A high-capacity Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode material, its preparation method, and its application.

Abstract

The application relates to the technical field of sodium ion battery positive electrode materials, in particular to a Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT positive electrode material.The Na2Fe2(SO4)3 and Na3Fe2(SO4)3F precursors are mixed and sintered, and are coated by CNT, so that the electrochemical performance of the Na2Fe2(SO4)3 is remarkably improved, the conductivity of the material is improved, the material can be used as a sodium ion battery positive electrode material, has a high discharge specific capacity, and has cycle performance and rate performance.

Description

Technical Field

[0001] This invention relates to the technical field of electrode materials, specifically to a high-capacity Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode material, its preparation method, and its application. Background Technology

[0002] In recent years, the development of new energy storage systems based on rechargeable batteries has become crucial. Sodium-ion batteries, due to the abundance and low cost of sodium resources, are expected to become an important tool in these new energy storage systems. The cathode material, as a vital component of the battery system, determines the power density and energy density of sodium-ion batteries. Considering the low voltage and poor cycle stability of layered materials, sodium iron sulfate, a polyanionic material, has attracted widespread attention due to its high operating voltage, high energy density, and excellent cycle stability, and is considered a promising candidate for cathode materials in sodium-ion batteries.

[0003] However, for Na2Fe2(SO4)3 materials, incomplete reactions during sintering lead to the formation of FeSO4 impurities, and poor conductivity result in low actual reversible specific capacity and poor cycle performance. Chinese patents CN 110931784 A and CN 111063871 A, in order to suppress irreversible oxidation of iron and the formation of the Fe(SO4)3 impurity phase during preparation and improve the electrochemical performance of sodium ferric sulfate, use F as a dopant element to change the electron cloud arrangement of sodium ferric sulfate, thus preparing sodium fluoroferric sulfate with a high discharge specific capacity. However, by comparing the XRD patterns and charge-discharge curves of sodium ferric sulfate and sodium fluoroferric sulfate, it was found that sodium ferric sulfate doped with F lost its original characteristic plateau, and the doping of F altered the crystal structure of sodium ferric sulfate. For sodium ferric sulfate, the low discharge specific capacity and poor cycle stability of the original material limit its development. In order to maintain the original bulk phase and charge / discharge plateau of sodium ferric sulfate and improve the sodium storage performance of the material, this invention provides a method to improve the discharge specific capacity of Na2Fe2(SO4)3. By combining it with Na3Fe2(SO4)3F, which has good conductivity and high sodium storage capacity, Na3Fe2(SO4)3F is embedded in the bulk phase of Na2Fe2(SO4)3. The resulting Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode material has the characteristics of both in terms of charge / discharge plateau and XRD, indicating that the material can inherit the advantages of both and exhibit high discharge specific capacity and excellent stability. Summary of the Invention

[0004] To address the issues of low reversible specific capacity and poor conductivity in sodium ferric sulfate cathode materials, this invention proposes a method for preparing a sodium ferric sulfate composite sodium fluoride ferric sulfate cathode material.

[0005] This invention is achieved through the following technical solution:

[0006] This invention proposes a method for preparing a high-capacity Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode material, comprising:

[0007] Preparation of Na2Fe2(SO4)3: Sodium source and iron source were added to pure water in stoichiometric ratio and mixed. Acid and carbon nanotube aqueous slurry were added. The mixed slurry was stirred and ultrasonically treated. The resulting suspension was spray-dried to obtain Na2Fe2(SO4)3 precursor powder.

[0008] Preparation of Na3Fe2(SO4)3F: Sodium source, iron source and fluorine source are added to pure water in stoichiometric ratio and mixed. Acid and carbon nanotube aqueous slurry are added. The mixed slurry is stirred and ultrasonically treated. The resulting suspension is spray-dried to obtain Na3Fe2(SO4)3F precursor powder.

[0009] Preparation of Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode material: Na2Fe2(SO4)3 precursor powder and Na3Fe2(SO4)3F precursor powder were ball-milled and mixed at a mass ratio of 1:X, and then sintered and ground to obtain Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode material, wherein X is 0.5-2.

[0010] Furthermore, the preparation of Na2Fe2(SO4)3 includes the following steps:

[0011] Step S1: Prepare a mixed aqueous solution containing sodium and iron sources at a stoichiometric ratio of 1:2, ensuring the solid content of the solution is 20 wt%. This is necessary to maintain the particle size of the obtained precursor powder and prevent excessive viscosity from causing it to stick to the walls.

[0012] Step S2: Add acid to the mixed solution to adjust the pH value of the solution to ≤4 to prevent the oxidation of ferrous iron;

[0013] Step S3: Stir the mixed solution at a speed of 300-500 rpm for 1-2 hours to keep the solution clear and transparent;

[0014] Step S4: Add carbon source slurry to the mixed solution in step S3 and sonicate for 30-60 minutes to obtain a black and uniform suspension;

[0015] Step S5: Spray dry the suspension to obtain Na2Fe2(SO4)3 precursor powder.

[0016] Furthermore, the preparation of Na3Fe2(SO4)3F includes the following steps:

[0017] Step S1: Prepare a mixed aqueous solution containing sodium source, iron source and fluorine source according to a stoichiometric ratio of 1:2:1, and ensure that the solid content of the solution is 20 wt%.

[0018] Step S2: Add acid to the mixed solution to adjust the pH value of the solution to ≤4 to prevent the oxidation of ferrous iron;

[0019] Step S3: Stir the mixed solution at a speed of 300-500 rpm for 1-2 hours to keep the solution clear and transparent;

[0020] Step S4: Add carbon source slurry to the mixed solution in step S3 and sonicate for 30-60 minutes to obtain a black and uniform suspension;

[0021] Step S5: Spray dry the suspension to obtain Na3Fe2(SO4)3F precursor powder.

[0022] Furthermore, the preparation process of the Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode material includes: physically mixing the prepared Na2Fe2(SO4)3 and Na3Fe2(SO4)3F precursor powders in a ball mill jar at a mass ratio of 1:X, where X = 0.5, 1, and 2; the ball milling speed is 300-500 rpm, and the time is 30-60 min; the obtained mixed precursor powder is transferred to a tube furnace and sintered under a protective atmosphere to obtain the Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode material, wherein the protective atmosphere is at least one of argon, nitrogen, or a hydrogen-argon mixture; the sintering process program settings are: heating rate of 2℃ / min, pre-sintering stage of 200℃, sintering time of 2 h, final sintering temperature of 400℃, and sintering time of 6 h.

[0023] Furthermore, in step S1, the sodium source is at least one of sodium sulfate, sodium carbonate, or sodium bisulfate; the iron source is at least one of anhydrous ferrous sulfate, ferrous sulfate monohydrate, or ferrous sulfate heptahydrate; and the fluorine source is sodium fluoride.

[0024] Furthermore, the acid mentioned in step S2 is at least one of ascorbic acid or acetic acid.

[0025] Furthermore, in step S4, the carbon source slurry is an aqueous slurry with a mass fraction of 9-10 wt% carbon nanotubes.

[0026] Furthermore, in step S5, the spray drying settings are as follows: inlet air temperature is 160℃ and outlet air temperature is 85-95℃.

[0027] On the other hand, this invention proposes a high-capacity Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode material prepared by the above method, and its application in sodium-ion batteries.

[0028] The principle of this invention is as follows: F doping improves the electrochemical performance of the original Na2Fe2(SO4)3, providing additional sodium storage space. However, excessive F alters the crystal structure of Na2Fe2(SO4)3, forming a separate phase structure independent of Na2Fe2(SO4)3. Therefore, to stabilize the Na2Fe2(SO4)3 phase structure and charge / discharge characteristic plateau, the introduction of Na3Fe2(SO4)3F, a material with good conductivity, solves the problems of poor conductivity and low specific capacity of Na2Fe2(SO4)3. The strategy of combining Na2Fe2(SO4)3 with Na3Fe2(SO4)3F not only improves the conductivity of the material but also increases the sodium storage capacity, thereby improving the actual discharge capacity.

[0029] The beneficial effects of this invention are as follows: the preparation method described in this invention has the advantages of simple process, low cost, and easy industrialization. Furthermore, the composite of two different cathode materials allows the final cathode material to possess the advantages of both, providing a reference for the subsequent development of high-capacity sodium-ion battery cathode materials. Attached Figure Description

[0030] Figure 1 The XRD pattern of the positive electrode prepared in Example 1 of this invention;

[0031] Figure 2 The XRD pattern of the positive electrode prepared in Comparative Example 1 of this invention;

[0032] Figure 3 The XRD pattern of the positive electrode prepared in Comparative Example 2 of this invention;

[0033] Figure 4 This is the first-cycle performance diagram of the positive electrode prepared in Example 1 of the present invention;

[0034] Figure 5 This is the first-cycle performance diagram of the positive electrode prepared in Example 2 of the present invention;

[0035] Figure 6 This is the first-cycle performance diagram of the positive electrode prepared in Example 3 of the present invention;

[0036] Figure 7 The first-cycle performance diagram of the positive electrode prepared in Comparative Example 1 of this invention is shown.

[0037] Figure 8 The first-cycle performance diagram of the positive electrode prepared in Comparative Example 2 of this invention is shown. Detailed Implementation

[0038] The technical solutions in the embodiments of the present invention will be clearly and completely described below. The described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] Example 1

[0040] Sodium sulfate and ferrous sulfate heptahydrate were weighed according to a sodium-to-iron molar ratio of 1:2, and ascorbic acid (5 wt% of the total mass of the combined materials) was added and dissolved in deionized water to ensure a solid content of 20 wt%. The mixture was stirred at 300 rpm for 10 min to obtain a clear and transparent mixed solution. Subsequently, CNT aqueous slurry (5 wt% of the total mass of the combined materials) was added, and the mixture was stirred at 500 rpm for 1 h, followed by ultrasonication for 30 min to obtain a black suspension. The obtained suspension was spray-dried (inlet air temperature 160℃, outlet air temperature 90℃) to obtain sodium ferric sulfate precursor powder.

[0041] Sodium sulfate, ferrous sulfate heptahydrate, and sodium fluoride were weighed according to a sodium source, iron source, and fluorine source molar ratio of 1:2:1, and sodium fluoride was obtained by following the same steps as described above to obtain sodium ferric sulfate precursor powder.

[0042] Sodium ferric sulfate precursor and sodium ferric fluorosulfate precursor were physically mixed at a ratio of 2:1. The precursor powders were placed in a high-energy ball mill jar, vacuum-treated, and mixed at 400 rpm for 1 h. The mixed precursor powders were placed in a crucible and placed in a tube furnace filled with argon atmosphere. The temperature was increased to 200℃ at a rate of 2℃ / min and held for 2 h. Then the temperature was increased to 400℃ and held for 6 h. After sintering, Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode material was obtained.

[0043] A CR2032 coin cell was assembled from Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT positive electrode material, carbon black, and binder in an 8:1:1 ratio. The positive electrode was prepared by homogenizing and coating. Metallic sodium was used as the counter electrode, glass fiber as the separator, and a 1 mol / L NaClO4 propylene carbonate (PC) solution as the electrolyte. The positive electrode, sodium sheet, separator, gasket, and spring were then placed in a coin cell to form the CR2032 coin cell. Finally, the coin cell was tested for electrochemical performance using a Newway testing system. The electrochemical performance test parameters were set as follows: voltage range 2.0V-4.5V, and discharge specific capacity test at 0.1C.

[0044] Example 2

[0045] Sodium sulfate and ferrous sulfate heptahydrate were weighed according to a sodium-to-iron molar ratio of 1:2, and ascorbic acid (5 wt% of the total mass of the combined materials) was added and dissolved in deionized water to ensure a solid content of 20 wt%. The mixture was stirred at 300 rpm for 10 min to obtain a clear and transparent mixed solution. Subsequently, CNT aqueous slurry (5 wt% of the total mass of the combined materials) was added, and the mixture was stirred at 500 rpm for 1 h, followed by ultrasonication for 30 min to obtain a black suspension. The obtained suspension was spray-dried (inlet air temperature 160℃, outlet air temperature 90℃) to obtain sodium ferric sulfate precursor powder.

[0046] Sodium sulfate, ferrous sulfate heptahydrate, and sodium fluoride were weighed according to a sodium source, iron source, and fluorine source molar ratio of 1:2:1, and sodium fluoride was obtained by following the same steps as described above to obtain sodium ferric sulfate precursor powder.

[0047] Sodium ferric sulfate precursor and sodium ferric fluorosulfate precursor were physically mixed in a 1:1 ratio. The precursor powders were placed in a high-energy ball mill jar, vacuum-treated, and mixed at 400 rpm for 1 h. The mixed precursor powders were placed in a crucible and placed in a tube furnace filled with argon atmosphere. The temperature was increased to 200℃ at a rate of 2℃ / min and held for 2 h. Then the temperature was increased to 400℃ and held for 6 h. After sintering, Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode material was obtained.

[0048] A CR2032 coin cell was assembled from Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT positive electrode material, carbon black, and binder in an 8:1:1 ratio. The positive electrode was prepared by homogenizing and coating. Metallic sodium was used as the counter electrode, glass fiber as the separator, and a 1 mol / L NaClO4 propylene carbonate (PC) solution as the electrolyte. The positive electrode, sodium sheet, separator, gasket, and spring were then placed in a coin cell to form the CR2032 coin cell. Finally, the coin cell was tested for electrochemical performance using a Newway testing system. The electrochemical performance test parameters were set as follows: voltage range 2.0V-4.5V, and discharge specific capacity test at 0.1C.

[0049] Example 3

[0050] Sodium sulfate and ferrous sulfate heptahydrate were weighed according to a sodium-to-iron molar ratio of 1:2, and ascorbic acid (5 wt% of the total mass of the combined materials) was added and dissolved in deionized water to ensure a solid content of 20 wt%. The mixture was stirred at 300 rpm for 10 min to obtain a clear and transparent mixed solution. Subsequently, CNT aqueous slurry (5 wt% of the total mass of the combined materials) was added, and the mixture was stirred at 500 rpm for 1 h, followed by ultrasonication for 30 min to obtain a black suspension. The obtained suspension was spray-dried (inlet air temperature 160℃, outlet air temperature 90℃) to obtain sodium ferric sulfate precursor powder.

[0051] Sodium sulfate, ferrous sulfate heptahydrate, and sodium fluoride were weighed according to a sodium source, iron source, and fluorine source molar ratio of 1:2:1, and sodium fluoride was obtained by following the same steps as described above to obtain sodium ferric sulfate precursor powder.

[0052] Sodium ferric sulfate precursor and sodium ferric fluorosulfate precursor were physically mixed at a ratio of 1:2. The precursor powders were placed in a high-energy ball mill jar, vacuum-treated, and mixed at 400 rpm for 1 h. The mixed precursor powders were placed in a crucible and placed in a tube furnace filled with argon atmosphere. The temperature was increased to 200℃ at a rate of 2℃ / min and held for 2 h. Then the temperature was increased to 400℃ and held for 6 h. After sintering, Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode material was obtained.

[0053] A CR2032 coin cell was assembled from Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT positive electrode material, carbon black, and binder in an 8:1:1 ratio. The positive electrode was prepared by homogenizing and coating. Metallic sodium was used as the counter electrode, glass fiber as the separator, and a 1 mol / L NaClO4 propylene carbonate (PC) solution as the electrolyte. The positive electrode, sodium sheet, separator, gasket, and spring were then placed in a coin cell to form the CR2032 coin cell. Finally, the coin cell was tested for electrochemical performance using a Newway testing system. The electrochemical performance test parameters were set as follows: voltage range 2.0V-4.5V, and discharge specific capacity test at 0.1C.

[0054] Comparative Example 1

[0055] Sodium sulfate and ferrous sulfate heptahydrate were weighed according to a sodium-to-iron molar ratio of 1:2, and ascorbic acid (5 wt% of the total mass of the combined materials) was added and dissolved in deionized water to ensure a solid content of 20 wt%. The mixture was stirred at 300 rpm for 10 min to obtain a clear and transparent mixed solution. Subsequently, CNT aqueous slurry (5 wt% of the total mass of the combined materials) was added and stirred at 500 rpm for 1 h, followed by ultrasonication for 30 min to obtain a black suspension. The obtained suspension was spray-dried (inlet air temperature 160℃, outlet air temperature 90℃) to obtain sodium ferric sulfate precursor powder, which was transferred to a crucible and placed in a tube furnace filled with argon atmosphere. The temperature was increased to 200℃ at a rate of 2℃ / min and held for 2 h, then increased to 400℃ and held for 6 h. After sintering, Na2Fe2(SO4)3@CNT cathode material was obtained.

[0056] A CR2032 coin cell was assembled from Na2Fe2(SO4)3@CNT positive electrode material, carbon black, and binder in an 8:1:1 ratio using homogenized slurry coating. Metallic sodium was used as the counter electrode, glass fiber as the separator, and a 1 mol / L NaClO4 propylene carbonate (PC) solution as the electrolyte. The positive electrode, sodium sheet, separator, gasket, and spring were then placed in a coin cell to form the CR2032 coin cell. Finally, the coin cell was tested for electrochemical performance using a Newway testing system. The electrochemical performance test parameters were set as follows: voltage range 2.0V-4.5V, and discharge specific capacity test at 0.1C.

[0057] Comparative Example 2

[0058] Sodium sulfate, ferrous sulfate heptahydrate, and sodium fluoride were weighed according to a sodium, iron, and fluorine source molar ratio of 1:2:1. Ascorbic acid (5 wt% of the total mass of the combined materials) was added and dissolved in deionized water to ensure a solid content of 20 wt%. The mixture was stirred at 300 rpm for 10 min to obtain a clear and transparent mixed solution. Then, CNT aqueous slurry (5 wt% of the total mass of the combined materials) was added and stirred at 500 rpm for 1 h. Following this, the mixture was sonicated for 30 min to obtain a black suspension. The obtained suspension was spray-dried (inlet air temperature 160℃, outlet air temperature 90℃) to obtain sodium ferric fluorosulfate precursor powder. This powder was transferred to a crucible and placed in a tube furnace filled with argon atmosphere. The temperature was increased to 200℃ at a rate of 2℃ / min and held for 2 h. The temperature was then increased to 400℃ and held for 6 h. After sintering, Na3Fe2(SO4)3F@CNT cathode material was obtained.

[0059] A CR2032 coin cell was assembled from Na3Fe2(SO4)3F@CNT positive electrode material, carbon black, and binder in an 8:1:1 ratio using homogenized slurry coating. Metallic sodium was used as the counter electrode, glass fiber as the separator, and a 1 mol / L NaClO4 propylene carbonate (PC) solution as the electrolyte. The positive electrode, sodium sheet, separator, gasket, and spring were then placed in a coin cell to form the CR2032 coin cell. Finally, the coin cell was tested for electrochemical performance using a Newway testing system. The electrochemical performance test parameters were set as follows: voltage range 2.0V-4.5V, and discharge specific capacity test at 0.1C.

[0060] Table 1. Performance comparison of Samples from Examples 1-3 and Comparative Examples 1-2

[0061]

[0062] As can be seen from Table 1, the synthesized Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode material, synthesized by mixing sodium ferric sulfate and sodium ferric fluoride sulfate, has a high discharge specific capacity.

[0063] In summary, this invention synthesizes a series of Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode materials with different proportions by preparing sodium ferric sulfate and sodium ferric fluorosulfate precursors respectively via spray drying. The addition of sodium ferric fluorosulfate improves the conductivity of the material, leading to the release of more sodium ions. The prepared cathode materials possess the stability of sodium ferric sulfate and the high specific capacity of sodium ferric fluorosulfate, showing potential for large-scale application.

[0064] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several equivalent substitutions and improvements can be made without departing from the technical principles of the present invention, and these equivalent substitutions and improvements should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a high-capacity Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode material, characterized in that, include: Preparation of Na2Fe2(SO4)3: Step S1: Prepare a mixed solution containing sodium and iron sources at a stoichiometric ratio of 1:2, ensuring the solid content of the solution is 20 wt%. Step S2: Add acid to the mixed solution to adjust the pH value of the solution to ≤4 to prevent the oxidation of ferrous iron; Step S3: Stir the mixed solution at a speed of 300-500 rpm for 1-2 hours to keep the solution clear and transparent; Step S4: Add carbon nanotube aqueous slurry to the mixed solution in step S3 and sonicate for 30-60 minutes to obtain a black and uniform suspension. Step S5: Spray dry the suspension to obtain Na2Fe2(SO4)3 precursor powder; Preparation of Na3Fe2(SO4)3F: Step S1: Prepare a mixed solution containing sodium source, iron source and fluorine source according to a stoichiometric ratio of 1:2:1, and ensure that the solid content of the solution is 20 wt%. Step S2: Add acid to the mixed solution to adjust the pH value of the solution to ≤4 to prevent the oxidation of ferrous iron; Step S3: Stir the mixed solution at a speed of 300-500 rpm for 1-2 hours to keep the solution clear and transparent; Step S4: Add carbon nanotube aqueous slurry to the mixed solution in step S3 and sonicate for 30-60 minutes to obtain a black and uniform suspension. Step S5: Spray-dry the suspension to obtain Na3Fe2(SO4)3F precursor powder; Preparation of Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT: The prepared Na2Fe2(SO4)3 and Na3Fe2(SO4)3F precursor powders were physically mixed in a ball mill jar at a mass ratio of 1:X, where X = 0.5, 1, or 2; the ball milling speed was 300-500 rpm, and the time was 30-60 min; the obtained mixed precursor powder was transferred to a tube furnace and sintered under a protective atmosphere to obtain Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode material, wherein the protective atmosphere was at least one of argon, nitrogen, or a hydrogen-argon mixture; the sintering process was programmed as follows: heating rate of 2℃ / min, pre-sintering stage of 200℃, sintering time of 2 h, final sintering temperature of 400℃, and sintering time of 6 h.

2. The method for preparing the cathode material according to claim 1, characterized in that, The sodium source is at least one of sodium sulfate, sodium carbonate, or sodium bisulfate; the iron source is at least one of anhydrous ferrous sulfate, ferrous sulfate monohydrate, or ferrous sulfate heptahydrate; and the fluorine source is sodium fluoride.

3. The method for preparing the cathode material according to claim 1, characterized in that, The acid is at least one of ascorbic acid or acetic acid.

4. The method for preparing the cathode material according to claim 1, characterized in that, The mass fraction of the carbon nanotube aqueous slurry is 9-10 wt%.

5. The method for preparing the cathode material according to claim 1, characterized in that, Spray drying settings: inlet air temperature is 160℃, outlet air temperature is 85-95℃.

6. A high-capacity Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode material, characterized in that, The cathode material is prepared by the preparation method according to any one of claims 1-5.

7. The application of a high-capacity Na2Fe2(SO4)3@Na3Fe2(SO4)3F@CNT cathode material in sodium-ion batteries, characterized in that, The cathode material is the cathode material according to claim 6, or the cathode material prepared by the preparation method according to any one of claims 1-5.

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