an inlet temperature of 250°C and an outlet temperature of 120°C to obtain a sodium ferric sulfate precursor.

[0058] In a N2 atmosphere, the sodium ferric sulfate precursor prepared above was sintered at 350°C for 16 h to obtain sodium ferric sulfate cathode material.

[0059] Comparative Example 2 Sodium sulfate and ferrous sulfate heptahydrate in a molar ratio of 1.5:1 were weighed and dissolved in deionized water, resulting in a total concentration of 2.5 mol / L. Citric acid in a molar ratio of 1:10 to ferrous sulfate was then added and stirred. CNTs were then added and uniformly dispersed to obtain a mixture containing 2% CNTs. The mixture was then spray-dried at an inlet temperature of 250°C and an outlet temperature of 120°C to obtain a sodium ferric sulfate precursor.

[0060] A mixture of sodium ferric sulfate precursor and Na4Fe3(PO4)2P2O7 was mechanically fused in a high-speed mixer to form a composite material. The mass of Na4Fe3(PO4)2P2O7 was 1 wt% of the sodium ferric sulfate precursor, and the median particle size D50 of Na4Fe3(PO4)2P2O7 was 1 μm. The mechanical fusion process parameters were: high-speed mixer linear velocity of 25 m / s and time of 15 min. The mechanically fused mixture was then sintered in a nitrogen atmosphere at 350 °C for 16 h to obtain the composite cathode material.

[0061] Comparative Example 3 The only difference between Comparative Example 3 and Example 1 is that the median particle size D50 of Na4Fe3(PO4)2P2O7 used in Comparative Example 3 is 10 μm. The other preparation steps are the same as those in Example 1 and will not be repeated here.

[0062] Comparing Example 1 and Comparative Example 3, it was found that because the particle size of Na4Fe3(PO4)2P2O7 in Comparative Example 3 was too large, the prepared composite cathode material formed a mixture of two polyanions. The surfaces of these two polyanion particles were coated with Al2O3, which could not form the gradient coating structure in this invention.

[0063] Comparative Example 4 The only difference between Comparative Example 4 and Example 1 is that Comparative Example 4 does not use a high-speed mixer for mechanical fusion; instead, the materials are simply mixed uniformly. The rest of the process is the same as in Example 1 and will not be repeated here.

[0064] Compared to Example 1, in the composite material prepared in Comparative Example 4, sodium ferric sulfate and sodium ferric pyrophosphate exist in a mixture. Sodium ferric pyrophosphate cannot form an effective coating structure on the surface of sodium ferric sulfate particles, and the coating effect of the nano-conductive compound on the particle surface is not good, resulting in poor stability in air.

[0065] The positive electrode materials obtained in the examples and comparative examples were used to fabricate coin cells, with the electrode components in the following weight ratio: positive electrode material: conductive agent (conductive carbon black): binder (PVDF) = 93:3:4; the negative electrode used a sodium sheet, and the electrolyte was 1 mol / L NaClO4 + DMC:EC:EMC (1:1:1 vol%). A glass fiber separator was used as the separator. The cells were assembled into CR2032 type coin cells in a glove box. The coin cells were tested at 25°C, and their charge-discharge performance was tested under conditions of 2~4.5V and 0.1C.

[0066] Table 1. Battery performance of cathode materials prepared based on examples and comparative examples. sample Charging specific capacity (mAh / g) Discharge specific capacity (mAh / g) Discharge specific capacity (mAh / g) after 7 days of exposure to external environment Example 1 98.3 92.5 90.1 Example 2 104.2 97.2 93.8 Example 3 101.1 94.3 91.7 Example 4 92.3 83.5 82.4 Example 5 98.6 92.3 89.6 Comparative Example 1 70.3 60.1 20.7 Comparative Example 2 95.7 86.9 59.1 Comparative Example 3 97.8 91.6 85.8 Comparative Example 4 97.6 91.2 30.3 In summary, this invention employs a mechanical fusion method to prepare a polyanionic composite cathode material. By modifying the material surface through coating and sintering processes, a coating layer containing sodium ferric sulfate pyrophosphate and nano-conductive compounds is formed on the surface of the sodium ferric sulfate particles. Simultaneously, a coating layer of nano-conductive compounds is also present on the surface of the sodium ferric sulfate pyrophosphate, thereby improving the air stability of the composite material. Furthermore, this method improves electrical conductivity, addressing the relatively low conductivity of sodium ferric sulfate. Simultaneously, the resulting composite material exhibits high specific capacity, good rate performance, and significantly improved electrical properties. The preparation method provided by this invention is simple to operate and suitable for commercial production.

[0067] All aspects, embodiments, features, and examples of this invention are to be regarded as illustrative in all respects and are not intended to limit the invention, the scope of which is defined only by the claims. Other embodiments, modifications, and uses will become apparent to those skilled in the art without departing from the spirit and scope of the invention as claimed.

[0068] In addition, the inventors of this case also conducted experiments with other raw materials, process operations, and process conditions described in this specification, referring to the aforementioned embodiments, and obtained relatively ideal results in all cases.

[0069] Although the invention has been described with reference to illustrative embodiments, those skilled in the art will understand that various other changes, omissions, and / or additions can be made without departing from the spirit and scope of the invention, and that elements of the embodiments can be substituted with substantially equivalents. Furthermore, many modifications can be made without departing from the scope of the invention to adapt particular situations or materials to the teachings of the invention. Therefore, this invention is not intended to be limited to the specific embodiments disclosed for carrying out the invention, but rather is intended to encompass all embodiments falling within the scope of the appended claims. Moreover, unless specifically stated otherwise, any use of the terms first, second, etc., does not indicate any order or importance, but is used to distinguish one element from another.

Claims

1. A polyanion composite cathode material for sodium-ion batteries, characterized in that, include: The core and the coating structure encapsulating the core, the core comprising sodium ferric sulfate particles, and the coating structure comprising sodium ferric phosphate pyrophosphate and a nano-conductive compound; The particle size of the sodium ferric sulfate particles is larger than that of the sodium ferric pyrophosphate, and the particle size of the sodium ferric pyrophosphate is larger than that of the nano-conductive compound. The coating structure includes a first coating layer and a second coating layer arranged sequentially in a direction away from the core. The first coating layer includes sodium iron phosphate pyrophosphate and a first portion of a nano-conductive compound, the first portion of which is bonded to the surface of the sodium iron sulfate particles. The second coating layer includes a second portion of a nano-conductive compound, the second portion of which is coated on the surface of the sodium iron phosphate pyrophosphate in the first coating layer.

2. The polyanion composite cathode material according to claim 1, characterized in that: The median particle size of the sodium ferric sulfate particles is 15~30μm, the median particle size of the sodium ferric pyrophosphate is less than 10μm, preferably less than 2μm, and the particle size of the nano-conductive compound is 1~100 nm. And / or, the polyanion composite cathode material is a spherical particle with a median particle size of 15~40μm; And / or, the ratio of the thickness of the first coating layer to the thickness of the second coating layer is 200:1 to 20:1; And / or, the chemical formula of the polyanion composite cathode material is: Na x Fe(SO4) y @Na a Fe b (PO4) c P2O7 / M d O e , where 1≤x≤3, y=(x+2) / 2, 3≤a≤4.5, 2≤b≤3, 1≤c≤2, 1≤d≤2, 1≤e≤3; And / or, the mass of the sodium ferric pyrophosphate is 0.1-10 wt% of the sodium ferric sulfate particles, and the total mass of the nano-conductive compound is 0.1%-10 wt% of the sodium ferric sulfate particles; And / or, the nano-conductive compound includes one or more of aluminum oxide, aluminum fluoride, copper oxide, titanium dioxide, or silver oxide; And / or, the second part of the nano-conductive compound accounts for 2 to 95% of the total mass of the nano-conductive compound; And / or, the sodium ferric sulfate particles contain 0.1 to 10 wt% of conductive material, the conductive material including carbon material, preferably, the sodium ferric sulfate particles contain 2 to 5 wt% of conductive material.

3. A method for preparing a polyanion composite cathode material for sodium-ion batteries, characterized in that, include: The sodium ferric sulfate precursor material, powdered sodium ferric pyrophosphate, and nano-conductive compound were mechanically fused to obtain a mixture. Under a protective atmosphere, the mixture is sintered at high temperature to obtain a polyanion composite cathode material.

4. The preparation method according to claim 3, characterized in that, The mechanical fusion is carried out in a high-speed mixer with a linear velocity of 10 m / s to 40 m / s, and the mechanical fusion time is 10 to 30 minutes. And / or, sinter the mixture at 300~400℃ for 5~24h to obtain the polyanion composite cathode material.

5. The preparation method according to claim 3, characterized in that: The protective atmosphere includes one or a combination of two of argon and nitrogen, or the protective atmosphere includes a hydrogen-argon mixture or a nitrogen-hydrogen mixture. Preferably, the volume percentage of hydrogen in the argon-hydrogen mixture is less than 5%; the volume percentage of hydrogen in the nitrogen-hydrogen mixture is less than 5%.

6. The preparation method according to claim 3, characterized in that: The mass of sodium ferric pyrophosphate in the mixture is 0.1-10 wt% of the sodium ferric sulfate precursor material, and the mass of the nano-conductive compound is 0.1-10 wt% of the sodium ferric sulfate precursor material. And / or, the median particle size of the sodium iron pyrophosphate is less than 10 μm, preferably less than 2 μm; And / or, the particle size of the nano-conductive compound is 1~100 nm.

7. The preparation method according to claim 3, characterized in that, The preparation method of the sodium ferric sulfate precursor material includes: dissolving sodium sulfate and ferrous sulfate in a solvent, and uniformly dispersing an antioxidant and a conductive material in the solvent to obtain a mixture; spray drying the mixture to obtain the sodium ferric sulfate precursor material. Preferably, the molar ratio of sodium sulfate to ferrous sulfate is 1:1 to 3:1; Preferably, the antioxidant includes one or a combination of more than one of ascorbic acid, citric acid, oxalic acid or sodium sulfite; Preferably, the molar ratio of the antioxidant to ferrous sulfate is 2:10~100; Preferably, the conductive material includes one or a combination of carbon nanotubes, carbon fibers, reduced graphene oxide, graphene, and conductive carbon black. Preferably, the amount of the conductive material is 0.1~10wt% of the mixture, more preferably 2~5wt%; Preferably, the inlet air temperature of the spray dryer is 150~250℃ and the outlet air temperature is 80~120℃.

8. A polyanion composite cathode material for sodium-ion batteries, characterized in that: It is prepared by the method described in any one of claims 3 to 7.

9. A positive electrode for a sodium-ion battery, characterized in that: The positive electrode includes a current collector and a positive electrode active material layer formed on the current collector, wherein the positive electrode active material layer includes the polyanion composite positive electrode material for sodium-ion batteries as described in any one of claims 1, 2, and 8.

10. A sodium-ion battery, comprising a positive electrode, a negative electrode, and an electrolyte, characterized in that: The positive electrode is the positive electrode for a sodium-ion battery according to claim 9.

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