Positive plate, preparation method thereof and sodium ion battery

Abstract

The invention provides a positive plate, a preparation method thereof and a sodium ion battery, and relates to the technical field of batteries. The positive plate provided by the invention comprises a current collector and a positive active material layer coated on the current collector, the positive active material layer comprises a positive active material, a binder, a conductive agent and a film-forming agent; the positive electrode active material comprises ferric sodium pyrophosphate and lithium iron phosphate. According to the invention, NFPP and LFP positive electrode materials are compounded, so that the consumption of a positive electrode sodium source can be reduced, the stability of the positive electrode material is improved, a layer of compact and stable Na / Li mixed solid electrolyte interface film rich in inorganic components can be formed on the negative electrode side, and the cycle performance of the battery is improved.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a positive electrode sheet and its preparation method, and a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries are considered a potential alternative to lithium-ion batteries due to abundant sodium resources, considerable cost-effectiveness, and similar working principles. However, the peak energy density of sodium-ion batteries remains below 160 Wh / kg, significantly limiting their applicability in applications requiring higher energy densities. Anode-less batteries utilize current collectors as the anode to replace excess metallic sodium, eliminating the need for anode active materials and significantly reducing the overall thickness and weight of the battery, thereby increasing its volumetric and gravimetric energy density. However, the core challenge of anode-less structures lies in the fact that their sodium source is entirely dependent on the cathode material. During the first charge cycle, sodium ions released from the cathode form an unstable SEI film during the formation of metallic sodium at the anode, accompanied by dendrite growth and the generation of "dead sodium," resulting in extremely low coulombic efficiency (typically <80%) in the first cycle, leading to a sharp decline in battery capacity.

[0003] In view of this, the present invention is hereby proposed. Summary of the Invention

[0004] The primary objective of this invention is to provide a positive electrode to solve the aforementioned technical problems.

[0005] The second objective of this invention is to provide a method for preparing the above-mentioned positive electrode sheet.

[0006] A third objective of this invention is to provide a sodium-ion battery.

[0007] To achieve the above objectives, the following technical solution is adopted: In a first aspect, the present invention provides a positive electrode sheet, comprising a current collector and a positive electrode active material layer coated on the current collector; The positive electrode active material layer comprises a positive electrode active material, a binder, a conductive agent, and a film-forming agent; the mass percentage of the positive electrode active material in the positive electrode active material layer is 93.5%-96.0%. The positive electrode active material includes sodium iron pyrophosphate and lithium iron phosphate; sodium iron pyrophosphate has a D50 of 3.0-7.0 μm and a specific surface area of ​​8-15 g / cm³. 3 Lithium iron phosphate: D50 is 0.5-1.5 μm, and specific surface area is 10-15 g / cm³. 3 The mass percentage of lithium iron phosphate in the positive electrode active material is 1%-10%.

[0008] As a further technical solution, the content of magnetic material in the sodium ferric pyrophosphate is less than 0.5 ppm; The lithium iron phosphate has a magnetic material content of less than 1.0 ppm.

[0009] As a further technical solution, the positive electrode active material layer comprises, by mass percentage, 1.5%-2.0% binder, 1.5%-2.5% conductive agent, and 1.0%-2.0% film-forming agent, with the balance being positive electrode active material.

[0010] As a further technical solution, the adhesive includes at least one of PVDF, PTFE, PVC or PMMA.

[0011] As a further technical solution, the conductive agent includes at least one of conductive carbon black, conductive graphite, graphene, or carbon nanotubes.

[0012] As a further technical solution, the film-forming agent includes at least one of FEC, VC, PS, or DTD.

[0013] Secondly, the present invention provides the above-mentioned preparation method, comprising the following steps: A slurry is prepared by mixing positive electrode active material, binder, conductive agent, film-forming agent and solvent, then coating it onto current collector, and drying it to obtain a positive electrode sheet.

[0014] As a further technical solution, the solvent is an organic solvent, including at least one of NMP, DMSO or DMF.

[0015] Thirdly, the present invention provides a sodium-ion battery, including the above-mentioned positive electrode sheet.

[0016] Compared with the prior art, the present invention has the following beneficial effects: The positive electrode sheet provided by this invention, by combining NFPP and LFP positive electrode materials, can reduce the consumption of sodium source in the positive electrode and improve the stability of the positive electrode material, and can also form a dense, stable Na / Li mixed solid electrolyte interface film rich in inorganic components on the negative electrode side, thereby improving the cycle performance of the battery. Detailed Implementation

[0017] The embodiments and examples of the present invention will be described in detail below. However, those skilled in the art will understand that the following embodiments and examples are for illustrative purposes only and should not be considered as limiting the scope of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention. Unless otherwise specified, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0018] In a first aspect, the present invention provides a positive electrode sheet, comprising a current collector and a positive electrode active material layer coated on the current collector; The positive electrode active material layer includes a positive electrode active material, a binder, a conductive agent, and a film-forming agent; the mass percentage of the positive electrode active material in the positive electrode active material layer is 93.5%-96.0%, for example, but not limited to 93.5%, 95.0%, or 96.0%; The positive electrode active material includes sodium iron pyrophosphate and lithium iron phosphate; sodium iron pyrophosphate has a D50 of 3.0-7.0 μm (e.g., it can be, but is not limited to, 3.0 μm, 5.0 μm or 7.0 μm) and a specific surface area of ​​8-15 g / cm³. 3 (For example, it can be, but is not limited to, 8 g / cm³) 3 10 g / cm 3 Or 15 g / cm 3 Lithium iron phosphate: D50 is 0.5-1.5 μm (e.g., it can be, but is not limited to, 0.5 μm, 1.0 μm, or 1.5 μm), and specific surface area is 10-15 g / cm³. 3 (For example, it can be, but is not limited to, 10 g / cm³) 3 12 g / cm 3 Or 15 g / cm 3 The mass percentage of lithium iron phosphate in the positive electrode active material is 1%-10%, for example, but not limited to 1%, 3% or 10%.

[0019] This invention uses NFPP and LFP of different particle sizes to form a specific pore structure. This structure affects the ion insertion / extraction kinetics on the positive electrode side, and thus affects the deposition morphology of sodium ions on the negative electrode side. Furthermore, the lithium iron phosphate content is ≤10%, and the Li... + Excessive sodium ion content can clog sodium ion channels and corrode the negative electrode interface, leading to performance degradation; insufficient sodium ion content will prevent the formation of a synergistic effect.

[0020] The sodium / lithium mixed-ion composite cathode constructed in this invention can simultaneously release Na+ during the first charge. + and Li +Lithium ions preferentially deposit on the surface of the negative electrode current collector, forming nanoscale lithium metal nucleation sites, rather than sodium ions. These lithium nuclei, acting as heterogeneous nucleation seeds, significantly reduce the nucleation overpotential of sodium ions and promote the uniform and stable deposition of sodium ions; furthermore, Li + It also participates in the formation of the SEI film, which is rich in lithium-containing inorganic phases such as LiF and Li3N. Compared with traditional sodium-based SEI films, this lithium-sodium composite SEI film has both high mechanical strength (contributed by LiF) and high ionic conductivity (contributed by Li3N), which can effectively inhibit dendrite growth and promote Na+ growth. + Rapid and uniform deposition. This hybrid SEI film significantly improves the reversibility of subsequent sodium metal deposition / dissolution, thereby greatly improving the first-cycle coulombic efficiency and cycle stability of the battery. Secondly, LFP is a material with excellent structural stability, which helps to suppress structural distortion or collapse of NFPP materials during deep sodium removal, improves the structural reversibility and cycle life of the cathode, and thus ensures the long-term stability of the sodium source.

[0021] In some alternative embodiments, the magnetic material content of the sodium ferric pyrophosphate is less than 0.5 ppm; The lithium iron phosphate has a magnetic material content of less than 1.0 ppm.

[0022] By making the above choices, we can control battery safety risks and prevent safety problems caused by battery short circuits.

[0023] In some alternative embodiments, the positive electrode active material layer comprises, by weight percentage, 1.5%-2.0% binder (e.g., but not limited to 1.5%, 1.7%, or 2.0%), 1.5%-2.5% conductive agent (e.g., but not limited to 1.5%, 2.0%, or 2.5%), and 1.0%-2.0% film-forming agent (e.g., but not limited to 1.0%, 1.5%, or 2.0%), with the balance being the positive electrode active material.

[0024] In some alternative embodiments, the adhesive includes, but is not limited to, at least one of PVDF, PTFE, PVC or PMMA.

[0025] In some alternative embodiments, the conductive agent includes, but is not limited to, at least one of: conductive carbon black, conductive graphite, graphene, or carbon nanotubes.

[0026] In some alternative embodiments, the film-forming agent includes, but is not limited to, at least one of FEC, VC, PS, or DTD.

[0027] Film-forming additives are pre-dispersed in the positive electrode material layer to avoid contact with the electrolyte and prevent side reactions during long-term storage. Furthermore, they are released in a controlled manner during cycling. When the battery cycles to a certain number of cycles and the SEI film on the negative electrode side cracks or thickens, the additives loaded on the positive electrode side can still continuously dissolve and migrate to the negative electrode to repair the damaged SEI film, thereby extending the battery's cycle life.

[0028] Secondly, the present invention provides the above-mentioned preparation method, comprising the following steps: A slurry is prepared by mixing positive electrode active material, binder, conductive agent, film-forming agent and solvent, then coating it onto current collector, and drying it to obtain a positive electrode sheet.

[0029] The preparation method is simple and convenient.

[0030] In some alternative embodiments, the solvent is an organic solvent, including but not limited to at least one of NMP, DMSO or DMF.

[0031] Thirdly, the present invention provides a sodium-ion battery, including the above-mentioned positive electrode sheet.

[0032] The present invention provides a sodium-ion battery with good first-cycle coulombic efficiency and cycle stability.

[0033] In some alternative implementations, the negative electrode of a sodium-ion battery can be made directly using current collectors such as aluminum foil, copper foil, carbon-coated aluminum foil, or carbon-coated copper foil. Alternatively, an active material, such as hard carbon, can be coated onto the surface of the current collector to form an ultra-thin active layer to create the negative electrode.

[0034] The present invention will be further illustrated below with specific embodiments and comparative examples. However, it should be understood that these embodiments are merely for the purpose of more detailed illustration and should not be construed as limiting the present invention in any way.

[0035] Example 1 A positive electrode sheet includes a current collector and a positive electrode active material layer coated on the current collector; The positive electrode active material layer includes positive electrode active material, binder PVDF, conductive agent conductive carbon black and film-forming agent FEC, with a mass ratio of 94.7:1.8:2.0:1.5; The positive electrode active materials include sodium iron pyrophosphate and lithium iron phosphate; sodium iron pyrophosphate has a D50 of 4.9 μm and a specific surface area of ​​11.2 g / cm³. 3 The magnetic material content is less than 0.5 ppm; lithium iron phosphate: D50 is 0.9 μm, and specific surface area is 12.1 g / cm³. 3 The content of magnetic materials is less than 1.0 ppm; the mass ratio of lithium iron phosphate in the positive electrode active material is 4.9%.

[0036] The preparation method is as follows: A slurry is prepared by mixing positive electrode active material, binder, conductive agent, film-forming agent and solvent, then coating it onto current collector, and drying it to obtain a positive electrode sheet.

[0037] Example 2 A positive electrode sheet includes a current collector and a positive electrode active material layer coated on the current collector; The positive electrode active material layer includes positive electrode active material, binder PVDF, conductive agent conductive carbon black and film-forming agent FEC, with a mass ratio of 94.7:1.8:2.0:1.5; The positive electrode active materials include sodium iron pyrophosphate and lithium iron phosphate; sodium iron pyrophosphate has a D50 of 3.0 μm and a specific surface area of ​​8 g / cm³. 3 The magnetic material content is less than 0.5 ppm; lithium iron phosphate: D50 is 0.5 μm, and specific surface area is 10 g / cm³. 3 The content of magnetic materials is less than 1.0 ppm; the mass ratio of lithium iron phosphate in the positive electrode active material is 1%.

[0038] The preparation method is the same as in Example 1.

[0039] Example 3 A positive electrode sheet includes a current collector and a positive electrode active material layer coated on the current collector; The positive electrode active material layer includes positive electrode active material, binder PVDF, conductive agent conductive carbon black and film-forming agent FEC, with a mass ratio of 94.7:1.8:2.0:1.5; The positive electrode active materials include sodium iron pyrophosphate and lithium iron phosphate; sodium iron pyrophosphate has a D50 of 7.0 μm and a specific surface area of ​​15 g / cm³. 3 The content of magnetic materials is less than 0.5 ppm; lithium iron phosphate: D50 is 1.5 μm, and specific surface area is 15 g / cm³. 3 The content of magnetic materials is less than 1.0 ppm; the mass ratio of lithium iron phosphate in the positive electrode active material is 10%.

[0040] The preparation method is the same as in Example 1.

[0041] Comparative Example 1 A positive electrode sheet, which differs from Example 1 in that the mass percentage of lithium iron phosphate in the positive electrode active material is 20%.

[0042] Comparative Example 2 A positive electrode sheet differs from Example 1 in that the amount of positive electrode active material is the same, but lithium iron phosphate is not added.

[0043] Comparative Example 3 A positive electrode sheet differs from Example 1 in that the sodium iron pyrophosphate and lithium iron phosphate have the same particle size, both being 2.0 μm.

[0044] Comparative Example 4 A positive electrode, differing from Example 1, has a D50 of 1.0 μm and a specific surface area of ​​12.0 g / cm³ for sodium iron pyrophosphate. 3 The lithium iron phosphate has a D50 of 5.0 μm and a specific surface area of ​​10.0 g / cm³. 3 .

[0045] Comparative Example 5 A positive electrode sheet, which differs from Example 1 in that no film-forming agent is added.

[0046] Experimental Example 1 The positive and negative electrode sheets, separator, and electrolyte provided in the above embodiments and comparative examples were stacked in a Z-shape to form a battery. The performance of the obtained battery was tested, and the results are as follows: Negative electrode sheet: It consists of a current collector and an active material layer coated on the current collector. In the negative electrode active material layer, the mass ratio of hard carbon is 94.5%; the mass ratio of negative electrode conductive agent is 2.0%; and the mass ratio of negative electrode binder is 3.5%.

[0047] Diaphragm: PP diaphragm.

[0048] The sodium-ion batteries prepared using the various embodiments and comparative examples were tested for their initial coulombic efficiency and cycle performance. The specific test methods are as follows: First Coulomb efficiency test of the battery: After the assembled battery was left to stand for 10 hours, it was charged to 3.5V at a constant current and constant voltage of 0.2C, then left to stand for another 0.5 hours, and then discharged to 2.0V at a constant current of 0.5C. The ratio of the battery's discharge capacity to its charge capacity was calculated as CE. Cycle life test: After the assembled battery is left to rest for 10 hours, it is charged to 3.5V at 0.5C constant current and constant voltage at room temperature of 25℃. After resting for 0.5 hours, it is discharged to 2.0V at 1C constant current. The cycle is repeated 100 times and the capacity retention rate is recorded. Volume change test: After the battery was formed and capacity tested, it was placed at room temperature (25°C) for 10 hours. The initial volume of the battery was measured by the water displacement method and recorded as V1. Then it was placed in a high-temperature room at 45°C for 7 days. After that, it was taken out and cooled to room temperature (25°C). The volume after storage was measured by the water displacement method and recorded as V2. The volume change rate after 7 days of storage at 45°C is ΔV=(V2-V1) / V1*100%.

[0049] The performance of the obtained batteries was tested, and the results are as follows:

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A positive electrode plate, characterized in that, Includes a current collector and a positive electrode active material layer coated on the current collector; The positive electrode active material layer comprises a positive electrode active material, a binder, a conductive agent, and a film-forming agent; the mass percentage of the positive electrode active material in the positive electrode active material layer is 93.5%-96.0%. The positive electrode active material includes sodium iron pyrophosphate and lithium iron phosphate; sodium iron pyrophosphate has a D50 of 3.0-7.0 μm and a specific surface area of ​​8-15 g / cm³. 3 ; Lithium iron phosphate: D50 is 0.5-1.5 μm, and specific surface area is 10-15 g / cm³. 3 The mass percentage of lithium iron phosphate in the positive electrode active material is 1%-10%.

2. The positive electrode sheet according to claim 1, characterized in that, The content of magnetic material in the sodium ferric pyrophosphate is less than 0.5 ppm; The content of magnetic material in the lithium iron phosphate is less than 1.0 ppm.

3. The positive electrode sheet according to claim 1, characterized in that, By mass percentage, the positive electrode active material layer comprises 1.5%-2.0% binder, 1.5%-2.5% conductive agent, and 1.0%-2.0% film-forming agent, with the balance being positive electrode active material.

4. The positive electrode sheet according to claim 1, characterized in that, The adhesive includes at least one of PVDF, PTFE, PVC or PMMA.

5. The positive electrode sheet according to claim 1, characterized in that, The conductive agent includes at least one of conductive carbon black, conductive graphite, graphene, or carbon nanotubes.

6. The positive electrode sheet according to claim 1, characterized in that, The film-forming agent includes at least one of FEC, VC, PS, or DTD.

7. The method for preparing the positive electrode sheet according to any one of claims 1-6, characterized in that, Includes the following steps: A slurry is prepared by mixing positive electrode active material, binder, conductive agent, film-forming agent and solvent, then coating it onto current collector, and drying it to obtain a positive electrode sheet.

8. The preparation method according to claim 7, characterized in that, The solvent is an organic solvent, including at least one of NMP, DMSO or DMF.

9. A sodium-ion battery, characterized in that, Includes the positive electrode sheet as described in any one of claims 1-6.

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