A binder composition for a sodium-ion battery positive electrode, a positive electrode sheet, and a sodium-ion battery

Abstract

This invention provides a binder composition for a sodium-ion battery positive electrode, a positive electrode sheet, and a sodium-ion battery. At 25°C, the binder composition has a storage elastic modulus of 1000 MPa to 2000 MPa and a tensile strength of 15 MPa to 45 MPa. The binder composition includes a first binder and a second binder, wherein the storage elastic modulus of the first binder is less than that of the second binder, and the tensile strength of the first binder is less than that of the second binder. This invention, by compounding a first binder and a second binder with different mechanical property advantages, forms a "rigid-flexible" interpenetrating network structure, preventing the generation or propagation of cracks in the positive electrode sheet, avoiding electrode breakage or interface peeling, ensuring an effective electron transport path, and improving the structural stability of the positive electrode sheet and the cycle performance of the sodium-ion battery.

Description

Technical Field

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

[0002] In recent years, sodium-ion batteries have been gradually replacing lead-acid batteries due to their advantages such as better power characteristics, wide temperature range adaptability, high safety and no over-discharge problem, and self-controllable resources. They can also serve as a useful supplement and partial replacement for lithium-ion batteries in some application scenarios.

[0003] Currently, the binder widely used in sodium-ion battery cathodes is still the traditional polyvinylidene fluoride (PVDF), but it has limitations in many aspects, which seriously affect electrode performance and cycle stability, as follows:

[0004] (1) The chain has high rigidity and insufficient flexibility, making it prone to interfacial peeling.

[0005] PVDF's molecular chains are too rigid, making it difficult to adapt to high-capacity cathode materials (such as layered oxides like Na). x The significant volume change of MO2 during charging and discharging can cause cracks or even fractures in the electrode sheets, disrupting the electron conduction path, reducing the stability of the electrode structure, and the volume change of the positive electrode material during cycling can reduce the peel strength between the material layer and the current collector, exacerbating interfacial peeling, causing contact failure between the active material and the current collector, and hindering electron transport.

[0006] (2) The interfacial bonding force is weak, and electrode pulverization is likely to occur.

[0007] The -CF2- groups in PVDF and the cathode material (such as layered oxide Na) x The weak bonding between oxygen atoms on the surface of MO2 results in insufficient interfacial adhesion between the binder and the cathode material, making it prone to electrode pulverization after cycling.

[0008] (3) Poor chain segment mobility at low temperatures.

[0009] PVDF has a high degree of crystallinity and a regular molecular chain structure. Under low temperature conditions, the ability of molecular chain segments to move decreases significantly, and flexibility is lost. It cannot effectively buffer the stress changes of the cathode material during the sodium ion insertion / extraction process, which further promotes the generation and propagation of cracks.

[0010] These factors together lead to electrode structure damage, contact failure, and electrochemical performance degradation, limiting the high-performance development of sodium-ion batteries. Summary of the Invention

[0011] The purpose of this invention is to provide a binder composition for the positive electrode of a sodium-ion battery, a positive electrode sheet, and a sodium-ion battery, so as to solve the problems in the background art.

[0012] The technical solution adopted in this invention includes: a binder composition for a sodium-ion battery positive electrode, wherein at 25°C: the energy storage elastic modulus of the binder composition is 1000MPa~2000MPa, and the tensile strength is 15MPa~45MPa; the binder composition includes a first binder and a second binder, wherein the energy storage elastic modulus of the first binder is less than that of the second binder, and the tensile strength of the first binder is less than that of the second binder.

[0013] Preferably, the first adhesive contains at least one of hydroxyl, carboxyl, and ester groups.

[0014] Preferably, at 25°C: the storage elastic modulus of the first adhesive is 1 MPa to 10 MPa, the storage elastic modulus of the second adhesive is 1000 MPa to 3000 MPa; the tensile strength of the first adhesive is 1 MPa to 10 MPa, and the tensile strength of the second adhesive is 40 MPa to 60 MPa.

[0015] Preferably, the first adhesive comprises at least polytrimethylene carbonate, the weight-average molecular weight of which is 50,000 to 300,000 Da.

[0016] Preferably, the second adhesive comprises at least polyvinylidene fluoride, wherein the weight-average molecular weight of polyvinylidene fluoride is 300,000 to 1,000,000 Da.

[0017] Preferably, the mass ratio of the first adhesive to the second adhesive is 0.1:(1-5).

[0018] The technical solution of the present invention also includes: a positive electrode sheet for a sodium-ion battery, the positive electrode sheet comprising a current collector and a positive electrode material layer, the positive electrode material layer being disposed on the surface of the current collector, wherein it contains a positive electrode active material, a conductive agent and the above-mentioned binder composition for the sodium-ion battery positive electrode, the binder composition accounting for 2% to 10% of the total mass of the positive electrode material layer.

[0019] Preferably, the positive electrode active material includes Na x MO2 is a layered oxide cathode material, wherein M includes at least one of Fe, Mn, Ni, Cu, Co and Mg, and 0.5≤x≤1.

[0020] The technical solution of the present invention also includes: a sodium-ion battery, the sodium-ion battery comprising a positive electrode, a negative electrode and an electrolyte of the above-mentioned sodium-ion battery, wherein the swelling rate of the binder assembly in the positive electrode in the electrolyte is ≤5%.

[0021] Preferably, the sodium-ion battery is a pouch battery, a prismatic battery, or a cylindrical battery.

[0022] The beneficial effects of this invention include at least the following:

[0023] By compounding a first binder and a second binder with different mechanical properties, the first binder and the second binder are blended at the molecular level to form an interpenetrating network structure that combines rigidity and flexibility. The high mechanical strength of the second binder is used to maintain the structural integrity of the sodium-ion battery cathode, while the high viscoelasticity of the first binder is used to alleviate the volume stress of the sodium-ion battery cathode material during cycling, improve the flexibility of the cathode, avoid the generation or propagation of cathode cracks, avoid electrode breakage or interface peeling, ensure an effective electron transport path, and improve the structural stability of the cathode and the cycle performance of the sodium-ion battery.

[0024] By selecting a first binder with special functional groups to form hydrogen bonds with the hydroxyl groups in the layered cathode oxide material of sodium-ion batteries, the interfacial bonding force between the binder composition and the cathode active material can be enhanced, preventing electrode pulverization after cycling, thereby improving the structural stability of the cathode sheet throughout the entire life cycle of sodium-ion batteries. Attached Figure Description

[0025] Figure 1 These are the adhesion test results of the positive electrode sheets prepared in Example 1 and Comparative Example 1 of this invention;

[0026] Figure 2 The results are the low-temperature discharge performance test results of sodium-ion batteries assembled using the positive electrode sheets prepared in Example 1 and Comparative Example 1 of this invention at -30°C.

[0027] Figure 3 The results are the charge-discharge cycle performance test results of sodium-ion batteries assembled using the positive electrode sheets prepared in Example 1 and Comparative Example 1 of this invention at room temperature 1C / 1C. Detailed Implementation

[0028] The embodiments of the present invention are described in detail below.

[0029] This invention provides a binder composition for the positive electrode of a sodium-ion battery, a positive electrode sheet of a sodium-ion battery containing such a binder composition, and a sodium-ion battery containing such a positive electrode sheet, to solve the problems of high elastic modulus and poor electrode flexibility of PVDF in the prior art, improve the structural stability of the positive electrode, and enhance the electrochemical performance of the sodium-ion battery.

[0030] The binder composition for the sodium-ion battery cathode provided in this embodiment includes a first binder and a second binder. At 25°C: the energy storage elastic modulus of the first binder is less than that of the second binder, that is, the viscoelasticity of the first binder is better than that of the second binder; the tensile strength of the first binder is less than that of the second binder, that is, the mechanical strength of the second binder is better than that of the first binder.

[0031] The above technical solution combines a first binder and a second binder with different mechanical properties to form an interpenetrating network structure that combines rigidity and flexibility. The high mechanical strength of the second binder maintains the structural integrity of the sodium-ion battery cathode, while the high viscoelasticity of the first binder alleviates the volume stress of the sodium-ion battery cathode material during cycling, improves the flexibility of the cathode, avoids the generation or propagation of cracks, avoids electrode breakage or interface peeling, ensures an effective electron transport path, and improves the structural stability of the cathode and the cycle performance of the sodium-ion battery.

[0032] Even better: The first binder contains at least one of hydroxyl, carboxyl and ester groups. These groups can form hydrogen bonds with the hydroxyl groups in the layered positive electrode oxide material of sodium-ion batteries, enhance the interfacial bonding force between the binder composition and the positive electrode active material, and prevent electrode pulverization after cycling, thereby improving the structural stability of the positive electrode sheet throughout the entire life cycle of the sodium-ion battery. In addition, this technical solution also plays a certain positive role in improving the processing performance of the electrode sheet.

[0033] Optionally, the first binder can be polytrimethylene carbonate with a weight-average molecular weight of 50,000 to 300,000 Da, and the second binder can be polyvinylidene fluoride with a weight-average molecular weight of 300,000 to 1,000,000 Da. The ester groups in the polytrimethylene carbonate can form hydrogen bonds with the hydroxyl groups in the layered positive electrode oxide material of the sodium-ion battery, enhancing the interfacial bonding force between the binder composition and the positive electrode active material. At 25°C, the polytrimethylene carbonate has a storage elastic modulus of 1 MPa to 10 MPa and a tensile strength of 1 MPa to 10 MPa, exhibiting high viscoelasticity. The polyvinylidene fluoride has a storage elastic modulus of 1000 MPa to 3000 MPa and a tensile strength of 40 MPa to 60 MPa, exhibiting high mechanical strength. The combination of the two binders balances the flexibility and mechanical strength of the positive electrode sheet, thereby improving the structural stability of the positive electrode sheet and enhancing the electrochemical performance of the sodium-ion battery.

[0034] In further research, the inventors discovered that when the mass ratio of the above-mentioned polytrimethylene carbonate to polyvinylidene fluoride is 0.1:(1-5), the energy storage elastic modulus of the binder composition can be controlled at 1000MPa-2000MPa, and the tensile strength of the binder composition can be controlled at 15MPa-45MPa, thereby comprehensively and efficiently improving the flexibility and mechanical strength of the positive electrode sheet.

[0035] Embodiments of the present invention also provide a positive electrode sheet for a sodium-ion battery. The positive electrode sheet includes a current collector and a positive electrode material layer. The positive electrode material layer is disposed on the surface of the current collector and contains a positive electrode active material, a conductive agent, and the above-mentioned binder composition. The binder composition accounts for 2% to 10% of the total mass of the positive electrode material layer.

[0036] Positive electrode active materials include those with the chemical formula Na x The layered oxide cathode material is MO2, where M includes at least one of Fe, Mn, Ni, Cu, Co, and Mg, and 0.5 ≤ x ≤ 1. Existing conventional conductive agents can be used, such as acetylene black, SuperP, and carbon nanotubes. The mass ratio of the cathode active material and the conductive agent in the cathode material layer can be adjusted according to the specific capacity and electrochemical performance requirements of the sodium-ion battery; no further restrictions are imposed here.

[0037] Optionally, the compaction density of the above-mentioned positive electrode sheet is 2.5–3.5 g / cm³. 3 Surface density ≥14mg / cm³ 2 .

[0038] Embodiments of the present invention also provide a sodium-ion battery, including but not limited to pouch cells, prismatic cells, and cylindrical cells. This sodium-ion battery comprises a casing, an electrode assembly inside the casing, and an electrolyte. The electrode assembly includes a negative electrode, a separator, and the aforementioned positive electrode. The selection of the negative electrode material and the formulation of the electrolyte can be adjusted and designed according to the specific capacity and electrochemical performance requirements of the sodium-ion battery; no further limitations are imposed here. As an example, a 1M NaPF6 / PC / EMC system can be used as the electrolyte, and the swelling rate of the aforementioned binder composition in this electrolyte is ≤5%.

[0039] The following are specific embodiments and comparative examples of the present invention.

[0040] Example 1

[0041] (1) Polytrimethylene carbonate with a weight average molecular weight of 50,000 to 300,000 Da and polyvinylidene fluoride with a weight average molecular weight of 300,000 to 1,000,000 Da are mixed at a mass ratio of 0.5:5 to obtain an adhesive composition.

[0042] (2) Take the positive electrode active material (Na) according to the mass ratio of 95:0.8:1.2:3. 2 / 3 Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 The above-mentioned binder composition (O2), acetylene black conductive agent, carbon nanotube conductive agent, and binder are then stirred and mixed with solvent (NMP) to form a uniform and stable positive electrode slurry.

[0043] (3) The above-mentioned positive electrode slurry was coated onto the front and back surfaces of a 10μm aluminum foil current collector using a coating process, with a coating amount of 28mg / cm². 2 Then, it is placed in an oven and dried at 110°C to obtain the positive electrode sheet for sodium-ion batteries.

[0044] Example 2

[0045] (1) Polytrimethylene carbonate with a weight average molecular weight of 50,000 to 300,000 Da and polyvinylidene fluoride with a weight average molecular weight of 300,000 to 1,000,000 Da are mixed at a mass ratio of 1:5 to obtain an adhesive composition.

[0046] (2) Take the positive electrode active material (Na) according to the mass ratio of 95.5:0.8:1.2:2.5. 2 / 3 Ni 1 / 10 Fe 1 / 10 Mn 3 / 5Cu 1 / 10 Mg 1 / 10 The above-mentioned binder composition (O2), acetylene black conductive agent, carbon nanotube conductive agent, and binder are then stirred and mixed with solvent (NMP) to form a uniform and stable positive electrode slurry.

[0047] (3) The above-mentioned positive electrode slurry was coated onto the front and back surfaces of the 12μm aluminum foil current collector using a coating process, with a coating amount of 30mg / cm². 2 Then, it is placed in an oven and dried at 110°C to obtain the positive electrode sheet for sodium-ion batteries.

[0048] Example 3

[0049] (1) Polytrimethylene carbonate with a weight average molecular weight of 50,000 to 300,000 Da and polyvinylidene fluoride with a weight average molecular weight of 300,000 to 1,000,000 Da are mixed at a mass ratio of 1:5 to obtain an adhesive composition.

[0050] (2) Take the positive electrode active material (Na) according to the mass ratio of 94.5:0.8:1.2:3.5. 2 / 3 Ni 1 / 10 Fe 1 / 10 Mn 3 / 5Cu1 / 10 Mg 1 / 10 The above-mentioned binder composition (O2), acetylene black conductive agent, carbon nanotube conductive agent, and binder are then stirred and mixed with solvent (NMP) to form a uniform and stable positive electrode slurry.

[0051] (3) The above-mentioned positive electrode slurry was coated onto the front and back surfaces of the 12μm aluminum foil current collector using a coating process, with a coating amount of 35mg / cm². 2 Then, it is placed in an oven and dried at 110°C to obtain the positive electrode sheet for sodium-ion batteries.

[0052] Comparative Example 1

[0053] (1) Take the positive electrode active material (Na) according to the mass ratio of 95:0.8:1.2:3. 2 / 3 Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 O2), acetylene black conductive agent, carbon nanotube conductive agent and binder (PVDF), and then mixed with solvent (NMP) to form a uniform and stable positive electrode slurry.

[0054] (2) The above-mentioned positive electrode slurry was coated onto the front and back surfaces of a 10μm aluminum foil current collector using a coating process, with a coating amount of 28mg / cm². 2 Then, it is placed in an oven and dried at 110°C to obtain the positive electrode sheet for sodium-ion batteries.

[0055] Test case

[0056] (1) The adhesion strength of the positive electrode sheets of the sodium-ion batteries prepared in Example 1 and Comparative Example 1 was tested using the 180° peel force test method. The test results are shown in Appendix. Figure 1 .

[0057] (2) The positive electrode sheets of sodium-ion batteries prepared in Example 1 and Comparative Example 1 were rolled, sheared, die-cut, dusted and dried, and then assembled with hard carbon negative electrode sheets and ceramic separators in a stacking manner to form a cell bare electrode assembly; the cell bare electrode assembly was subjected to tab welding, encapsulation, 1M NaPF6 / PC / EMC based electrolyte injection, formation and sorting to finally obtain sodium-ion batteries.

[0058] The low-temperature discharge performance of the two groups of sodium-ion batteries at -30℃ and their charge-discharge cycle performance at room temperature (1C / 1C) were tested respectively. The test results are shown in the attached table. Figure 2 and attached Figure 3 .

[0059] Comparison Appendix Figure 1-3 The bonding strength of the positive electrode sheet prepared in Example 1, the low-temperature discharge performance of the sodium-ion battery, and the room-temperature cycling performance are all better than those of Comparative Example 1.

[0060] It should be understood that the specific embodiments described above are merely illustrative or explanatory of the principles of the invention and do not constitute a limitation thereof. Therefore, any modifications, equivalent substitutions, improvements, etc., made without departing from the spirit and scope of the invention should be included within the protection scope of the invention. Furthermore, the appended claims are intended to cover all variations and modifications falling within the scope and boundaries of the claims, or equivalent forms of such scope and boundaries.

Claims

1. A binder composition for a sodium-ion battery positive electrode, characterized in that, At 25°C: the storage modulus of the adhesive composition is 1000 MPa to 2000 MPa, and the tensile strength is 15 MPa to 45 MPa; the adhesive composition includes a first adhesive and a second adhesive, wherein the storage modulus of the first adhesive is less than that of the second adhesive, and the tensile strength of the first adhesive is less than that of the second adhesive; the first adhesive includes at least polytrimethylene carbonate, the weight-average molecular weight of which is 50,000 to 300,000 Da; the second adhesive includes at least polyvinylidene fluoride, the weight-average molecular weight of which is 300,000 to 1,000,000 Da, and the mass ratio of polytrimethylene carbonate to polyvinylidene fluoride is 0.1:(1 to 5).

2. The binder composition for the positive electrode of a sodium-ion battery according to claim 1, characterized in that, The first adhesive contains at least one of hydroxyl, carboxyl, and ester groups.

3. The binder composition for the positive electrode of a sodium-ion battery according to claim 1 or 2, characterized in that, At 25°C: the storage modulus of the first adhesive is 1 MPa to 10 MPa, and the storage modulus of the second adhesive is 1000 MPa to 3000 MPa; the tensile strength of the first adhesive is 1 MPa to 10 MPa, and the tensile strength of the second adhesive is 40 MPa to 60 MPa.

4. A positive electrode sheet for a sodium-ion battery, characterized in that, The positive electrode sheet includes a current collector and a positive electrode material layer. The positive electrode material layer is disposed on the surface of the current collector and contains a positive electrode active material, a conductive agent, and a binder composition of the sodium-ion battery positive electrode according to any one of claims 1-3. The binder composition accounts for 2% to 10% of the total mass of the positive electrode material layer.

5. The positive electrode of the sodium-ion battery according to claim 4, characterized in that, The positive electrode active material includes Na x MO2 is a layered oxide cathode material, wherein M includes at least one of Fe, Mn, Ni, Cu, Co and Mg, and 0.5≤x≤1.

6. A sodium-ion battery, characterized in that, The sodium-ion battery includes a positive electrode, a negative electrode, and an electrolyte as described in claim 4 or 5, wherein the swelling rate of the binder assembly in the positive electrode in the electrolyte is ≤5%.

7. The sodium-ion battery according to claim 6, characterized in that, The sodium-ion battery is a pouch battery, a prismatic battery, or a cylindrical battery.

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