Sodium ferric phosphate pyrophosphate positive electrode material, method for preparing sodium ferric phosphate pyrophosphate positive electrode material by taking iron black as iron source and application of sodium ferric phosphate pyrophosphate positive electrode material

Abstract

The invention discloses a sodium ferric phosphate pyrophosphate positive electrode material, a method for preparing the sodium ferric phosphate pyrophosphate positive electrode material by taking iron black as an iron source and application of the sodium ferric phosphate pyrophosphate positive electrode material. Iron black with relatively strong reaction activity under an acidic condition is selected as the iron source, and is heated and stirred in an acid-containing mixed solution to react and generate a new iron-containing compound; the synthesis method is simple, and the finished product has high purity and high raw material atom utilization rate. The particle size of the newly generated iron-containing compound is lower than that of a finished product raw material, so that uniform dispersion is facilitated, and high capacity can be achieved under the condition that sanding treatment is not carried out. In addition, after sintering, due to evaporation of water in a positive electrode precursor and thermal decomposition of a carbon source, the product has a loose and porous structure, so that the positive electrode material can be soaked in an electrolyte in a battery, and the sodium ion transmission rate is improved. Through verification, the positive electrode material has the advantages of high specific capacity and cycle stability when being used for a battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of active materials for secondary batteries, and in particular to a sodium iron pyrophosphate phosphate cathode material, a method for preparing the same using iron black as an iron source, and an application thereof. Background Art

[0002] In the field of energy storage technology, lithium-ion batteries are the most widely studied and applied. However, due to the limited lithium reserves, rising costs, and increasingly strict requirements for safety, cycle performance, rate performance, etc., it is necessary to deeply study new battery systems with high safety, high efficiency, and excellent performance. Sodium-ion batteries have the same working principle as lithium-ion batteries and have the characteristics of rich reserves, high safety, and excellent high and low temperature performance, making them one of the best routes for the development of large-scale energy storage technology in the future.

[0003] In sodium-ion batteries, the cathode material usually determines the energy density and power density of the system. Since sodium ions have a larger radius and higher mass than lithium ions, the kinetics of the material interface and bulk phase are slow. Therefore, studying and improving the structural stability and conductivity of the cathode material is one of the main directions to promote the application of sodium-ion batteries.

[0004] The crystal structure of the sodium iron pyrophosphate phosphate material formed under a certain sodium-iron-phosphorus ratio is of the NASICON type, which is conducive to the rapid transmission and insertion / extraction of sodium ions. The sodium iron pyrophosphate phosphate cathode material was first reported in 2012. Kim et al. obtained it by solid-phase mixing and high-temperature sintering of ferrous oxalate dihydrate, sodium pyrophosphate, ammonium dihydrogen phosphate, and high molecular polymers such as paraffin. Its structure is Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 O. At present, this element ratio is still used in most studies, but there have been many different designs in recent years for the selection of raw materials, synthesis methods, and improving conductivity. For raw materials (mainly iron sources), using insoluble iron salts as raw materials usually cannot achieve highly uniform mixing, resulting in poor electrochemical performance, poor repeatability, and long time consumption; using soluble iron salts as raw materials is likely to introduce ions such as NO3- and SO42-, and harmful gases are generated during sintering. For synthesis methods, sol-gel, spray drying, freeze drying, electrospinning, etc. have all been reported. For improving conductivity, it is improved by introducing different conductive materials into the system to construct a conductive network or by doping and modification, thereby increasing the charge-discharge capacity of the material.

[0005] In recent years, there have been some reports on the synthesis of sodium iron pyrophosphate phosphate electrode materials, and new explorations have been made in the raw materials used and preparation methods. For example:

[0006] CN117374257A weighs ferrous oxalate dihydrate and sodium dihydrogen phosphate according to the molar ratio of iron to sodium element of 2:3, fully mixes the iron source, part of the sodium phosphorus source and the conductive material in a vertical mixer, compacts in a corundum crucible, heat treats at 160°C in a hydrogen-argon atmosphere for 6 hours, fully mixes the obtained intermediate material with the remaining sodium dihydrogen phosphate and the sodium supplement, heat treats at 300°C for 1 hour in the same manner, and then heat treats at 550°C for 10 hours to obtain the finished product. This patent adopts the solid phase mixing method for preparation, which is simple to operate, but the material mixing uniformity is slightly poor, and the 0.1C discharge specific capacity is 95mAh / g.

[0007] CN117393750A adopts the method of promoting carbonization of carbon source by concentrated acid under vacuum state for synthesis, the iron source used is ferric nitrate or ferric sulfate, the sodium source and phosphorus source are sodium dihydrogen phosphate or sodium phosphate, the reducing agent is ascorbic acid, the carbon source is citric acid, and the concentrated acid introduced is concentrated phosphoric acid or concentrated sulfuric acid. After the material is assembled into a half-cell, the 0.1C reversible specific capacity is 109 mAh / g. It reduces the time required for the later sintering chemical reaction, that is, shortens the time consumption of high-temperature sintering, but the early process of this method is complicated, and the harmful gases released by the sintering of the soluble iron source are not conducive to environmental protection.

[0008] CN117486186A uses iron oxide as the iron source, and improves the dispersion uniformity of the suspension by adding a dispersing aid PEG-4000 and high-speed sand milling, while shortening the sand milling time and greatly simplifying the sand milling process. However, the obtained material contains impurity peaks of sodium iron pyrophosphate phase, and the half-cell charge and discharge voltage platform is short, and obvious voltage rise and fall occurs after 80mAh / g. Summary of the invention

[0009] The object of the present invention is to provide a sodium iron phosphate pyrophosphate positive electrode material, a method for preparing the same using iron black as an iron source, and an application thereof. The above method is simple, safe, and pollution-free, and the synthesized sodium iron phosphate pyrophosphate positive electrode material has a high purity and is used for lithium / sodium ion batteries, and has a high charge and discharge specific capacity and stable cycle performance.

[0010] The technical solution of the present invention is as follows:

[0011] A method for preparing sodium iron pyrophosphate positive electrode material using iron black as an iron source comprises the following steps:

[0012] (1) heating a phosphoric acid solution to 40-90° C., then adding black iron powder to obtain a black suspension, maintaining the system temperature, and stirring until the suspension first changes from black to brown and then to white; wherein the molar ratio of phosphorus in the phosphoric acid solution to iron in the black iron powder is 2.8-3.2:2;

[0013] (2) When white appears in the suspension, add organic acid carbon source and sodium source in sequence, maintain the system temperature, and stir until the color of the suspension turns grayish green; wherein, the mass ratio of the organic acid carbon source to iron black is 0.3-2:1; the sodium source is a single sodium source compound without heteroatoms such as S and Cl, and the molar ratio of sodium element in the sodium source to phosphorus element in the phosphoric acid solution is 1:1;

[0014] (3) Add conductive slurry to the suspension, disperse evenly, and then perform spray drying to obtain the cathode precursor. The addition amount of the conductive carbon material in the conductive slurry is 0.1-5% of the total mass of the iron source, organic acid carbon source, sodium source and phosphoric acid;

[0015] (4) Sinter the cathode precursor under an inert atmosphere to obtain the sodium iron pyrophosphate phosphate cathode material.

[0016] In some preferred embodiments, the heating temperature in step (1) is 50-80 °C.

[0017] In some preferred embodiments, the concentration of the phosphoric acid solution is 20-60 wt%. Further preferably, the concentration of the phosphoric acid solution is 30-50 wt%, which can be obtained by diluting 85 wt% concentrated phosphoric acid.

[0018] In some preferred embodiments, the stirring method is mechanical stirring or magnetic stirring. Exemplarily, the stirring speed is 20-500 rpm, which is specifically set according to the preparation scale and the equipment used.

[0019] In some preferred embodiments, the mass ratio of the organic acid carbon source to iron black is 0.8-1.5:1.

[0020] In some preferred embodiments, the addition amount of the conductive carbon material in the conductive slurry is 0.5-1.2% of the total mass of the iron source, organic acid carbon source, sodium source and phosphoric acid.

[0021] In some preferred embodiments, the organic acid carbon source is at least one of citric acid, malic acid, tartaric acid, sub-tartaric acid, gluconic acid, glycolic acid and lactic acid.

[0022] In some preferred embodiments, the sodium source is at least one of sodium acetate, sodium formate, sodium carbonate, sodium oxalate, sodium citrate, sodium gluconate, sodium glycolate, sodium tartrate, sodium bicarbonate, sodium hydroxide and sodium peroxide.

[0023] In some preferred embodiments, the conductive slurry is an aqueous dispersion of a conductive carbon material, and the conductive carbon material is at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, Super P and Ketjen black, and its concentration is 5-20 wt%.

[0024] In some preferred embodiments, the operation of uniform dispersion in step (4) is to first stir for 5 - 10 min and then perform ultrasonic treatment for 0.5 - 2 h.

[0025] In some preferred embodiments, the inlet temperature of spray drying in step (4) is 150 - 250 °C. Further preferably, the inlet temperature is 180 - 220 °C.

[0026] In some preferred embodiments, the specific operation of sintering in step (5) is as follows: heat at a rate of 3 - 5 °C / min to 300 - 400 °C, heat for 2 - 5 h, after naturally cooling to room temperature, grind and crush the agglomerated intermediate material, then compact it into strips for secondary sintering, and heat at a rate of 3 - 5 °C / min to 500 - 600 °C for 6 - 12 h during the secondary sintering.

[0027] In some preferred embodiments, the inert atmosphere is an argon atmosphere or a hydrogen - argon atmosphere.

[0028] In some preferred embodiments, the method further includes step (6): grinding the sodium iron pyrophosphate phosphate cathode material into ellipsoidal sponge - like particles with a diameter of 5 - 25 μm.

[0029] A sodium iron pyrophosphate phosphate cathode material is prepared by the above - mentioned method.

[0030] Application of the sodium iron pyrophosphate phosphate cathode material prepared by the above - mentioned method in the preparation of lithium / sodium ion batteries.

[0031] A lithium / sodium ion battery, wherein the raw material of the positive electrode includes the sodium iron pyrophosphate phosphate cathode material prepared by the above - mentioned method.

[0032] The present invention has at least the following beneficial effects:

[0033] 1. The method provided by the present invention selects iron black with slightly stronger reactivity under acidic conditions as the iron source, heats and stirs it in a mixed solution containing acid to react and generate a new iron - containing compound. The particle size of the newly generated iron - containing compound is lower than that of the finished product raw material, which is beneficial to uniform dispersion and can also exhibit a high capacity without sanding treatment. In addition, after sintering, due to the evaporation of moisture in the cathode precursor and the thermal decomposition of organic acid - based carbon sources, the product has a porous structure, which is beneficial for the material to soak the electrolyte in the battery and improve the sodium ion transmission rate.

[0034] 2. Compared with the existing similar synthesis processes, it reduces the dependence on high - speed sanding, simplifies the pretreatment link of the mixed solution, the method is simple and the cost is low; using this method to synthesize iron phosphate has no impurity elements introduced, the finished product has high purity and high raw material atom utilization rate; in addition, since iron, sodium, and phosphorus come from different raw materials, it is convenient to control the element ratio and there is no need to consider the error caused by the crystal water of the raw material during weighing.

[0035] 3. After testing, the positive electrode sheet prepared from the sodium iron pyrophosphate phosphate positive electrode material provided by the present invention has an initial discharge specific capacity of 105 mAh / g at 0.1C, and the capacity retention rate is 99.7% after 260 cycles at 1C, with almost no attenuation, having the advantages of high specific capacity and stable cycling. Description of the Drawings

[0036] Figure 1 is a scanning electron microscope image of the sodium iron pyrophosphate phosphate material prepared in Example 1 of the present invention;

[0037] Figure 2 is an X-ray powder diffraction pattern of the sodium iron pyrophosphate phosphate material prepared in Example 1 of the present invention;

[0038] Figure 3 is the first charge-discharge curve of the sodium iron pyrophosphate phosphate material prepared in Example 1 of the present invention;

[0039] Figure 4 is the cycling performance graph of the sodium iron pyrophosphate phosphate material prepared in Example 1 of the present invention;

[0040] Figure 5 is an X-ray powder diffraction pattern of the sodium iron pyrophosphate phosphate material prepared in Comparative Example 1 of the present invention;

[0041] Figure 6 is the first charge-discharge curve of the sodium iron pyrophosphate phosphate material prepared in Comparative Example 1 of the present invention; Detailed Description of the Invention

[0042] The technical solutions of the present invention will be further described and described below through specific embodiments.

[0043] In the following examples, the water used can be one or more of distilled water, purified water, and drinking water; unless otherwise specified, the detection methods in the following examples are all conventional detection methods; unless otherwise specified, the reagents in the following examples are all purchased from commercial channels. Lithium / sodium ion batteries refer to lithium ion batteries or sodium ion batteries.

[0044] Example 1

[0045] In this example, tartaric acid was used as an organic acid carbon source, iron black as an iron source, sodium carbonate as a sodium source, and an aqueous dispersion of multi-walled carbon nanotubes (concentration 10 wt%) as a conductive paste for the preparation of a sodium iron pyrophosphate phosphate positive electrode material, where sodium carbonate, iron black, and phosphoric acid were weighed according to the molar ratio of Na, Fe, and P of 3:2:3, and the addition amount of tartaric acid was the same as the total mass of sodium carbonate, iron black, and phosphoric acid. The specific steps are as follows:

[0046] (1) Weigh 85 wt% of concentrated phosphoric acid, dilute it with water to 40 wt%, stir and heat it to 60 °C. First, slowly add iron black to the phosphoric acid aqueous solution, continue to stir and maintain the system temperature until the black suspension gradually turns brown and finally begins to turn white.

[0047] (2) Add tartaric acid, and then slowly add sodium carbonate. After the bubbles completely disappear, maintain the system temperature. After 2 h, the white suspension turns gray-green.

[0048] (3) Add conductive paste to the gray-green suspension. The mass of the conductive carbon material in the conductive paste is 1% of the total mass of other solid materials. After ultrasonic dispersion for 1 h, spray drying is carried out. The inlet temperature of the spray is 200 °C, and a yellow-green cathode material precursor is obtained after drying.

[0049] (4) Place the cathode material precursor in a quartz tube, introduce argon, heat it to 300 °C at a rate of 3 °C / min, take it out after annealing for 5 h, grind and crush the massive material and then press it into a strip shape, and then place it back in the quartz tube and perform secondary annealing at 600 °C for 8 h under the same conditions to obtain sodium iron pyrophosphate phosphate cathode material. As Figure 1 shown, the sodium iron pyrophosphate phosphate cathode material is ellipsoidal sponge-like particles with a diameter of 5 - 25 μm.

[0050] Comparative Example 1

[0051] In this example, glycolic acid is used as the organic acid carbon source, iron powder as the iron source, sodium carbonate as the sodium source, and multi-walled carbon nanotube dispersion (concentration 10 wt%) as the conductive paste for the preparation of the comparative cathode material. Among them, sodium carbonate, iron black and phosphoric acid are weighed according to the molar ratio of Na, Fe and P of 3:2:3, and the addition amount of glycolic acid is 50% of the total mass of sodium carbonate, iron black and phosphoric acid. The specific steps are as follows:

[0052] (1) Weigh 85 wt% of concentrated phosphoric acid and dilute it with water to 50 wt%. Since iron powder has stronger magnetism than iron black and it is difficult to make it react completely by magnetic stirring, first slowly add iron powder to the phosphoric acid aqueous solution, stir and react at 80 °C for 1 h, then transfer the iron-phosphorus mixture to a reaction kettle and seal it, and keep it at 150 °C for 8 h to obtain a white milky iron phosphate salt;

[0053] (2) Add glycolic acid and an appropriate amount of water, heat and stir at 80 °C to make the milky iron phosphate salt disperse evenly, and then slowly add sodium carbonate, and continue to stir and heat for 2 h to obtain a light yellow-green suspension.

[0054] (3) Add conductive paste to the yellow-green suspension. The mass of the conductive carbon material in the conductive paste is 1% of the total mass of other solid materials. After ultrasonic dispersion for 1 h, spray drying is carried out. The inlet temperature of the spray drying is 200 °C to obtain a comparative precursor.

[0055] (4) Place the comparative precursor in a quartz tube, introduce argon, heat it to 300 °C at a rate of 3 °C / min, take it out after annealing for 5 h, grind and crush the bulk material and then compact it into a strip, place it back in the quartz tube, and perform secondary annealing at 600 °C for 8 hours under the same conditions to obtain the comparative cathode material.

[0056] Performance Test

[0057] 1. Product Structure

[0058] Perform X-ray powder diffraction test and analysis on the sodium iron pyrophosphate phosphate cathode material prepared in Example 1 and the comparative cathode material prepared in Comparative Example 1. The results are as Figure 2 and Figure 5 shown.

[0059] From Figure 2 it can be seen that the cathode material prepared in Example 1 is a mixed phase of sodium iron pyrophosphate phosphate and partial sodium pyrophosphate iron, with a relatively high crystal phase purity. Further, through inductively coupled plasma chromatography test, it is known that its element ratio is sodium: iron: phosphorus = 3.01: 1.97: 2.98.

[0060] Figure 5 shows the characteristic peaks of sodium iron pyrophosphate phosphate. In addition, it also contains partial characteristic peaks belonging to sodium pyrophosphate iron and a small amount of characteristic peaks belonging to the inactive sodium iron phosphate phase. Through inductively coupled plasma chromatography test, it is known that its element ratio is sodium: iron: phosphorus = 3.08: 1.90: 2.95.

[0061] 2. Electrochemical Performance

[0062] Use the sodium iron pyrophosphate phosphate cathode material prepared in Example 1 and the comparative cathode material prepared in Comparative Example 1 as the active substances, coat and make a positive electrode sheet and assemble a half-cell for cyclic charge and discharge tests. The specific steps are as follows:

[0063] (1) Using N-methylpyrrolidone as the solvent, mix the active substance, acetylene black and polyvinylidene fluoride (PVDF) in a mass ratio of 8: 1: 1 and stir to make a slurry for electrode coating;

[0064] (2) Coat the prepared slurry on the current collector carbon-coated aluminum foil and dry it for 8 hours to remove the solvent to obtain the positive electrode sheet;

[0065] (3) Use a 2016-type battery case, a sodium metal negative electrode, a Whatman glass fiber separator, 1M NaClO 4 (pure PC + 5% FEC) electrolyte and the positive electrode sheet prepared in step (2) for battery assembly;

[0066] Leave it standing for 6 h, and then perform cyclic charge-discharge tests according to the set charge-discharge process. The parameters are as follows: the magnification for the first 3 cycles is 0.1C, the magnification for the 4th to 300th cycles is 1C, the voltage range is 1.5V - 3.9V. In addition, the first cycle charging is carried out with constant voltage charging at 3.9V for 6 h.

[0067] The charge-discharge curves and cyclic performance curves are as Figure 3 and Figure 4 shown. Through electrochemical tests, it can be known that the 0.1C first-cycle discharge specific capacity of the positive electrode material of Example 1 is 105 mAh / g, the Coulomb efficiency is 92.6%, and the capacity retention rate after 260 cycles at 1C is 99.7%, with almost no capacity decay, showing excellent cyclic stability. The 0.1C first-cycle discharge specific capacity of the comparative positive electrode material is 100 mAh / g, and the Coulomb efficiency is 94.3%.

[0068] The discharge specific capacity of the positive electrode material prepared in Example 1 is higher than that of Comparative Example 1, CN117374257A, and CN117486186A, and is equivalent to that of CN117393750A. However, the preparation method of Example 1 is simplified compared with Comparative Example 1 and the above patent applications. In summary, the present invention simplifies the preparation process, and the prepared product has a high charge-discharge specific capacity and stable cyclic performance, and can be applied to sodium-ion batteries as their positive electrode material.

[0069] The above is only the preferred embodiment of the present invention, and thus the scope of implementation of the present invention cannot be limited thereby. That is, equivalent changes and modifications made according to the scope of the present invention patent and the content of the specification should still fall within the scope covered by the present invention.

Claims

1. A method for preparing sodium iron pyrophosphate positive electrode material using iron black as an iron source, characterized in that: The steps include: (1) heating a phosphoric acid solution to 40-90° C., then adding black iron powder to obtain a black suspension, maintaining the system temperature, and stirring until the suspension first changes from black to brown and then to white; wherein the molar ratio of phosphorus in the phosphoric acid solution to iron in the black iron powder is 2.8-3.2:2; (2) when white appears in the suspension, adding an organic acid carbon source and a sodium source in sequence, maintaining the system temperature, and stirring until the color of the suspension changes to gray-green; wherein the mass ratio of the organic acid carbon source to the iron black is 0.3 to 2:1; the sodium source is a monosodium source compound free of heteroatoms, and the molar ratio of the sodium element in the sodium source to the phosphorus element in the phosphoric acid solution is 1:1; (3) adding a conductive slurry to the suspension, dispersing it evenly, and then spray drying it to obtain a positive electrode precursor, wherein the amount of the conductive carbon material added to the conductive slurry is 0.1 to 5% of the total mass of the iron source, the organic acid carbon source, the sodium source and the phosphoric acid; (4) Sintering the positive electrode precursor under an inert atmosphere to obtain the sodium iron phosphate pyrophosphate positive electrode material.

2. The method according to claim 1, characterized in that The heating temperature in step (1) is 50-80°C.

3. The method according to claim 1, characterized in that The concentration of the phosphoric acid solution is 20-60 wt %.

4. The method according to claim 1, characterized in that The mass ratio of the organic acid carbon source to the iron black is 0.8 to 1.5:1; And / or, the amount of the conductive carbon material added to the conductive paste is 0.5-1.2% of the total mass of the iron source, the organic acid carbon source, the sodium source and the phosphoric acid.

5. The method according to claim 1, characterized in that The organic acid carbon source is at least one of citric acid, malic acid, tartaric acid, tartaric acid, gluconic acid, glycolic acid and lactic acid; and / or, the sodium source is at least one of sodium acetate, sodium formate, sodium carbonate, sodium oxalate, sodium citrate, sodium gluconate, sodium glycolate, sodium tartrate, sodium bicarbonate, sodium hydroxide and sodium peroxide; And / or, the conductive slurry is an aqueous dispersion of the conductive carbon material, the conductive carbon material is at least one of single-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, Super P and Ketjen black, and the concentration thereof is 5-20wt%.

6. The method according to claim 1, characterized in that The specific operation of sintering in step (5) is as follows: heating to 300-400°C at 3-5°C / min, heating for 2-5h, cooling naturally to room temperature, grinding and crushing the agglomerated intermediate material, and then compacting it into strips for secondary sintering. The secondary sintering is heated to 500-600°C at 3-5°C / min and heated for 6-12h.

7. The method according to claim 1, characterized in that The method further comprises step (6): grinding the sodium iron phosphate pyrophosphate positive electrode material into ellipsoidal sponge-like particles with a diameter of 5 to 25 μm.

8. A sodium iron phosphate pyrophosphate positive electrode material, characterized in that: The method is prepared by any one of claims 1 to 7.

9. Use of the sodium iron phosphate pyrophosphate positive electrode material prepared by the method according to any one of claims 1 to 7 in the preparation of lithium / sodium ion batteries.

10. A lithium / sodium ion battery, characterized in that: The raw material of the positive electrode comprises the sodium iron phosphate pyrophosphate positive electrode material prepared by the method described in any one of claims 1 to 7.

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