Positive electrode slurry and sodium ion battery

Abstract

The invention provides positive electrode slurry and a sodium ion battery, and belongs to the technical field of battery materials. The invention provides positive electrode slurry. The positive electrode slurry comprises basic slurry, sodium difluorophosphate (NaPO2F2) and perfluorononenoxy sodium benzenesulfonate (OBS). The invention also provides a positive pole piece containing the positive pole slurry and a sodium ion battery. According to the invention, a proper amount of OBS and NaPO2F2 are added into the positive electrode slurry so as to improve the reversible specific capacity and the first effect of the existing positive electrode material and solve the problem of large resistance of the existing battery; oBS can increase the compatibility of the electrolyte and the positive electrode material, and OBS and NaPO2F2 also enable the positive electrode material to be rich in anionic compound NaF, so that the mass-charge transfer resistance of the battery is effectively reduced; in the positive electrode slurry provided by the invention, phosphoryl ions in NaPO2F2 are weakly acidic, so that the alkaline value of the slurry can be effectively reduced, and the stability of the positive electrode slurry is further facilitated, so that the problems of high coating difficulty, difficulty in consistency of dressing, poor adhesive force of the slurry and the like in the prior art are solved.

Description

Technical Field

[0001] The present invention relates to the technical field of battery materials, and more specifically, to a positive electrode paste and a sodium-ion battery. Background Art

[0002] Sodium-ion batteries (SIBs) have the advantages of low cost, long cycle life, high safety, etc., and have become one of the most promising candidates for large-scale energy storage systems. All the Na + extracted from the cathode needs to be reversibly inserted into the anode after passing through the electrolyte. In fact, due to the formation of the solid electrolyte interface (SEI) film and some unknown irreversible reactions, a certain amount of active Na + is inevitably consumed at the anode, which greatly reduces the energy density of SIBs. Therefore, a sodium supplement can well make up for these sodium losses.

[0003] Sodium supplementation can be divided into positive electrode sodium supplementation and negative electrode pre-sodiation. By using the pre-sodiation strategy to improve the initial charge-discharge efficiency (ICE) of hard carbon negative electrode materials, due to the limitations of problems such as harsh processing conditions, reagent residues, and complex process flows caused by the use of sodium metal, it is very difficult to implement the pre-sodiation technology of negative electrode materials on a large scale. Positive electrode sodium supplementation can be well compatible with the battery manufacturing process, and positive electrode sodium supplementation materials are easy to synthesize and have relatively stable chemical properties. Therefore, low-cost positive electrode sodium supplementation materials provide a new solution for the commercial application of sodium supplementation technology.

[0004] As is well known, polyanionic compounds have become one of the most promising electrode materials for SIBs due to their stability, safety, and low cost. However, their low conductivity and relatively low capacity still limit their development. Currently, positive electrode sodium supplementation usually adopts an electrochemical method, by directly adding a sodium supplement in the preparation of the positive electrode paste. When the battery is charged, the sodium supplement decomposes and releases active sodium at a suitable potential to make up for the irreversible active sodium loss consumed in the formation of the negative electrode SEI. Some sodium supplementation additives such as NaN3, Na2CO3, NaCrO2, Na3C6H5O7, Na2C2O4, Na2C4O4, etc. have been developed. These low-cost, environmentally friendly, and efficient self-sacrificing additives do play a role in sodium supplementation. However, their own alkalinity such as Na2CO3, NaCrO2, Na3C6H5O7, Na2C2O4, Na2C4O4, etc. will exacerbate the instability of the positive electrode paste, resulting in great challenges in subsequent coating, and the large battery internal resistance has not been improved at the same time.

[0005] Based on this, it is necessary to propose a positive electrode paste and a sodium-ion battery to solve the problems existing in the prior art. Summary of the Invention

[0006] To achieve the invention purpose, the present invention provides a positive electrode paste and a sodium ion battery to improve problems such as the reversible specific capacity of existing positive electrode materials being low, the battery resistance being large, and the paste being prone to gelation. By appropriately adding a multifunctional positive electrode sodium supplement agent, the energy loss inside the battery is reduced and the processing performance such as subsequent coating of the paste is improved. The present invention appropriately adds OBS and NaPO2F2 to the positive electrode paste to improve the reversible specific capacity and initial efficiency of existing positive electrode materials and solve the problem of large battery resistance; OBS can increase the compatibility between the electrolyte and the positive electrode materials, and OBS and NaPO2F2 also make the positive electrode materials rich in anionic compound NaF, thereby effectively reducing the charge transfer resistance of the battery; in the positive electrode paste provided by the present invention, the phosphoryl group ions in NaPO2F2 are weakly acidic, which can effectively reduce the alkalinity value of the paste, and thus is beneficial to the stability of the positive electrode paste, so as to improve problems such as difficult coating, inconsistent dressing, and poor paste adhesion in the existing technology.

[0007] To achieve the above purpose, the technical solution of the present invention is as follows: In the first aspect, the present invention provides a positive electrode paste, which includes sodium perfluorononenyloxybenzenesulfonate (OBS) and sodium difluorophosphate (NaPO2F2).

[0008] Further, the positive electrode paste further includes a base paste, and the base paste includes a binder, conductive carbon, and a positive electrode material.

[0009] Further, the binder is selected from at least one of polyvinylidene fluoride (PVDF), polyvinyl alcohol, polyacrylamide, polytetrafluoroethylene, sodium alginate, sodium carboxymethyl cellulose, styrene-butadiene rubber, polyethyl methacrylate hydroxyethyl ester, and a water-soluble copolymer containing polyhydroxy groups; preferably PVDF 5130 type.

[0010] Further, the conductive carbon is selected from at least one of graphene oxide (GO), multi-walled carbon nanotubes (WMCNT), nano-scale graphite, amorphous carbon, fullerene carbon nanotubes, carbon nanofibers, graphene, lamellar graphite, carbon nanoflowers, porous activated carbon, Ketjen black, acetylene black (Kappa100), and conductive carbon black.

[0011] Further, the positive electrode material is at least one of sodium iron phosphate (NFPP), vanadium-based phosphate, iron-based phosphate, manganese-based phosphate, lithium iron phosphate, single crystal nickel cobalt manganese ternary material, sodium vanadium fluoride phosphate, sodium vanadium phosphate, P2-type transition metal oxide, O3-type transition metal oxide, and Prussian blue.

[0012] Further, the positive electrode paste includes polyvinylidene fluoride (PVDF), graphene oxide GO, multi-walled carbon nanotubes, acetylene black, and composite sodium iron phosphate (NFPP).

[0013] Further, in the positive electrode paste, by weight percentage, sodium perfluorononenyloxybenzenesulfonate accounts for 0.5 - 8%, NaPO2F2 accounts for 0.5 - 8%, sodium composite phosphate iron accounts for 75 - 85%, graphene oxide accounts for 0.5 - 2%, multi-walled carbon nanotubes account for 0.5 - 2%, acetylene black accounts for 5 - 10%, and polyvinylidene fluoride accounts for 5 - 15%.

[0014] Further, in the positive electrode paste, by weight percentage, sodium perfluorononenyloxybenzenesulfonate accounts for 0.5 - 2%, NaPO2F2 accounts for 0.8 - 4%, sodium composite phosphate iron accounts for 75 - 80%, graphene oxide accounts for 0.9 - 1%, multi-walled carbon nanotubes account for 0.9 - 1%, acetylene black accounts for 7 - 8%, and polyvinylidene fluoride accounts for 9 - 10%.

[0015] Further, in the positive electrode paste, by weight percentage, sodium composite phosphate iron accounts for 77.5% - 78%, 70%, 75%, 76.92%, 77.29%, 77.67%, 78.05%, 78.43%, 78.82%, 79.21%, 80%, 85% or 90%.

[0016] Further, in the positive electrode paste, by weight percentage, graphene oxide accounts for 0.95% - 0.98%, 0.5%, 0.8%, 0.95%, 0.96%, 0.97%, 0.98%, 0.99%, 1%, 1.5% or 2%.

[0017] Further, in the positive electrode paste, by weight percentage, multi-walled carbon nanotubes account for 0.95% - 0.98%, 0.5%, 0.8%, 0.95%, 0.96%, 0.97%, 0.98%, 0.99%, 1%, 1.5% or 2%.

[0018] Further, in the positive electrode paste, by weight percentage, acetylene black accounts for 7.5 - 7.8%, 5%, 7%, 7.5%, 7.73%, 7.8%, 8%, 10%, 12%, 14% or 15%.

[0019] Further, in the positive electrode paste, by weight percentage, polyvinylidene fluoride accounts for 9.5 - 10%, 5%, 8%, 9.62%, 9.66%, 9.71%, 9.76%, 9.80%, 9.85%, 9.90%, 10%, 12%, 14% or 15%.

[0020] Further, in the positive electrode slurry, by weight percentage, perfluorononenyloxybenzenesulfonic acid sodium salt (OBS) accounts for 0.8 - 1%, NaPO2F2 accounts for 0.8 - 4%, sodium iron phosphate composite accounts for 75.5 - 78%, graphene oxide accounts for 0.95 - 1%, multi-walled carbon nanotubes account for 0.95 - 1%, acetylene black accounts for 7.5 - 8%, and polyvinylidene fluoride accounts for 9.5 - 10%.

[0021] Further, in the positive electrode slurry, by weight percentage, perfluorononenyloxybenzenesulfonic acid sodium salt (OBS) accounts for 0.5 - 8%, preferably 0.5 - 2%, 0.95% - 1%, 0.8%, 0.97%, 0.99%, 1%, 1.48%, 1.5%, 1.96%, 2%, 2.44%, 2.5%, 2.91%, 3%, 3.38%, 3.5%, 3.85%, 4%, 6% or 8%.

[0022] Further, in the positive electrode slurry, by weight percentage, sodium difluorophosphate accounts for 0.5 - 8%, preferably 0.8 - 4%, 1.5% - 3%, 1.5% - 2.5%, 2.4 - 2.5%, 0.8%, 0.99%, 1%, 1.48%, 1.5%, 1.96%, 2%, 2.42%, 2.44%, 2.5%, 2.91%, 3%, 3.38%, 3.5%, 3.85%, 4%, 6% or 8%.

[0023] Further, in the positive electrode slurry, by weight percentage, perfluorononenyloxybenzenesulfonic acid sodium salt (OBS) accounts for 0.8 - 1%, NaPO2F2 accounts for 1.46 - 2.88%, sodium iron phosphate composite accounts for 75.5 - 78%, graphene oxide accounts for 0.95 - 1%, multi-walled carbon nanotubes account for 0.95 - 1%, acetylene black accounts for 7.5 - 8%, and polyvinylidene fluoride accounts for 9.5 - 10%.

[0024] Further, in the positive electrode slurry, by weight percentage, perfluorononenyloxybenzenesulfonic acid sodium salt accounts for 0.97%, NaPO2F2 accounts for 2.42%, sodium iron phosphate composite accounts for 77.28%, graphene oxide accounts for 0.97%, multi-walled carbon nanotubes account for 0.97%, acetylene black accounts for 7.73%, and polyvinylidene fluoride accounts for 9.66%.

[0025] In the third aspect, the present invention provides a preparation method of the positive electrode slurry described in the second aspect, including the following steps: S1: Pretreat the binder, conductive carbon, positive electrode material, and positive electrode sodium supplement agent; S2: Grind the conductive carbon, positive electrode material, and positive electrode sodium supplement agent to obtain a mixture; S3: Add the binder to the mixture and stir evenly to obtain the product.

[0026] Further, the preparation method includes the following steps: S1: Pretreatment: Bake polyvinylidene fluoride, sodium composite iron phosphate, graphene oxide, multi-walled carbon nanotubes, acetylene black, sodium perfluorononenyloxybenzenesulfonate, and sodium difluorophosphate respectively; prepare a polyvinylidene fluoride solution; S2: Grind sodium composite iron phosphate, graphene oxide, multi-walled carbon nanotubes, acetylene black, and sodium difluorophosphate to obtain a mixture; S3: Add the polyvinylidene fluoride solution obtained in step S1 to the mixture obtained in step S2, and stir evenly to obtain the positive electrode slurry.

[0027] Further, the binder, conductive carbon, and positive electrode material all need to be pretreated by baking. Further, the baking temperature is 110 - 125 °C, and the baking time is 4 - 6 h. Preferably, the baking temperature is 110 °C, 115 °C, 120 °C, 125 °C, and the baking time is 4 h, 4.5 h, 5 h, 5.5 h, 6 h.

[0028] Further, the positive electrode sodium supplement agent needs to be pretreated by baking. Further, the baking temperature is 70 - 85 °C, and the baking time is 4 - 6 h. Preferably, the baking temperature is 70 °C, 75 °C, 80 °C, 85 °C, and the baking time is 4 h, 4.5 h, 5 h, 5.5 h, 6 h.

[0029] Further, the grinding time is 10 - 15 min, preferably 10 min, 12 min, 14 min, 15 min. Further, the grinding is carried out in a mortar.

[0030] Further, the binder needs to be formulated into a binder solution with a solvent. Further, the mass fraction of the binder solution is 6 - 8%, preferably 6%, 7%, 8%.

[0031] Further, the binder solution is a PVDF solution with a mass fraction of 6 - 8% prepared with N-methylpyrrolidone (NMP) as the solvent, preferably 6%, 7% or 8%.

[0032] Further, in S3, a solvent needs to be added so that the solid content in the slurry is 25 - 40%, preferably 25%, 30%, 35%, 40%. Further preferably, the solvent is at least one of N-methylpyrrolidone, N,N-dimethylformamide, water, ethanol, and toluene.

[0033] Fourthly, the present invention provides a method for preparing a positive electrode sheet, including the following steps: Coating the positive electrode slurry described in the second aspect of the present invention or the positive electrode slurry prepared by the preparation method described in the third aspect on a current collector, drying, and slicing to obtain.

[0034] Further, the current collector is at least one of aluminum foil, composite aluminum foil, copper foil, steel wire mesh, and nickel foam.

[0035] Further, the drying temperature is 95~105°C, preferably 95°C, 100°C, and 105°C.

[0036] Further, the drying time is 6~7h, preferably 6h, 6.5h, and 7h.

[0037] In a fifth aspect, the present invention provides a sodium-ion battery, which includes the positive electrode paste or positive electrode sheet described in the present invention.

[0038] In a sixth aspect, the present invention provides the use of a sodium-ion battery in an energy storage device.

[0039] In a seventh aspect, the present invention provides an energy storage device, which includes the sodium-ion battery described in the present invention.

[0040] Beneficial effects: In the prior art, due to the lower energy density and compaction of the positive electrode material phosphate compared to layered oxides, its applications are mostly in large-scale energy storage power stations, and the application market is limited. By adding the positive electrode sodium supplement agents - OBS and NaPO2F2 to the positive electrode material, the present invention can effectively improve the reversible specific capacity of the phosphate in the positive electrode material, and can broaden the market application of the positive electrode material.

[0041] In the prior art, in addition to the lower reversible specific capacity, the positive electrode material phosphate also has the problem of large battery resistance. By adding specific proportions of the positive electrode sodium supplement agents - OBS and NaPO2F2 to the positive electrode material, the present invention can regulate the interfacial film structure of the positive electrode material, make the positive electrode material rich in the anionic compound NaF, and thus effectively reduce the charge transfer resistance of the battery, and can improve the rate performance and cycle stability of the battery, etc.

[0042] In the prior art, NaPO2F2 is mostly used as a film-forming additive in the electrolyte in the battery. It can be seen that NaPO2F2 itself has good ionic conductivity and low resistance. Based on this, by adding a specific proportion of the positive electrode sodium supplement agent - NaPO2F2 to the positive electrode material, the present invention can make Na + transfer better between the positive electrode materials, thereby reducing the energy loss inside the battery. The complementary OBS contains an organic chain. According to the "like dissolves like" principle, it can make the electrolyte rich in organic components added with FEC better penetrate into the positive electrode material, which is beneficial to the electric conduction at the interface between the electrolyte and the positive electrode, and further makes the positive electrode and the electrolyte better compatible. Its carbon chain is rich in F with strong electronegativity. During the charge and discharge process of the battery, the C-F bond is easily broken to generate NaF, improving the interfacial film of the positive electrode.

[0043] In the prior art, due to the high pH value of the positive electrode slurry and the free Na + makes the slurry prone to gel during coating, increasing the difficulty of coating, resulting in poor consistency of the dressing weight on the electrode sheet, chaotic NP ratios of the positive and negative electrodes, and increasing the risk of sodium deposition on the negative electrode sheet during cycling; the high alkalinity of the slurry will also cause partial failure of the positive electrode binder PVDF to release HF, affecting the adhesion of the slurry to the electrode sheet, thereby affecting the electrical performance of the battery. However, the phosphoryl group ions in the positive electrode sodium supplement agent - NaPO2F2 added to the positive electrode material in the present invention are weakly acidic, which can not only effectively reduce the alkalinity value of the slurry, but also be beneficial to the stability of the positive electrode slurry, so as to improve the problems of large coating difficulty, difficult dressing consistency, and poor slurry adhesion existing in the prior art. The positive electrode sodium supplement agent - perfluorononeneoxybenzenesulfonate added to the positive electrode material in the present invention can improve the voltage platform of the positive electrode slurry in the NFPP slurry, which is beneficial to optimizing the electronic structure and electrochemical performance of the positive electrode interface film.

[0044] The test results of the present invention show that NaPO2F2 and OBS can effectively increase the specific capacity of the NFPP positive electrode material, indicating that the battery prepared from the positive electrode sheet added with NaPO2F2 and OBS has a high energy density, better chemical stability, and can broaden the market application of the positive electrode material; NaPO2F2 and OBS can effectively improve the initial efficiency of the NFPP positive electrode material, indicating that the positive electrode material has low irreversible loss, good chemical stability, and good compatibility with the electrolyte during the first charge and discharge process. Brief Description of the Drawings

[0045] Figure 1 It is the initial charge and discharge curve diagram of each sample in Example 1 of the effect at 25°C and 0.1C of the battery.

[0046] Figure 2 It is the Nyquist curve diagram of the sample in Example 2 of the effect.

[0047] Figure 3 It is the Bode diagram of the sample in Example 2 of the effect. Among them, Figure a is the amplitude-frequency diagram, and Figure b is the phase-frequency diagram. Figure 3 -69.5° and -53.6° on the curve in Figure b represent the phase angles. The phase margin is the difference between the phase angle and -180°. The more positive the value, the more stable the battery system.

[0048] Figure 4 It is the effect comparison diagram of the NFPP electrode sheet and the NFPP-4 electrode sheet in Examples 1 and 2. Among them, Figure A is the initial charge and discharge curve diagram, and Figure B is the amplitude-frequency diagram.

[0049] Figure 5 It is the viscosity change diagram of two positive electrode slurries within 2 hours in Example 3 of the effect.

[0051] The present invention will be further described below in conjunction with specific embodiments. These embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. Specific Embodiments

[0052] The present invention will be further described below in conjunction with specific embodiments. These embodiments are only used to illustrate the present invention and not to limit the scope of the present invention.

[0053] I. Chemicals and Instruments The positive electrode material is sodium iron phosphate composite (NFPP) (particle size 10 - 50 µm, specific surface area 10 - 15 m 2 / g), 2032# button electrode case, acetylene black (99.9 wt%, Kappa100), polyvinylidene fluoride (99.9 wt%, PVDF 5130 binder), glass fiber separator (GF / D Whatman), and sodium sheet negative electrode (15.6 mm × 0.45 mm) are all provided by Dongguan Keld New Energy Co., Ltd.; Graphene oxide (GO, 99 wt%, number of layers < 10, specific surface area 50 - 100 m 2 / g) and multi - walled carbon nanotubes (WMCNT, 95 wt%) are provided by Shenzhen Suiheng Graphene Technology Co., Ltd.; N - methyl - 2 - pyrrolidone (AR 99%) and sodium perfluorononenyloxybenzenesulfonate (95%) are provided by Shanghai Macklin Biochemical Co., Ltd.; 1M sodium hexafluorophosphate (NaPF6) sodium - ion electrolyte (NP - 026, containing sodium hexafluorophosphate, solvent and additives, where the solvent is ethylene carbonate EC and propylene carbonate PC (volume ratio 1:1); the additive is 5 wt% fluoroethylene carbonate FEC) and sodium difluorophosphate NaPO2F2 (99.7 wt%, water < 100 ppm) powder are both purchased from Duoduo Chemical Technology Co., Ltd.

[0054] The vacuum drying oven (DZF - 6020B) is purchased from Shanghai Yixin Scientific Instruments Co., Ltd.; The vacuum mixer (SFM - 7 150 500) is purchased from Hefei Kejing Materials Technology Co., Ltd.; The rotational viscometer (NDJ - 8S) is purchased from Hunan Lichen Instrument Technology Co., Ltd.; The charge - discharge performance test of the battery at room temperature at 0.1C is all carried out on a Neware test cabinet (CT - 4008Tn); The electrochemical impedance is measured on an electrochemical workstation (CHI660E) of Shanghai Chenhua Instrument Co., Ltd., and the XRD material test is carried out on a diffractometer (XRD 6100) of Shimadzu, Japan.

[0055] II. The routine operations are as follows: (1) Preparation of the positive electrode sheet 1) Preparation of the NFPP slurry and the electrode sheet Pretreatment: PVDF, graphene oxide (GO), multi-walled carbon nanotubes (WMCNT), acetylene black (Kappa100), and the positive electrode material sodium iron phosphate composite (NFPP) are baked at 110 - 125 °C for 4 - 6 h respectively and then reserved. A PVDF solution with a mass fraction of 6 - 8% (PVDF binder, model 5130) prepared with N-methylpyrrolidone (NMP) as the solvent is prepared and reserved.

[0056] Prepare the positive electrode paste according to the mass ratio of NFPP:PVDF:GO:WMCNT:Kappa100 = 80:10:1:1:8. The detailed operation is as follows: Weigh the conductive carbon (GO, WMCNT, and Kappa100) and the positive electrode material NFPP according to the ratio, grind them in a mortar for 10 - 15 min, transfer them to a 5 - 10 ml beaker, add the required amount of PVDF, add NMP as appropriate, and stir evenly to obtain the NFPP slurry. The addition amount of NMP is such that the solid content of the obtained NFPP slurry is 25 - 40%.

[0057] Use a scraper to evenly coat the positive electrode slurry on the current collector aluminum foil, then dry it in a vacuum oven at 95 - 105 °C for 6 - 7 h, then punch it into a circular electrode sheet, and finally weigh the mass of the electrode sheet and reserve it to obtain the NFPP positive electrode sheet.

[0058] When preparing the NFPP slurry by machine automation: Weigh the PVDF, conductive carbon, and the positive electrode material sodium iron phosphate composite (NFPP) according to the ratio, add NMP as appropriate. The addition amount of NMP is such that the solid content of the obtained NFPP slurry is 35 - 40%, the vacuum degree is -1 bar, the mixing time is set to 90 - 120 min, and it is filtered through a 200 - mesh sieve after discharging to obtain the basic slurry (i.e., the NFPP slurry).

[0059] 2) Preparation of the NFPP sodium - supplementing slurry and the electrode sheet Sodium difluorophosphate powder (NaPO2F2) and perfluorononenyloxybenzenesulfonate powder are baked at 75 - 85 °C for 4 - 6 h. During pulp making, the perfluorononenyloxybenzenesulfonate powder and the sodium difluorophosphate powder are added together with the positive electrode material (the addition amount of the perfluorononenyloxybenzenesulfonate powder is fixed at 1% of the total weight of the basic slurry, and the addition amount of the sodium difluorophosphate powder is 1 - 4% of the total weight of the basic slurry). Other steps are the same as those in the preparation of "1) Preparation of the NFPP slurry and the electrode sheet" above, and finally the NFPP sodium - supplementing slurry is obtained.

[0060] Referring to the previous text “1) Preparation of NFPP Slurry and Electrodes”, the positive electrode was prepared.

[0061] (2) Assembly of 2032# Coin Cells Assemble in the order of positive electrode, separator, electrolyte, composite sodium sheet, gasket, shrapnel, and negative electrode case. The oxygen content in the glove box is <0.1 ppm, the water content is <0.1 ppm, the electrolyte is 1M NaPF6 (EC / PC = 1 / 1 V / V, 5% FEC), and the battery sealing pressure is 500 - 750 MPa.

[0062] (3) Battery Testing 1) Charge and Discharge Testing After the battery is clamped on the Neware test cabinet, set the test steps as follows: stand for 5 h, charge at a constant current of 0.1C to 4V, stand for 10 min, and then discharge at a constant current of 0.1C to 2V.

[0063] 2) EIS Testing Input the open - circuit voltage of the battery, and the frequency test range is 100 KHz - 0.001 Hz.

[0064] III. Explanation of the Natural Properties of the Sodium - Supplementing Agent 1) Explanation of the Relevant Information about Sodium Difluorophosphate Powder: The sodium difluorophosphate powder used in the present invention is a crystal. The NaPO2F2 crystal belongs to the Pī triclinic crystal system space group with C2v symmetry. According to the molecular geometric structure deduced from the crystallographic data, there is no disorder phenomenon in the PO2F2 - anion. Two independent PO2F2 - ions in the crystallographic structure are found in each unit cell. The lengths of the P - F bonds are: (1.537(2), 1.548(2), 1.559(3), and 1.549(2) Å), and the lengths of the P–O bonds are (1.476(3), 1.472(3), 1.471(3), and 1.468(3) Å). It can be seen that the P - F bond is more likely to break than the P - O bond, which is beneficial to the formation of NaF and improves the ionic transport ability of the positive electrode interface film; PO2F2 -It can be ionized or hydrolyzed. However, the P-F bond is more likely to break, and relatively speaking, the P-O bond is not easily formed. As a result, its degree of ionization is greater than that of hydrolysis, so it is acidic. As is well known, in a strongly alkaline environment, the C-H and C-F of PVDF will undergo an elimination reaction to form an unsaturated C=C double bond, resulting in chemical cross-linking and the appearance of by-product HF. Moreover, the regularity of the PVDF chain is very high, and it is prone to continuous HF elimination reactions (the reason for the bubbles appearing after the positive electrode slurry stands still for a period of time). The double bond is formed at the end of the chain and ends at the beginning of the chain, which leads to the formation of a conjugated double bond structure in the entire slurry. Since the formed double bond is unstable, it will break during the stirring process and cross-link with adjacent molecules, increasing the viscosity of the slurry and causing gelation, resulting in the slurry forming lumps. When coating, the slurry cannot be evenly coated on the current collector well, leading to a series of coating processing problems, and further causing the NP ratio of the positive and negative electrodes to be disordered. During the electrode cycling, sodium deposition will occur in some areas of the negative electrode sheet, affecting the electrical performance and safety performance of the battery. NaPO2F2 can improve the alkalinity of the slurry and thus enhance the coating processing performance of the positive electrode material.

[0065] 2) Related information description about sodium perfluorononenyloxybenzenesulfonate: The structural formula of sodium perfluorononenyloxybenzenesulfonate is shown as the following formula (I): Formula (I) The principle of self α-elimination of sodium perfluorononenyloxybenzenesulfonate is shown as the following formula (II): Formula (II) As shown in formula (I), the structure of sodium perfluorononenyloxybenzenesulfonate contains perfluorononenyloxy group, benzene ring and sodium sulfonate, etc. Among them, the perfluorononenyloxy group consists of nine perfluorinated carbon atoms to form a straight-chain structure. As a connecting group, the planar rigid structure of the benzene ring helps the molecules to be oriented and arranged at the interface. The high polarity of the sulfonic acid group in sodium perfluorononenyloxybenzenesulfonate makes it show excellent solubility in polar solvents, and the ion pair formed with sodium ions further enhances the polar dispersion ability of the compound.

[0066] During the application process, on the one hand, the addition of sodium perfluorononeneoxybenzenesulfonate can significantly reduce the surface tension of organic reagents. Adding sodium perfluorononeneoxybenzenesulfonate to the positive electrode slurry is beneficial to the wettability and miscibility of the surface liquid; to a certain extent, it increases the interfacial compatibility between the positive electrode and the electrolyte. Due to the strong electron-withdrawing effect of fluorine atoms in sodium perfluorononeneoxybenzenesulfonate, the acidity of fluorine-containing compounds will increase, so sodium perfluorononeneoxybenzenesulfonate will not increase the alkalinity of the slurry. Due to the low polarizability of fluorine atoms, the force between H-F in sodium perfluorononeneoxybenzenesulfonate is weak. Fluorine-related hydrogen bonding generally only exists within the molecule, and intramolecular hydrogen bonding will weaken the dispersion force and also reduce the melting and boiling points, and will not increase the viscosity of the electrolyte. On the other hand, sodium perfluorononeneoxybenzenesulfonate itself will undergo an α-elimination reaction (shown in Equation (II)): when there are heteroatoms such as fluorine at the α-position of the carbanion, due to the negative hyperconjugation effect, it will undergo α-elimination by itself to obtain a carbene. For example, trifluoromethyl anion itself is easily converted into difluorocarbene. (Just as: TMSCF3 is a good difluorocarbene reagent in the presence of NaI, and the reagent FSO2CF2COOMe can also achieve efficient trifluoromethylation reaction under the catalysis of Cu.). After sodium perfluorononeneoxybenzenesulfonate undergoes α-elimination by itself, it will produce negatively charged fluoride ions. As is well known, introducing F into NFPP can effectively improve the voltage platform of the material, thereby increasing its energy density, and the NaF generated at the positive electrode interface can further optimize the electronic structure and electrochemical performance of the positive electrode interface film. In summary, the F generated by sodium perfluorononeneoxybenzenesulfonate in the NFPP slurry can improve the voltage platform of the positive electrode slurry, which is beneficial to optimizing the electronic structure and electrochemical performance of the positive electrode interface film.

[0067] Example 1: Preparation of positive electrode sheet (1) Preparation of NFPP slurry and electrode sheet S1: Pretreatment: PVDF, graphene oxide (GO), multi-walled carbon nanotubes (WMCNT), acetylene black (Kappa100), and the positive electrode material NFPP are baked at 120 °C for 5 h respectively; PVDF 5130 is formulated into a PVDF solution with a mass fraction of 7% using N-methylpyrrolidone (NMP) (i.e., polyvinylidene fluoride PVDF 5130 binder); S2: Preparation of positive electrode slurry: Weigh graphene oxide, multi-walled carbon nanotubes, acetylene black, and the positive electrode material NFPP according to the weight ratio of NFPP:PVDF 5130:GO:WMCNT:Kappa100 = 80:10:1:1:8. Mix these components, grind for 12 min, add the required PVDF solution (formulated into a PVDF solution as required) to obtain a mixture, and then add an appropriate amount of NMP and stir until the slurry is uniform to obtain the positive electrode slurry (i.e., the basic slurry), where the addition amount of NMP is such that the solid content of the positive electrode slurry is 30%; S3: Preparation of the positive electrode sheet: The slurry was evenly coated on the current collector aluminum foil with a scraper. After drying at 100 °C for 6 h, it was punched into a circular electrode sheet. Finally, the mass of the electrode sheet was weighed and reserved to obtain the NFPP electrode sheet.

[0068] (2) Preparation of the NFPP sodium-supplemented slurry and its electrode sheet The preparation process is the same as that in (1), specifically as follows: S1: Pretreatment: Sodium difluorophosphate powder (NaPO2F2) and sodium perfluorononeneoxybenzenesulfonate powder (OBS) were baked at 80 °C for 5 h respectively; PVDF, graphene oxide (GO), multi-walled carbon nanotubes (WMCNT), acetylene black (Kappa100), and the positive electrode material NFPP were baked at 120 °C for 5 h respectively. S2: Preparation of the positive electrode slurry: Graphene oxide, multi-walled carbon nanotubes, acetylene black, and the positive electrode material NFPP were weighed according to the weight ratio of NFPP:PVDF 5130:GO:WMCNT:Kappa100 = 80:10:1:1:8. Then, the sodium difluorophosphate powder and sodium perfluorononeneoxybenzenesulfonate powder obtained in step S1 were mixed with graphene oxide, multi-walled carbon nanotubes, acetylene black, and the positive electrode material NFPP (the addition amount of sodium difluorophosphate powder was 1 - 4% of the total weight of the base slurry (NFPP, PVDF 5130, GO, WMCNT, and Kappa100), and the addition amount of sodium perfluorononeneoxybenzenesulfonate powder was fixed at 1% of the total weight of the base slurry) to obtain a mixture. Then, an appropriate amount of NMP was added and stirred until the slurry was uniform to obtain the NFPP sodium-supplemented slurry, where the addition amount of NMP was such that the solid content of the NFPP sodium-supplemented slurry was 30%. S3: Preparation of the positive electrode sheet: The NFPP sodium-supplemented slurry was evenly coated on the current collector aluminum foil with a scraper. After drying at 100 °C for 6 h, it was punched into a circular electrode sheet. Finally, the mass of the electrode sheet was weighed and reserved.

[0069] Different electrode sheets were prepared according to the proportion of the added sodium difluorophosphate powder, specifically as shown in Table 1 below: Table 1 Effect Example 1: Material specific capacity test Based on the 8 positive electrode sheets prepared in Example 1, the effects on the specific capacity and initial efficiency of the battery were explored. 3 parallel samples were set up for each group of experiments, and charge-discharge tests were carried out on the test cabinet. Specifically, the operation was carried out with reference to "II. The conventional operations are as follows: (3) Testing of the battery" in the previous text. One sample was taken from each group to draw a charge-discharge curve.

[0070] The results are as Figure 1 shown in Table 2.

[0071] Table 2 0.1C charge and discharge conditions of each electrode Result analysis: (1) As can be seen from Figure 1 and Table 2, the specific capacity and initial efficiency of the battery prepared with the positive electrode sheet added with NaPO2F2 and OBS are both larger than those of the battery prepared with the NFPP electrode sheet. Among them, the battery prepared with the positive electrode sheet numbered NFPP-4 (2.5% NaPO2F2 and 1% OBS based on the total weight of the base slurry) has the largest reversible capacity and initial efficiency, with a reversible capacity of 133.44 mAhg -1 , and the initial efficiency is 102.03%; while for the NFPP electrode sheet without the sodium supplement agent (the reversible capacity is 102.08 mAhg -1 and the initial efficiency is 94.41%); the reversible capacity and initial efficiency of the battery prepared with the positive electrode sheet added with only a single additive NaPO2F2 or OBS are significantly lower than those of the battery prepared with the positive electrode sheet numbered NFPP-2; for the battery prepared with the positive electrode sheet numbered NFPP-7 (4% NaPO2F2 and 1% OBS based on the total weight of the base slurry), its reversible capacity and initial efficiency start to decline compared with the positive electrode sheet numbered NFPP-6 (3.5% NaPO2F2 and 1% OBS based on the total weight of the base slurry). The test results show that NaPO2F2 and OBS can effectively improve the specific capacity of the NFPP positive electrode material, indicating that the battery prepared with the positive electrode sheet added with NaPO2F2 and OBS has a high energy density, better chemical stability, and can broaden the market application of the positive electrode material; NaPO2F2 and OBS can effectively improve the initial efficiency of the NFPP positive electrode material, indicating that this positive electrode material has low irreversible loss, good chemical stability, and good compatibility with the electrolyte during the first charge and discharge process.

[0072] (2) When the addition amount of OBS is 1 wt% of the total weight of the base slurry, the addition amount of NaPO2F2 is preferably 1.5 wt% - 3 wt% of the total weight of the base slurry, more preferably 1.5 wt% - 2.5 wt%, which is more conducive to improving the reversible capacity, initial efficiency, and capacity retention rate of the obtained battery, and has unexpected technical effects.

[0073] (3) The present invention adopts the combination of OBS and NaPO2F2. OBS and NaPO2F2 cooperate with each other, unexpectedly improving the specific capacity of the positive electrode material, and also improving the energy density and chemical stability of the prepared battery, which is superior to the simple superposition of single NaPO2F2 and single OBS, and has unexpected synergistic technical effects.

[0074] Effect Example 2: EIS test To investigate the influence of the addition of sodium supplement NaPO2F2 and OBS on the electrochemical reactions inside the battery, the batteries prepared using NFPP electrodes and NFPP-4 electrodes respectively (hereinafter referred to as NFPP batteries and NFPP-4 batteries respectively) were subjected to electrochemical impedance tests, and Nyquist plots (see Figure 2 ) and Bode plots (see Figure 3 ) were obtained.

[0075] Result analysis: In the Nyquist plot, an arc in the high- to medium-frequency range can characterize the charge transfer resistance (Rct) related to the Faraday redox reaction of the electrode. The smaller the diameter of the arc, the lower the charge transfer resistance. From Figure 2 the Nyquist plot, it can be found that the semicircle of the NFPP electrode curve is significantly larger than that of the NFPP-4 electrode, indicating that the charge transfer resistance is significantly greater than that of the NFPP-4 electrode. It can be seen that the addition of 1% OBS and 2.5% NaPO2F2 based on the total weight of the base slurry can effectively reduce Rct.

[0076] In the Nyquist plot, the oblique line appearing in the low-frequency region represents the Warburg impedance on the electrode, that is, the impedance when ions in the electrolyte diffuse to the electrode surface. The closer the line is to the horizontal axis, the greater the diffusion resistance. From Figure 2 the Nyquist plot, it can be found that in the low-frequency region, the oblique line, that is, the tail appearing after the impedance semicircle, has a smaller slope for the NFPP electrode compared to the NFPP-4 electrode, and has a larger electrode diffusion resistance, indicating that the addition of 1% OBS and 2.5% NaPO2F2 based on the total weight of the base slurry can effectively reduce the battery Warburg impedance.

[0077] In summary, the appropriate addition of OBS and NaPO2F2 is beneficial to reducing the internal resistance of the battery reaction.

[0078] As Figure 3 (a) shows, in the entire frequency range, the curve of the NFPP-4 electrode is below the curve of the NFPP electrode, indicating that with the addition of 1% OBS based on the total weight of the base slurry, the appropriate addition of NaPO2F2 is beneficial to reducing the overall impedance of the NFPP-4 electrode.

[0079] In the Bode plot, the change of the phase angle with frequency can reflect the phase difference between voltage and current. The calculation of the phase margin is the difference between the current phase value and -180°. The more positive the value, the stronger the stability of the system. As Figure 3As shown in (b), the phase margin of the NFPP electrode is 110.5°, which is significantly smaller than that of the NFPP-4 electrode (the margin is 126.4°), indicating that OBS with an addition amount of 1% of the total weight of the base slurry and the appropriately added NaPO2F2 are beneficial to increasing the stability of the battery system containing the NFPP-4 electrode.

[0080] The impedance test of this effect example shows that the electrode using OBS with an addition amount of 1% of the total weight of the base slurry and adding NaPO2F2 with an addition amount of 2.5% of the total weight of the base slurry can effectively reduce the internal resistance of the battery. It may be that OBS and NaPO2F2 in the slurry regulate the interfacial film of the positive electrode. Since OBS itself will generate negatively charged fluoride ions after α-elimination, regulating the electron cloud density on the positive electrode interfacial film and enhancing its electronegativity, which is beneficial to the entry and exit of sodium ions; at the same time, due to the natural properties of NaPO2F2, there is a strong Π form of bonding between the P=O bond and NFPP in the positive electrode material, increasing the electron delocalization of the NFPP-4 electrode, resulting in a decrease in the powder resistance; the P-F bond breaks to generate NaF, making the interfacial film rich in anionic components, and then promoting the easier charge transfer of the CEI of the positive electrode interfacial film of the NFPP-4 electrode; it shows that the electrode coated with the positive electrode slurry containing OBS and NaPO2F2 prepared by the present invention can participate in the electrochemical reaction more efficiently during the operation of the battery, while reducing unnecessary energy loss.

[0081] Effect Example 3 In order to further explore the influence of the addition of the sodium supplement agent OBS and NaPO2F2 on the stability of the positive electrode slurry, the positive electrode slurry corresponding to the NFPP electrode (NFPP slurry) and the positive electrode slurry corresponding to the NFPP-4 electrode (NFPP-4 slurry) were respectively subjected to an amplification experiment. The positive electrode slurry was prepared using a vacuum stirrer, and the slurry was transferred to a stainless steel cup. The viscosity probe of the rotational viscometer was inserted into the slurry, and the viscosity of the slurry was recorded every 20 min. The results are as Figure 4 and Figure 5 shown.

[0082] Figure 4 is the viscosity change of the two positive electrode slurries within 2 h. It can be seen from the viscosity diagram that as time goes by, the viscosity of the positive electrode slurry shows an upward trend. Among them, the change of the NFPP slurry is the largest, rising from the initial 3680 mPa s to 10200 mPa s, and the viscosity difference is 6520 mPa s. The initial viscosity of the NFPP-4 slurry is 3560 mPa s, and the viscosity difference is 1970 mPa s. It can be seen that the appropriate addition of OBS and NaPO2F2 can effectively stabilize the slurry. At the same time, from Figure 5In it, it can be found that wrinkling appears on the surface of the NFPP slurry, and there are also many bubbles of different sizes, indicating that the NFPP slurry has obviously gelled and formed lumps after 2 h. The surface of NFPP-4 is smooth, indicating that the addition of OBS and NaPO2F2 can effectively stabilize the slurry and improve the processing of the slurry, that is, it is beneficial to improve the quality uniformity of the slurry on the current collector during coating and is beneficial to improve the adhesion of the glue, thereby further improving the electrical performance and safety performance of the battery.

[0083] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A positive electrode paste, characterized in that, It includes a base slurry, NaPO2F2, and perfluorononenyloxybenzenesulfonate; wherein the base slurry includes sodium complex phosphate iron, graphene oxide, multi-walled carbon nanotubes, acetylene black, and polyvinylidene fluoride. By weight percentage, in the positive electrode slurry, perfluorononenyloxybenzenesulfonate accounts for 0.5 - 8%, NaPO2F2 accounts for 0.5 - 8%, sodium complex phosphate iron accounts for 75 - 85%, graphene oxide accounts for 0.5 - 2%, multi-walled carbon nanotubes account for 0.5 - 2%, acetylene black accounts for 5 - 10%, and polyvinylidene fluoride accounts for 5 - 15%.

2. The positive electrode paste according to claim 1, characterized in that, In the positive electrode slurry, by weight percentage, perfluorononenyloxybenzenesulfonate accounts for 0.5 - 2%, NaPO2F2 accounts for 0.8 - 4%, sodium complex phosphate iron accounts for 75 - 80%, graphene oxide accounts for 0.9 - 1%, multi-walled carbon nanotubes account for 0.9 - 1%, acetylene black accounts for 7 - 8%, and polyvinylidene fluoride accounts for 9 - 10%. Or, in the positive electrode slurry, by weight percentage, perfluorononenyloxybenzenesulfonate (OBS) accounts for 0.8 - 1%, NaPO2F2 accounts for 0.8 - 4%, sodium complex phosphate iron accounts for 75.5 - 78%, graphene oxide accounts for 0.95 - 1%, multi-walled carbon nanotubes account for 0.95 - 1%, acetylene black accounts for 7.5 - 8%, and polyvinylidene fluoride accounts for 9.5 - 10%.

3. The positive electrode slurry according to claim 1 or 2, wherein In the positive electrode slurry, by weight percentage, perfluorononenyloxybenzenesulfonate (OBS) accounts for 0.8 - 1%, NaPO2F2 accounts for 1.46 - 2.88%, sodium complex phosphate iron accounts for 75.5 - 78%, graphene oxide accounts for 0.95 - 1%, multi-walled carbon nanotubes account for 0.95 - 1%, acetylene black accounts for 7.5 - 8%, and polyvinylidene fluoride accounts for 9.5 - 10%. Or, for the positive electrode slurry, by weight percentage, perfluorononenyloxybenzenesulfonate accounts for 0.97%, NaPO2F2 accounts for 2.42%, sodium complex phosphate iron accounts for 77.28%, graphene oxide accounts for 0.97%, multi-walled carbon nanotubes account for 0.97%, acetylene black accounts for 7.73%, and polyvinylidene fluoride accounts for 9.66%.

4. A method for preparing the positive electrode paste according to any one of claims 1 to 3, characterized in that, It includes the following steps: S1: Pretreatment: Bake polyvinylidene fluoride, sodium complex phosphate iron, graphene oxide, multi-walled carbon nanotubes, acetylene black, perfluorononenyloxybenzenesulfonate, and sodium difluorophosphate respectively; prepare a polyvinylidene fluoride solution. S2: Grind sodium complex phosphate iron, graphene oxide, multi-walled carbon nanotubes, acetylene black, and sodium difluorophosphate to obtain a mixture. S3: Add the polyvinylidene fluoride solution obtained in step S1 to the mixture obtained in step S2 and stir evenly to obtain the positive electrode slurry.

5. The preparation method according to claim 4, wherein The polyvinylidene fluoride solution is a PVDF solution with a mass fraction of 6 - 8% prepared with N-methylpyrrolidone as the solvent; and / or in S3, further include adding N-methylpyrrolidone to make the solid content in the positive electrode slurry 25 - 40%.

6. A method for preparing a positive electrode plate, characterized in that, It includes the following steps: Coating the positive electrode slurry according to any one of claims 1 to 3 or the positive electrode slurry obtained by the preparation method according to claims 4 to 5 on a current collector, drying and making into sheets to obtain the positive electrode sheet; Optionally, the current collector is at least one of aluminum foil, composite aluminum foil, copper foil, steel wire mesh, and nickel foam; and / or The drying time is 6 to 7 h; and / or The drying temperature is 95 to 105 °C.

7. A positive electrode sheet, characterized in that, Obtained by the preparation method according to claim 6.

8. A sodium-ion battery, characterized in that, It includes the positive electrode slurry according to any one of claims 1 to 4, the positive electrode sheet prepared by the method according to claim 6, or the positive electrode sheet according to claim 7.

9. Use of the sodium ion battery according to claim 8 in a energy storage device.

10. A energy storage device, characterized in that, It includes the sodium ion battery according to claim 8.

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