Carbon-coated polyanion-doped sulfate positive electrode material as well as preparation method and application thereof

Abstract

The invention discloses a carbon-coated polyanion-doped sulfate positive electrode material as well as a preparation method and application thereof. Relates to the technical field of sodium ion battery cathode materials. Aiming at the problems of poor sodium storage capacity, cycle performance and water resistance of current sodium ferric sulfate under high magnification, the preparation method comprises the following steps: dispersing a metal salt, a doping source, sodium sulfate, ascorbic acid and carbon nanotubes in water according to a certain proportion, uniformly mixing, and carrying out spray drying to obtain precursor particles; and finally, sintering for several hours at 200 DEG C and 400 DEG C in an inert atmosphere to obtain the carbon-coated polyanion-doped sulfate positive electrode material. The positive electrode material provided by the invention has the advantages of simple and convenient preparation method, low cost, good air stability and rate capability, long cycle life and the like, and has relatively strong market competitiveness in the field of large-scale energy storage application.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of positive electrode materials of sodium ion batteries, in particular to a carbon-coated polyanion-doped sulfate positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] Sodium ion batteries are gradually becoming an ideal choice in the fields of large-scale energy storage and new energy vehicles due to rich raw materials, low cost and excellent safety. As a key component, the positive electrode material directly determines the energy density, rate performance and cycle life of the battery. At present, the positive electrode of the sodium ion battery mainly includes three systems of transition metal layered oxides, prussian blue analogues and polyanion compounds: the layered oxides have a relatively high capacity, but are prone to irreversible phase transition in long cycle, which affects the stability due to the relatively high cost; the prussian blue analogues have low cost and controllable structure, but the crystal water is difficult to completely remove, and the preparation process is limited; compared with the above two, sodium iron sulfate (Na2Fe2(SO2)3) in the polyanion compound has a high working potential of about 3.8V, a theoretical capacity of 120mAh g -1 and stable cycle characteristics. However, the intrinsic electronic conductivity of sodium iron sulfate is extremely poor, which limits the efficiency in high-rate charging and discharging; at the same time, the strong hygroscopicity of sodium iron sulfate can quickly form a hydrated side phase in air or electrolyte, which damages the crystal structure and interface stability; in addition, SO4 2- is easy to decompose to produce SO2 in the calcination process above 450 DEG C, which also puts higher requirements on the process window and safety of the material. Therefore, how to improve the electrical conductivity and rate performance through carbon coating, conductive additives or multi-anion doping interface and structure engineering, and on the other hand, inhibit the hygroscopicity and side reaction, has become the core issue to realize the large-scale application of sodium iron sulfate in sodium ion batteries. SUMMARY

[0003] In order to solve at least one of the above technical problems, a long-life and high-efficiency positive electrode material of a sodium ion battery is developed, and the application provides a carbon-coated polyanion-doped sulfate positive electrode material and a preparation method and application thereof.

[0004] On one hand, the application provides a carbon-coated polyanion-doped sulfate positive electrode material, which adopts carbon-composite polyanion-doped sulfate, and the chemical formula of the polyanion-doped sulfate is: Na 6-a(2-y)+(b-2)x M 2-y a (SO4) 3-x N x b ;

[0005] Where M is a metal ion and N is a polyanion; M is composed of active ions that can undergo redox reactions and inert ions that cannot undergo redox reactions. The active ions contribute capacity as the charge compensation host, and the inert ions serve as the support to stabilize the structure. The molar ratio of active ions to inert ions ranges from 1:1 to (2-y).

[0006] Where a is the average valence state of M, 2≤|a|≤3; b is the average valence state of N, 2≤|b|<5;

[0007] Where x is the polyanion doping substitution number, 0 < x ≤ 1; y is the number of metal ions, 0 ≤ y ≤ 0.5.

[0008] Optionally, the carbon content in the composite is 5 wt% to 8 wt%.

[0009] Optionally, the active ion is selected from Fe. 2+ Co 2+ Ni 2+ Mn 2+ Cr 3+ V 3+ One or more;

[0010] The inert ions are selected from Mg 2+ Cu 2+ Zn 2+ Al 3+ ,Sc 3+ Mn 2+ Sb 3+ Mo 3+ Nd 3+ Ce 4+ W 4+ Sn 4+ Zr 4+ Bi 5+ One or more of the following;

[0011] The polyanion is selected from CO3. 2- SiO4 4- WO4 2- VO4 3- Cr2O7 2- MoO4 2- V2O7 4- BO3 3- One or more of them.

[0012] Secondly, this application provides a method for preparing the above-mentioned cathode material, including the following steps:

[0013] S1. Add M source, sodium source, sulfur source, carbon source and N source to deionized water, and heat and stir to completely dissolve the raw materials to obtain solution A;

[0014] S2. Use spray drying to convert solution A into precursor powder B;

[0015] S3. The precursor powder B is calcined under an inert atmosphere, and the resulting product is crushed and ground to obtain carbon-coated polyanion-doped sodium iron sulfate cathode material.

[0016] Optionally, the molar ratio of the M source, sodium source, sulfur source, carbon source, and N source is (1.0-2.5):(1.0-2.5):(1.0-2.5):(0.1-1.0):(0.01-1).

[0017] Optionally, the carbon source is a mixture of carbon source I and carbon source II, wherein carbon source I is carbon nanotubes and carbon source II is one or both of citric acid and ascorbic acid;

[0018] The sodium source is any one or more of anhydrous sodium sulfate, sodium oxalate, sodium bicarbonate, anhydrous sodium acetate, sodium citrate, sodium sulfide, sodium salicylate, and sodium hydroxide.

[0019] The sulfur source is any one or more of ferrous sulfate heptahydrate, anhydrous sodium sulfate, potassium sulfate, and ammonium sulfate.

[0020] Optionally, in step S1, the heating and stirring method involves adding the raw materials sequentially to ionized water at 60°C and stirring for 80–130 minutes, followed by ultrasonic dispersion for 20–60 minutes to obtain solution A.

[0021] Optionally, in step S2, the inlet temperature of the spray dryer is 200–280°C, the outlet temperature is 100–120°C, and the feed rate is 20–25%.

[0022] Optionally, in step S3, the inert gas protective atmosphere for calcination is one of argon, an argon-hydrogen mixture, and nitrogen; the calcination conditions are: first, raise the temperature to 200-250°C at a heating rate of 2-5°C / min, hold for 2-4 hours, then raise the temperature to 350-400°C at a heating rate of 2-5°C / min, and hold for 8-12 hours.

[0023] Thirdly, this application provides the application of the above-mentioned cathode material in the field of sodium-ion batteries.

[0024] In summary, the present invention has at least one of the following beneficial technical effects:

[0025] (1) By utilizing the local electronic structure changes induced by polyanions, some O2- in SO42- groups are delocalized and migrated, optimizing the electron distribution, thereby effectively reducing electrochemical polarization and improving the specific capacity and rate performance of the material.

[0026] (2) Introducing polyanion doping increases the O–Na–O interlayer spacing and widens the sodium ion diffusion channels, thereby reducing Na2+ diffusion. + The insertion / extraction barrier and diffusion resistance enhance the migration rate of sodium ions within the material, thereby improving cycling performance at high rates.

[0027] (3) Inert atoms enhance the rigidity of the crystal framework, alleviate stress accumulation during charging and discharging, and can effectively prevent structural collapse, thereby improving the structural stability and cycle life of the material.

[0028] (4) Carbon coating can form a continuous conductive path on the particle surface, thereby significantly improving the electronic conductivity of the cathode material, accelerating the charge transfer rate, and improving the overall reaction kinetics.

[0029] (5) The carbon coating on the surface is hydrophobic, which can isolate the material from direct contact with moisture in the air; at the same time, polyanion doping can reduce the adsorption tendency of surface active sites for water molecules, significantly improve the stability of the material in a humid environment, and effectively inhibit the formation of hydrated impurities.

[0030] (6) By using spray drying combined with calcination process, the synergistic preparation of polyanion doping and carbon coating is achieved. The process is efficient and fast, suitable for industrial production, and the resulting particles have uniform distribution and controllable morphology. Attached Figure Description

[0031] Figure 1 The rate performance diagrams show the sodium-ion batteries prepared using the cathode materials of Examples 1-2 and Comparative Examples 1-2.

[0032] Figure 2 Here is a SEM image of the cathode material from Example 1;

[0033] Figure 3 Here is a SEM image of the cathode material in Comparative Example 1;

[0034] Figure 4 The image shows the XRD pattern of the cathode material in Example 1.

[0035] Figure 5 The first charge-discharge curve of the sodium-ion battery prepared using the cathode material of Example 1 at 0.1C is shown.

[0036] Figure 6 The graph shows the 5C cycle performance of the cathode materials prepared in Example 1 and Comparative Example 2. Detailed Implementation

[0037] The present application will be further described in detail below with reference to the accompanying drawings and embodiments.

[0038] This application designs a method for preparing carbon-coated polyanion-doped sulfate cathode materials and their applications. In sulfate cathode materials, the introduction of polyanion doping and surface carbon coating can achieve synergistic effects, improving both internal and external performance. Polyanion doping involves replacing some SO42- with larger, more strongly bonded multi-component anions (such as P2O7). 4- MoO4 2- or VO4 3- At the atomic level, a more rigid covalent framework is formed, which not only suppresses the collapse of the local framework during charging and discharging, thus buffering electrochemical stress, but also "expands" Na + The diffusion channels and reduced insertion / extraction barriers significantly improve ion diffusion kinetics and cycle efficiency. Simultaneously, carbon coating employs a dual-carbon source spray drying and calcination process using carbon nanotubes and ascorbic acid to construct a highly conductive network on the particle surface, enhancing electron transport and forming a protective film at the electrode / electrolyte interface. This effectively suppresses side reactions, moisture erosion, and capacity decay. The synergistic effect of these two processes enhances both the intrinsic conductivity and environmental stability of sodium iron sulfate while ensuring capacity retention under high rate and long cycle conditions, providing strong support for the long-life and high-efficiency application of sodium-ion battery cathode materials. Some examples are listed below. Specific Implementation

[0040] Example 1

[0041] Carbon-coated polyanion-doped Na 2.4 Fe 1.9 Sn 0.1 (SO4) 2.8 (P2O7) 0.2 Preparation of cathode materials:

[0042] Step 1: Add ferrous sulfate heptahydrate, tin sulfate, anhydrous sodium sulfate, sodium pyrophosphate, carbon nanotubes, and ascorbic acid sequentially to 60 mL of deionized water at 60 °C in a molar ratio of 2.2:0.2:1:0.25:0.1:0.05. Stir for 2 hours to completely dissolve the raw materials and obtain a uniform, light green, clear solution A.

[0043] Step 2: Use spray drying to convert the uniform solution A into precursor powder B. The specific parameters are: air inlet rate 80%, air inlet temperature 280℃, air outlet temperature 90℃, and feed rate 25% (the maximum acceptable feed rate of the equipment is 100% of the maximum pump speed).

[0044] Step 3: The precursor powder B was transferred to a tube furnace for calcination. The calcination conditions were as follows: under an argon atmosphere, the temperature was first increased to 250°C at a rate of 5°C / min and held for 2 hours, then increased to 400°C at a rate of 5°C / min and held for 10 hours. The calcined product was then pulverized, ground, and sieved to obtain carbon-coated polyanion-doped Na. 2.4 Fe 1.9 Sn 0.1 (SO4) 2.8 (P2O7) 0.2 Positive electrode material.

[0045] Example 2

[0046] Carbon-coated polyanion-doped Na 2.6 Fe 1.8 Ce 0.1 (SO4) 2.7 (P2O7) 0.3 Preparation of cathode materials:

[0047] Step 1: Add ferrous sulfate heptahydrate, cerium sulfate, anhydrous sodium sulfate, sodium pyrophosphate, carbon nanotubes, and ascorbic acid sequentially to 60 mL of deionized water at 60 °C in a molar ratio of 1.7:0.1:1:0.3:0.1:0.05. Stir for 2 hours to completely dissolve the raw materials and obtain a uniform, light green, clear solution A.

[0048] Step 2: Use spray drying to convert the uniform solution A into precursor powder B. The specific parameters are: air inlet rate 80%, air inlet temperature 280℃, air outlet temperature 90℃, and feed rate 25% (the maximum acceptable feed rate of the equipment is 100% of the maximum pump speed).

[0049] Step 3: The precursor powder B was transferred to a tube furnace for calcination. The calcination conditions were as follows: under an argon atmosphere, the temperature was first increased to 250°C at a rate of 5°C / min and held for 2 hours, then increased to 400°C at a rate of 5°C / min and held for 10 hours. The calcined product was then pulverized, ground, and sieved to obtain carbon-coated polyanion-doped Na. 2.6 Fe 1.8 Ce 0.1 (SO4) 2.7 (P2O7) 0.3 Positive electrode material.

[0050] Example 3

[0051] Carbon-coated polyanion-doped Na 2.6 Fe 1.9 Sn 0.1 (SO4) 2.7 (MoO4) 0.3 Preparation of cathode materials:

[0052] Step 1: Add ferrous sulfate heptahydrate, tin sulfate, anhydrous sodium sulfate, sodium molybdate, carbon nanotubes, and ascorbic acid sequentially to 60 mL of deionized water at 60 °C in a molar ratio of 1.6:0.15:1:0.3:0.1:0.05. Stir for 2 hours to completely dissolve the raw materials and obtain a uniform, light green, clear solution A.

[0053] Step 2: Use spray drying to convert the uniform solution A into precursor powder B. The specific parameters are: air inlet rate 80%, air inlet temperature 280℃, air outlet temperature 90℃, and feed rate 25% (the maximum acceptable feed rate of the equipment is 100% of the maximum pump speed).

[0054] Step 3: The precursor powder B was transferred to a tube furnace for calcination. The calcination conditions were as follows: under an argon atmosphere, the temperature was first increased to 250°C at a rate of 5°C / min and held for 2 hours, then increased to 400°C at a rate of 5°C / min and held for 10 hours. The calcined product was then pulverized, ground, and sieved to obtain carbon-coated polyanion-doped Na. 2.6 Fe 1.9 Sn 0.1 (SO4) 2.7 (MoO4) 0.3 Positive electrode material.

[0055] Example 4

[0056] This embodiment differs from Embodiment 1 in that the amounts of carbon nanotubes and ascorbic acid added in Embodiment 1 are changed.

[0057] Carbon-coated polyanion-doped Na 2.4 Fe 1.9 Sn 0.1 (SO4) 2.8 (P2O7) 0.2 Preparation of cathode materials:

[0058] Step 1: Add ferrous sulfate heptahydrate, anhydrous sodium sulfate, sodium pyrophosphate, carbon nanotubes, and ascorbic acid sequentially to 60 mL of deionized water at 60 °C in a molar ratio of 2.2:0.2:1:0.25:0.15:0.075. Stir for 2 hours to completely dissolve the raw materials and obtain a uniform, light green, clear solution A.

[0059] Step 2: Use spray drying to convert the uniform solution A into precursor powder B. The specific parameters are: air inlet rate 80%, air inlet temperature 280℃, air outlet temperature 90℃, and feed rate 25% (the maximum acceptable feed rate of the equipment is 100% of the maximum pump speed).

[0060] Step 3: The precursor powder B was transferred to a tube furnace for calcination. The calcination conditions were as follows: under an argon atmosphere, the temperature was first increased to 250°C at a rate of 5°C / min and held for 2 hours, then increased to 400°C at a rate of 5°C / min and held for 10 hours. The calcined product was then pulverized, ground, and sieved to obtain carbon-coated polyanion-doped Na. 2.4 Fe 1.9 Sn 0.1 (SO4) 2.8 (P2O7) 0.2 Positive electrode material.

[0061] Example 5

[0062] In this embodiment, ascorbic acid is replaced with citric acid, which is different from that in Example 1.

[0063] Carbon-coated polyanion-doped Na 2.4 Fe 1.9 Sn 0.1 (SO4) 2.8 (P2O7) 0.2 Preparation of cathode materials:

[0064] Step 1: Add ferrous sulfate heptahydrate, tin sulfate, anhydrous sodium sulfate, sodium pyrophosphate, carbon nanotubes, and citric acid sequentially to 60 mL of deionized water at 60 °C in a molar ratio of 2.2:0.2:1:0.25:0.1:0.05. Stir for 2 hours to completely dissolve the raw materials and obtain a uniform, light green, clear solution A.

[0065] Step 2: Use spray drying to convert the uniform solution A into precursor powder B. The specific parameters are: air inlet rate 80%, air inlet temperature 280℃, air outlet temperature 90℃, and feed rate 25% (the maximum acceptable feed rate of the equipment is 100% of the maximum pump speed).

[0066] Step 3: The precursor powder B was transferred to a tube furnace for calcination. The calcination conditions were as follows: under an argon atmosphere, the temperature was first increased to 250°C at a rate of 5°C / min and held for 2 hours, then increased to 400°C at a rate of 5°C / min and held for 10 hours. The calcined product was then pulverized, ground, and sieved to obtain carbon-coated polyanion-doped Na. 2.4 Fe 1.9 Sn 0.1 (SO4) 2.8 (P2O7) 0.2 Positive electrode material.

[0067] Example 6

[0068] Carbon-coated polyanion-doped Na 2.5 Fe 1.8 Mn 0.1(SO4) 2.7 (Cr2O7) 0.3 Preparation of cathode materials:

[0069] Step 1: Add ferrous sulfate heptahydrate, manganese sulfate, anhydrous sodium sulfate, sodium dichromate, carbon nanotubes, and ascorbic acid sequentially to 60 mL of deionized water at 60 °C in a molar ratio of 1.8:0.1:1:0.3:0.1:0.05. Stir for 2 hours to allow the raw materials to fully dissolve and form a light orange clear solution A.

[0070] Step 2: Use spray drying to convert the uniform solution A into precursor powder B. The specific parameters are: air inlet rate 80%, air inlet temperature 280℃, air outlet temperature 90℃, and feed rate 25% (the maximum acceptable feed rate of the equipment is 100% of the maximum pump speed).

[0071] Step 3: The precursor powder B was transferred to a tube furnace for calcination. The calcination conditions were as follows: under an argon atmosphere, the temperature was first increased to 250°C at a rate of 5°C / min and held for 2 hours, then increased to 400°C at a rate of 5°C / min and held for 10 hours. The calcined product was then pulverized, ground, and sieved to obtain carbon-coated polyanion-doped Na. 2.5 Fe 1.8 Mn 0.1 (SO4) 2.7 (Cr2O7) 0.3 Positive electrode material.

[0072] Example 7

[0073] Carbon-coated polyanion-doped Na 2.3 Fe 1.8 V 0.2 (SO4) 2.8 (P2O7) 0.2 Preparation of cathode materials:

[0074] Step 1: Add ferrous sulfate heptahydrate, vanadium sulfate, anhydrous sodium sulfate, sodium pyrophosphate, carbon nanotubes, and ascorbic acid sequentially to 60 mL of deionized water at 60 °C in a molar ratio of 1.8:0.2:1:0.2:0.1:0.05. Stir for 2 hours to allow the raw materials to fully dissolve and form a uniform, transparent green solution A.

[0075] Step 2: Use spray drying to convert the uniform solution A into precursor powder B. The specific parameters are: air inlet rate 80%, air inlet temperature 280℃, air outlet temperature 90℃, and feed rate 25% (the maximum acceptable feed rate of the equipment is 100% of the maximum pump speed).

[0076] Step 3: The precursor powder B was transferred to a tube furnace for calcination. The calcination conditions were as follows: under an argon atmosphere, the temperature was first increased to 250°C at a rate of 5°C / min and held for 2 hours, then increased to 400°C at a rate of 5°C / min and held for 10 hours. The calcined product was then pulverized, ground, and sieved to obtain carbon-coated polyanion-doped Na. 2.3 Fe 1.8 V 0.2 (SO4) 2.8 (P2O7) 0.2 Positive electrode material.

[0077] Example 8

[0078] Carbon-coated polyanion-doped Na2Mn2(SO4) 2.8 (P2O7) 0.2 Preparation of cathode materials:

[0079] Step 1: Add manganese sulfate, anhydrous sodium sulfate, sodium pyrophosphate, carbon nanotubes and ascorbic acid sequentially to 60 mL of deionized water at 60 °C in a molar ratio of 2.0:1:0.2:0.1:0.05, and stir for 2 h to fully dissolve the raw materials and form a light pink clear solution A.

[0080] Step 2: Use spray drying to convert the uniform solution A into precursor powder B. The specific parameters are: air inlet rate 80%, air inlet temperature 280℃, air outlet temperature 90℃, and feed rate 25% (the maximum acceptable feed rate of the equipment is 100% of the maximum pump speed).

[0081] Step 3: The precursor powder B was transferred to a tube furnace for calcination. The calcination conditions were as follows: under an argon atmosphere, the temperature was first increased to 250°C at a rate of 5°C / min and held for 2 hours, then increased to 400°C at a rate of 5°C / min and held for 10 hours. The calcined product was then pulverized, ground, and sieved to obtain carbon-coated polyanion-doped Na₂Mn₂(SO₄). 2.8 (P2O7) 0.2 Positive electrode material.

[0082] Example 9

[0083] Carbon-coated polyanion-doped Na2Co 1.9 Mg 0.1 (SO4) 2.7 (MoO4) 0.3 Preparation of cathode materials:

[0084] Step 1: Add cobalt sulfate, magnesium sulfate, anhydrous sodium sulfate, sodium molybdate, carbon nanotubes and ascorbic acid sequentially to 60 mL of deionized water at 60 °C in a molar ratio of 1.9:0.1:1:0.3:0.1:0.05, and stir for 2 h to fully dissolve the raw materials and form a purple-red solution A.

[0085] Step 2: Use spray drying to convert the uniform solution A into precursor powder B. The specific parameters are: air inlet rate 80%, air inlet temperature 280℃, air outlet temperature 90℃, and feed rate 25% (the maximum acceptable feed rate of the equipment is 100% of the maximum pump speed).

[0086] Step 3: The precursor powder B was transferred to a tube furnace for calcination. The calcination conditions were as follows: under an argon atmosphere, the temperature was first increased to 250°C at a rate of 5°C / min and held for 2 hours, then increased to 400°C at a rate of 5°C / min and held for 10 hours. The calcined product was then pulverized, ground, and sieved to obtain carbon-coated polyanion-doped Na₂Co. 1.9 Mg 0.1 (SO4) 2.7 (MoO4) 0.3 Positive electrode material.

[0087] Example 10

[0088] Carbon-coated polyanion-doped Na2Ni 1.8 Sn 0.2 (SO4) 2.8 (P2O7) 0.2 Preparation of cathode materials:

[0089] Step 1: Add nickel sulfate, tin sulfate, anhydrous sodium sulfate, sodium pyrophosphate, carbon nanotubes and ascorbic acid sequentially to 60 mL of deionized water at 60 °C in a molar ratio of 1.8:0.2:1:0.2:0.1:0.05, and stir for 2 h to fully dissolve the raw materials and form green solution A.

[0090] Step 2: Use spray drying to convert the uniform solution A into precursor powder B. The specific parameters are: air inlet rate 80%, air inlet temperature 280℃, air outlet temperature 90℃, and feed rate 25% (the maximum acceptable feed rate of the equipment is 100% of the maximum pump speed).

[0091] Step 3: The precursor powder B was transferred to a tube furnace for calcination. The calcination conditions were as follows: under an argon atmosphere, the temperature was first increased to 250°C at a rate of 5°C / min and held for 2 hours, then increased to 400°C at a rate of 5°C / min and held for 10 hours. The calcined product was then pulverized, ground, and sieved to obtain carbon-coated polyanion-doped Na₂Ni. 1.8 Sn 0.2 (SO4)2.8 (P2O7) 0.2 Positive electrode material.

[0092] Comparative Example 1

[0093] This comparative example was not doped. Preparation of carbon-coated Na₂Fe₂(SO₄)₃ cathode material:

[0094] Step 1: Add ferrous sulfate heptahydrate, anhydrous sodium sulfate, carbon nanotubes, and ascorbic acid sequentially to 60 mL of deionized water at 60 °C in a molar ratio of 2:1:0.1:0.05. Stir for 2 hours to completely dissolve the raw materials and obtain a uniform, light green, clear solution A.

[0095] Step 2: Use spray drying to convert the uniform solution A into precursor powder B. The specific parameters are: air inlet rate 80%, air inlet temperature 280℃, air outlet temperature 90℃, and feed rate 25% (the maximum acceptable feed rate of the equipment is 100% of the maximum pump speed).

[0096] Step 3: The precursor powder B was transferred to a tube furnace for calcination. The calcination conditions were as follows: under an argon atmosphere, the temperature was first increased to 250°C at a rate of 5°C / min and held for 2 hours, then increased to 400°C at a rate of 5°C / min and held for 10 hours. The calcined product was then pulverized, ground, and sieved to obtain carbon-coated Na2Fe2(SO4)3 cathode material.

[0097] Comparative Example 2

[0098] This comparative example was not doped or carbon-coated.

[0099] Preparation of Na2Fe2(SO4)3 cathode material:

[0100] Step 1: Add ferrous sulfate heptahydrate and anhydrous sodium sulfate to 60 mL of deionized water at 60 °C in a molar ratio of 2:1. Stir for 2 hours to completely dissolve the raw materials and obtain a uniform, light green, clear solution A.

[0101] Step 2: Use spray drying to convert the uniform solution A into precursor powder B. The specific parameters are: air inlet rate 80%, air inlet temperature 280℃, air outlet temperature 90℃, and feed rate 25% (the maximum acceptable feed rate of the equipment is 100% of the maximum pump speed).

[0102] Step 3: The precursor powder B was transferred to a tube furnace for calcination. The calcination conditions were as follows: under an argon atmosphere, the temperature was first increased to 250°C at a heating rate of 5°C / min and held for 2 hours, then increased to 400°C at a heating rate of 5°C / min and held for 10 hours. The calcined product was then pulverized, ground, and sieved to obtain the Na2Fe2(SO4)3 cathode material.

[0103] Comparative Example 3

[0104] This comparative example adds carbon nanotubes as a carbon source to the comparative example 2 for carbon coating.

[0105] Preparation of carbon-coated Na2Fe2(SO4)3 cathode material:

[0106] Step 1: Add ferrous sulfate heptahydrate, anhydrous sodium sulfate, and carbon nanotubes sequentially to 60 mL of deionized water at 60 °C in a molar ratio of 2:1:0.1. Stir for 2 hours to completely dissolve the raw materials and obtain a uniform, light green, clear solution A.

[0107] Step 2: Use spray drying to convert the uniform solution A into precursor powder B. The specific parameters are: air inlet rate 80%, air inlet temperature 280℃, air outlet temperature 90℃, and feed rate 25% (the maximum acceptable feed rate of the equipment is 100% of the maximum pump speed).

[0108] Step 3: The precursor powder B was transferred to a tube furnace for calcination. The calcination conditions were as follows: under an argon atmosphere, the temperature was first increased to 250°C at a rate of 5°C / min and held for 2 hours, then increased to 400°C at a rate of 5°C / min and held for 10 hours. The calcined product was then pulverized, ground, and sieved to obtain carbon-coated Na2Fe2(SO4)3 cathode material.

[0109] Comparative Example 4

[0110] This comparative example adds ascorbic acid as a carbon source to the comparative example 2 for carbon coating.

[0111] Preparation of carbon-coated Na2Fe2(SO4)3 cathode material:

[0112] Step 1: Add ferrous sulfate heptahydrate, anhydrous sodium sulfate, and ascorbic acid sequentially to 60 mL of deionized water at 60 °C in a molar ratio of 2:1:0.05. Stir for 2 hours to completely dissolve the raw materials and obtain a uniform, light green, clear solution A.

[0113] Step 2: Use spray drying to convert the uniform solution A into precursor powder B. The specific parameters are: air inlet rate 80%, air inlet temperature 280℃, air outlet temperature 90℃, and feed rate 25% (the maximum acceptable feed rate of the equipment is 100% of the maximum pump speed).

[0114] Step 3: The precursor powder B was transferred to a tube furnace for calcination. The calcination conditions were as follows: under an argon atmosphere, the temperature was first increased to 250°C at a rate of 5°C / min and held for 2 hours, then increased to 400°C at a rate of 5°C / min and held for 10 hours. The calcined product was then pulverized, ground, and sieved to obtain carbon-coated Na2Fe2(SO4)3 cathode material.

[0115] Comparative Example 5

[0116] This comparative example maintains the same amount of carbon nanotubes added as in Example 1, but adds an excessive amount of ascorbic acid.

[0117] Carbon-coated polyanion-doped Na 2.4 Fe 1.9 Sn 0.1 (SO4) 2.8 (P2O7) 0.2 Preparation of cathode materials:

[0118] Step 1: Add ferrous sulfate heptahydrate, tin sulfate, anhydrous sodium sulfate, sodium pyrophosphate, carbon nanotubes, and ascorbic acid sequentially to 60 mL of deionized water at 60 °C in a molar ratio of 2.2:0.2:1:0.25:0.1:0.1. Stir for 2 hours to completely dissolve the raw materials and obtain a uniform, light green, clear solution A.

[0119] Step 2: Use spray drying to convert the uniform solution A into precursor powder B. The specific parameters are: air inlet rate 80%, air inlet temperature 280℃, air outlet temperature 90℃, and feed rate 25% (the maximum acceptable feed rate of the equipment is 100% of the maximum pump speed).

[0120] Step 3: The precursor powder B was transferred to a tube furnace for calcination. The calcination conditions were as follows: under an argon atmosphere, the temperature was first increased to 250°C at a rate of 5°C / min and held for 2 hours, then increased to 400°C at a rate of 5°C / min and held for 10 hours. The calcined product was then pulverized, ground, and sieved to obtain carbon-coated Na. 2.4 Fe 1.9 Sn 0.1 (SO4) 2.8 (P2O7) 0.2 Positive electrode material.

[0121] Performance testing

[0122] The final products of Examples 1-10 and Comparative Examples 1-5 were mixed with a conductive agent (SuperP) and a binder (polyvinylidene fluoride) in a mass ratio of 7:2:1, and then diluted with 750 μL of N-methylpyrrolidone (NMP) to prepare a uniform slurry. The slurry was evenly coated onto an aluminum current collector and vacuum dried at 110°C for 12 hours. It was then cut into 1.2 cm diameter discs using a cutting machine to serve as the positive electrode for sodium-ion batteries. A glass microfiber separator was used, and 1 mol / L of a solution of ethylene carbonate / diethyl carbonate (EC / DEC, volume ratio 1:1) was used. -1 NaClO4 and 5% fluoroethylene carbonate (FEC) were used as electrolytes.

[0123] A half-cell was assembled using a sodium sheet of the same size as the positive electrode as the counter electrode. Electrochemical tests were then performed after standing for 6 hours at a constant temperature of 25°C.

[0124] like Figure 1 The figures show the rate performance of Examples 1 and 2, and Comparative Examples 1 and 2 as cathode materials for sodium-ion batteries. The data clearly demonstrates that the carbon-coated polyanion-doped cathode material exhibits significantly better capacity at different rates than Comparative Example 1 and the undoped carbon-coated cathode, and Comparative Example 2 and the undoped and uncoated cathode.

[0125] like Figure 2 The image shown is of carbon-coated polyanion-doped Na prepared in Example 1. 2.4 Fe 1.9 Sn 0.1 (SO4) 2.8 (P2O7) 0.2 SEM image of the cathode material. From Figure 2 It can be seen that the spray-dried particles exhibit an irregular, near-spherical shape, with no obvious adhesion between the particles.

[0126] like Figure 3 The image shown is a SEM image of the carbon-coated Na2Fe2(SO4)3 cathode material prepared in Comparative Example 1. Similar to the SEM image of Example 1, it can be seen that pyrophosphate doping does not affect the morphology.

[0127] like Figure 4 The image shown is of carbon-coated polyanion-doped Na prepared in Example 1. 2.4 Fe 1.9 Sn 0.1 (SO4) 2.8 (P2O7) 0.2 XRD pattern of the cathode material. The upper half of the horizontal line represents carbon-coated polyanion-doped Na. 2.4 Fe 1.9 Sn 0.1 (SO4)2.8 (P2O7) 0.2 The XRD patterns of the samples correspond one-to-one with the standard Na2Fe2(SO4)3 in the lower half, confirming that trace doping does not change the overall structure of Na2Fe2(SO4)3.

[0128] like Figure 5 The image shown is of carbon-coated polyanion-doped Na prepared in Example 1. 2.4 Fe 1.9 Sn 0.1 (SO4) 2.8 (P2O7) 0.2 The initial charge-discharge curve of the cathode material at 0.1C. The capacity of the first charge cycle at 0.1C is 113 mAh·g. -1 The initial discharge capacity is 106 mAh·g. -1 .

[0129] like Figure 6 The image shown is of carbon-coated polyanion-doped Na prepared in Example 1. 2.4 Fe 1.9 Sn 0.1 (SO4) 2.8 (P2O7) 0.2 The 5C cycle performance of the cathode material and comparative example 2, carbon-coated Na2Fe2(SO4)3 cathode material as the cathode of sodium-ion battery, shows that pyrophosphate doping can significantly improve battery capacity and cycle stability.

[0130] Table 1

[0131]

[0132]

[0133] As shown in Table 1, the following conclusions can be drawn: First, the initial discharge specific capacities of Examples 1, 2, and 3 at 0.1C, 1C, and 3C reached 105 / 85 / 64, 95 / 79 / 60, and 92 / 81 / 63 mAh / g, respectively, which were significantly higher than those of Comparative Example 1 (78 / 66 / 42 mAh / g) and Comparative Example 2 (64 / 50 / 33 mAh / g), indicating that the synergistic effect of "polyanion doping + carbon nanotubes + ascorbic acid dual carbon source" can effectively improve ion migration kinetics and electronic conductivity. In all examples, the carbon source ratio of 0.02:0.01 was optimal. Although excess carbon source (Example 4) or replacement of reducing agent (Example 5) slightly optimized the low-rate capacity, the performance at high rates was not better than the 0.02:0.01 combination due to increased interface impedance. Second, Example 6 (Mn 2+ +Cr2O7 2- ) and Example 7 (V 3+ +P₂O₇4- Under the same conditions, the capacity reached 107 / 87 / 66 and 104 / 84 / 63 mAh / g respectively, with a capacity retention exceeding 62%, further verifying the dual benefits of a small amount of active metal + polyanion doping on structural rigidity and channel expansion. Furthermore, even with the complete omission of Fe, Example 8 (pure Mn) 2+ ) can still achieve 98 / 78 / 58mAh / g, Example 9 (Co 2+ +Mg 2+ +MoO42-) and Example 10 (Ni 2+ +Sn 4+ +P₂O₇ 4- The results also achieved 102 / 81 / 61 and 100 / 79 / 60 mAh / g, respectively, far exceeding the comparative examples. This demonstrates that by precisely combining active / inert metal ions with polyanions, both capacity and cycle stability can be achieved at high rates. Finally, while the performance of comparative examples 3–5 (CNTs only, ascorbic acid only, or excess carbon source) was better than that of uncoated and undoped comparative example 2 at various rates, they consistently lagged behind any of the example series, further highlighting the necessity and superiority of the dual carbon source and doping strategy. These results systematically demonstrate that by precisely controlling the types and ratios of metal ions, the types and contents of polyanions, and the carbon nanotube + ascorbic acid carbon source ratio (0.02:0.01), sodium-ion battery cathode materials with high capacity, high conductivity, and excellent structural stability can be obtained under high rate and high cycle conditions.

[0134] Table 2

[0135]

[0136] Based on the comparison of capacity retention of each sample at different cycle counts in Table 2, the following points can be further illustrated: First, the capacity retention rates of Examples 1-3 were 98%, 97%, and 95% respectively after 100 cycles, and maintained high stability of 89%, 86%, and 82% after 1000 cycles, all significantly better than Comparative Example 1 (74%) and Comparative Example 2 (63%), demonstrating the significant advantages of polyanion doping and dual carbon source coating in high-rate long-term cycling. Among them, Example 1 not only had a high initial capacity (105 mAh / g) but also maintained 89% capacity during long-term cycling, making it the sample with the best overall performance. Examples 2 and 3 were slightly lower than Example 1, suggesting that different dopant ions (Sn) 2+ Mn 2+The effects of different carbon sources on crystal stability control are slightly different. Secondly, regarding the optimization of the carbon source ratio, Example 4 (CNT:ascorbic acid = 0.03:0.02) achieved a retention rate of 92% after 1000 cycles, higher than Example 1, indicating that moderately increasing the carbon source can further enhance the integrity of the carbon layer and the stability of the electronic network; however, Example 5 (citric acid instead of ascorbic acid) only achieved 80%, lower than the sample using ascorbic acid, indicating that the type of reducing agent has a significant impact on the carbon coating quality, with ascorbic acid resulting in denser carbon and better electronic conductivity. Thirdly, Examples 6-7 (doped with Mn, respectively) 2+ +Cr2O7 2- With V 3+ +P₂O₇ 4- After 1000 cycles, the retention rates were 89% and 81% respectively, with overall performance close to that of Examples 1-2, indicating that different polyanion combinations also have strong structure retention capabilities during long-term operation. Examples 8-10 (Fe-free system) had retention rates of 77%, 79%, and 75% after 1000 cycles, which, although slightly lower than the Fe-based system, were still significantly better than Comparative Examples 3-5 (69%, 72%, and 68% respectively), verifying that even if Fe is completely discarded, a cathode material system with good cycle stability can be constructed by optimizing the combination of metal ions and anions. Finally, Comparative Example 2 had the lowest capacity retention rate among the three groups (only 70% after 100 cycles and dropping to 63% after 1000 cycles), making it the baseline system for uncoated and undoped systems. Although Comparative Examples 3–5 used a single carbon source or an excess of carbon source, their retention rates after 1000 cycles were still lower than those of the Example series, further confirming that the synergistic effect of "polyanion doping + CNT / ascorbic acid dual carbon source" in constructing a continuous conductive network, buffering structural stress, and stabilizing the lattice framework is irreplaceable.

[0137] In summary, Examples 1-10 all exhibited excellent capacity retention under high-rate and long-life cycling conditions. Particularly with the combination of polyanion doping and CNT+ ascorbic acid carbon coating, the materials effectively balance high conductivity, ion diffusion, and structural stability, providing a solid experimental basis and design reference for constructing high-performance sodium-ion battery cathode materials. The various embodiments in this specification are described in a progressive manner; similar or identical parts between embodiments can be referred to interchangeably. Each embodiment focuses on highlighting its differences from other embodiments.

[0138] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A carbon-coated polyanion-doped sulfate cathode material, characterized in that, Carbon-based composite polyanion-doped sulfates are used. The chemical formula of the polyanion-doped sulfate is: Na 6-a(2-y)+(b-2)x M 2-y a (SO4) 3-x N x b ; Where M is a metal ion and N is a polyanion; M is composed of active ions that can undergo redox reactions and inert ions that cannot undergo redox reactions. The active ions contribute capacity as the charge compensation host, and the inert ions serve as the support to stabilize the structure. The molar ratio of active ions to inert ions ranges from 1:1 to (2-y). Where a is the average valence state of M, 2≤|a|≤3; b is the average valence state of N, 2≤|b|<5; Where x is the polyanion doping substitution number, 0 < x ≤ 1; y is the number of metal ions, 0 ≤ y ≤ 0.

5.

2. The cathode material according to claim 1, characterized in that, The carbon content is 5 wt% to 8 wt%.

3. The cathode material according to claim 1, characterized in that, The active ions are selected from Fe. 2+ Co 2+ Ni 2+ Mn 2 + Cr 3+ V 3+ One or more; The inert ions are selected from Mg 2+ Cu 2+ Zn 2+ Al 3+ ,Sc 3+ Mn 2+ Sb 3+ Mo 3+ Nd 3+ Ce 4+ W 4+ Sn 4+ Zr 4+ Bi 5+ One or more of the following; The polyanion is selected from CO3. 2- SiO4 4- WO4 2- VO4 3- Cr2O7 2- MoO4 2- V2O7 4- BO3 3- One or more of them.

4. A method for preparing a positive electrode material as described in any one of claims 1 to 3, characterized in that, Includes the following steps: S1. Add M source, sodium source, sulfur source, carbon source and N source to deionized water, and heat and stir to completely dissolve the raw materials to obtain solution A; S2. Use spray drying to convert solution A into precursor powder B; S3. The precursor powder B is calcined under an inert atmosphere, and the resulting product is crushed and ground to obtain carbon-coated polyanion-doped sodium iron sulfate cathode material.

5. The preparation method according to claim 4, characterized in that, The molar ratio of the M source, sodium source, sulfur source, carbon source, and N source is (1.0–2.5):(1.0–2.5):(1.0–2.5):(0.1–1.0):(0.01–1).

6. The preparation method according to claim 4, characterized in that, The carbon source is a mixture of carbon source I and carbon source II, where carbon source I is carbon nanotubes and carbon source II is one or both of citric acid and ascorbic acid. The sodium source is any one or more of anhydrous sodium sulfate, sodium oxalate, sodium bicarbonate, anhydrous sodium acetate, sodium citrate, sodium sulfide, sodium salicylate, and sodium hydroxide. The sulfur source is any one or more of ferrous sulfate heptahydrate, anhydrous sodium sulfate, potassium sulfate, and ammonium sulfate.

7. The preparation method according to claim 4, characterized in that, In step S1, the heating and stirring method involves adding the raw materials sequentially to ionized water at 60°C and stirring for 80–130 minutes, followed by ultrasonic dispersion for 20–60 minutes to obtain solution A.

8. The preparation method according to claim 4, characterized in that, In step S2, the inlet temperature of the spray dryer is 200–280°C, the outlet temperature is 100–120°C, and the feed rate is 20–25%.

9. The preparation method according to claim 4, characterized in that, In S3, the inert gas protective atmosphere for calcination is one of argon, argon-hydrogen mixture and nitrogen; the calcination conditions are: first, raise the temperature to 200-250℃ at a heating rate of 2-5℃ / min, hold for 2-4 hours, then raise the temperature to 350-400℃ at a heating rate of 2-5℃ / min, and hold for 8-12 hours.

10. The application of the cathode material according to claim 1 in the field of sodium-ion batteries.

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