Polyanionic sodium ion battery positive electrode material

Abstract

The invention discloses a polyanionic sodium ion battery positive electrode material, and belongs to the technical field of sodium ion batteries. The chemical formula of the polyanion type sodium ion battery positive electrode material is Na4FeX (PO4) 2P2O7 (at) C * CODs, x is larger than or equal to 2.6 and smaller than or equal to 3, (at) C represents a carbon composite material, and CODs are carbon quantum dots. The positive electrode material provided by the invention is a carbon quantum dot doped polyanion type ferric sodium pyrophosphate positive electrode material, and the overall conductivity of the material is improved due to carbon quantum dot doping, so that the material has ultrahigh rate capability and cycling stability, and can be used as a sodium ion battery positive electrode material.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion batteries, and in particular to a polyanion type sodium ion battery positive electrode material. Background Art

[0002] Energy is the driving force behind the rapid development of human civilization, and fossil energy is currently the main source of energy for humanity. However, the storage of fossil energy is limited, and its widespread use will cause large amounts of carbon dioxide emissions, leading to global warming. Sodium-ion batteries are classified as secondary rechargeable batteries. As one of the energy storage systems with the highest energy conversion efficiency, secondary batteries have received increasing attention. Among the many secondary battery systems, sodium-ion batteries have the advantages of abundant reserves of raw sodium sources, wide distribution, and low cost. Sodium-ion batteries have excellent rate performance and high and low temperature performance, and have the advantages of not catching fire or exploding during safety tests and having good safety performance. Therefore, when the energy density requirements are not high, sodium-ion batteries are an excellent alternative to lithium-ion batteries.

[0003] The cathode material is a key component of sodium ion batteries and the main place to store sodium ions. It has a decisive influence on whether the battery is safe and reliable, whether the electrochemical performance is excellent, and whether the battery has a future development. At present, the cathode materials of sodium ion batteries include metal layered oxides, polyanions, Prussian blue, etc. Among them, polyanion cathode materials (A x M y [(XO m ) n ] z ) is a tetrahedral anion unit (XO4) n- (X = S, P, Si, or W) and MO with strong covalent bonds x A class of polyhedral compounds (M represents a transition metal). The presence of XO₄ in their structure increases the effective redox potential of the transition metal. The strong XO covalent bond significantly improves the stability of oxygen in the crystal lattice, thereby enhancing the material's safety. Therefore, these materials are considered to be the most promising class of cathode materials. However, their low ionic and electronic conductivity leads to poor rate performance, and their electrochemical performance still falls far short of the requirements of their production and application.

[0004] Therefore, how to effectively improve the ionic conductivity and electronic conductivity of the positive electrode material of polyanion sodium ion battery, thereby improving its rate performance and charge-discharge cycle stability, is a technical problem that needs to be solved urgently in this field. Summary of the Invention

[0005] The present invention addresses the problem of low ionic and electronic conductivity in existing polyanionic sodium-ion battery cathode materials, resulting in poor rate performance and poor cycling stability. The present invention provides a polyanionic sodium-ion battery cathode material. The cathode material provided by the present invention is a polyanionic sodium ferric pyrophosphate cathode material doped with carbon quantum dots. The carbon quantum doping improves the overall conductivity of the material, resulting in ultra-high rate performance and cycling stability, making it suitable for use as a sodium-ion battery cathode material.

[0006] The present invention first provides a polyanion type sodium ion battery positive electrode material, the chemical formula of which is Na4Fe X (PO4)2P2O7@C·

CODs

[0007] The present invention further provides a method for preparing a polyanionic sodium ion battery positive electrode material, comprising the following steps: mixing a sodium source compound, an iron source compound, a phosphorus source compound, a carbon source and carbon quantum dots in liquid phase and then sand-milling the mixture to obtain a precursor; and calcining the precursor to obtain the polyanionic sodium ion battery positive electrode material.

[0008] The preparation method of the present invention comprises the following steps: preparing a mixture of carbon quantum dots (which have good electrical conductivity), an iron source, a sodium source and a carbon source, sand-milling, spray-drying and then calcining to obtain a carbon quantum dot-doped polyanion type sodium ion battery positive electrode material; the carbon quantum dots are uniformly mixed with the raw materials, thereby improving the ionic conductivity and electronic conductivity of the carbon quantum dot-doped polyanion type sodium ion battery positive electrode material without affecting the electrochemical properties of the polyanion type sodium ion battery positive electrode material itself, thereby effectively improving the rate performance and cycle stability of the carbon quantum dot-doped polyanion type sodium ion battery positive electrode material.

[0009] In the above-mentioned method for preparing the polyanion sodium ion battery positive electrode material, the sodium source compound is selected from at least one of sodium carbonate, sodium hydroxide, sodium nitrate, sodium oxalate and sodium acetate; The iron source compound is at least one of ferric phosphate, ferric nitrate, ferrous oxalate, ferric citrate, ferric acetate, ferric chloride and ferric oxide; The phosphorus source compound is at least one of phosphoric acid, sodium pyrophosphate, pyrophosphoric acid, ammonium dihydrogen phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate and disodium hydrogen phosphate; The carbon source is selected from at least one of glucose, sucrose, polyethylene glycol, cyclodextrin, phenolic resin, oxalic acid, ascorbic acid, formaldehyde, acetaldehyde, n-butyraldehyde, lactic acid, citric acid, malic acid, oxalic acid, adipic acid, soluble starch and ethylenediaminetetraacetic acid.

[0010] In the above-mentioned method for preparing the polyanion sodium ion battery positive electrode material, the carbon source is glucose.

[0011] In the above-mentioned method for preparing the polyanion sodium ion battery positive electrode material, the size of the carbon quantum dots is less than or equal to 20 nm; specifically, it can be 2 to 20 nm; more specifically, it can be 2 nm, 7 nm or 13 nm.

[0012] In the above-mentioned method for preparing the polyanion sodium ion battery positive electrode material, the molar ratio of sodium, iron and phosphorus in the sodium source compound, iron source compound and phosphorus source compound is 4:x:4, wherein 2.6≦x≦3; The mass of the carbon source is 0.1% to 15% of the mass of the iron source compound, specifically 5% or 10%; The mass of the carbon quantum dots is 0.1% to 5% of the mass of the iron source compound, specifically 2% or 3%.

[0013] In the above-mentioned method for preparing the polyanion sodium ion battery positive electrode material, the calcination is carried out in an inert atmosphere; specifically, it can be a nitrogen or argon atmosphere; The calcination temperature is 350-1000°C, specifically 500°C; the calcination time is 4-35h, specifically 10h; The heating rate of the calcination is 0.1-10°C / min, specifically 2°C / min or 10°C / min.

[0014] In the above-mentioned method for preparing the polyanion sodium ion battery positive electrode material, the sand grinding is performed to a material particle size D50 of 400-650 nm, specifically 500 nm; After the sand grinding, there is a drying step to obtain the precursor; Specifically, the drying is spray drying; after spray drying, the moisture content of the material is less than 1000 ppm.

[0015] In the above-mentioned method for preparing the polyanion sodium ion battery positive electrode material, the liquid phase mixing is mixed in water.

[0016] Third, the present invention provides a polyanion sodium ion battery positive electrode material prepared by the above preparation method.

[0017] Finally, the application of the above-mentioned polyanion sodium ion battery positive electrode material in the preparation of sodium ion battery positive electrode also falls within the protection scope of the present invention.

[0018] The polyanion sodium ion battery positive electrode material of the present invention is made into a pole piece, and the 1C discharge capacity of the pole piece is preferably above 100 mAh / g, and the 1C 50-week cycle capacity retention rate is preferably above 95%.

[0019] Compared with the prior art, the present invention has the following advantages: (1) The raw materials proposed in the present invention have a wide range of sources, a simple preparation method, low production cost, and are easy to mass-produce. By mixing with carbon quantum dots and grinding in one step, a polyanion-type positive electrode material precursor with iron phosphate as the skeleton and carbon quantum dots on the surface is obtained, and then calcined to obtain a polyanion-type sodium ion battery positive electrode material with high rate performance and long cycle life. (2) The present invention utilizes carbon quantum dots to dope polyanion sodium ion battery positive electrode materials. The highly dispersed carbon quantum dots also provide electron migration tunnels for the polyanion sodium ion battery positive electrode materials, thereby reducing the polarization of the battery. At the same time, the rate performance of the polyanion sodium ion battery positive electrode materials can be improved, and the cycle performance can be improved, so that the positive electrode material has ultra-high rate performance and cycle stability, and can be used as a positive electrode material for sodium ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is an SEM image of the carbon quantum dot-doped polyanion sodium ion battery positive electrode material prepared in Example 1. DETAILED DESCRIPTION

[0021] The present invention will be further described in detail below in conjunction with specific embodiments. The examples given are only for illustrating the present invention, not for limiting the scope of the present invention.

[0022] The experimental methods in the following examples are conventional methods unless otherwise specified.

[0023] The quantitative tests in the following examples were all repeated three times, and the results were averaged.

[0024] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0025] The present invention provides a polyanion type sodium ion battery positive electrode material, the chemical formula of which is Na4Fe X (PO4)2P2O7@C·

CODs

[0026] The preparation method of the polyanion type sodium ion battery positive electrode material of the present invention comprises the following steps: mixing a sodium source compound, an iron source compound, a phosphorus source compound, a carbon source and carbon quantum dots in liquid phase and then sand-milling the mixture to obtain a precursor; and calcining the precursor to obtain the polyanion type sodium ion battery positive electrode material.

[0027] The polyanion-type sodium ion battery cathode material prepared in the present invention was used as the positive electrode material to manufacture button-type batteries. The discharge capacity per gram of the button-type batteries was measured at current densities of 0.2C, 0.33C, and 1C, respectively. The 1C cycling stability was also measured by cycling at a current density of 1C. The results showed that the polyanion-type sodium ion battery cathode material prepared in the present invention exhibited excellent rate performance and cycling stability.

[0028] Example 1 Sodium carbonate, ferric phosphate, phosphoric acid, glucose, and carbon quantum dots were mixed in deionized water in a liquid phase and then sand-milled. The molar ratio of sodium, iron, and phosphorus in the sodium carbonate, ferric phosphate, and phosphoric acid was 4:3:4, the glucose content accounted for 10% of the ferric phosphate weight, and the carbon quantum dots accounted for 3% of the ferric phosphate weight. The carbon quantum dots had a size of 2 nm. The mixture was sand-milled to a particle size D50 of 500 nm and then spray-dried (moisture content after spraying was less than 1000 ppm; atomizer frequency was 30 Hz, and inlet and outlet air temperatures were 300°C and 110°C, respectively) to obtain a precursor. The precursor was then calcined at 500°C for 10 h in a nitrogen atmosphere at a heating rate of 2°C / min. The calcined material was removed, crushed, and sieved to obtain a carbon quantum dot-doped polyanionic sodium-ion battery cathode material.

[0029] Figure 1 The following is an SEM image of the carbon quantum dot-doped polyanion sodium ion battery cathode material prepared in this embodiment. Figure 1 It can be seen that some spheres in the material are broken, which increases the packing density of the material and thus improves the compaction of the material. In addition, some spherical particles are broken after coarse crushing to form a graded combination of large and small particles, and the surface is coated with a conductive network of carbon.

[0030] Example 2 The preparation method of the carbon quantum dot-doped polyanion sodium ion battery positive electrode material in this embodiment is the same as that in Example 1, except that the amount of carbon quantum dots added is 2% of the mass of the iron phosphate.

[0031] Example 3 The preparation method of the carbon quantum dot-doped polyanion sodium ion battery positive electrode material in this embodiment is the same as that in Example 1, except that the amount of glucose input accounts for 5% of the mass of the iron phosphate.

[0032] Example 4 The preparation method of the carbon quantum dot-doped polyanion sodium ion battery positive electrode material in this embodiment is the same as that in Example 1, except that the glucose in Example 1 is replaced by sucrose, and the input amount accounts for 5% of the mass of the iron phosphate.

[0033] Example 5 The preparation method of the carbon quantum dot-doped polyanion sodium ion battery positive electrode material in this embodiment is the same as that in Example 1, except that the molar ratio of sodium, iron, and phosphorus in sodium carbonate, iron phosphate, and phosphoric acid is 4:2.6:4.

[0034] Example 6 The preparation method of the carbon quantum dot-doped polyanion sodium ion battery positive electrode material in this embodiment is the same as that in Example 1, except that the size of the carbon quantum dots is 7 nm.

[0035] Example 7 The preparation method of the carbon quantum dot-doped polyanion sodium ion battery positive electrode material in this embodiment is the same as that in Example 1, except that the size of the carbon quantum dots is 13 nm.

[0036] Example 8 The preparation method of the carbon quantum dot-doped polyanion sodium ion battery positive electrode material in this embodiment is the same as that in Example 1, except that the heating rate of the calcination heating in Example 1 is set to 10° C. / min.

[0037] Comparative Example 1 The preparation method of the polyanionic sodium ion battery positive electrode material in this comparative example is the same as that in Example 1, except that the amount of carbon quantum dots added accounts for 0% of the mass of the iron phosphate.

[0038] Comparative Example 2 The preparation method of the polyanionic sodium ion battery positive electrode material in this comparative example is the same as that in Example 1, except that the amount of glucose input accounts for 0% of the mass of the iron phosphate.

[0039] Comparative Example 3 The preparation method of the polyanion sodium ion battery positive electrode material in this comparative example is the same as that in Example 1, except that glucose and carbon quantum dots are not added.

[0040] The 2016 type button cell battery was prepared as follows: The positive electrode sheet is prepared by mixing the positive electrode material, conductive carbon black and binder polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1, using N-methylpyrrolidone (NMP) as a solvent to form a slurry, and then evenly coating it on aluminum foil. After drying at 120°C, the slurry is rolled to a thickness of 300μm and vacuum-dried at 120°C for 12 hours to obtain the positive electrode sheet. The positive electrode materials are respectively prepared by the examples or comparative examples. The negative electrode uses a metallic sodium sheet, the separator is a polypropylene porous membrane, and the electrolyte is 1 mol / L NaPF6 / EC+DEC+DMC (EC:DEC:DMC=1:1:1 volume ratio).

[0041] The discharge capacity of the prepared button-type battery was measured at current densities of 0.2C, 0.33C, and 1C, and its cycling stability was determined by cycling for 50 cycles at a current density of 1C. The test results are shown in Table 1.

[0042] Table 1 Charge and discharge rate and cycle data of cathode materials

[0043] As can be seen from Table 1, compared with Comparative Examples 1-3, the button-type battery of the carbon quantum dot-doped polyanion-type sodium ion battery positive electrode material prepared in Example 1 exhibits excellent rate performance and cycle stability, maintaining 96.64% of the cycle stability after 50 cycles at a 1C rate; indicating that the carbon quantum dot-doped polyanion-type sodium ion battery positive electrode material provided by the present invention is particularly suitable as a cyclic sodium ion battery positive electrode material; from the comparison of the data of Example 1 and Comparative Example 1, it can be seen that the method of coating the carbon quantum dots has a significant effect on the electrical properties of the positive electrode material (after adding carbon quantum dots, the first charge of the material is increased from 97.2 to 108.5, and the 1C rate performance is increased from 92 to 101.7 mAh / g). Comparing Example 1 with Example 2, the carbon quantum dots with a content of 3% have better rate performance than the carbon quantum dots with an addition amount of 2%, and the cycle performance is similar. Comparing Example 1 and Example 4, it can be seen that when glucose is used as the carbon source, the material's first charge capacity is 108.5, which is about 7 mAh / g higher than 101.3 mAh / g when sucrose is used as the carbon source. The first efficiency is similar, but the rate performance and cycle performance are better. In addition, when Example 1 is compared with Example 3, the addition of 10% glucose has better electrical performance than the addition of 5% glucose. Therefore, the type and amount of carbon source added are also key to improving the conductive properties of the material. Compared with the 7nm and 13nm carbon quantum dots in Examples 6-7, the first charge capacity of Example 1 is 108.5, 105.3, and 101.3 mAh / g, respectively, and the 1C discharge capacity is 101.7, 97.8, and 95.8 mAh / g, respectively. Small particles and nano-sized particles help improve the material's rate performance. Compared with Example 8, the 1C discharge capacity of the material prepared in Example 8 is only 91.0 mAh / g. The heating rate of 2°C / min is more conducive to the sintering of the material than the heating rate of 10°C / min. The low heating rate helps the stable growth of the material lattice, reduces the material lattice defects during the sintering process, and is conducive to the diffusion and transmission of lithium ions.

Claims

1. A polyanion sodium ion battery positive electrode material, characterized in that: The chemical formula of the polyanion type sodium ion battery positive electrode material is Na4Fe X (PO4)2P2O7@C·【CODs】, where 2.6≦x≦3, @C represents carbon composite material, and CODs represents carbon quantum dots.

2. A method for preparing a positive electrode material for a polyanion sodium ion battery, characterized in that: The method comprises the following steps: mixing a sodium source compound, an iron source compound, a phosphorus source compound, a carbon source and carbon quantum dots in liquid phase and then sand-milling the mixture to obtain a precursor; and calcining the precursor to obtain the polyanion sodium ion battery positive electrode material.

3. The method for preparing the positive electrode material of a polyanion type sodium ion battery according to claim 2, characterized in that: The sodium source compound is selected from at least one of sodium carbonate, sodium hydroxide, sodium nitrate, sodium oxalate and sodium acetate; The iron source compound is at least one of ferric phosphate, ferric nitrate, ferrous oxalate, ferric citrate, ferric acetate, ferric chloride and ferric oxide; The phosphorus source compound is at least one of phosphoric acid, sodium pyrophosphate, pyrophosphoric acid, ammonium dihydrogen phosphate, ammonium dihydrogen phosphate, sodium dihydrogen phosphate and disodium hydrogen phosphate; The carbon source is selected from at least one of glucose, sucrose, polyethylene glycol, cyclodextrin, phenolic resin, oxalic acid, ascorbic acid, formaldehyde, acetaldehyde, n-butyraldehyde, lactic acid, citric acid, malic acid, oxalic acid, adipic acid, soluble starch and ethylenediaminetetraacetic acid.

4. The method for preparing the positive electrode material of a polyanion sodium ion battery according to claim 3, characterized in that: The carbon source is glucose.

5. The method for preparing the positive electrode material of polyanion type sodium ion battery according to claim 2, characterized in that: The size of the carbon quantum dots is less than 20 nm, and may be specifically 2-20 nm.

6. The method for preparing the positive electrode material of a polyanion sodium ion battery according to claim 2, characterized in that: The molar ratio of sodium, iron and phosphorus in the sodium source compound, iron source compound and phosphorus source compound is 4:x:4, wherein 2.6≦x≦3; The mass of the carbon source is 0.1% to 15% of the mass of the iron source compound; The mass of the carbon quantum dots is 0.1% to 5% of the mass of the iron source compound.

7. The method for preparing the positive electrode material of a polyanion sodium ion battery according to claim 2, characterized in that: The calcination is carried out in an inert atmosphere; The calcination temperature is 350-1000°C and the calcination time is 4-35h; The heating rate of the calcination is 0.1-10°C / min.

8. The method for preparing the positive electrode material of a polyanion sodium ion battery according to claim 2, characterized in that: The sand grinding is performed until the material particle size D50 is 400-650nm; After the sand grinding, there is a drying step, and the precursor is obtained after drying.

9. The polyanion type sodium ion battery positive electrode material prepared by the preparation method according to any one of claims 2 to 8.

10. Use of the polyanion sodium ion battery positive electrode material according to claim 9 in preparing a positive electrode for a sodium ion battery.

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