Positive electrode and sodium-ion battery and preparation method thereof

Abstract

The application provides a positive electrode and a sodium ion battery and a preparation method thereof. The sodium ion battery comprises a positive electrode, a negative electrode and an electrolyte; the slurry of the positive electrode comprises a positive electrode active material, a multifunctional additive, a binder and conductive carbon; the multifunctional additive accounts for 0.2-1.4% of the total mass of the positive electrode slurry; the multifunctional additive is a combination of methyl benzotriazole sodium salt and sodium 1,1-difluoroethane sulfinate, and the mass ratio of the two is (1-5):(1-5). The application effectively improves the reversible capacity and the initial efficiency of the sodium ion positive electrode material, improves the mass-charge transfer resistance of the positive electrode surface to improve the rate and the room temperature cycle performance of the battery, and also improves the charging and discharging capacity at low temperature of-20 DEG C. The addition of the multifunctional additive optimizes the slurry formula of the positive electrode, and further benefits the electrical performance of the battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of battery materials, more particularly, to a positive electrode and a sodium ion battery and a preparation method thereof. BACKGROUND

[0002] Lithium ion batteries, as an electrochemical energy storage device with excellent specific energy and excellent durability, have dominated the mobile electronic products, new energy transportation, energy storage systems and aerospace fields in the past three decades. However, due to the limited reserves of lithium ore, the highly concentrated geographical distribution and the continuously rising mining cost, the popularization and application of lithium ion battery technology is facing significant challenges, especially in the application of large-scale energy storage field. Therefore, it is urgent to develop a low-cost energy storage device that can utilize abundant natural resources.

[0003] Sodium has similar chemical properties to lithium and is widely distributed and stored in the earth's crust, which makes sodium ion batteries expected to become a complementary alternative to lithium ion batteries in some fields, especially sodium ion batteries with polyanion as the positive electrode, which have long cycle life, good rate performance, high working voltage, good stability, high safety and wide temperature range (-40℃ ~ 55℃). However, the polyanion positive electrode has obvious disadvantages in capacity and energy density. At present, various organic / inorganic sodium salts are developed as sodium supplement agents, such as Na2C2O4, NaN3H, NaNO2, Na3C6O7H5 and Na3P, etc. Although the gas generated by these sodium supplement agents can be removed after the formation and degassing process is completed during the manufacturing process of the battery, since the sodium supplement agent is uniformly distributed during homogenization, the remaining voids of the sodium supplement agent turned into gas during charging and discharging can still have adverse effects on the electrode structure and the electronic transmission path, resulting in the decrease of battery cycle performance, the increase of resistance or the deterioration of rate performance. In order to improve the adverse effects of sodium supplement agent gas production and voids, some measures have been proposed, such as double coating, coating the slurry with a sodium supplement layer on the outer layer to avoid the collapse of the electrode structure. However, this double coating makes the coating process more complex, which undoubtedly increases the manufacturing cost; NaN3H and Na3P also have toxicity and safety problems. Obviously, it will be more cost-effective and attractive to develop a multifunctional additive to improve the performance of the positive electrode and the sodium ion battery. SUMMARY

[0004] To achieve the above-mentioned purposes, the present application provides a positive electrode and a sodium ion battery and a preparation method thereof. By adding a multifunctional additive to the positive electrode, the present application can not only achieve the effect of supplementing sodium, promote the initial efficiency and energy density of the positive electrode, but also stabilize the positive electrode and reduce the specific resistance of the positive electrode surface, thereby improving the rate, room temperature cycle capacity and low temperature discharge performance of the battery.

[0005] To achieve the above-mentioned purposes, the technical solutions of the present application are as follows:

[0006] In a first aspect, the present application provides a sodium-ion battery, comprising a positive electrode, a negative electrode and an electrolyte;

[0007] The slurry of the positive electrode comprises a positive electrode active material, a multifunctional additive, a binder and conductive carbon; the multifunctional additive accounts for 0.2-1.4% of the total mass of the positive electrode slurry;

[0008] The multifunctional additive is a combination of methyl benzotriazole sodium salt and sodium 1,1-difluoroethane sulfinate, and the mass ratio of the two is (1-5):(1-5).

[0009] In some embodiments, the positive electrode active material is at least one of sodium iron phosphate, sodium pyrophosphate, and sodium phosphate pyrophosphate.

[0010] In some embodiments, the slurry of the positive electrode comprises, by weight percentage, 75-96% of the positive electrode active material, 0.5-10% of the binder, 0.5-10% of the conductive carbon and 0.2-1.4% of the multifunctional additive.

[0011] In some embodiments, the slurry of the positive electrode comprises, by weight percentage, 75-85% of the positive electrode active material, 5-10% of the binder, 5-10% of the conductive carbon and 0.2-1.4% of the multifunctional additive.

[0012] In some embodiments, the slurry of the positive electrode comprises, by weight percentage, 75%, 76%, 77%, 78%, 79%, 80%, 82%, 84%, 86%, 88%, 90%, 92%, 93%, 94%, 95%, 96% or any range formed by any two of the above values of the positive electrode active material.

[0013] In some embodiments, the slurry of the positive electrode comprises, by weight percentage, 0.5%, 1%, 2%, 3%, 4%, 6%, 7%, 8%, 8.5%, 9%, 9.5%, 9.6%, 9.7%, 9.8%, 9.9%, 10% or any range formed by any two of the above values of the binder.

[0014] In some embodiments, the slurry of the positive electrode comprises, by weight percentage, 0.5%, 1%, 2%, 2.5%, 3%, 3.5%, 4%, 5%, 6%, 7%, 8%, 10% or any range formed by any two of the above values of the conductive carbon.

[0015] In some embodiments, the multifunctional additive accounts for 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, and any range formed by any two of the above values, by weight percentage, in the slurry of the positive electrode.

[0016] In some embodiments, the multifunctional additive is a combination of sodium methazol and sodium 1,1-difluoroethanesulfinate. In the combination of sodium methazol C7H6N3Na and sodium 1,1-difluoroethanesulfinate, the mass ratio of the two is 1:1.

[0017] In some embodiments, the binder is at least one of polyvinylidene fluoride (PVDF), polyimide (PI), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), and polyamide-imide (PAI), and preferably, the polyvinylidene fluoride is at least one of PVDF5130 or PVDF HSV-900.

[0018] In some embodiments, the conductive carbon is at least one of conductive carbon black, conductive graphite, Ketjen black, carbon nanotubes, and graphene, and preferably, the conductive carbon is conductive carbon black kappa 100.

[0019] In some embodiments, the negative electrode is a sodium sheet.

[0020] In some embodiments, the electrolyte includes a solvent, a solute, and an electrolyte additive.

[0021] In some embodiments, the solvent is a cyclic and chain carbonate. In some embodiments, the solvent is a combination of ethylene carbonate (EC), propylene carbonate (PC), and methyl ethyl carbonate (EMC). In some embodiments, in the solvent, the volume ratio of EC, PC, and EMC is 1:5:8.

[0022] In some embodiments, the solute is sodium hexafluorophosphate. In some embodiments, the concentration of the solute sodium hexafluorophosphate in the electrolyte is 0.5-1 mol / L, and preferably, any one range formed by any two of 0.7 mol / L, 0.75 mol / L, 1 mol / L, and the above values.

[0023] In some embodiments, the electrolyte additive is a combination of sodium difluorophosphate (NaPO2F2), tetrahydrofuran, and 3,3,3-trifluoropropyl methyl ester.

[0024] In some embodiments, the proportion of NaPO2F2 in the electrolyte is 1-5%, preferably 1%, 2%, 3%, 4%, 5%, and any range formed by any two of the above values; the proportion of tetrahydrofuran is 1-5%, preferably 1%, 2%, 3%, 4%, 5%, and any range formed by any two of the above values; the proportion of methyl 3,3,3-trifluoropropionate is 1-5%, preferably 1%, 2%, 3%, 4%, 5%, and any range formed by any two of the above values.

[0025] In a second aspect, the present application provides a preparation method of the sodium ion battery.

[0026] (1) Dissolve NaPF6 in a mixed solvent, add an electrolyte additive, mix uniformly to prepare an electrolyte;

[0027] (2) Mix the positive electrode active material, the multifunctional additive, the binder and the conductive carbon with NMP as the solvent to prepare a positive electrode slurry;

[0028] (3) Coat the positive electrode slurry on a current collector, dry and slice to prepare a positive electrode;

[0029] (4) Use the obtained electrolyte, positive electrode and negative electrode to prepare a sodium ion battery.

[0030] In a third aspect, the present application provides a positive electrode of the present application, wherein the slurry of the positive electrode comprises a positive electrode active material, a multifunctional additive, a binder and conductive carbon; the multifunctional additive accounts for 0.2-1.4% of the total mass of the positive electrode slurry.

[0031] The multifunctional additive is a combination of methyl benzotriazole sodium salt and sodium 1,1-difluoroethane sulfinate, and the mass ratio of the two is (1-5):(1-5).

[0032] In some embodiments, the positive electrode active material is at least one of sodium iron phosphate, sodium pyrophosphate, and sodium phosphate pyrophosphate.

[0033] In some embodiments, the slurry of the positive electrode comprises, by weight percentage, 75-96% of the positive electrode active material, 0.5-10% of the binder, 0.5-10% of the conductive carbon, and 0.01-5% of the multifunctional additive.

[0034] In some embodiments, the slurry of the positive electrode comprises, by weight percentage, 75%, 76%, 77%, 78%, 79%, 80%, 82%, 84%, 86%, 88%, 90%, 92%, 93%, 94%, 95%, 96%, and any range formed by any two of the above values.

[0035] In some embodiments, the binder in the slurry of the cathode is in a range of 0.5%, 1%, 2%, 3%, 4%, 6%, 7%, 8%, 8.5%, 9%, 9.5%, 9.6%, 9.7%, 9.8%, 9.9%, 10% and any range between any two of the above values.

[0036] In some embodiments, the conductive carbon in the slurry of the cathode is in a range of 0.5%, 1%, 2%, 2.5%, 3%, 3.5%, 4%, 5%, 6%, 7%, 8%, 10% and any range between any two of the above values.

[0037] In some embodiments, the multifunctional additive in the slurry of the cathode is in a range of 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4% and any range between any two of the above values.

[0038] In some embodiments, the multifunctional additive is a mixture of methyl benzotriazole sodium salt C7H6N3Na and sodium 1,1-difluoroethane sulfinate, and the mass ratio of the two is (1~5):(1~5), preferably 1:1, 1:2, 1:3, 1:4, 1:5, 2:1, 2:3, 2:5, 3:1, 3:2, 3:4, 3:5, 4:1, 4:3, 4:5, 5:1, 5:2, 5:3, 5:4 and any range between any two of the above values.

[0039] In some embodiments, the binder is at least one of polyvinylidene fluoride (PVDF), polyimide (PI), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), and polyamide-imide (PAI), preferably, the polyvinylidene fluoride is at least one of PVDF5130 or PVDF HSV-900.

[0040] In some embodiments, the conductive carbon is at least one of conductive carbon black, conductive graphite, ketjen black, carbon nanotube and graphene, preferably, the conductive carbon black is kappa 100.

[0041] In a fourth aspect, the application provides a use of the sodium-ion battery or the cathode as described herein in an energy storage device.

[0042] In the present application, "low temperature" refers to a temperature in a range of -40℃ to 0℃, preferably -20℃ to 0℃.

[0043] "Sodium ion battery" refers to a sodium ion battery that can be charged and discharged at -40°C to 80°C, and in the present application, preferably -40°C to 60°C or -20°C to 55°C.

[0044] In the examples, "room temperature" refers to room temperature of 10-40°C, preferably 20-30°C or 25°C.

[0045] Compared with the prior art, one of the above technical solutions has the following advantages or beneficial effects:

[0046] The present application provides a positive electrode and a sodium ion battery and a preparation method thereof, which improves the reversible capacity and initial efficiency of the sodium ion positive electrode material, improves the surface charge transfer resistance of the positive electrode to improve the rate and room temperature cycle performance of the battery, and also improves the charge and discharge capacity at low temperature -20°C. The addition of multifunctional additives optimizes the slurry formulation of the positive electrode, thereby benefiting the electrical performance of the battery (for example, improving the rate of the battery, improving the cycle stability and resistance at high and low temperatures).

[0047] (1) The present application adds a multifunctional additive to the positive electrode, which can compensate for the irreversible sodium consumed in the formation of the SEI film during the first charge and discharge, so as to improve the reversible capacity and initial efficiency of the battery during the first charge and discharge.

[0048] (2) The multifunctional additive used in the present application can significantly improve the comprehensive performance of the battery. The multifunctional additive helps to improve the electrochemical performance of the battery, enhances the stability of the positive electrode structure, and effectively reduces the charge transfer resistance of the positive electrode interface, thereby optimizing the cycle performance of the battery under room temperature conditions. For example, some anion-type compounds (Na2CO3, Na2SO3, Na2SO4, Na2S, NaF, Na3N) form SEI films, which can improve the charge transfer rate between the electrode and the electrolyte, reduce the charge transfer resistance on the electrode surface, and thus improve the room temperature rate discharge and low temperature -20°C discharge performance. Sodium 1,1-difluoroethane sulfinate C2H2N3NaS and methyl benzene pyrrole triazole sodium salt C7H6N3Na form a high entropy state in the positive electrode, which can reduce the Gibbs free energy of the system and enhance the thermodynamic stability of the positive electrode material, effectively inhibiting the phase transition and structure degradation of the material. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 For the acid-base test of the multifunctional additive in Example 1, (a) shows the correspondence between the reagent bottle number and the material composition, and (b) shows the results of the acid-base test of the material in the reagent bottle using pH test paper.

[0050] Figure 2 For the impedance Nyquist plot of the No. 1 battery in Example 1 at room temperature.

[0051] Figure 3Impedance Nyquist plots of the 5 cells with single addition of C7H6N3Na in Example 1 at room temperature.

[0052] Figure 4 Impedance Nyquist plots of the 5 cells with single addition of C2H3F2NaO2S in Example 1 at room temperature.

[0053] Figure 5 Impedance Nyquist plots of the 4 cells with pairwise addition in Example 1 at room temperature.

[0054] Figure 6 Charge-discharge test plots of the two cells in Example 2: (a) is the time-current comparison plot of No. 1 cell and No. 3-3 cell, (b) is the cycle number-capacity retention comparison plot of No. 1 cell and No. 3-3 cell, (c) is the average capacity retention comparison plot of No. 1 cell and No. 3-3 cell.

[0055] Figure 7 Rate discharge dQ / dV curves of the two cells in Example 2: (a) is the rate discharge dQ / dV curve of No. 1 cell; (b) is the rate discharge dQ / dV curve of No. 3-3 cell.

[0056] Figure 8 Performance test plots of No. 1 cell and No. 3-3 cell at different temperatures: (a) is the charge-discharge curve at 55℃; (b) is the charge-discharge curve at 25℃; (c) is the charge-discharge curve at -20℃; (d) is the dQ / dV curve at 55℃; (e) is the dQ / dV curve at 25℃; (f) is the dQ / dV curve at -20℃.

[0057] The application will be further described in conjunction with specific examples. These examples are only used to illustrate the application and are not used to limit the scope of the application. DETAILED DESCRIPTION

[0058] The application will be further described in conjunction with specific examples. These examples are only used to illustrate the application and are not used to limit the scope of the application.

[0059] I. Chemicals and Instruments

[0060] Dimethyl carbonate (DMC, 99.9%) was purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd.; ethylene carbonate (EC, 99%), propylene carbonate (PC, 99.5%) were purchased from Shanghai Yinn Chemical Technology Co., Ltd.; sodium hexafluorophosphate (NaPF6, 99.9%), methyl ethyl carbonate (EMC, 98%) were purchased from Shanghai Aldrich Biochemical Technology Co., Ltd.

[0061] Battery tester (CT-4008-5VA-SI) was purchased from Shenzhen New Will Electronic Co., Ltd.; High and low temperature test chamber (LRHS-800B-L) was purchased from Shanghai Linpian Instrument Co., Ltd.; Electrochemical workstation (CHI660E) was purchased from Shanghai Chenhua Instrument Technology Co., Ltd.; Vacuum oven (DZF-6020BZ) was purchased from Shanghai Yixi Scientific Instrument Co., Ltd.; Vacuum stirrer (SFM-7) was purchased from Hefei Kejing Material Technology Co., Ltd.; Glass fiber (GF / D Whatman); CR2032 button cell complete accessories were purchased from Dongguan Keluode Innovation Technology Co., Ltd. Vacuum drying oven (DZF-6020B) was purchased from Shanghai Yixi Scientific Instrument Co., Ltd., the normal temperature 0.1C charge-discharge electrical performance test of the battery was carried out on the New Wei test cabinet (CT-4008Tn), the electrochemical impedance was carried out on the electrochemical workstation (CHI660E) of Shanghai Chenhua Instrument Technology Co., Ltd., the electrode punching machine (JK20140311) was purchased from Hefei Kejing Material Technology Co., Ltd.; Super purification glove box Super (1220 / 750 / 900) was purchased from Shanghai Micronora Mechanical and Electrical Technology Co., Ltd.; SZQ four-sided wet film coater 50-75-100-150 was purchased from Guangdong Huaguang Precision Detection Equipment Co., Ltd.; Button cell hydraulic sealing machine PX-HS-20 was purchased from Shenzhen Pengxiang Yunda Co., Ltd.

[0062] II. The routine operation is as follows:

[0063] 1. Preparation of positive electrode slurry and button cell

[0064] (1) Preparation of positive electrode sheet:

[0065] The positive active material, conductive carbon black and PVDF were baked in an oven at 80 °C for 12-24 h.

[0066] The mass ratio of positive active material, multifunctional additive, binder and conductive carbon was 8:(0.01-0.5):(0.5-0.99):1, respectively.

[0067] The positive active material is one or more of sodium iron phosphate, sodium pyrophosphate, sodium phosphate pyrophosphate, preferably sodium pyrophosphate; the multifunctional additive is one or both of methyl benzene pyrazole sodium salt C7H6N3Na and sodium 1,1-difluoroethane sulfinate C2H3F2NaO2S, preferably the multifunctional additive is a mixture of methyl benzene pyrazole sodium salt C7H6N3Na and sodium 1,1-difluoroethane sulfinate, and the mass fraction of the multifunctional additive is preferably 1%; the binder is one or more of polyvinylidene fluoride PVDF, polyimide PI, polytetrafluoroethylene PTFE, polyacrylonitrile PAN, and polyamide-imide PAI, preferably PVDF; and the conductive carbon is one or more of conductive carbon black, conductive graphite, ketjen black, carbon nanotubes, and graphene, preferably conductive carbon black kappa 100.

[0068] The positive active material, the multifunctional additive, the binder, and the conductive carbon are mixed in a mass ratio, dropped into a certain amount of NMP organic solvent, and stirred into a slurry in the form of mud, with a solid content of 48%. The slurry is coated on the surface of an aluminum foil, vacuum dried at a temperature of 100-110°C for 10-12 h, and then the electrode piece is cut into a circular piece with a diameter of 14 mm. The positive electrode piece is obtained by compaction at 3 MPa, and the amount of active material on each electrode piece is 2.5 mg.

[0069] (2) Preparation of electrolyte: The solvent of the electrolyte used is a mixed solvent of EC, PC, and EMC, with EC:PC:EMC=1:5:8 (v / v / v). The electrolyte additives are NaPO2F2, tetrahydrofuran, and 3,3,3-trifluoropropyl methyl carbonate, each accounting for 2% of the total mass, and the amount of electrolyte additive is 100-200 µL. In the prepared electrolyte, the concentration of sodium hexafluorophosphate is 0.75 mol / L.

[0070] (3) Battery assembly: In a glove box filled with high-purity argon gas (water, oxygen ≤0.1 ppm), the obtained positive electrode, sodium sheet, and glass fiber separator are assembled together into a CR2032 button cell. The accessories used include a gasket and a spring. The sodium sheet needs to have the thin film on both sides torn off, with the side facing the positive electrode sheet being the side with the colorless transparent film and the side in contact with the gasket being the side with the blue transparent film. The packaging pressure is 500 MPa, and the assembled battery is left to stand for 6-12 h before testing the electrochemical performance.

[0071] Comparative Example 1

[0072] The positive active material, conductive carbon black and PVDF were baked in an oven at 80°C for 15 h; the mass ratio of positive active material, binder, conductive carbon was 8:1:1, the positive active material was sodium iron pyrophosphate (NFPP); the binder was PVDF; the conductive carbon was conductive carbon black kappa 100. The positive active material, binder and conductive carbon were mixed according to the mass ratio, a certain amount of N-methyl pyrrolidone (NMP) organic solvent was added and stirred into a slurry (in order to facilitate clear comparison, the positive electrode slurry in Example 1 is numbered No. 1, and the corresponding battery is also numbered No. 1), the solid content was 48%, the slurry was coated on the surface of aluminum foil, vacuum dried at a temperature of 100°C for 10 h, and the positive electrode sheet was obtained by compaction under 3 MPa, and then the sheet was cut into a circular sheet with a diameter of 14 mm, and the mass of active material on each sheet was 2.5 mg.

[0073] Example 1

[0074] 1. pH test paper acidity test multifunctional additive effect on solvent

[0075] S1 Prepare test paper: tear off a piece of pH test paper, make sure it is dry and not contaminated.

[0076] S2 Sampling: use a clean glass rod to dip the liquid to be tested, drop it on the test paper, and observe the color change.

[0077] S3 Compare the color: compare the color of the test paper with the standard color spectrum to determine the pH value of the liquid to be tested. pH value less than 7 indicates acidity, equal to 7 indicates neutrality, and greater than 7 indicates alkalinity.

[0078] As shown in Figure 1 a, No. 1 reagent bottle only adds NMP; No. 2 reagent bottle is a mixed solution of 1,1-difluoroethane sulfinate sodium C2H3F2NaO2S and NMP; No. 3 reagent bottle is a mixed solution of methyl benzene pyrrole triazole sodium salt C7H6N3Na and NMP; No. 4 reagent bottle is a mixed solution of methyl benzene pyrrole triazole sodium salt, 1,1-difluoroethane sulfinate sodium and NMP. Among them, the amount of NMP in each reagent bottle is 2 mL; when the multifunctional additive added is a solid powder, the amount added is 2 mg; when the multifunctional additive added is a liquid reagent, the amount added is 200 µL. Figure 1b indicates that the color of the No. 1 reagent NMP on the pH test paper is close to the color of No. 7 color card, and the colors of the No. 2 reagent and the No. 3 reagent on the pH test paper are close to 7-8 after adding 1,1-difluoroethane sulfinate sodium and sodium methybenzotriazole salt respectively; the color of the No. 4 reagent after mixing 1,1-difluoroethane sulfinate sodium and sodium methybenzotriazole salt is also 7-8, which may be due to the increase of the concentration of sodium salt, and the color close to 8 has a larger range. However, the pH test paper shows that the multifunctional additive can be uniformly dissolved in the reagent NMP, and the obtained solution shows weak alkalinity.

[0079] 2. Preparation of positive electrode slurry and battery test:

[0080] (1) The positive active material, conductive carbon black and PVDF were baked in an oven at 80 ℃ for 15 h; the mass ratio of the positive active material, multifunctional additive, binder and conductive carbon was 8: (0.01-0.5): (0.5-0.99): 1, the positive active material was sodium iron pyrophosphate (NFPP); the multifunctional additive was a mixture of sodium methybenzotriazole salt (C7H6N3Na) and 1,1-difluoroethane sulfinate sodium (C2H3F2NaO2S), the binder was PVDF; the conductive carbon was conductive carbon black kappa 100. The positive active material, multifunctional additive, binder and conductive carbon were mixed according to the mass ratio, dropped into a certain amount of N-methyl pyrrolidone (NMP) organic solvent and stirred into a slurry, the solid content was 48%, the slurry was coated on the surface of aluminum foil, vacuum dried at 100 ℃ for 10 h, and the positive electrode sheet was obtained by pressing at 3 MPa, and then the sheet was cut into a circular sheet with a diameter of 14 mm, and the mass of the active material on each sheet was 2.5 mg. The specific component ratio of the positive electrode slurry is shown in Table 1:

[0081]

[0082] (2) Preparation of electrolyte: the solvent of the electrolyte used was a mixture of EC, PC and EMC, EC: PC: EMC = 1:5:8 (v / v / v), the electrolyte additives were NaPO2F2, tetrahydrofuran and 3,3,3-trifluoropropyl methyl acetate with a total mass of 2%, 2% and 2% respectively, and the amount of electrolyte additive was 180 µL. In the prepared electrolyte, the concentration of sodium hexafluorophosphate was 0.75 mol / L.

[0083] (3) Battery assembly: In a glove box filled with high-purity argon (water, oxygen ≤0.1 ppm), the obtained positive electrode, sodium sheet and glass fiber separator were assembled into a CR2032 button cell. The amount of electrolyte used was 180 μL; the accessories used included a gasket, a spring; the sodium sheet needed to tear off the film on both sides, and the side facing the positive electrode was the colorless transparent film side, and the side in contact with the gasket was the blue transparent film side; the packaging pressure was 500 MPa, and the assembled battery was left to stand for 6-12 h, and then the electrochemical performance was tested. Four parallel samples were set for each experiment, and the average value of the normal data was taken.

[0084] (4) Capacity test: The charge-discharge capacity test was performed on a battery tester, with a voltage range of 1.5-3.65 V, a charge-discharge rate of 0.1 C (0.1 mAh / g), a standard gram capacity of 98 mAh / g, and a charge-discharge current of 0.025 mA. The positive electrode sheet corresponding to the positive electrode slurry was prepared, and the corresponding battery was prepared, and the capacity of the battery was tested. The test results are shown in Table 2 below.

[0085]

[0086] As shown in Table 2, when the multifunctional additive in the batteries of No. 1-1, No. 1-2, No. 1-3, No. 1-4 and No. 1-5 is C7H6N3Na, the reversible capacity, initial efficiency and capacity retention rate of the positive electrode first increase and then decrease within the addition range of 0.2-2% of the total mass of the positive electrode slurry. The performance of the No. 1-4 battery prepared from the positive electrode slurry with a C2H3N3Na proportion of 1.5% is the best, and the reversible capacity, initial efficiency and 500-cycle cycle capacity retention rate are 105.69 mAh / g, 100.23%, and 90.28%, respectively. The reversible capacity, initial efficiency and 500-cycle cycle capacity retention rate of the No. 1 battery prepared from the positive electrode without any multifunctional additive are 98.12 mAh / g, 93.05% and 85.17%, respectively.

[0087] By comparing No. 1-4 battery and No. 1 battery, the reversible capacity of No. 1-4 battery is increased by 7.57 mAh / g, the initial efficiency is increased by 7.18%, and the 500-cycle cycle capacity retention rate is increased by 5.11%. The electrochemical charge-discharge test shows that the addition of multifunctional additive C7H6N3Na is beneficial to the reversible capacity, initial efficiency and cycle capacity retention rate of the positive electrode.

[0088] When the multifunctional additive is C2H3F2NaO2S, the reversible capacity, initial efficiency and capacity retention rate of the positive electrode in the batteries of Nos. 2-1, 2-2, 2-3, 2-4, 2-5 and 2-6 first increase and then decrease in the range of 0.5-3% of the total mass of the positive electrode slurry, and the performance of the battery of No. 2-5 prepared from the positive electrode slurry with 2.5% of C2H3F2NaO2S is the best, with the reversible capacity, initial efficiency and cycle capacity retention rate of 91.2% after 500 cycles being 109.31 mAh / g and 102.32%, respectively. The reversible capacity, initial efficiency and cycle capacity retention rate of the battery of No. 1 prepared from the positive electrode without any multifunctional additive are 98.12 mAh / g, 93.05% and 85.17%, respectively. The electrochemical charge and discharge test shows that the addition of the multifunctional additive C2H3F2NaO2S is beneficial to the reversible capacity, initial efficiency and cycle capacity retention rate of the positive electrode.

[0089] When the multifunctional additive is a mixed powder of C7H6N3Na and C2H3F2NaO2S, the reversible capacity, initial efficiency and capacity retention rate of the positive electrode in the batteries of Nos. 3-1, 3-2, 3-3 and 3-4 first increase and then decrease in the range of 0.2-1.4% of the total mass of the positive electrode slurry. The performance of the battery of No. 3-3 prepared from the positive electrode slurry with 1% of the multifunctional additive is the best, with the reversible capacity, initial efficiency and cycle capacity retention rate of 93.38% after 500 cycles being 116.23 mAh / g and 110.76%, respectively. Compared with the battery of No. 1 prepared from the positive electrode without any multifunctional additive, the battery of No. 3-3 can produce an additional discharge capacity of 18.11 mAh / g, the initial efficiency is increased by 17.71%, and the cycle capacity retention rate after 500 cycles is also increased by 8.21%.

[0090] It can be found that when either C7H6N3Na or C2H3F2NaO2S is added, the reversible capacity, initial efficiency and cycle capacity retention rate of the positive electrode can be effectively improved. However, when C7H6N3Na and C2H3F2NaO2S are added together, a smaller amount of the multifunctional additive, for example, 1% of the mass of the positive electrode slurry, has a better effect. When the multifunctional additive is added alone, the amount of C7H6N3Na is 1.5%, and the amount of C2H3F2NaO2S is 2.5%.

[0091] (5) The battery prepared in the present example was subjected to electrochemical impedance test: electrochemical impedance spectroscopy (EIS) was performed on an electrochemical workstation, and the frequency range was 100 KHz to 0.01 Hz.

[0092] The charge transfer resistance (Rct) is the Na ＋The addition of multifunctional additives can reduce the interfacial impedance of the positive electrode material and electrolyte and promote the Na ＋ fast transport. The Nyquist plot shows the electron transfer resistance (Rct) on the surface of the electrode, which is the same as the diameter of the semicircle part, and can be used to describe the characteristics of the interface between the electrode and the electrolyte. The Nyquist plot consists of two parts: the semicircle part in the high-frequency region corresponds to the electron transport limited process, and the diameter represents the electron transfer resistance Rct; the linear part in the low-frequency region corresponds to the diffusion limited process.

[0093] The EIS test at room temperature shows that the addition of multifunctional additives can slightly irregularize the measured semicircle of the battery, but can reduce the diameter of the measured semicircle. Surprisingly, with the addition of single components of multifunctional additives, the addition of two components, and the increase of the amount, the battery resistance all shows a trend of first increasing and then decreasing, which shows the importance of the appropriate amount of addition to the electrochemical impedance. Specifically, in the case of adding single component C7H6N3Na, with the increase of the amount, the Rct of the battery first increases and then decreases, and when the addition ratio of C7H6N3Na accounts for 1.5% of the total mass of the positive electrode powder, the Rct of the corresponding 1-4 battery is the smallest. The same is true for adding 2.5% C2H3F2NaO2S to the positive electrode, and the corresponding 2-5 battery shows the smallest semicircle. Adding 0.5% C7H6N3Na and C2H3F2NaO2S to the positive electrode respectively, the lowest interfacial impedance is obtained, which also proves the effectiveness of the multifunctional additives, especially the Rct of the 1-4 battery, the 2-5 battery and the 3-3 battery is much smaller than the Rct of the 1 battery in the figure (Fig. 2-5), which proves that the addition of multifunctional additives can effectively reduce the interfacial impedance and optimize the Na + transport process, which may promote the construction of SEI film by anionic compounds (Na2CO3, Na2SO3, Na2SO4, Na2S, NaF, Na3N), improve the charge transfer rate between the electrode and the electrolyte, and reduce the charge transfer resistance on the surface of the electrode. It is not difficult to find that the semicircle of the battery with two multifunctional additives is smaller than that of the battery with single multifunctional additive, and the total amount is also smaller. Therefore, the addition of appropriate amount of multifunctional additives and the collocation can make the battery show the optimal interfacial state. The 3-3 battery shows the smallest Rct, which means the best interfacial charge transport kinetics.

[0094] Example 2

[0095] On the basis of Example 1, the rate performance at room temperature, high and low temperature discharge performance of the 1 battery and the 3-3 battery were compared, and the test conditions were 55℃, 25℃ and -20℃.

[0096] The test results are shown in Table 1-Table 4. Figures 6-8 and Table 3-Table 4.

[0097] (1) The specific test method of battery overcharge and overdischarge is that the button cell is cycled for 5 weeks at 0.05C in the interval of 1.5-3.65V at room temperature, and then overcharge and overdischarge is carried out for 5 times at 0.1C / 0.2C / 0.5C / 1C / 2C / 5C / 10C, and the relevant data is recorded. Here, the 5-week cycle at 0.05C is a pre-cycle to promote the generation of protective interface film on the electrode interface, and the data of this step is not included in the analysis.

[0098]

[0099] Figure 6 and Table 3 is a comparison chart of overcharge and overdischarge of No. 1 battery and No. 3-3 battery. Figure 6 a shows that the running time of No. 3-3 battery is longer than that of No. 1 battery, which is because the capacity of No. 3-3 battery is higher than that of No. 1 battery, so the overcharge and overdischarge time of No. 3-3 battery is longer. Figure 6 b shows the capacity retention rate and stability of No. 1 battery and No. 3-3 battery at corresponding rates, and it is obvious that the overcharge and overdischarge capacity retention rate of No. 3-3 battery is higher than that of No. 1 battery, and the stability at high rates of 5C / 10C is also improved compared with No. 1 battery. The specific improvement can be found in Figure 6 c, the capacity retention rate of No. 1 battery at high rates of 5C and 10C is 77.82% and 66.81% respectively, while the capacity retention rate of No. 3-3 battery at high rates of 5C and 10C is 81.04% and 72.20% respectively, which is increased by 3.22% and 5.39% respectively, indicating that the multifunctional additive successfully improves the rate performance of the battery.

[0100] (2) In order to further explore the discharge behavior of the battery at high rates, the battery is additionally tested for overdischarge, and the specific operation is that the battery is cycled for 5 weeks at 0.05C in the interval of 1.5-3.65V at room temperature, and then charged at 0.1C, and discharged at respective rates, and the same is repeated for 5 times, and the average data is taken.

[0101] Figure 7 is a comparison chart of dQ / dV curves of two kinds of batteries at different discharge current densities. In Figure 7a shows that with the increase of discharge current, the peak intensity of the oxidation peak of No. 1 battery appears to weaken, and the peak value shifts to the right. The reduction peak is most affected. There is a sharp peak at 0.1C-1C, and the diffraction peak at 2C is flat. It shows that the electrochemical polarization generated by the 2C discharge rate has seriously deteriorated the electrode. And Figure 7 b shows that compared with No. 1 battery, the polarization voltage (ΔV) between adjacent oxidation and reduction peaks of No. 3-3 battery is effectively reduced with the increase of discharge rate, and the Na + migration kinetics. It shows that the addition of multifunctional additives in the positive slurry enhances the thermodynamic stability of the positive material, effectively inhibits phase transition and structural degradation, and thus effectively improves the rate performance of the battery.

[0102] (3) In order to further explore the adaptability of the battery to temperature, the battery was tested at high and low temperatures to analyze its electrochemical behavior during charging and discharging. The specific test method is to place the battery at the corresponding temperature for 6h, and then charge and discharge at 0.1C rate at 1.5-3.65V.

[0103]

[0104] From Table 4 and Figure 8 It can be seen that under the influence of high temperature 55℃ and low temperature-20℃, the capacity of the battery changes during charging and discharging. The specific charging and discharging behavior is shown in Figure 8 a-c, the first efficiency of No. 1 battery at high temperature 55℃ increases by 11.18%, and the first efficiency of No. 3-3 battery increases by 1.36%, but the first efficiency of No. 3-3 battery is still the largest, which is 108.12%. The reversible capacity of both is improved. At low temperature-20℃, the reversible capacity of No. 1 battery is 79.41mAh, and the first efficiency is 90.36%. The reversible capacity of No. 3-3 battery is 92.78mAh, and the first efficiency is 90.12%. No. 3-3 battery still has the largest reversible capacity and first efficiency.

[0105] Figure 8 a-c also shows that compared with room temperature 25℃, the capacity of No. 1 battery at high temperature 55℃ increases, and the capacity at low temperature-20℃ decreases, which is more obvious, indicating that temperature will affect the electrical performance of No. 1 battery. However, the resistance of No. 3-3 battery with multifunctional additives to high temperature 55℃ and low temperature-20℃ is obviously higher than that of ordinary No. 1 battery. The specific performance is Figure 8The change degree of the intensity of the diffraction peak, the deformation degree of the peak and the displacement size of the peak of the 3-3 battery are all smaller than those of the 1 battery in the dQ / dV curves corresponding to d-f, which may be that the high-entropy state energy of the multifunctional additives (sodium 1,1-difluoroethane sulfinate C2H2N3NaS and sodium methyl benzene triazole C7H6N3Na) in the positive electrode reduces the Gibbs free energy and enhances the thermodynamic stability of the positive electrode material, effectively inhibiting the phase change and structural degradation.

[0106] In particular Figure 8 In d-f, the 1 battery has only two reduction diffraction peaks at 25 DEG C, but three reduction diffraction peaks appear at 25 DEG C, which shows that the composition of the SEI film of the 1 battery changes and is unstable at high temperature 55 DEG C and low temperature-20 DEG C. Correspondingly, the 3-3 battery has three reduction diffraction peaks at 25 DEG C, and the change of the SEI film is not obvious at high temperature 55 DEG C and low temperature-20 DEG C. Figure 8 In e-f, the change of each peak is not obvious, which shows that the SEI film maintains the stability of the battery at high temperature 55 DEG C and low temperature-20 DEG C. Therefore, the tolerance and stability of the battery added with the multifunctional additive to high temperature 55 DEG C and low temperature-20 DEG C are obviously improved.

[0107] The above only describes the preferred embodiments of the present application and is not used to limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the protection scope of the present application.

Claims

1. A sodium-ion battery, characterized in that, The sodium-ion battery comprises a positive electrode, a negative electrode and an electrolyte. The slurry of the positive electrode comprises a positive electrode active material, a multifunctional additive, a binder and conductive carbon; the multifunctional additive accounts for 0.2-1.4% of the total mass of the positive electrode slurry. The multifunctional additive is a combination of methyl benzotriazole sodium salt and sodium 1,1-difluoroethane sulfinate, and the mass ratio of the two is (1-5):(1-5).

2. The sodium-ion battery of claim 1, wherein, The slurry of the positive electrode comprises, by weight percentage, 75-96% of the positive electrode active material, 0.5-10% of the binder, 0.5-10% of the conductive carbon and 0.2-1.4% of the multifunctional additive.

3. The sodium-ion battery of claim 1, wherein, The slurry of the positive electrode comprises, by weight percentage, 75-85% of the positive electrode active material, 5-10% of the binder, 5-10% of the conductive carbon and 0.2-1.4% of the multifunctional additive.

4. The sodium-ion battery of claim 1, wherein, The positive electrode active material is at least one of sodium iron phosphate, sodium pyrophosphate and sodium phosphate pyrophosphate. The binder is at least one of polyvinylidene fluoride, polyimide, polytetrafluoroethylene, polyacrylonitrile and polyamide-imide. The conductive carbon is at least one of conductive carbon black, conductive graphite, ketjen black, carbon nanotubes and graphene.

5. The sodium-ion battery of claim 1, wherein, The negative electrode is a sodium sheet.

6. The sodium-ion battery of claim 1, wherein, The electrolyte comprises a solvent, a solute and an electrolyte additive. The solvent is a combination of ethylene carbonate, propylene carbonate and methyl ethyl carbonate, and the volume ratio is 1:5:

8. The solute is sodium hexafluorophosphate; the concentration of the solute sodium hexafluorophosphate in the electrolyte is 0.5-1 mol / L. The electrolyte additive is a combination of sodium difluorophosphate, tetrahydrofuran and 3,3,3-trifluoropropyl methyl ester.

7. The sodium-ion battery of claim 6, wherein, The proportion of sodium difluorophosphate in the electrolyte is 1-5%, the proportion of tetrahydrofuran is 1-5%, and the proportion of 3,3,3-trifluoropropyl methyl ester is 1-5%.

8. A preparation method of the sodium-ion battery according to any one of claims 1-7, characterized in that, (1) sodium hexafluorophosphate is dissolved in a mixed solvent, an electrolyte additive is added, and the mixture is uniformly mixed to prepare an electrolyte; (2) the positive electrode active material, the multifunctional additive, the binder and the conductive carbon are mixed with NMP as the solvent to prepare a positive electrode slurry; (3) the positive electrode slurry is coated on a current collector, dried and sheeted to prepare a positive electrode; (4) the obtained electrolyte, positive electrode and negative electrode are used to prepare a sodium-ion battery.

9. A positive electrode, characterized by comprising: The slurry of the positive electrode comprises a positive electrode active material, a multifunctional additive, a binder and conductive carbon; the multifunctional additive accounts for 0.2-1.4% of the total mass of the positive electrode slurry. The multifunctional additive is a combination of methyl benzotriazole sodium salt and sodium 1,1-difluoroethane sulfinate, and the mass ratio of the two is (1-5):(1-5).

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