Sodium-ion battery positive electrode material and preparation method thereof

Abstract

The structural formula of the sodium ion battery positive electrode material is NaxFey (PO4) 2P2O7, x is more than or equal to 3.92 and less than or equal to 4.2, y is more than or equal to 2.8 and less than or equal to 3, and x-y is more than or equal to 1.1 and less than or equal to 1.4. The preparation method comprises the following steps: calcining a precursor and then cooling, or calcining the precursor and then quenching, or heating and calcining the precursor and then quenching and tempering; and S2, crushing the semi-finished product obtained by sintering in the step S1. The sintering method of the sodium ferric phosphate pyrophosphate is substantially improved, the heating-cooling process is controlled, the structure of the positive electrode material is improved, the structural formula of the positive electrode material is controlled to be NaxFey (PO4) 2P2O7, 3.92 < = x < = 4.2, 2.8 < = y < = 3 and 1.1 < = x-y < = 1.4, the tap density and the specific capacity of the sodium ferric phosphate pyrophosphate are improved, and the sodium ferric phosphate pyrophosphate has more excellent cell processing performance.

Description

Technical Field

[0001] The present invention relates to the field of sodium-ion batteries, and specifically relates to a cathode material for a sodium-ion battery and a preparation method thereof. Background Art

[0002] In recent years, the technology of lithium-ion batteries has developed very rapidly, and sodium-ion batteries with a working principle similar to that of lithium-ion batteries have also attracted extensive attention from researchers. The reserves of sodium resources in the earth's crust are about 400 times that of lithium resources, and the raw material cost is relatively low. Using low-cost and high-performance sodium-ion batteries can replace lithium-ion batteries in some cases.

[0003] Sodium iron pyrophosphate phosphate is the "pearl" of the polyanion material system. As one of the most widely studied materials, it has the advantages of low cost, small volume change in the sodium ion deintercalation / insertion structure, long cycle life, and environmental friendliness. However, in the synthesis process of sodium iron pyrophosphate phosphate, it is extremely easy to generate impurity phases, and it is also very difficult to synthesize highly dense particles.

[0004] CN119390031A discloses a method for synthesizing sodium iron pyrophosphate phosphate and preparing a cathode material for sodium iron pyrophosphate phosphate. The method for preparing the precursor of this cathode material has many steps, and there are problems such as large environmental pollution and potential safety hazards, which do not conform to the concept of green chemistry. Summary of the Invention

[0005] To solve the above problems, the purpose of the present invention is to provide a cathode material for a sodium-ion battery and a preparation method thereof. By substantially improving the sintering method of sodium iron pyrophosphate phosphate, controlling the heating-cooling process, optimizing the structural improvement of the cathode material, and controlling its chemical formula Na x Fe y (PO4)2P2O7, 3.92 ≤ x ≤ 4.2, 2.8 ≤ y ≤ 3, 1.1 ≤ x - y ≤ 1.4, the tap density and specific capacity of sodium iron pyrophosphate phosphate are improved.

[0006] The present invention is realized by the following technical solutions:

[0007] A cathode material for a sodium-ion battery, with a chemical formula of Na x Fe y (PO4)2P2O7, 3.92 ≤ x ≤ 4.2, 2.8 ≤ y ≤ 3, 1.1 ≤ x - y ≤ 1.4.

[0008] The chemical formula is Na 4.12 Fe 2.85 (PO4)2P2O7.

[0009] A cathode material for a sodium-ion battery, with the median particle size D50 ranging from 2.5 microns to 7 microns and a tap density of 0.6 - 1.2 g / cm3 。

[0010] An electrode sheet, comprising the sodium-ion battery cathode material as described above.

[0011] A battery, comprising the electrode sheet as claimed in claim 3.

[0012] A method for preparing a sodium-ion battery cathode material. S1: Calcine the precursor of the sodium-ion battery cathode material at 550 - 600 °C for a heat preservation time of 1 - 4 h, then cool it to 450 - 500 °C at a cooling rate of 5 - 10 °C / min and keep it warm for 8 - 12 h, or heat the precursor of the sodium-ion battery cathode material to 600 - 650 °C at a rate of 1 - 10 °C / min for calcination and then quench it, or heat the precursor of the sodium-ion battery cathode material to 600 - 700 °C at a rate of 1 - 10 °C / min for calcination and then quench and temper it, with the tempering temperature being 300 - 400 °C and the tempering time being 2 - 8 h; S2: Crush the semi-finished product obtained by sintering in S1 to obtain the sodium iron pyrophosphate cathode material.

[0013] The preparation of the precursor of the sodium-ion battery cathode material is as follows: Dissolve the sodium source, phosphorus source, iron source, and carbon source in pure water and stir well to form a slurry, then grind the slurry to a certain particle size, and then dry the slurry to obtain it.

[0014] The sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium acetate, sodium tartrate, sodium citrate, and sodium hydroxide. The phosphorus source is one or more of sodium dihydrogen phosphate, ammonium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, and phosphoric acid. The iron source is preferably one or more of ferrous oxalate, iron phosphate, ferrous acetate, and iron citrate.

[0015] The residual carbon rate of the carbon source ≤ 8%, and it is one or more of citric acid, PEG (molecular weight 200), polytetrafluoroethylene, paraffin, and polyvinyl alcohol. The added mass of the carbon source is 1 - 20% of the added mass of the iron source.

[0016] The drying method is spray drying, and the conditions are: inlet air temperature 160 - 280 °C, outlet air temperature 90 - 105 °C, and the median particle size of the slurry 0.15 ≤ D50 ≤ 0.35 microns.

[0017] When performing the crushing in S2 above, it can be jet milling, with the grinding air pressure being 0.3 - 0.4 Mpa and the classification wheel frequency being 100 - 150 Hz.

[0018] The inventor found that if x - y > 1.4, the activity of the sodium iron pyrophosphate / / air interface is enhanced, the air stability is poor, and it is extremely easy to form Na 3.12 Fe 2.44Insulators such as P2O7, Na2HPO4, and Na3PO4 increase the internal resistance of the battery, resulting in poor rate performance. More conductive carbon needs to be added during homogenization. If x - y < 1.1, sodium elements are easily consumed during the synthesis of the precursor. There are a large number of sodium ion vacancies in sodium iron pyrophosphate phosphate, and the material structure is extremely unstable, with a relatively low actual specific capacity. Therefore, the difference between the present invention and other technical solutions lies in that x - y is limited to an appropriate range, that is, 1.1 ≤ x - y ≤ 1.4.

[0019] In the preparation method, if the median particle size of the slurry > 0.45 microns, the bonding effect between the elements in the precursor is poor, and phase separation occurs after sintering, with obvious NaFePO4 impurity phases. If the median particle size of the slurry < 0.15 microns, the performance of the sodium iron pyrophosphate phosphate material does not improve significantly at this time, but the energy consumption during the material synthesis process increases. The feasible distribution range of the median particle size of the slurry in the present invention is wider, and no special modification of the sanding equipment is required.

[0020] Most of the current existing technologies are one-step sintering or two-step sintering. In the two-step sintering, the temperature of the first-step sintering is usually lower than 400 °C, and the second-step sintering is when sodium iron pyrophosphate phosphate is formed by reaction. The temperature of the first-step sintering in the solution of the present invention is higher than the sintering temperature of other existing technologies, and the raw materials fully react or decompose. By controlling the reaction time or changing the cooling process, NaFePO4 can be reversibly transformed into sodium iron pyrophosphate phosphate, and the tap density of the material is increased.

[0021] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0022] The present invention substantially improves the sintering method of sodium iron pyrophosphate phosphate, controls the heating-cooling process, optimizes the structure improvement of the cathode material, and controls its chemical formula Na x Fe y (PO4)2P2O7, 3.92 ≤ x ≤ 4.2, 2.8 ≤ y ≤ 3, 1.1 ≤ x - y ≤ 1.4, which improves the tap density and specific capacity of sodium iron pyrophosphate phosphate, and the capacity retention rate can reach more than 83% after being placed in the atmospheric environment for 10 days. Detailed implementation mode

[0023] To make the purpose, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the embodiments. The illustrative embodiments of the present invention and their descriptions are only used to explain the present invention and are not used to limit the present invention.

[0024] Example 1

[0025] 1. Dissolve 0.297 mol of sodium acetate, 0.285 mol of iron phosphate (i.e., 42.98 g of iron phosphate), 0.115 mol of sodium dihydrogen phosphate, and 4.298 g of citric acid in pure water and stir well to form a slurry. Then grind the slurry until the median particle size D50 = 0.25 microns. Subsequently, spray-dry the slurry at an inlet air temperature of 200 °C and an outlet air temperature of 95 °C to obtain the precursor powder.

[0026] 2. Heat the precursor powder obtained by drying in step 1 at a heating rate of 3 °C / min under a N2 atmosphere to 600 °C and sinter for 1 h. Then cool it at a cooling rate of 10 °C / min to 450 °C and sinter for 11 h. Then, perform air jet milling at a grinding air pressure of 0.3 Mpa and a classifier wheel frequency of 150 Hz to obtain a sodium iron pyrophosphate phosphate cathode material with D50 = 3.1 microns, and the structural formula is Na 4.12 Fe 2.85 (PO4)2P2O7 / C.

[0027] Example 2

[0028] The difference between Example 2 and Example 1 lies in step 2: Heat the precursor powder obtained by drying in step 1 at a heating rate of 3 °C / min under a N2 atmosphere to 600 °C and sinter for 1 h. Then cool it at a cooling rate of 10 °C / min to 500 °C and sinter for 11 h. Then, perform air jet milling at a grinding air pressure of 0.3 Mpa and a classifier wheel frequency of 150 Hz to obtain a sodium iron pyrophosphate phosphate cathode material with D50 = 3.1 microns, and the structural formula is Na 4.12 Fe 2.85 (PO4)2P2O7 / C.

[0029] Example 3

[0030] The difference between Example 3 and Example 1 lies in step 2: Heat the precursor powder obtained by drying in step 1 at a heating rate of 3 °C / min under a N2 atmosphere to 600 °C and sinter for 4 h. Then cool it at a cooling rate of 10 °C / min to 450 °C and sinter for 11 h. Then, perform air jet milling at a grinding air pressure of 0.3 Mpa and a classifier wheel frequency of 150 Hz to obtain a sodium iron pyrophosphate phosphate cathode material with D50 = 3.1 microns, and the structural formula is Na 4.12 Fe 2.85 (PO4)2P2O7 / C.

[0031] Example 4

[0032] Example 4 is different from Example 1 in Step 2: The precursor powder obtained by drying in Step 1 is heated at a heating rate of 3 °C / min under a N2 atmosphere to 600 °C and sintered for 1 h, then cooled to 450 °C at a cooling rate of 10 °C / min and sintered for 6 h, and then air-classified milled at a grinding air pressure of 0.3 Mpa and a classifier wheel frequency of 150 Hz to obtain a sodium iron pyrophosphate phosphate cathode material with D50 = 3.1 microns and a structural formula of Na 4.12 Fe 2.85 (PO4)2P2O7 / C.

[0033] Example 5

[0034] Example 5 is different from Example 1 in Step 2: The precursor powder obtained by drying in Step 1 is heated at a heating rate of 3 °C / min under a N2 atmosphere to 600 °C and sintered for 1 h, then cooled to 450 °C at a cooling rate of 5 °C / min and sintered for 11 h, and then air-classified milled at a grinding air pressure of 0.3 Mpa and a classifier wheel frequency of 150 Hz to obtain a sodium iron pyrophosphate phosphate cathode material with D50 = 3.1 microns and a structural formula of Na 4.12 Fe 2.85 (PO4)2P2O7 / C.

[0035] Example 6

[0036] Example 6 is different from Example 1 in Step 2: The precursor powder obtained by drying in Step 1 is heated at a heating rate of 3 °C / min under a N2 atmosphere to 550 °C and sintered for 1 h, then cooled to 450 °C at a cooling rate of 10 °C / min and sintered for 11 h, and then air-classified milled at a grinding air pressure of 0.3 Mpa and a classifier wheel frequency of 150 Hz to obtain a sodium iron pyrophosphate phosphate cathode material with D50 = 3.1 microns and a structural formula of Na 4.12 Fe 2.85 (PO4)2P2O7 / C.

[0037] Example 7

[0038] Example 7 is different from Example 1 in Step 2: The precursor powder obtained by drying in Step 1 is heated at a heating rate of 3 °C / min under a N2 atmosphere to 600 °C and then quenched, and then air-classified milled at a grinding air pressure of 0.3 Mpa and a classifier wheel frequency of 150 Hz to obtain a sodium iron pyrophosphate phosphate cathode material with D50 = 3.1 microns and a structural formula of Na 4.12 Fe 2.85 (PO4)2P2O7 / C.

[0039] Example 8

[0040] Example 8 differs from Example 1 in Step 2: The precursor powder obtained by drying in Step 1 is heated at a heating rate of 3 °C / min under a N2 atmosphere to 600 °C and then quenched, and then sintered at 600 °C for 11 h again, and then air-flow pulverized under a grinding air pressure of 0.3 Mpa and a classifier wheel frequency of 150 Hz to obtain a sodium iron pyrophosphate phosphate cathode material with D50 = 3.1 μm, and the structural formula is Na 4.12 Fe 2.85 (PO4)2P2O7 / C.

[0041] Comparative Example 1

[0042] Comparative Example 1 differs from Example 1 in Step 2: The precursor powder obtained by drying in Step 1 is heated at a heating rate of 3 °C / min under a N2 atmosphere to 600 °C and sintered for 11 h, and then air-flow pulverized under a grinding air pressure of 0.3 Mpa and a classifier wheel frequency of 150 Hz to obtain a sodium iron pyrophosphate phosphate cathode material with D50 = 3.1 μm, and the structural formula is Na 4.12 Fe 2.85 (PO4)2P2O7 / C.

[0043] Comparative Example 2

[0044] Comparative Example 2 differs from Example 1 in Step 2: The precursor powder obtained by drying in Step 1 is heated at a heating rate of 3 °C / min under a N2 atmosphere to 450 °C and sintered for 11 h, and then air-flow pulverized under a grinding air pressure of 0.3 Mpa and a classifier wheel frequency of 150 Hz to obtain a sodium iron pyrophosphate phosphate cathode material with D50 = 3.1 μm, and the structural formula is Na 4.12 Fe 2.85 (PO4)2P2O7 / C.

[0045] Comparative Example 3

[0046] Comparative Example 3 differs from Example 1 in Step 1: 0.345 mol of sodium acetate, 0.285 mol of iron phosphate, i.e., 42.98 g of iron phosphate, 0.115 mol of sodium dihydrogen phosphate, and 4.298 g of citric acid are dissolved in pure water and stirred well to form a slurry, and then the slurry is ground to a median particle size D50 = 0.25 μm, and then the slurry is spray-dried at an inlet air temperature of 200 °C and an outlet air temperature of 95 °C to obtain a precursor powder, and the structural formula is Na 4.6 Fe 2.85 (PO4)2P2O7 / C. That is, x - y = 1.75.

[0047] The difference between Comparative Example 4 and Example 1 lies in Step 1: Dissolve 0.25 mol of sodium acetate, 0.285 mol of iron phosphate (i.e., 42.98 g of iron phosphate), 0.115 mol of sodium dihydrogen phosphate, and 4.298 g of citric acid in pure water and stir well to form a slurry. Subsequently, grind the slurry to a median particle size D50 = 0.25 microns. Then, spray-dry the slurry at an inlet air temperature of 200 °C and an outlet air temperature of 95 °C to obtain a precursor powder with the structural formula Na 3.65 Fe 2.85 (PO4)2P2O7 / C. That is, x - y = 0.8.

[0048] Prepare batteries from the examples and comparative examples, and the method is as follows: Prepare a positive electrode plate from the sodium iron pyrophosphate phosphate positive electrode material, PVDF, and acetylene black according to a mass ratio of 8:1:1. Use metallic sodium as the negative electrode, and use 1 mol / L NaClO4 / EC + DEC (Vol% 1:1) as the electrolyte to form a button cell. Evaluate the battery performance of the positive electrode material through a Blue Power battery test system, and the battery charge and discharge range is 2 - 4V.

[0049] The test method for the median particle size D50 of the present invention is as follows: Dissolve the sample to be tested in a certain amount of deionized water and ultrasonically disperse it for 1 minute, then drop it into the sample cell of a Mastersizer 3000 particle size tester, and measure the median particle size of the sample according to the program.

[0050] The test method for the tapped density of the present invention is as follows: Place the sodium iron pyrophosphate phosphate positive electrode material in a tapped density tester and measure the tapped density of the powder. The specific test results are shown in Table 1.

[0051] Table 1 shows the electrical performance data and tapped density data of the examples and comparative examples of the present invention

[0052]

[0053] The present invention also tested the capacity retention rate of several samples after being placed in the atmospheric environment for 10 days. For example, the performance of Examples 2 and 3 is better than that of Comparative Examples 1 and 2.

[0054] The specific embodiments described above further elaborate on the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only the specific embodiments of the present invention and is not used to limit the protection scope of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A cathode material for a sodium-ion battery, characterized in that, The structural formula is Na x Fe y (PO4)2P2O7, 3.92 ≤ x ≤ 4.2, 2.8 ≤ y ≤ 3, 1.1 ≤ x - y ≤ 1.

4.

2. The cathode material for a sodium-ion battery according to claim 1, wherein The structural formula is Na 4.12 Fe 2.85 (PO4)2P2O7.

3. The positive electrode material of a sodium ion battery according to claim 1, wherein the median particle size D50 ranges from 2.5 micrometers to 7 micrometers, and the tapped density is 0.6 - 1.2 g / cm 3 .

4. An electrode sheet, comprising the sodium-ion battery cathode material as described in claim 1 or 2.

5. A battery, comprising the electrode sheet as described in claim 3.

6. The preparation method of a cathode material for a sodium-ion battery according to claim 1 or 2, characterized in that, S1. Calcine the precursor of the sodium-ion battery cathode material at 550 - 600 °C for a heat preservation time of 1 - 4 h, then cool it to 450 - 500 °C at a cooling rate of 5 - 10 °C / min and keep it warm for 8 - 12 h, or heat the precursor of the sodium-ion battery cathode material to 600 - 650 °C at a heating rate of 1 - 10 °C / min and then quench it, or heat the precursor of the sodium-ion battery cathode material to 600 - 700 °C at a heating rate of 1 - 10 °C / min and then quench and temper it, with the tempering temperature being 300 - 400 °C and the tempering time being 2 - 8 h; S2. Crush the semi-finished product obtained by sintering in S1 to obtain the sodium iron pyrophosphate cathode material.

7. The preparation method according to claim 5, wherein The preparation of the precursor of the sodium-ion battery cathode material is as follows: dissolve the sodium source, phosphorus source, iron source, and carbon source in pure water and stir well to form a slurry, then grind the slurry to a certain particle size, and then dry the slurry to obtain it.

8. The preparation method according to claim 6, characterized in that The sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium acetate, sodium tartrate, sodium citrate, and sodium hydroxide; the phosphorus source is one or more of sodium dihydrogen phosphate, ammonium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, and phosphoric acid; the iron source is one or more of ferrous oxalate, iron phosphate, ferrous acetate, and iron citrate.

9. The preparation method according to claim 7, wherein, The residual carbon rate of the carbon source is ≤ 8%, and it is one or more of citric acid, PEG, polytetrafluoroethylene, paraffin, and polyvinyl alcohol. The added mass of the carbon source is 1 - 20% of the added mass of the iron source.

10. The preparation method according to claim 6, characterized in that, The drying method is spray drying, and the conditions are: inlet air temperature 160 - 280 °C, outlet air temperature 90 - 105 °C, and the median particle size of the slurry 0.15 ≤ D50 ≤ 0.35 μm.

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