A sodium-ion battery

Abstract

This invention discloses a sodium-ion battery, characterized in that the sodium-ion battery comprises: a positive electrode, a negative electrode, a separator, and an electrolyte; the negative electrode active material used to prepare the negative electrode is a mixture of nano-sized hard carbon and micron-sized hard carbon; the negative electrode active material satisfies the following conditions: 3μm < D50 < 10μm, preferably 3μm < D50 < 7μm, 6μm < D50 < 10μm. 2 / g≤S≤10m 2 / g, 0.4 < D50 / S < 1, where D50 is the median particle size and S is the specific surface area; the negative electrode slurry used to prepare the negative electrode sheet is an oil-based slurry; the positive electrode slurry used to prepare the positive electrode sheet includes a first binder, which includes modified PVDF and first PVDF; the mass ratio of modified PVDF to first PVDF is 1:(3-5). The sodium-ion battery of the present invention can achieve comprehensive performance improvement with low internal resistance, high power, strong electrode bonding, and friendly positive electrode processing.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, and more specifically, relates to a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries have advantages such as abundant sodium resources, low cost, excellent safety performance, and compatibility with lithium-ion battery production equipment, and have broad application prospects in large-scale energy storage, low-speed electric vehicles, and engineering machinery.

[0003] However, existing sodium-ion batteries generally suffer from the following technical defects: (1) Sodium ions have a larger radius than lithium ions, resulting in greater solid-phase diffusion resistance, which leads to higher overall internal resistance of the cell and insufficient power and rate performance. (2) The hard carbon anode material has low bonding strength and weak peel force with the current collector, and high interfacial impedance, which seriously affects the battery's electrical performance and cycle life. (3) The pH value of layered oxide cathode materials is high (usually 12~13). Conventional polyvinylidene fluoride (PVDF) binders are prone to dehydrofluorination and cross-linking gelation, resulting in difficulty in homogenization, poor slurry stability, and low electrode consistency, making it difficult to meet the requirements of mass production. Summary of the Invention

[0004] The purpose of this invention is to provide a sodium-ion battery that achieves comprehensive performance improvements, including low internal resistance, high power, strong electrode bonding, and easy positive electrode processing.

[0005] To achieve the above objectives, the present invention provides a sodium-ion battery, which includes: a positive electrode, a negative electrode, a separator, and an electrolyte. The negative electrode active material used to prepare the negative electrode sheet is a mixture of nano-sized hard carbon and micron-sized hard carbon; the negative electrode active material satisfies the following conditions: 3μm < D50 < 10μm, preferably 3μm < D50 < 7μm, 6μm 2 / g≤S≤10m 2 / g, 0.4 < D50 / S < 1, where D50 is the median particle size and S is the specific surface area; The negative electrode slurry used to prepare the negative electrode sheet is an oil-based slurry; The positive electrode slurry used to prepare the positive electrode sheet includes a first binder, which includes modified PVDF and first PVDF; the mass ratio of the modified PVDF to the first PVDF is 1:(3-5).

[0006] In this invention, the first binder is a compound of modified PVDF and first PVDF, which can inhibit dehydrogenation crosslinking under high alkalinity cathode, completely solve the slurry gelation problem, and improve the uniformity of slurry and the stability of electrode.

[0007] In this invention, hard carbon particles of different sizes are used to fill the gaps, shortening the diffusion path of sodium ions and improving power performance; the surface area is moderate, which facilitates homogenization and coating.

[0008] According to the present invention, preferably, the negative electrode slurry comprises: a negative electrode active material, a second binder, a first solvent, a first conductive agent, and additives; Preferably, the mass percentage of the negative electrode active material, the second binder, and the first conductive agent is 89-96%, the mass percentage of the second binder is 2-8%, and the mass percentage of the first conductive agent is 0.5-2%, based on the sum of the masses of the negative electrode active material, the second binder, and the first conductive agent; the mass percentage of the additive is 0.5-3% of the total solid mass of the negative electrode slurry.

[0009] According to the present invention, preferably, the additive is anhydrous oxalic acid.

[0010] According to the present invention, preferably, the first solvent is N-methylpyrrolidone (NMP).

[0011] In this invention, the negative electrode slurry is an oil-based formulation to reduce the introduction of moisture; oxalic acid is added to slightly etch the foil surface, thereby improving peel strength and reducing interfacial resistance.

[0012] According to the present invention, preferably, the second adhesive is a second PVDF (polyvinylidene fluoride). The first conductive agent is at least one of acetylene black, carbon black, Ketjen black, carbon nanotubes, graphene, and carbon nanofibers.

[0013] According to the present invention, preferably, the modified PVDF is a modified PVDF containing difluoro and tetrafluoro copolymers.

[0014] According to the present invention, preferably, the positive electrode slurry further includes: a positive electrode active material, a second conductive agent, and a second solvent.

[0015] According to the present invention, preferably, the second conductive agent is a mixture of conductive carbon black and carbon nanotube slurry; The second solvent is N-methylpyrrolidone (NMP).

[0016] According to the present invention, preferably, the current collector used to prepare the positive electrode sheet is aluminum foil; The current collector used to prepare the negative electrode sheet is aluminum foil.

[0017] According to the present invention, preferably, the material of the separator is at least one selected from non-woven fabric, polyethylene, and polypropylene; The electrolyte is a sodium-ion battery electrolyte.

[0018] In this invention, preferably, the separator is a single-layer film or a multi-layer film.

[0019] In this invention, preferably, the sodium-ion battery electrolyte is a high-power sodium-ion battery electrolyte, comprising a sodium salt, an organic solvent, and an electrolyte additive; the sodium salt is NaPF6 and / or NaFSI; the solvent is at least two of EC, PC, DEC, and DMC; the electrolyte additive is at least one of FEC, VC, PS, and NaPO2F2; and the concentration of the sodium salt in the electrolyte is 1.0~1.5M.

[0020] In this invention, preferably, the method for preparing the sodium-ion battery includes the following steps: (1) Mix the negative electrode active material, the first conductive agent, the first binder and the first solvent evenly to form a slurry; and add additives during the homogenization process, and coat the mixed slurry on the upper and lower surfaces of the aluminum foil current collector; after coating and drying, roll and die-cut to obtain the negative electrode sheet; (2) Mix the positive electrode active material, the second conductive agent, the second binder and the second solvent evenly to form a slurry, and coat the mixed slurry on the upper and lower surfaces of the aluminum foil current collector; after coating and drying, roll and die-cut to obtain the positive electrode sheet; The positive electrode, negative electrode, and separator are stacked together to form a dry cell, which is then vacuum-sealed with aluminum-plastic film, baked to remove water, injected with electrolyte, and then formed and tested to obtain a sodium-ion battery.

[0021] The technical solution of the present invention has the following beneficial effects.

[0022] (1) The use of nano and micro hard carbon composites optimizes particle packing and sodium ion diffusion channels, significantly reducing internal resistance and improving power and rate performance; (2) The negative electrode oil system combined with oxalic acid additives effectively etches the surface of the current collector, greatly improves the negative electrode peel strength, and reduces the interface impedance; (3) The combination of two PVDFs inhibits the gelation of layered oxide cathode slurry, prolongs the slurry stabilization time, and improves the uniformity of the electrode and the processability for mass production; (4) The all-aluminum foil current collector design further reduces costs, is compatible with existing lithium-ion battery production equipment, and is easy to scale up production.

[0023] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0024] The above and other objects, features and advantages of the present invention will become more apparent from the more detailed description of exemplary embodiments of the invention in conjunction with the accompanying drawings, wherein the same reference numerals generally represent the same components in the exemplary embodiments of the invention.

[0025] Figure 1 A picture of the positive electrode slurry according to Embodiment 1 of the present invention is shown.

[0026] Figure 2 A picture of the positive electrode slurry according to Comparative Example 2 of the present invention is shown. Detailed Implementation

[0027] Preferred embodiments of the invention will now be described in more detail. While preferred embodiments of the invention are described below, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the invention will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0028] The present invention is further illustrated by the following examples: In the following embodiments and comparative examples: The modified PVDF used is a modified PVDF containing difluoro and tetrafluoro copolymers, specifically Daikin Fluorochemicals VT475; The first PVDF is Solvay PVDF5130; The second PVDF is Arkema Kynar 761A; The SBR (styrene-butadiene rubber) was purchased from JSR Corporation, with the grade TRD104A. The carbon nanotube slurry was made of Tiannai, an oil-based CNTs-LB 108-43. The nickel-iron-manganese layered oxide is tungsten, N3O3.

[0029] Example 1

[0030] 1) Preparation of negative electrode sheet

[0031] Negative electrode active material: a mixture of nano-hard carbon and micron-hard carbon (composite hard carbon). The parameters of the negative electrode active material are: D50 = 5 μm, specific surface area S = 8 m² / g, D50 / S = 0.625 (satisfying 0.4 < D50 / S < 1). The negative electrode slurry includes: compound hard carbon, conductive carbon black (first conductive agent), second PVDF (second binder), additives and NMP (first solvent). Based on the sum of the masses of the compound hard carbon, conductive carbon black, and the second PVDF, the mass percentage of the compound hard carbon is 94%, the mass percentage of the conductive carbon black is 1%, and the mass percentage of the second PVDF is 5%. Additive: Anhydrous oxalic acid, added at a rate (mass percentage) of 1.5% of the total solid mass of the negative electrode slurry; Preparation method: The negative electrode active material, the first conductive agent, the second binder, and the first solvent are mixed evenly to form a slurry; additives are added during the homogenization process. The mixed slurry is then coated on the upper and lower surfaces of the aluminum foil current collector, with an areal density (excluding foil) of 86 g / m² on each side. 2 After coating and drying, the electrode is rolled and die-cut into negative electrode sheets of a certain size.

[0032] 2) Preparation of positive electrode sheet

[0033] Positive electrode active material: nickel-iron-manganese layered oxide (layered oxide); The positive electrode slurry includes: positive electrode active material, second conductive agent (a mixture of conductive carbon black and carbon nanotube slurry), first binder (composite PVDF), and second solvent (NMP). Based on the sum of the masses of the positive electrode active material, the second conductive agent, and the first binder, the mass percentage of layered oxide is 95%, the mass percentage of conductive carbon black is 2%, the mass percentage of carbon nanotube slurry is 0.8%, and the mass percentage of compounded PVDF is 2.2%. The compound PVDF includes modified PVDF and first PVDF, with a mass ratio of modified PVDF to first PVDF of 1:4; Preparation method: Layered oxide positive electrode active material, conductive carbon black, carbon nanotube slurry, first binder and second solvent are mixed evenly to form a slurry. The mixed slurry is coated on the upper and lower surfaces of the aluminum foil current collector, with an areal density (excluding foil) of 160 g / m² on each side. 2 After coating and drying, the material is rolled and die-cut into positive electrode sheets of a certain size.

[0034] 3) Battery assembly

[0035] The positive electrode, negative electrode, and separator are stacked together to form a dry cell, which is then vacuum-sealed with an aluminum-plastic film, baked to remove moisture, and injected with sodium-ion battery electrolyte. After formation and capacity testing, a high-power sodium-ion battery (7Ah cell capacity) is obtained. The separator is made of polyethylene, specifically Gionee GM09 separator; the sodium-ion battery electrolyte is Faenlet FT-NA0407-3.

[0036] Example 2

[0037] 1) Negative electrode plate

[0038] The negative electrode sheet was prepared according to the method of Example 1, with the only difference being that the parameters of the mixture of nano-hard carbon and micron-hard carbon (composite hard carbon) were: D50=6μm, S=9 m² / g, D50 / S=0.67; and anhydrous oxalic acid was added at a rate (mass percentage) of 2.0% of the total solid mass of the negative electrode slurry. 2) Positive electrode sheet: Same as in Example 1; 3) Assembly: Assemble the battery according to the method of Example 1, the only difference being the use of the negative electrode sheet prepared in this example.

[0039] Example 3

[0040] 1) Negative electrode plate

[0041] The negative electrode sheet was prepared according to the method of Example 1, with the only difference being that the parameters of the mixture of nano-hard carbon and micron-hard carbon (composite hard carbon) were: D50=4μm, S=7 m² / g, D50 / S=0.57; and anhydrous oxalic acid was added at a rate (mass percentage) of 3.0% of the total solid mass of the negative electrode slurry. 2) Positive electrode sheet: Same as in Example 1; 3) Assembly: Assemble the battery according to the method of Example 1, the only difference being the use of the negative electrode sheet prepared in this example.

[0042] Example 4

[0043] 1) Negative electrode plate

[0044] The negative electrode sheet was prepared according to the method in Example 1, except that oxalic acid was not added. 2) Positive electrode sheet: Same as in Example 1; 3) Assembly: Assemble the battery according to the method of Example 1, the only difference being the use of the negative electrode sheet prepared in this example.

[0045] Comparative Example 1

[0046] 1) Negative electrode plate

[0047] The negative electrode sheet was prepared according to the method of Example 1, with the only difference being that the negative electrode active material was a single micron-sized hard carbon, and the parameters of the negative electrode active material were: D50=12μm, S=8m² / g, D50 / S=1.5. 2) Positive electrode sheet: Same as in Example 1; 3) Assembly: Assemble the battery according to the method of Example 1, except that the negative electrode sheet prepared in this comparative example is used.

[0048] Comparative Example 2

[0049] 1) Negative electrode sheet: Same as in Example 1; 2) Positive electrode sheet: The positive electrode sheet was prepared according to the method of Example 1, except that the first binder was the first PVDF; 3) Assembly: Assemble the battery according to the method of Example 1, the only difference being that the positive electrode sheet prepared in this comparative example is used.

[0050] Comparative Example 3

[0051] 1) Negative electrode plate

[0052] The negative electrode sheet was prepared according to the method of Example 1, with the only difference being that the first solvent was water, the second binder was SBR and sodium carboxymethyl cellulose (CMC), and oxalic acid was not added; based on the sum of the masses of the compound hard carbon, conductive carbon black, SBR, and sodium carboxymethyl cellulose (CMC), the mass percentage of the compound hard carbon was 94%, the mass percentage of the conductive carbon black was 1%, the mass percentage of the SBR was 3.7%, and the mass percentage of the sodium carboxymethyl cellulose (CMC) was 1.3%; 2) Positive electrode sheet: Same as in Example 1; 3) Assembly: Assemble the battery according to the method of Example 1, except that the negative electrode sheet prepared in this comparative example is used.

[0053] Comparative Example 4

[0054] 1) Negative electrode plate

[0055] The negative electrode sheet was prepared according to the method of Example 1, with the only difference being that the parameters of the mixture of nano hard carbon and micron hard carbon (composite hard carbon) were: D50 = 5μm, S = 4 m² / g, D50 / S = 5 / 4 = 1.25; 2) Positive electrode sheet: Same as in Example 1; 3) Assembly: Assemble the battery according to the method of Example 1, except that the negative electrode sheet prepared in this comparative example is used.

[0056] Test Example 1

[0057] The performance of the batteries prepared in the above embodiments and comparative examples was tested, and the specific test results are shown in the table below.

[0058] The DCR test method is as follows: a) At an ambient temperature of 25℃±2℃, let stand for 30 minutes, then charge at a constant current of 0.33C until the charging cutoff voltage is reached, then switch to constant voltage charging, and stop charging when the charging current drops to 0.05C. b) Under an ambient temperature of 25℃±2℃, adjust the load to 50% SOC; c) Allow the sample to stand at the target ambient temperature for 0.5 hours, then perform a pulse charge / discharge test using a pulse ammeter; The discharge retention rate test method is as follows: a) Let stand for 30 minutes at an ambient temperature of 25℃±2℃; b) At an ambient temperature of 25℃±2℃, the test sample was discharged at a constant current of 1C until the specified discharge cutoff condition was met, and then left to stand for 30 minutes. c) At an ambient temperature of 25℃±2℃, the test sample was charged at a current of 1C. When the individual cell voltage reached the specified charging cutoff condition, the charging was switched to constant voltage charging. Charging was stopped when the charging termination current dropped to 0.05C. The sample was then left to stand for 30 minutes. d) Record the changes in cell voltage, current and surface temperature during charging, and calculate the charging capacity (Ah) and charging energy (Wh). e) Repeat steps a) to d), where the discharge current in step c) is 1C and 10C respectively.

[0059] Table 1. Influence of negative electrode particle parameters on power and internal resistance

[0060] Test Example 2

[0061] The peel strength, impedance, and moisture content of the negative electrode sheets prepared in the above embodiments and comparative examples were tested; the specific test results are shown in the table below. The electrode sheet peel strength test was conducted in accordance with the domestic standard GB / T 2792-2014 "Test Method for Peel Strength of Adhesive Tapes"; the interfacial impedance was tested using an electrode resistance meter, specifically a Hiochi RM2610; the moisture content was tested using a Mettler C30s Karl Fischer coulometric titrator, with the drying temperature set at 170℃, the airflow rate at 80 mL / min, and the cutoff condition: heating time of 600 s.

[0062] Table 2 Effects of oxalic acid on peel strength, resistance, and moisture content of the system

[0063] Test Example 3

[0064] The gelation of the positive electrode slurry in the above embodiments and comparative examples was observed. The stability time of the positive electrode slurry and the areal density difference of the positive electrode sheet were tested. The specific test methods are as follows, and the test results are shown in the table below.

[0065] Stabilization time: Stir with a spoon every 30 minutes and observe its fluidity. If the slurry falls in the form of threads or droplets rather than in clumps or blocks, it is considered that the slurry is stable and has not gelled. That is, the time during which the slurry remains stable without gelling is the stabilization time.

[0066] Areal density range: Use an areal density sampler to take electrodes at different coating locations, with a sample quantity of ≥9. Weigh each electrode individually using a 0.01% balance and calculate the areal density of each electrode. Areal density range = ((maximum areal density - minimum areal density) / average areal density) 100.

[0067] Table 3. Effect of the PVDF cathode system on slurry stability

[0068] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments.

Claims

1. A sodium-ion battery, characterized in that, The sodium-ion battery includes: a positive electrode, a negative electrode, a separator, and an electrolyte; The negative electrode active material used to prepare the negative electrode sheet is a mixture of nano-sized hard carbon and micron-sized hard carbon; the negative electrode active material satisfies the following conditions: 3μm < D50 < 10μm, preferably 3μm < D50 < 7μm, 6μm 2 / g≤S≤10m 2 / g, 0.4 < D50 / S < 1, where D50 is the median particle size and S is the specific surface area; The negative electrode slurry used to prepare the negative electrode sheet is an oil-based slurry; The positive electrode slurry used to prepare the positive electrode sheet includes a first binder, which includes modified PVDF and first PVDF; the mass ratio of the modified PVDF to the first PVDF is 1:(3-5).

2. The sodium-ion battery according to claim 1, wherein, The negative electrode slurry comprises: a negative electrode active material, a second binder, a first solvent, a first conductive agent, and additives; Preferably, the mass percentage of the negative electrode active material, the second binder, and the first conductive agent is 89-96%, the mass percentage of the second binder is 2-8%, and the mass percentage of the first conductive agent is 0.5-2%, based on the sum of the masses of the negative electrode active material, the second binder, and the first conductive agent; the mass percentage of the additive is 0.5-3% of the total solid mass of the negative electrode slurry.

3. The sodium-ion battery according to claim 2, wherein, The additive is anhydrous oxalic acid.

4. The sodium-ion battery according to claim 2, wherein, The first solvent is N-methylpyrrolidone.

5. The sodium-ion battery according to claim 2, wherein, The second adhesive is a second PVDF; The first conductive agent is at least one of acetylene black, carbon black, Ketjen black, carbon nanotubes, graphene, and carbon nanofibers.

6. The sodium-ion battery according to claim 1, wherein, The modified PVDF is a modified PVDF containing difluoro and tetrafluoro copolymers.

7. The sodium-ion battery according to claim 1, wherein, The positive electrode slurry also includes: a positive electrode active material, a second conductive agent, and a second solvent.

8. The sodium-ion battery according to claim 7, wherein, The second conductive agent is a mixture of conductive carbon black and carbon nanotube slurry; The second solvent is N-methylpyrrolidone.

9. The sodium-ion battery according to claim 1, wherein, The current collector used to prepare the positive electrode sheet is aluminum foil; The current collector used to prepare the negative electrode sheet is aluminum foil.

10. The sodium-ion battery according to claim 1, wherein, The material of the isolation membrane is at least one of non-woven fabric, polyethylene, and polypropylene; The electrolyte is a sodium-ion battery electrolyte.

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