Positive electrode material and preparation method and application thereof

Abstract

The invention belongs to the field of batteries, and particularly discloses a positive electrode material and a preparation method and application thereof.The positive electrode material comprises a ferric sodium phosphate pyrophosphate material and a carbon layer coating the surface of the ferric sodium phosphate pyrophosphate material, and the chemical formula of the ferric sodium phosphate pyrophosphate material is Na < 4-2x > Fe < 3 + x > (PO4) 2 (P2O7); 0 lt; and x is less than or equal to 0.3. The positive electrode material is rich in iron, the surplus Fe occupies part of Na2 sites, on one hand, Fe replaces part of inactive Na which cannot be deintercalated, and the charge transfer number is not affected; on the other hand, the conductivity of the positive electrode material is greatly enhanced through Fe substitution; meanwhile, Fe replaces Na, so that a better structure supporting effect can be achieved, and the cycling stability and the rate capability of the positive electrode material are improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of batteries, and particularly relates to a positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] Sodium-ion batteries rely on the intrinsic advantages of alkali metal homologous elements (the abundance of sodium in the earth's crust is 2.3%, which is 354 times that of lithium), and exhibit unique technical and economic properties: the raw material supply chain is free from the shackles of strategic metals, the electrolyte system is highly compatible with lithium batteries, and has excellent low-temperature performance and safety performance.

[0003] Current technical research focuses on the development of key electrode materials, among which iron-based phosphate positive electrode materials are highly regarded due to the advantages of element abundance (Fe: 6.3%, P: 0.1%) and technical maturity. In particular, composite sodium iron phosphate (Na4Fe3(PO4)2(P2O7), hereinafter referred to as NFPP) with a NASICON structure has developed into a new generation of high-performance sodium-ion battery positive electrode material system, with moderate working voltage, 129 mAh / g theoretical specific capacity and cycle stability due to the three-dimensional ion diffusion channel. + Fast diffusion channel, Na4 forms Na + Fast diffusion channel through point sharing, Na3 has a small coordination number around it, and has a low deintercalation barrier, which can also ensure Na + Fast diffusion. At the same time, Na2 is tightly wrapped by FeO6, which has a very high deintercalation barrier and is difficult to effectively deintercalate sodium ions without affecting the crystal structure, so it is difficult for sodium ions at the Na2 site to migrate and diffuse.

[0004] The electronic conductivity of NFPP is poor, and there are two main strategies for improving the conductivity of NFPP positive electrode materials. One is to add additional conductive additives, such as a carbon coating. The conductive additive as a non-active component reduces the overall energy density of the battery; the other is to ion-dope and lattice-regulate NFPP, such as introducing defects to increase the carrier concentration by using high-valence doping ions, however, these high-valence ions usually cannot transfer electrons, reducing the number of transferable charges, and thus reducing the overall energy density of the material. The above two improvement strategies both affect the overall energy density of the NFPP material, and thus affect the performance of the battery, limiting the popularization and application of the NFPP material. SUMMARY

[0005] In order to overcome at least one technical problem existing in the prior art, one of the purposes of the present application is to provide a positive electrode material.

[0006] The second object of the present application is to provide a preparation method of the positive electrode material.

[0007] The third object of the present application is to provide a battery.

[0008] To achieve the above objects, the technical solution adopted by the present application is as follows: The first aspect of the present application provides a positive electrode material, comprising a sodium iron pyrophosphate phosphate material and a carbon layer coated on the surface of the sodium iron pyrophosphate phosphate material, the chemical formula of the sodium iron pyrophosphate phosphate material is Na 4-2x Fe 3+x (PO4)2(P2O7); 0 < x ≤ 0.3.

[0009] In some embodiments of the present application, the x is any one of the values of 0.001, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30 or a range value formed by any two of them.

[0010] In some embodiments of the present application, 0.06 ≤ x ≤ 0.2.

[0011] In some embodiments of the present application, the mass of carbon in the carbon layer is 0.3-3% of the total mass of the positive electrode material. The present application adopts a carbon layer to coat the sodium iron pyrophosphate phosphate material, thereby improving the conductivity of the positive electrode material, so that the prepared positive electrode material can have higher specific capacity and capacity retention rate at high rate.

[0012] In some embodiments of the present application, the mass of carbon in the carbon layer is any one of the values of 0.3%, 0.5%, 0.7%, 0.9%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.2%, 2.4%, 2.6%, 2.8%, 3% or a range value formed by any two of them.

[0013] The second aspect of the present application provides a preparation method of the positive electrode material of the first aspect of the present application, comprising the following steps: Mixing raw materials including a pyrophosphate source, a phosphate source, a sodium source and an iron source, then mixing with a carbon source precursor solution, pre-sintering after freeze-drying, cooling to obtain a precursor; Formal sintering of the precursor under pressure to prepare the positive electrode material.

[0014] In preparing the cathode material, this invention first mixes raw materials including pyrophosphate, phosphate, sodium, and iron sources, and then mixes them with a carbon source precursor solution. The carbon source precursor solution can coat the surface of the reactants, forming carbon coating or carbon doping during subsequent sintering. In some embodiments of this invention, the pre-sintering is carried out under a protective atmosphere; the protective atmosphere includes at least one of nitrogen, argon, hydrogen, and helium.

[0015] In some embodiments of the present invention, the pre-sintering temperature is any value of 200°C, 250°C, 300°C, 350°C, 400°C, 450°C, 500°C, or a range formed by any two of them; in some embodiments of the present invention, the pre-sintering temperature is 250~400°C.

[0016] In some embodiments of the present invention, the heating rate of the pre-sintering is 0.5~10℃ / min; in some embodiments of the present invention, the heating rate of the pre-sintering is any value or a range formed by any two of the following: 0.5℃ / min, 1℃ / min, 1.5℃ / min, 2℃ / min, 2.5℃ / min, 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, 5℃ / min, 5.5℃ / min, 6℃ / min, 6.5℃ / min, 7℃ / min, 7.5℃ / min, 8℃ / min, 8.5℃ / min, 9℃ / min, 9.5℃ / min, 10℃ / min; in some embodiments of the present invention, the heating rate of the pre-sintering is 1~8℃ / min; in some embodiments of the present invention, the heating rate of the pre-sintering is 3~8℃ / min.

[0017] In some embodiments of the present invention, the pre-sintering holding time is 1 to 24 hours; in some embodiments of the present invention, the pre-sintering holding time is any value or a range formed by any two of the following: 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, 21 hours, 22 hours, 23 hours, and 24 hours; in some embodiments of the present invention, the pre-sintering holding time is 1 to 12 hours; in some embodiments of the present invention, the pre-sintering holding time is 1 to 5 hours.

[0018] In some embodiments of the present invention, the formal sintering is carried out under a protective atmosphere; the protective atmosphere includes at least one of nitrogen, argon, hydrogen, and helium.

[0019] In some embodiments of the present invention, the formal sintering temperature is any value or a range formed by any two of 400°C, 450°C, 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, and 800°C; in some embodiments of the present invention, the formal sintering temperature is 450~600°C.

[0020] In some embodiments of the present invention, the heating rate of the formal sintering is 0.5~10℃ / min; in some embodiments of the present invention, the heating rate of the formal sintering is any value or a range formed by any two of the following: 0.5℃ / min, 1℃ / min, 1.5℃ / min, 2℃ / min, 2.5℃ / min, 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, 5℃ / min, 5.5℃ / min, 6℃ / min, 6.5℃ / min, 7℃ / min, 7.5℃ / min, 8℃ / min, 8.5℃ / min, 9℃ / min, 9.5℃ / min, 10℃ / min; in some embodiments of the present invention, the heating rate of the formal sintering is 1~8℃ / min; in some embodiments of the present invention, the heating rate of the formal sintering is 3~8℃ / min.

[0021] In some embodiments of the present invention, the holding time for formal sintering is 1 to 24 hours; in some embodiments of the present invention, the holding time for formal sintering is any value or a range formed by any two of the following: 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, 21 hours, 22 hours, 23 hours, and 24 hours; in some embodiments of the present invention, the holding time for formal sintering is 3 to 15 hours; in some embodiments of the present invention, the holding time for formal sintering is 6 to 12 hours.

[0022] In some embodiments of the present invention, the pressure of the formal sintering is 1~25 GPa; in some embodiments of the present invention, the pressure of the formal sintering is any value or a range formed by any two of the following: 1 GPa, 2 GPa, 3 GPa, 4 GPa, 5 GPa, 6 GPa, 7 GPa, 8 GPa, 9 GPa, 10 GPa, 11 GPa, 12 GPa, 13 GPa, 14 GPa, 15 GPa, 16 GPa, 17 GPa, 18 GPa, 19 GPa, 20 GPa, 21 GPa, 22 GPa, 23 GPa, 24 GPa, 25 GPa; in some embodiments of the present invention, the pressure of the formal sintering is 3~10 GPa.

[0023] In some embodiments of the present invention, both pre-sintering and formal sintering require cooling to 20~40°C. There are no particular limitations on the cooling process. The conventional method is to turn off the heating mode of the sintering equipment and allow it to cool naturally.

[0024] In some embodiments of the present invention, the formal sintering step is performed in a multi-faceted anvil press.

[0025] In some embodiments of the present invention, the pyrophosphate source includes at least one of pyrophosphate, sodium pyrophosphate, and disodium dihydrogen pyrophosphate.

[0026] In some embodiments of the present invention, the phosphate source includes at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, phosphorus pentoxide, sodium ammonium hydrogen phosphate, sodium phosphate, and ammonium phosphate; in some embodiments of the present invention, the phosphate source includes at least one of ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

[0027] In some embodiments of the present invention, the sodium source includes at least one of sodium acetate, sodium hydroxide, sodium carbonate, sodium oxalate, sodium nitrite, disodium hydrogen phosphate, sodium bicarbonate, sodium citrate, anhydrous sodium sulfate, sodium stearate, sodium oleate, sodium tartrate, sodium alginate, sodium carboxymethyl cellulose, sodium lactate, sodium gluconate, and sodium ascorbate; in some preferred embodiments of the present invention, the sodium source includes at least one of sodium acetate, sodium carbonate, and sodium oxalate.

[0028] In some embodiments of the present invention, the iron source includes at least one of ferrous sulfate, ferrous oxalate, ferrous oxide, ferrous chloride, ferrous sulfate monohydrate, ferrous sulfate tetrahydrate, ferrous sulfate heptahydrate, ferrous hydroxide, ferric oxide, ferric chloride, and ferric hydroxide; in some embodiments of the present invention, the iron source includes at least one of ferrous oxalate, ferrous oxide, and ferrous chloride.

[0029] In some embodiments of the present invention, the carbon source precursor in the carbon source precursor solution includes at least one of glucose, citric acid, polyvinylpyrrolidone, polyethylene glycol, ascorbic acid, isoascorbic acid, sodium isoascorbate, tea polyphenols, and hydroquinone; in some preferred embodiments of the present invention, the carbon source precursor in the carbon source precursor solution includes at least one of citric acid and polyvinylpyrrolidone.

[0030] In some embodiments of the present invention, the solvent in the carbon source precursor solution includes at least one of water, methanol, ethanol, propanol, acetone, ethylene glycol, and pyridine; in some preferred embodiments of the present invention, the solvent in the carbon source precursor solution includes at least one of water and methanol.

[0031] In some embodiments of the present invention, the concentration of the carbon source precursor solution is 0.001~1 kg / L; in some embodiments of the present invention, the concentration of the carbon source precursor solution is 0.001 kg / L, 0.01 kg / L, 0.02 kg / L, 0.03 kg / L, 0.04 kg / L, 0.05 kg / L, 0.06 kg / L, 0.07 kg / L, 0.08 kg / L, 0.09 kg / L, 0.1 kg / L, 0.15 kg / L, 0.2 kg / L, 0.25 kg / L, 0.3 kg / L, 0. The concentration of the carbon source precursor solution is any value or a range formed by any two of the following: 35 kg / L, 0.4 kg / L, 0.45 kg / L, 0.5 kg / L, 0.55 kg / L, 0.6 kg / L, 0.65 kg / L, 0.7 kg / L, 0.75 kg / L, 0.8 kg / L, 0.85 kg / L, 0.9 kg / L, 0.95 kg / L, and 1 kg / L. In some embodiments of the present invention, the concentration of the carbon source precursor solution is 0.01 to 0.1 kg / L.

[0032] In some embodiments of the present invention, the carbon source precursor solution is prepared by dissolving the carbon source precursor in a solvent.

[0033] In some embodiments of the present invention, the molar ratio of the pyrophosphate source, phosphate source, sodium source, and iron source is 1:2:(1-x):(3+x), 0 <x≤0.3。

[0034] In some embodiments of the present invention, the freeze-drying temperature is -80 ℃ to -20 ℃; in some embodiments of the present invention, the freeze-drying temperature is any value or a range formed by any two of -80 ℃, -70 ℃, -60 ℃, -50 ℃, -40 ℃, -30 ℃, and -20 ℃.

[0035] In some embodiments of the present invention, the freeze-drying time is 4 to 48 hours; in some embodiments of the present invention, the freeze-drying time is any value or a range formed by any two of the following: 4 hours, 6 hours, 8 hours, 10 hours, 12 hours, 14 hours, 16 hours, 18 hours, 20 hours, 22 hours, 24 hours, 26 hours, 28 hours, 30 hours, 32 hours, 34 hours, 36 hours, 38 hours, 40 hours, 42 hours, 44 hours, 46 hours, and 48 hours.

[0036] In some embodiments of the present invention, the freeze-drying is performed under vacuum conditions. After the freeze-drying process, the solvent completely evaporates, yielding a powder.

[0037] A third aspect of the present invention provides a battery comprising a positive electrode containing the positive electrode material described in the first aspect of the present invention or the positive electrode material prepared by the preparation method described in the second aspect of the present invention.

[0038] In some embodiments of the present invention, the battery is a sodium-ion battery, a sodium metal battery, a solid sodium-ion battery, or a sodium metal battery without a negative electrode.

[0039] In some embodiments of the present invention, the battery includes a button cell, a flat cell, or a cylindrical cell.

[0040] The beneficial effects of this invention are: the cathode material of this invention is rich in iron, and the excess Fe occupies part of the Na2 sites. On the one hand, Fe replaces part of the inactive Na that cannot be inserted or removed, without affecting the charge transfer number, and the energy density remains basically unchanged; on the other hand, the substitution of Fe greatly enhances the conductivity of the cathode material, and Fe replacing Na can play a better structural support role, thereby improving the cycle stability and rate performance of the cathode material.

[0041] The preparation method of this invention involves sintering the precursor under high pressure and high temperature, thereby allowing the excess Fe in the precursor to occupy part of the Na2 sites in NFPP. The preparation method is simple, the material cost is low, and it is suitable for industrial production.

[0042] The battery of this invention contains the aforementioned iron-rich cathode material, exhibiting high specific capacity, high cycle stability, and high capacity retention at a high rate of 20C, with a specific capacity of 79.49~93.52 mAh·g. -1 The capacity retention rate after 1000 laps is 83.9-94.7%. Attached Figure Description

[0043] Figure 1 The image shows the XRD pattern of the cathode material prepared in Example 1.

[0044] Figure 2 This is a transmission electron microscope image of the cathode material prepared in Example 1.

[0045] Figure 3 The graph shows the rate performance of a sodium-ion battery assembled from the cathode material obtained in Example 1.

[0046] Figure 4 The image shows the electrochemical impedance spectroscopy of a sodium-ion battery assembled from the cathode materials obtained in Example 1 and Comparative Example 1. Detailed Implementation

[0047] The following further details the specific implementation of the present invention in conjunction with the accompanying drawings and examples. However, the implementation and protection of the present invention are not limited thereto. It should be noted that for the processes not specifically described in detail below, those skilled in the art can implement or understand them by referring to the prior art. Reagents or instruments without indicating the manufacturer can be obtained as conventional products through commercial purchase.

[0048] The present invention provides a cathode material, which includes sodium iron pyrophosphate phosphate material. The chemical formula of the sodium iron pyrophosphate phosphate material is Na 4-2x Fe 3+x (PO4)2(P2O7); 0 < x ≤ 0.3. In the sodium iron pyrophosphate phosphate material, the content of Fe is relatively high, and the surplus Fe occupies part of the Na2 sites. On the one hand, Fe replaces some inactive Na that cannot be deintercalated, without affecting the charge transfer number, and the energy density is basically the same as that of the stoichiometric ratio of Na4Fe3(PO4)2(P2O7). On the other hand, the substitution of Fe greatly enhances the conductivity of the cathode material. At the same time, the substitution of Fe for Na can play a better structural support role, improving the cycle stability and rate performance of the cathode material. The radius of the ferrous ion (0.76 Å) is slightly smaller than the radius of the sodium ion (0.95 Å), and the common coordination numbers of the iron-oxygen polyhedron and the sodium-oxygen polyhedron have an overlapping range (5 - 6). However, it is found that by using the conventional heat sintering treatment method, the excess iron is difficult to enter the NFPP lattice, but instead forms inactive m-NaFePO4 impurities in the product. In the present invention, sintering is carried out under high pressure and high temperature. The high pressure promotes the plastic flow and diffusion mass transfer between particles, increases the chemical reaction between raw material particles, and thus promotes the entry of excess iron into the lattice sites of sodium.

[0049] From a microscopic perspective, after doping iron occupies the inactive sodium sites, the microscopic arrangement of the surrounding atoms is adjusted, making the Fe-O- Na -O-Fe that was originally difficult to conduct electrons become Fe-O- Fe -O-Fe with good electron conduction ability, thus improving the overall electron conductivity of the material. At the same time, the doped iron has a stronger affinity for the surrounding oxygen, causing the oxygen to shift towards the position of the doped iron, thereby increasing the distance between the active sodium (Na1, Na3, Na4) and the surrounding oxygen, making the active sodium easier to transport.

[0050] The following further details the specific implementation of the present invention in conjunction with specific examples.

[0051] Example 1 This example provides a cathode material, including sodium iron pyrophosphate phosphate material and a carbon layer coated on the surface of the sodium iron pyrophosphate phosphate material; the chemical formula of the sodium iron pyrophosphate phosphate material is Na 3.76 Fe 3.12(PO4)2(P2O7); calculated based on the total mass of the positive electrode material, the positive electrode material contains 1.2% by mass of carbon (the carbon content is obtained by testing with a sulfur-carbon analyzer).

[0052] The positive electrode material in this example is prepared by a method including the following steps: (1) Dissolve 0.04 kg of polyvinylpyrrolidone in 1 L of ethanol to form a homogeneous solution A.

[0053] (2) Take 0.2 mol of disodium dihydrogen pyrophosphate, 0.4 mol of ammonium dihydrogen phosphate, 0.176 mol of sodium carbonate, and 0.624 mol of iron oxalate, mix them, dissolve them in solution A, stir evenly, and then perform vacuum freeze-drying (equipment model: LGJ-60A freeze dryer, freezing temperature -37°C, duration 24 hours). Subsequently, put the powder into a heating furnace, introduce an argon atmosphere, and raise the temperature to T1 = 300°C at a heating rate of 5°C / min and hold for t1 = 3 h, then cool down to obtain a precursor powder.

[0054] (3) Put the precursor powder into a multi-anvil press, introduce an argon atmosphere, pressurize to 5 GPa, and raise the temperature to T2 = 500°C at a heating rate of 5°C / min and hold for t2 = 10 h. After completion, perform furnace cooling to obtain the positive electrode material in this example.

[0055] Examples 2 to 16 The difference between the positive electrode materials in Examples 2 to 16 and Example 1 is only that: the chemical formula of the sodium iron pyrophosphate phosphate material is different, and the chemical formula of the sodium iron pyrophosphate phosphate material is Na[[ID=十七]] 4-2x Fe 3+x (PO4)2(P2O7), (0 < x ≤ 0.3), the specific chemical formula of each example is shown in Table 1 below.

[0056] The difference between the preparation methods of the positive electrode materials in Examples 2 to 16 and Example 1 is only that: the raw material dosages are different, and the specific raw material dosages of each example are shown in Table 1 below.

[0057] Table 1 Raw material dosages and chemical formula of NFPP in Examples 2 to 16

[0058] Example 16 The difference between the preparation method of the positive electrode material in this example and Example 1 is only that: in step (1) of this example, methanol is used to replace ethanol in Example 1.

[0059] Example 17 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that in step (1) of this example, citric acid is used to replace polyvinylpyrrolidone in Example 1.

[0060] Example 18 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the amount of polyvinylpyrrolidone used in step (1) of this example is 0.01 kg.

[0061] Example 19 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the amount of polyvinylpyrrolidone used in step (1) of this example is 0.02 kg.

[0062] Example 20 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the amount of polyvinylpyrrolidone used in step (1) of this example is 0.06 kg.

[0063] Example 21 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the amount of polyvinylpyrrolidone used in step (1) of this example is 0.08 kg.

[0064] Example 22 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the amount of polyvinylpyrrolidone used in step (1) of this example is 0.1 kg.

[0065] The only difference between the preparation methods of the cathode materials in Examples 18-22 and Example 1 is the amount of polyvinylpyrrolidone used. Therefore, compared with Example 1, the chemical formula of the sodium iron pyrophosphate material in the cathode materials prepared in Examples 18-22 is exactly the same, but the carbon content is different from that in Example 1. The carbon content (the mass percentage of carbon calculated based on the total mass of the cathode material) in the cathode materials prepared in Examples 18-22 was tested using a sulfur-carbon analyzer. The carbon content of each example is shown in Table 2 below.

[0066] Table 2. Test results of carbon content in the cathode materials of Examples 18-22

[0067] As shown in Table 2, the carbon content of the cathode materials prepared in Examples 18-22 is 0.3-3%.

[0068] The cathode materials obtained in Examples 23-60 are the same as those in Example 1, except that the raw materials or preparation parameters differ, as detailed below: Example 23 The difference between the preparation method of the positive electrode material in this example and that in Example 1 is only that: in step (2) of this example, pyrophosphate is used to replace sodium dihydrogen pyrophosphate in Example 1, and the amount of each raw material is: 0.2 mol pyrophosphate, 0.4 mol ammonium dihydrogen phosphate, 0.376 mol sodium carbonate, and 0.624 mol ferrous oxalate.

[0069] Example 24 The difference between the preparation method of the positive electrode material in this example and that in Example 1 is only that the amount of each raw material used in step (2) of this example is: 0.2 mol pyrophosphate, 0.4 mol sodium dihydrogen phosphate, 0.176 mol sodium carbonate, and 0.624 mol ferrous oxalate.

[0070] Example 25 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that in step (2) of this example, diammonium hydrogen phosphate is used instead of ammonium dihydrogen phosphate in Example 1.

[0071] Example 26 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that in step (2) of this example, 0.352 mol sodium acetate is used to replace 0.176 mol sodium carbonate in Example 1.

[0072] Example 27 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that step (2) in this example uses an equimolar amount of sodium oxalate to replace sodium carbonate in Example 1.

[0073] Example 28 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that step (2) in this example uses equimolar ferrous oxide to replace ferrous oxalate in Example 1.

[0074] Example 29 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that step (2) in this example uses equimolar ferrous chloride to replace ferrous oxalate in Example 1.

[0075] Example 30 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that nitrogen is used instead of argon in step (3) of this example.

[0076] Example 31 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that in step (3) of this example, a hydrogen-argon mixture (volume ratio 5:95) is used to replace the argon gas in Example 1.

[0077] Example 32 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the pressure in step (3) of this example is 3 GPa.

[0078] Example 33 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the pressure in step (3) of this example is 4 GPa.

[0079] Example 34 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the pressure in step (3) of this example is 6 GPa.

[0080] Example 35 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the pressure in step (3) of this example is 7 GPa.

[0081] Example 36 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the pressure in step (3) of this example is 8 GPa.

[0082] Example 37 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the pressure in step (3) of this example is 9 GPa.

[0083] Example 38 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the pressure in step (3) of this example is 10 GPa.

[0084] Example 39 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the heating rates v1 and v2 in steps (2) and (3) in this example are both 3℃ / min.

[0085] Example 40 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the heating rates v1 and v2 in steps (2) and (3) in this example are both 4℃ / min.

[0086] Example 41 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the heating rate v1 in step (2) is 3℃ / min and the heating rate v2 in step (3) is 6℃ / min.

[0087] Example 42 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the heating rate v1 in step (2) is 3℃ / min and the heating rate v2 in step (3) is 7℃ / min.

[0088] Example 43 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the heating rates v1 and v2 in steps (2) and (3) in this example are both 8℃ / min.

[0089] Example 44 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that T1 in step (2) of this example is 250°C.

[0090] Example 45 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that T1 in step (2) of this example is 275°C.

[0091] Example 46 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that T1 in step (2) of this example is 325°C.

[0092] Example 47 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that T1 in step (2) of this example is 350°C.

[0093] Example 48 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that T1 in step (2) of this example is 375°C.

[0094] Example 49 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that T1 in step (2) of this example is 400°C.

[0095] Example 50 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that t1 in step (2) of this example is 1 h.

[0096] Example 51 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that t1 in step (2) of this example is 5 h.

[0097] Example 52 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the T2 in step (3) of this example is 450°C.

[0098] Example 53 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the T2 in step (3) of this example is 475°C.

[0099] Example 54 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the T2 in step (3) of this example is 525°C.

[0100] Example 55 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the T2 in step (3) of this example is 550°C.

[0101] Example 56 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the T2 in step (3) of this example is 575°C.

[0102] Example 57 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that the T2 in step (3) of this example is 600°C.

[0103] Example 58 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that t2 in step (3) of this example is 6 h.

[0104] Example 59 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that t2 in step (3) of this example is 8 h.

[0105] Example 60 The only difference between the preparation method of the positive electrode material in this example and that in Example 1 is that t2 in step (3) of this example is 12 h.

[0106] Comparative Example 1 This example provides a cathode material comprising sodium iron pyrophosphate material and a carbon layer coated on the surface of sodium iron pyrophosphate material. The chemical formula of sodium iron pyrophosphate material is Na4Fe3(PO4)2(P2O7). Based on the total mass of the cathode material, the cathode material contains 1.2% carbon by mass (the carbon content is obtained by testing with a sulfur and carbon analyzer).

[0107] The preparation method of sodium iron phosphate cathode material in this example differs from that in Example 1 in that the amount of each raw material is different, specifically: 0.2 mol disodium dihydrogen pyrophosphate, 0.4 mol ammonium dihydrogen phosphate, 0.2 mol sodium carbonate, and 0.6 mol ferrous oxalate.

[0108] Comparative Example 2 This example provides a positive electrode material comprising a low-iron sodium iron pyrophosphate material and a carbon layer coated on the surface of the low-iron sodium iron pyrophosphate material. The chemical formula of the low-iron sodium iron pyrophosphate material is Na₄Fe₂O₃. 2.98 (PO4)2(P2O7) contains 1.2% carbon by mass based on the total mass of the cathode material (the carbon content is obtained by testing with a sulfur and carbon analyzer).

[0109] The difference between the preparation method of the iron-poor sodium iron phosphate cathode material in this example and that in Example 1 is only that the amount of each raw material is different, specifically: 0.2 mol disodium dihydrogen pyrophosphate, 0.4 mol ammonium dihydrogen phosphate, 0.2 mol sodium carbonate, and 0.596 mol ferrous oxalate.

[0110] Performance testing: The positive electrode materials prepared in Examples 1-60 and Comparative Examples 1-2 were mixed with conductive carbon black and polyvinylidene fluoride binder at a mass ratio of 8:1:1, and an appropriate amount of N-methylpyrrolidone was added. The mixture was thoroughly mixed to form a slurry, which was then uniformly coated onto aluminum foil, dried, and cut into small discs to obtain sodium-ion battery positive electrode sheets. The loading of the positive electrode material on the positive electrode sheet was approximately 3 mg / cm³. 2 .

[0111] The positive electrode sheet prepared by the above method was used as the positive electrode, and a sodium sheet was cut as the negative electrode. A 1 mol / L sodium hexafluorophosphate solution (1 mol of sodium hexafluorophosphate dissolved in 1 L of propylene carbonate) was used as the electrolyte. Whatman GF / D glass fiber was used as the separator, and sodium-ion coin cell half-cells were assembled. After the resulting coin cells were left to stand for 12 hours, constant current charge-discharge tests were performed at an ambient temperature of 25 ℃. The test voltage range was 1.7~3.8 V, and 1C = 119 mA / g. Cyclic performance tests were conducted on all cells corresponding to the examples at a high rate of 20C. Specific capacity refers to the specific capacity at the first discharge at 20C, and capacity retention refers to the capacity retention at 20C after 1000 cycles. Specific test results are shown in Table 3.

[0112] Table 3 Battery test results of Examples 1-60 and Comparative Examples 1-2

[0113] As shown in Table 3, compared with Comparative Examples 1-2, the cathode materials prepared in Examples 1-60 have higher specific capacity and higher capacity retention, with a first-cycle discharge specific capacity of 79.49-93.52 mAh·g at 20C. -1 The capacity retention rate after 1000 cycles at 20°C is 83.9%~94.7%. Among them, Example 1 has better specific capacity and cycle stability than other examples, as detailed below: By comparing Examples 1-15 with Comparative Examples 1-2, it can be seen that the iron-rich material (the cathode material of Examples 1-15 of this invention) has better rate performance and cycle stability than the isostometric material (Comparative Example 1) and the iron-poor material (Comparative Example 2). Through high-temperature and high-pressure synthesis, excess Fe occupies some Na2 sites. On the one hand, Fe replaces some of the inactive Na that cannot be extracted or extracted, without affecting the charge transfer number; on the other hand, the substitution of Fe greatly enhances the conductivity of the material, effectively improving the rate performance of the battery. At the same time, Fe replacing Na can provide better structural support, which is beneficial to improving the cycle stability of the battery.

[0114] Comparative examples 16-31 show that different raw materials have a slight impact on the properties of the synthesized materials.

[0115] Comparative examples 32-37 show that the cathode material prepared at a synthesis pressure of 5 GPa has better performance than cathode materials synthesized under other pressures. Both excessively high and excessively low pressures are not conducive to the precise synthesis of cathode materials in accordance with the specified proportions.

[0116] Comparative examples 38-60 show that, under the conditions of heating rates v1 and v2 both being 5℃ / min, T1 being 300℃, t1 being 3h, T2 being 500℃, and t2 being 10h, the synthesized cathode material has better performance than cathode materials prepared under other sintering conditions.

[0117] The XRD pattern of the cathode material prepared in Example 1 was measured using an X-ray diffractometer, as shown in the figure below. Figure 1 As shown, by Figure 1 It can be seen that the phase of the cathode material prepared in Example 1 is very pure, and there are no diffraction peaks of impurities.

[0118] The cathode material prepared in Example 1 was observed using a transmission electron microscope, specifically as follows: Figure 2 As shown. By Figure 2 It can be seen that the sodium iron pyrophosphate material particles prepared in Example 1 are coated with a layer of carbon.

[0119] After the batteries corresponding to Example 1 and Comparative Examples 1-2 were left to rest for 12 hours, their rate performance was tested. The rate performance test graph of the battery in Example 1 is shown below. Figure 3 As shown. By Figure 3It can be seen that Example 1 exhibits high specific capacity at different rate settings, specifically: 114.2 mAh / g, 107.5 mAh / g, and 103.8 mAh / g at 0.2C, 1C, and 5C, respectively. In comparison, Comparative Example 1 exhibits specific capacities of 104.2 mAh / g, 102.4 mAh / g, and 80.3 mAh / g at 0.2C, 1C, and 5C, respectively; and Comparative Example 2 exhibits specific capacities of 102.4 mAh / g, 99.5 mAh / g, and 78.3 mAh / g at 0.2C, 1C, and 5C, respectively.

[0120] After the coin cells obtained in Example 1 and Comparative Example 1 were left to stand for 12 hours, electrochemical impedance spectroscopy was performed. The test conditions were: room temperature, frequency range of 0.1 Hz to 10 kHz, amplitude of 1 mV, and the test results are as follows. Figure 4 As shown. By Figure 4 It can be seen that the impedance of the positive electrode material prepared in Example 1 (about 76Ω) is significantly smaller than that of Comparative Example 1 (about 120Ω).

[0121] The embodiments of the present invention have been described in detail above. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and the features thereof can be combined with each other unless otherwise specified.

Claims

1. A cathode material, characterized in that: It includes a sodium iron pyrophosphate material and a carbon layer coating the surface of the sodium iron pyrophosphate material, wherein the chemical formula of the sodium iron pyrophosphate material is Na. 4-2x Fe 3+x (PO4)2(P2O7); 0 <x≤0.3。 2. The cathode material according to claim 1, characterized in that: 0.06≤x≤0.2。 3. The cathode material according to claim 1, characterized in that: The mass of carbon in the carbon layer is 0.3 to 3% of the total mass of the cathode material.

4. The method for preparing the cathode material according to any one of claims 1 to 3, characterized in that: Includes the following steps: Raw materials including pyrophosphate source, phosphate source, sodium source and iron source are mixed, then mixed with carbon source precursor solution, freeze-dried and pre-sintered, and cooled to obtain precursor; The precursor is sintered under pressure to obtain the cathode material.

5. The method for preparing the cathode material according to claim 4, characterized in that: The pre-sintering has at least one of the following characteristics: (a1) The pre-sintering is carried out under a protective atmosphere; the protective atmosphere includes at least one of nitrogen, argon, hydrogen, and helium; (a2) The heating rate of the pre-sintering is 0.5~10℃ / min; (a3) The pre-sintering temperature is 200~500℃; (a4) The heat preservation time for the pre-sintering is 1~24h.

6. The method for preparing the cathode material according to claim 4, characterized in that: The formal sintering has at least one of the following characteristics: (b1) The formal sintering is carried out under a protective atmosphere; the protective atmosphere includes at least one of nitrogen, argon, hydrogen, and helium; (b2) The heating rate for the formal sintering is 0.5~10℃ / min; (b3) The formal sintering temperature is 400~800℃; (b4) The holding time for the formal sintering is 1~24h; (b5) The pressure for formal sintering is 1~25 GPa.

7. The method for preparing the cathode material according to claim 4, characterized in that: The pyrophosphate source includes at least one of pyrophosphate, sodium pyrophosphate, and disodium dihydrogen pyrophosphate. And / or, the phosphate source includes at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, phosphorus pentoxide, sodium ammonium hydrogen phosphate, sodium phosphate, and ammonium phosphate; And / or, the sodium source includes at least one of sodium acetate, sodium hydroxide, sodium carbonate, sodium oxalate, sodium nitrite, disodium hydrogen phosphate, sodium bicarbonate, sodium citrate, anhydrous sodium sulfate, sodium stearate, sodium oleate, sodium tartrate, sodium alginate, sodium carboxymethyl cellulose, sodium lactate, sodium gluconate, and sodium ascorbate. And / or, the iron source includes at least one of ferrous sulfate, ferrous oxalate, ferrous oxide, ferrous chloride, ferrous sulfate monohydrate, ferrous sulfate tetrahydrate, ferrous sulfate heptahydrate, ferrous hydroxide, ferric oxide, ferric chloride, and ferric hydroxide.

8. The method for preparing the cathode material according to claim 4, characterized in that: The carbon source precursor solution has at least one of the following characteristics: (c1) The carbon source precursor in the carbon source precursor solution includes at least one of glucose, citric acid, polyvinylpyrrolidone, polyethylene glycol, ascorbic acid, isoascorbic acid, sodium isoascorbate, tea polyphenols, and hydroquinone. (c2) The solvent in the carbon source precursor solution includes at least one of water, methanol, ethanol, propanol, acetone, ethylene glycol, and pyridine; (c3) The concentration of the carbon source precursor solution is 0.001~1 kg / L.

9. The method for preparing the cathode material according to claim 4, characterized in that: The molar ratio of the pyrophosphate source, phosphate source, sodium source, and iron source is 1:2:(1-x):(3+x), 0 <x≤0.3。 10. A battery, characterized in that: The cathode includes a positive electrode containing the positive electrode material according to any one of claims 1 to 3 or the positive electrode material prepared by the preparation method according to any one of claims 4 to 9.

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