Iron-based sodium pyrophosphate / C composites, their preparation, and their application in sodium-ion batteries

Abstract

This invention belongs to the field of sodium-ion battery cathode materials, specifically disclosing a method for preparing a sodium iron pyrophosphate / C composite material. The method involves pre-reacting iron source A and iron source B to obtain a composite iron source, then adding sodium, phosphorus, and carbon sources for mechanical mixing, followed by solvent removal to obtain precursor powder. The precursor powder is then subjected to high-temperature calcination to obtain the sodium iron pyrophosphate / C composite material. This invention also includes the material prepared by the described method and its applications. The material prepared by this method exhibits excellent fast-charging performance and long-cycle stability.

Description

Technical fields:

[0001] This invention belongs to the field of sodium-ion battery material technology, specifically relating to an iron-based composite phosphate cathode material for sodium-ion batteries. Background technology:

[0002] The reserves and price of lithium resources influence the widespread application of green energy. Green energy, represented by new power generation devices such as wind and solar power, urgently needs the support of large-scale energy storage equipment to break through its current development scale. Sodium-ion batteries, due to their low price advantage, have become a hot topic in academic and industrial research, and the development of energy storage also brings new development opportunities to sodium-ion batteries.

[0003] As a crucial component of sodium-ion batteries, the capacity of the cathode material limits the improvement of its energy density and also determines its cost. Iron-based sodium phosphate pyrophosphates, such as the commonly used Na3Fe2PO4P2O7 and Na4Fe3(PO4)2P2O7, possess high theoretical specific capacity and high discharge voltage, making them among the most commercially promising cathode materials for sodium-ion batteries.

[0004] Iron-based sodium pyrophosphate is mainly obtained by one-pot sintering of iron, sodium, and phosphorus sources. Among these, the iron source is a crucial raw material affecting the material's properties. For example, Chinese patent document CN116565165A discloses a bicontinuous phase-coated sodium iron pyrophosphate cathode material and its preparation method, where the described iron source includes one or more of ferrous oxalate, ferric oxalate, ferrous sulfate, ferrous ammonium sulfate, magnetite, iron oxide, and iron phosphate. As another example, Chinese patent document CN117393750A discloses a sodium iron pyrophosphate material, with the described iron source being ferric nitrate or ferric sulfate.

[0005] In summary, although existing processes can successfully synthesize iron-based phosphate sodium pyrophosphate and, with continuous technological evolution, can effectively control its impurity phases, the control of impurities and grain boundaries in existing processes still needs further improvement. The material's excessively large specific surface area makes it difficult to adapt well to complex electrochemical environments and application requirements under high-rate and fast-charging conditions. Summary of the Invention:

[0006] To address the issues of unsatisfactory rate capability and fast-charging stability of existing iron-based sodium pyrophosphate composites, this invention provides a method for preparing iron-based sodium pyrophosphate / C composite materials, aiming to improve the rate capability and fast-charging stability of the prepared iron-based sodium pyrophosphate / C composite materials.

[0007] The second objective of this invention is to provide an iron-based sodium pyrophosphate / C composite material prepared by the aforementioned method and its application in sodium-ion batteries.

[0008] A third objective of this invention is to provide a sodium-ion battery comprising the aforementioned iron-based sodium pyrophosphate / C composite material, as well as its positive electrode and positive electrode material.

[0009] To address the shortcomings of existing methods for preparing iron-based sodium pyrophosphate / C composite materials, such as difficulties in effectively resolving issues related to impurities and grain boundary matching, and in meeting the requirements for fast-charging stability at high rates, this invention provides the following improvements:

[0010] A method for preparing an iron-based sodium pyrophosphate / C composite material involves combining an iron source, a phosphorus source, a sodium source, and a carbon source to obtain a precursor, which is then calcined to obtain the final product.

[0011] The iron source includes iron source A and iron source B, wherein iron source A is Fe4(P2O7)3, and iron source B includes at least one of ferrous oxalate, ferric oxide, ferrous oxide, ferric oxide, ferric phosphate, and iron powder, wherein the iron in iron source A is 40-70% of the total iron molar amount.

[0012] This invention innovatively uses Fe4(P2O7)3 as iron source A and other iron-containing components as iron source B, and innovatively combines iron source A and iron source B. This unexpectedly achieves synergy, shortens the reaction process, reduces the occurrence of side reactions, and can fundamentally control the impurity phase of the synthesized material and control the grain boundary matching degree of the material. This unexpectedly improves the high-rate stability of the prepared material, thereby improving its fast-charging performance.

[0013] In this invention, the chemical formula of the iron-based phosphate sodium pyrophosphate is Na. a Fe b (PO4) c (P2O7) d Wherein, a is 2-5, b is 1-4, c is 1-3, d is 1-2, and a / b / c / d satisfies positive and negative charge balance. For example, as a typical embodiment, the iron-based phosphate sodium pyrophosphate includes at least one of Na4Fe3(PO4)2P2O7, Na3Fe2PO4P2O7, and Na5Fe4(PO4)3P2O7.

[0014] In this invention, the iron source, phosphorus source, and sodium source are formulated according to the stoichiometric ratio of Na, Fe, and P elements in the iron-based phosphate sodium pyrophosphate salt.

[0015] In this invention, the iron source A is a composition known in the industry, and its D50 is preferably 150-500nm, and can be further 200-400nm.

[0016] Preferably, the iron source B is at least one selected from ferric oxide, ferrous oxide, magnetite, ferric phosphate, and iron powder. Preferably, the D50 of the iron source B is 200-1000 nm, and more preferably 400-850 nm.

[0017] Preferably, in the iron source, the D50 of iron source A is 200-400 nm, and the D50 of iron source B is 1.9-2.5 times that of iron source A. This invention unexpectedly demonstrates that using a gradation combination of iron sources A and B with preferred particle sizes can further synergistically improve the phase composition and crystal structure of the product, contributing to further improvement in its fast-charging stability.

[0018] Preferably, in the present invention, the iron in the iron source A is 45-60% of the total iron molar amount.

[0019] In a further preferred embodiment of the present invention, iron source A and iron source B are pre-mixed and pre-treated at a temperature of 200–300°C (more preferably 230–270°C), and then compounded with positive electrode raw materials of phosphorus source and sodium source and carbon source to obtain the precursor. In the present invention, the pretreatment time can be 1–5 hours (more preferably 2–4 hours).

[0020] This invention demonstrates that pretreatment of iron sources A and B can synergistically improve the phase composition and crystal structure of the product, thereby contributing to further improvement in its fast-charging stability.

[0021] In this invention, the sodium source can be a sodium-containing material known in the industry that can be used to prepare cathode materials, such as, but not limited to, at least one of sodium carbon source, sodium bicarbonate, sodium acetate, sodium dihydrogen phosphate, disodium monohydrogen phosphate, sodium hydroxide, sodium citrate, and sodium pyrophosphate.

[0022] In this invention, the phosphorus source can be a phosphorus-containing material known in the industry that can be used to prepare cathode materials, such as, but not limited to, at least one of ammonium dihydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, disodium dihydrogen pyrophosphate, and diammonium hydrogen phosphate.

[0023] In this invention, the carbon source can be an organic carbon material known in the industry that can be used to prepare cathode materials, such as, but not limited to, at least one of glucose, sucrose, polyethylene glycol, lauric acid, phenolic resin, soluble starch, citric acid, and oxalic acid.

[0024] In this invention, the amount of carbon source can be adjusted based on conventional industry knowledge, for example, it can be 5 to 15% of the designed weight of iron-based phosphate sodium pyrophosphate.

[0025] In this invention, the precursor is obtained by mixing the raw materials in a solid phase, or by mixing them in a liquid phase and then drying them.

[0026] In this invention, the solvent for liquid-phase mixing includes at least one of water and alcohols such as ethanol;

[0027] In this invention, the solid-phase mixing and liquid-phase mixing processes are carried out with mechanical assistance.

[0028] The mechanical aids mentioned are, for example, ball milling, sand milling, grinding, etc., and there are no special requirements for the specific processing technology. For example, it can be sand milling, and the rotation speed can be 1000-2000 rpm, or even 1400-1800 rpm.

[0029] In this invention, the sanding time can be 5 to 60 minutes;

[0030] In this invention, the drying method can be conventional, such as spray drying. There are no special requirements for the conditions of spray drying; for example, the inlet temperature of the spray drying stage is 130-240℃, the outlet temperature is 70-130℃, and the spray rate is 10-100 L / h.

[0031] In this invention, the atmosphere during roasting is nitrogen or argon.

[0032] Preferably, the calcination temperature is 450–630°C, more preferably 500–570°C;

[0033] Preferably, the roasting time is 5-16 hours, more preferably 8-12 hours;

[0034] Preferably, the heating rate during calcination is 1–10 °C / min.

[0035] The present invention provides an example of a method for preparing a Na4Fe3(PO4)2P2O7 / C composite cathode material, wherein sodium source, iron source A and iron source B, phosphorus source and carbon source are prepared by sand milling according to the stoichiometric ratio of Na / Fe / P to obtain a nanoscale slurry, which is then spray-dried to obtain a precursor powder, and combined with a one-step high-temperature sintering to obtain the Na4Fe3(PO4)2P2O7 / C composite material.

[0036] The present invention also provides an iron-based sodium pyrophosphate / C composite material prepared by the preparation method described above.

[0037] In this invention, the preparation method described herein can endow the obtained product with special microscopic physicochemical characteristics, and the iron-based sodium pyrophosphate / C composite material with the aforementioned characteristics obtained by the preparation method has excellent high-rate fast charging stability.

[0038] In this invention, the content of the carbon coating layer in the iron-based sodium pyrophosphate / C composite material is 1-5 wt%.

[0039] In this invention, the particle size of the iron-based sodium pyrophosphate / C composite material is 50–300 nm.

[0040] The present invention also provides a positive electrode material for a sodium-ion battery, including an active material, wherein the active material includes an iron-based sodium pyrophosphate / C composite material prepared by the preparation method described in the present invention.

[0041] In this invention, the content of the iron-based sodium pyrophosphate / C composite material in the active material is above 50 wt.%; more specifically, it can be an iron-based sodium pyrophosphate / C composite material.

[0042] In this invention, the positive electrode material further comprises a conductive agent and / or a binder. The conductive agent and binder can be components known in the industry, and their content can be adjusted as needed.

[0043] In this invention, the content of the positive electrode active material in the positive electrode material is 70-90 wt.%. The content of the conductive agent and binder can be adjusted according to common industry practices, for example, less than or equal to 15 wt%, and more specifically, 1-10 wt%.

[0044] The present invention also provides a positive electrode for a sodium-ion battery, comprising a current collector and a positive electrode material composited thereon, wherein the positive electrode material is a positive electrode material comprising the iron-based sodium pyrophosphate / C composite material described in the present invention.

[0045] The present invention also provides a sodium-ion battery comprising the positive electrode comprising the iron-based sodium pyrophosphate / C composite material described in the present invention.

[0046] The sodium-ion battery and its positive electrode and positive electrode material described in this invention, except for the iron-based sodium pyrophosphate / C composite material described in this invention, can have other conventional components and parts.

[0047] This invention has the following significant features:

[0048] (1) Innovatively, this invention uses Fe4(P2O7)3 as iron source A and further combines it with iron source B. This unexpectedly achieves synergy, improves the crystal phase and crystal structure of the product, and unexpectedly significantly improves the fast charging stability of the prepared material at high rates.

[0049] (2) In this invention, under the combined use of iron source A and iron source B, the particle size distribution or pretreatment of the components is further improved, which can unexpectedly improve the synergy between the two, help to further optimize the crystal phase and crystal structure of the product, and unexpectedly significantly improve the fast charging stability of the prepared material at high rates. Attached Figure Description

[0050] Figure 1 The XRD pattern of the Na4Fe3(PO4)2P2O7 / C composite material prepared in Example 1 of this invention.

[0051] Figure 2 The image shows the SEM image of the Na4Fe3(PO4)2P2O7 / C composite material prepared in Example 1 of this invention. Detailed Implementation

[0052] Example 1

[0053] (1) Iron source, ammonium dihydrogen phosphate, and sodium carbonate are prepared according to the molar ratio of Na, Fe, and P elements of 4:3:4. The iron source is iron source A (Fe4(P2O7)3, D50 is 395nm) and iron source B (ferric oxide, D50 is 403nm). Iron in iron source A accounts for 45% of the total iron element molar amount. Then, it is mixed with carbon source (citric acid, the weight of carbon source is 10% of the theoretical sodium iron pyrophosphate). Water is added as a solvent (liquid-solid ratio is 1-2 ml / g). The mixture is then sand-milled at a speed of 1500-2000 rpm for 40-60 min. After that, it is transferred to the spray drying process with an inlet temperature of 200℃, an outlet temperature of 120℃, and a spray rate of 25 L / h to prepare the precursor powder.

[0054] (2) The precursor powder in step (1) is placed in a box furnace and calcined at high temperature under nitrogen protection. The calcination temperature is 500℃, the time is 10h, and the heating rate is 2℃ / min. High specific capacity, pure phase Na4Fe3(PO4)2P2O7 / C composite material (active material) can be obtained.

[0055] Example 2

[0056] Compared to Example 1, the only difference is that the type of iron source B is changed, and the experimental groups are as follows:

[0057] Group A: Iron source B is ferric oxalate;

[0058] Group B: Iron source B is iron phosphate;

[0059] Other operations and parameters are the same as in Example 1.

[0060] Example 3

[0061] Compared with Example 1, the only difference is that the particle sizes of iron source A and iron source B are changed, specifically:

[0062] Group A: The D50 particle size of iron source A is 201 nm;

[0063] Group B: The D50 particle size of iron source B is 204 nm;

[0064] Group C: The D50 particle size of iron source B is 801 nm;

[0065] Other operations and parameters are the same as in Example 1.

[0066] Example 4

[0067] Compared with Example 1, the only difference is that after the iron source A and B solid phases are mixed, they are pretreated in air atmosphere at 250°C for 2 hours, and then mixed and prepared according to the method of Example 1.

[0068] Other operations and parameters are the same as in Example 1.

[0069] Example 5

[0070] Compared with Example 1, the only difference is that the iron content in iron source A is adjusted to 60% of the total iron element molar amount.

[0071] Other operations and parameters are the same as in Example 1.

[0072] Example 6

[0073] Compared with Example 1, the only difference is that in step 1, the inlet temperature of the spray drying process is 230°C, the outlet temperature is 110°C, the spray rate is 30L / h, and the precursor powder is prepared; and in step 2, the calcination temperature is 570°C, the heating rate is 4°C / min, and the calcination holding time is 8h. Other operations and parameters are the same as in Example 1.

[0074] Comparative Example 1

[0075] Compared to Example 1, the only difference is that iron source A is missing, and the required amount of iron is provided by iron source B. All other operations and parameters are the same as in Example 1.

[0076] Comparative Example 2

[0077] Compared with Example 1, the only difference is that iron source A is replaced with iron phosphate, wherein the molar amount of iron in iron phosphate is the same as that in iron source A, and all other operations and parameters are the same as in Example 1.

[0078] Comparative Example 3

[0079] Compared to Example 1, the only difference is that iron source B is missing, and all required iron is provided through iron source A. All other operations and parameters are the same as in Example 1.

[0080] Comparative Example 4

[0081] Compared with Example 1, the only difference is that the molar proportion of iron source A in the iron source is adjusted to 20%, and all other operations and parameters are the same as in Example 1.

[0082] The electrical properties of sodium iron pyrophosphate prepared in each embodiment and comparative example were tested:

[0083] The main steps of the test are as follows:

[0084] (1) Using a 2032 model battery case, the positive electrode is a sodium ferrous fluorophosphate electrode, the current collector is aluminum foil, the active material (in each example and comparative example): conductive carbon (acetylene black): PVDF weight ratio = 9:0.5:0.5, the negative electrode is sodium metal, using a fiber separator (model Whatman Grade GF / D), and the electrolyte is 1M NaPF6 (EC / PC+5% FEC) to pack the battery;

[0085] (2) Set the resting time for 10 hours, set a charge-discharge program with a rate of 10C (1C = 129mA / g), a voltage range of 1.7V-4.3V, and 3000 cycles (the temperature during the cycle is 25℃).

[0086] (3) The theoretical specific capacity of sodium iron pyrophosphate is 129 mAh / g;

[0087] The test results are shown in Table 1:

[0088] Table 1

[0089]

[0090]

[0091] By combining the iron sources A and B in the aforementioned ratio, a material with fast charging stability can be unexpectedly obtained through sintering. Furthermore, a comparison of Examples 3A, 3C, and 3B shows that using D50... 铁源B / D50 铁源A The gradation method of 1.9 to 2.5 times, especially the control of the range of iron source A (3A and 3C), can unexpectedly further synergistically improve the fast-charging stability of the prepared material. Moreover, the present invention also shows that the pre-treatment of iron source A and B (Example 4) can further synergistically improve the fast-charging stability of the prepared material.

Claims

1. A method for preparing an iron-based sodium pyrophosphate / C composite material, comprising combining an iron source, a phosphorus source, a sodium source, and a carbon source to obtain a precursor, followed by calcination treatment to obtain the final product; characterized in that, The iron source includes iron source A and iron source B, wherein iron source A is Fe4(P2O7)3, and iron source B includes at least one of ferrous oxalate, ferric oxide, ferrous oxide, ferric oxide, ferric phosphate, and iron powder, wherein the iron in iron source A accounts for 40-70% of the total iron molar amount; The iron-based phosphate sodium pyrophosphate salt comprises Na4Fe3(PO4)2P2O7; the iron source, phosphorus source, and sodium source are formulated according to the stoichiometric ratio of Na, Fe, and P elements in the iron-based phosphate sodium pyrophosphate salt. The D50 of iron source A is 200~400nm, and the D50 of iron source B is 1.9~2.5 times that of iron source A, or the D50 of iron source B is 400~850nm. Iron source A and iron source B are pre-mixed and pre-treated at a temperature of 200~300℃, and then combined with positive electrode raw materials of phosphorus source and sodium source and carbon source to obtain the precursor.

2. The preparation method of the iron-based sodium pyrophosphate / C composite material as described in claim 1, characterized in that, The pretreatment time is 1 to 5 hours.

3. The method for preparing the iron-based sodium pyrophosphate / C composite material as described in claim 1, characterized in that, The sodium source includes at least one of sodium carbonate, sodium bicarbonate, sodium acetate, sodium dihydrogen phosphate, disodium monohydrogen phosphate, sodium hydroxide, sodium citrate, and sodium pyrophosphate. The phosphorus source includes at least one of ammonium dihydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, disodium dihydrogen phosphate, and diammonium hydrogen phosphate. The carbon source includes at least one of glucose, sucrose, polyethylene glycol, lauric acid, phenolic resin, soluble starch, citric acid, and oxalic acid. The carbon source is 5-15% of the designed weight of iron-based phosphate sodium pyrophosphate.

4. The method for preparing the iron-based sodium pyrophosphate / C composite material as described in claim 3, characterized in that, The precursor is obtained by mixing the raw materials in a solid phase, or by mixing them in a liquid phase and then drying them. The solvent for the liquid phase mixture includes at least one of water and alcohol; Solid-phase mixing and liquid-phase mixing processes are carried out with mechanical assistance; The drying method described is spray drying.

5. The method for preparing the iron-based sodium pyrophosphate / C composite material as described in claim 1, characterized in that, The roasting temperature is 450~630℃; The roasting time is 5-16 hours; The heating rate during calcination is 1~10℃ / min; The atmosphere during roasting is nitrogen or argon.

6. The method for preparing the iron-based sodium pyrophosphate / C composite material as described in claim 1, characterized in that, The roasting temperature is 500~570℃; The roasting time is 8~12 hours.

7. An iron-based sodium pyrophosphate / C composite material prepared by the preparation method according to any one of claims 1 to 6.

8. A positive electrode material for a sodium-ion battery, comprising an active material, characterized in that, The active material includes the iron-based sodium pyrophosphate / C composite material prepared by the preparation method according to any one of claims 1 to 6.

9. The positive electrode material of the sodium-ion battery as described in claim 8, characterized in that, In the active material, the content of the iron-based sodium pyrophosphate / C composite material is above 50 wt.%.

10. The positive electrode material of the sodium-ion battery as described in claim 8 or 9, characterized in that, It also contains conductive agents and / or binders; In the aforementioned cathode material, the content of the active material is 70~90 wt.%.

11. A positive electrode for a sodium-ion battery, comprising a current collector and a positive electrode material composited thereon, characterized in that, The cathode material is the cathode material according to any one of claims 8 to 10.

12. A sodium-ion battery, characterized in that, It includes the positive electrode as described in claim 11.

Images