A sodium supplement additive, positive electrode sheet and sodium ion battery

Abstract

The application belongs to the technical field of sodium ion batteries, and relates to a sodium supplementing additive, a positive electrode sheet and a sodium ion battery. In order to solve the problem of excessively high decomposition potential of the existing positive electrode sodium supplementing additive, sodium chromate and an organic sodium supplementing agent (such as sodium formate) are treated through a molten salt compounding process (calcination under inert atmosphere after tabletting) to prepare the sodium supplementing additive. The molten salt compounding process improves the interface contact and compounding uniformity of the inorganic additive and the organic sodium supplementing agent, so that, in the first charging process, the high-valence Cr 4+ generated by the decomposition of sodium chromate can efficiently catalyze the decomposition of the organic sodium supplementing agent, thereby significantly reducing the decomposition potential. The compounding sodium supplementing agent of the application is compatible with the existing electrode process, and can effectively improve the initial coulomb efficiency and the full battery energy density of the sodium ion battery. The compounding sodium supplementing additive prepared by the application can be directly added to the positive electrode slurry, is compatible with the existing electrode process, and has good industrial application prospect.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium ion batteries, and particularly relates to a sodium supplementing additive, a positive electrode sheet and a sodium ion battery. BACKGROUND

[0002] With the acceleration of global energy transformation process, efficient energy storage technology has become a key pillar to support the development of renewable energy. Sodium ion batteries are considered as the most potential alternative or complementary technology to lithium ion batteries in large-scale energy storage field due to the significant advantages of abundant sodium resource reserves, low cost and wide distribution. However, the commercialization process of sodium ion batteries faces a key bottleneck. In the first cycle of sodium ion batteries, the sodium ions released from the positive electrode will form a solid electrolyte interface film on the surface of the hard carbon negative electrode, which irreversibly consumes a large amount of active sodium ions. This problem seriously restricts the improvement of the energy density of sodium ion batteries and limits their practical application prospects.

[0003] The sodium supplementing technology is essentially an active sodium compensation strategy. By introducing an additional sodium source into the battery system, the irreversible loss in the initial cycle is precisely compensated, so that more active sodium ions can continuously participate in the reversible electrochemical reaction. This not only improves the initial performance of the battery, but also has a positive effect on the cycle stability and life extension.

[0004] Current sodium ion battery sodium supplementing technology research mainly focuses on three directions: positive electrode sodium supplementing additive, negative electrode pre-sodium and new compensation strategy, forming diversified technical routes. Among them, the positive electrode sodium supplementing additive is the most promising technical direction, which mainly compensates for the initial loss by decomposing and releasing sodium ions at high voltage. The decomposition voltage of many sodium supplementing agents is too high, close to or even exceeding the oxidation decomposition potential of the electrolyte, which will cause serious side reactions. The high decomposition voltage applied to the positive electrode material may also force the positive electrode material itself to undergo irreversible phase change or lattice oxygen loss. For example, sodium ferrite releases sodium ions in the voltage range of 3.8V-4.1V, accompanied by oxygen generation. These gases will cause the internal pressure of the battery to rise, swell and deform, and even cause safety hazards.

[0005] Therefore, developing a sodium supplementing additive that can reduce the decomposition potential of the sodium supplementing agent is the key to improving the performance of sodium ion batteries and ensuring the safety of the batteries. SUMMARY

[0006] To solve the problem of high decomposition potential of the positive electrode supplementing sodium agent in the prior art, the application provides a sodium supplementing additive, a positive electrode sheet and a sodium ion battery. The sodium supplementing additive is prepared by compounding sodium chromate, a layered transition metal oxide, with an organic sodium salt through a molten salt method, so as to reduce the decomposition potential of the organic sodium supplementing agent. The prepared battery has high capacity and low decomposition potential, excellent cycle stability, and effectively reduces the cost of raw materials.

[0007] The technical scheme for achieving the object of the present application is as follows:

[0008] The first aspect of the present application provides a preparation method of a sodium supplementing additive, comprising the following steps:

[0009] (1) Synthesizing sodium chromate by sol-gel method: continuously stirring sodium source, chromium source and chelating agent until wet gel is formed, then performing freeze-drying, grinding and calcination to obtain sodium chromate;

[0010] (2) Ball-milling the organic sodium supplementing agent and sodium chromate at a mass ratio of 1-3:1 to obtain a mixture;

[0011] (3) Tabletting the mixture and calcining under inert atmosphere for 8-15 hours to perform molten salt compounding, the calcination temperature is 250-350℃, the heating rate is 1-5℃ / min, to obtain a molten salt sodium supplementing additive.

[0012] Preferably, the molten salt compounding is high-temperature calcination after tabletting to promote the interface fusion of inorganic and organic components. Step (3) promotes the interface fusion and compounding uniformity of sodium chromate and the organic sodium supplementing agent, reduces the decomposition potential of the organic sodium supplementing agent, and improves the sodium compensation efficiency.

[0013] Preferably, the specific steps of synthesizing sodium chromate by sol-gel method in step (1) are as follows: continuously stirring sodium source, chromium source and chelating agent at 60-80℃ until wet gel is formed, then freeze-drying the wet gel, grinding, calcining at 400-500℃ for 2-5 hours, the heating rate is 1-5℃ / min, cooling to room temperature, tabletting the product, calcining at 850-1000℃ under inert atmosphere for 8-15 hours, the heating rate is 1-5℃ / min, the calcination product is sodium chromate, the particle size is significantly reduced compared with that of sodium chromate synthesized by conventional solid-phase method, which is more conducive to contact with sodium formate. Further preferably, the sodium source is one or more than two of sodium carbonate, sodium bicarbonate and sodium acetate; the chromium source is chromium trioxide or chromium dioxide, and the chelating agent is oxalic acid or citric acid.

[0014] Preferably, the organic sodium supplementing agent is one of sodium oxalate, sodium formate and sodium squarate. The organic sodium supplementing agent is recrystallized (solvent / anti-solvent method) before use, and the aqueous solution of the organic sodium supplementing agent is added dropwise into an ethanol solution to reduce the particle size and make it better compounded with conductive carbon black and sodium chromate.

[0015] Preferably, the inert atmosphere is nitrogen or argon.

[0016] Preferably, the sodium chromate is a layered transition metal oxide, which undergoes oxidation reaction after undergoing an electrochemical process, and the high-valence transition metal ions produced can catalyze the organic sodium supplementing agent, the two sodium supplementing agents are in full contact, and the decomposition potential of the organic sodium supplementing agent can be reduced.

[0017] The second aspect of the present application provides a sodium supplementing additive prepared by the preparation method.

[0018] The third aspect of the present application provides a positive electrode tab, which is prepared by coating a positive electrode slurry on the surface of a current collector, wherein the positive electrode slurry comprises the sodium supplementing additive, a conductive agent, a binder and a solvent.

[0019] Preferably, the mass ratio of the sodium supplementing additive, the conductive agent and the binder is 40-80:10-40:10-20, and the sodium supplementing additive accounts for 0.5-10 % of the total mass of the positive electrode slurry.

[0020] Preferably, the positive electrode tab further comprises a positive electrode active material. The positive electrode active material is at least one of transition metal layered oxides, tunnel phase oxides and polyanion types. Specifically, the transition metal layered oxides are Na 0.9 Ni 0.4 Fe 0.1 Mn 0.5 O2 or Na 0.8 Na 0.9 Ni 0.4 Fe 0.2 Mn 0.4 O2, the tunnel phase oxides are Na 0.44 MnO2, and the polyanion types are Na4Fe3(PO4)2P2O7.

[0021] Further preferably, the mass ratio of the positive electrode active material, the sodium supplementing additive, the conductive agent and the binder is 65-75:5-15:10-15:10-15, and the sodium supplementing additive accounts for 1-8 % of the mass of the positive electrode slurry.

[0022] Preferably, the preparation method of the positive electrode tab is as follows: dissolving the positive electrode active material and / or the sodium supplementing additive, the conductive agent and the binder in a solvent, uniformly stirring under vacuum to obtain a positive electrode slurry, uniformly coating the positive electrode slurry on an aluminum foil current collector, and drying under vacuum at 70-90℃ to obtain the positive electrode tab.

[0023] Preferably, the conductive agent is Super P, the binder is PVDF, and the solvent is NMP.

[0024] The fourth aspect of the present application provides a sodium ion battery comprising the positive electrode tab.

[0025] Preferably, the assembly method of the sodium ion battery is as follows: assembling the positive electrode tab, a separator, an electrolyte and a negative electrode tab in a glove box atmosphere to finally obtain the sodium ion battery.

[0026] Preferably, the electrolyte solute is 1M NaPF6, and the solvent is G2 (diethylene glycol dimethyl ether). The separator is Glass fiber. The negative electrode sheet is a metal sodium sheet or hard carbon.

[0027] The fifth aspect of the present application provides the application of the sodium supplementing additive in a sodium ion battery.

[0028] The advantages and beneficial effects of the present application are:

[0029] 1. The present application uses sol-gel method to prepare small particle size and high activity sodium chromate (NaCrO2) as an inorganic additive, and then composites it with an organic sodium supplementing agent (such as sodium formate) through a specific molten salt process, thereby constructing a high-efficiency synergistic sodium supplementing system. The selected sodium chromate additive is oxidized at a low potential to generate Cr 4+ , which has catalytic activity and can catalyze the decomposition reaction of the organic sodium supplementing agent, thereby reducing the decomposition potential and achieving sodium compensation. At the same time, the structure collapse of sodium chromate provides additional sodium compensation capacity and reduces the pores formed by the decomposition of the organic sodium salt. This process utilizes the catalytic activity of Cr 4+ , and realizes the complete decomposition of sodium chromate and sodium formate through a multi-stage reaction, which can effectively reduce the decomposition potential of the organic sodium supplementing agent, improve the initial coulombic efficiency, and has broad research prospects and application value.

[0030] 2. The sodium supplementing additive of the present application can be directly added during the stirring of the positive electrode, and is compatible with the existing homogenizing coating process. This method is simple to operate, has a short process flow, rapid reaction, high safety, and is very easy to industrialize. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 is the first cycle charge-discharge curve of the positive electrode sheet half-cell prepared in Example 1;

[0032] Figure 2 is the first cycle charge-discharge curve of the positive electrode sheet full-cell prepared in Example 2;

[0033] Figure 3 is the first cycle charge-discharge curve of the positive electrode sheet half-cell prepared in Comparative Example 1;

[0034] Figure 4 is the first cycle charge-discharge curve of the positive electrode sheet half-cell prepared in Comparative Example 2;

[0035] Figure 5 is the first cycle charge-discharge curve of the positive electrode sheet full-cell prepared in Comparative Example 3;

[0036] Figure 6 is the first cycle charge-discharge curve of the positive electrode sheet full-cell prepared in Comparative Example 4;

[0037] Figure 7is the first circle charge-discharge curve of the full battery of the positive electrode sheet prepared in Comparative Example 5;

[0038] Figure 8 is the first circle charge-discharge curve of the half battery of the positive electrode sheet prepared in Comparative Example 6;

[0039] Figure 9 is the first circle charge-discharge curve of the half battery of the positive electrode sheet prepared in Comparative Example 7;

[0040] Figure 10 is the first two circle charge-discharge curve of the half battery of the positive electrode sheet prepared in Comparative Example 8;

[0041] Figure 11 is the scanning electron microscope image of sodium chromate synthesized by sol-gel method;

[0042] Figure 12 is the scanning electron microscope image of sodium chromate synthesized by solid phase method. DETAILED DESCRIPTION

[0043] The application will be described in detail below with reference to the accompanying drawings, but these examples are not meant to limit the scope of protection of the application. The reagents used in the examples are commercially available or obtained by conventional synthesis in the art, unless otherwise specified.

[0044] Example 1

[0045] A method for preparing a sodium supplementing additive, comprising the following steps:

[0046] (1) Synthesis of sodium chromate, sodium chromate material is synthesized by sol-gel method. The sodium source (sodium carbonate), chromium source (chromium trioxide) and chelating agent (citric acid) are continuously stirred at 70°C until a wet gel is formed. Then the wet gel is freeze-dried, ground, and then calcined at a temperature of 450°C under a nitrogen atmosphere, with a heating rate of 5°C / min, and a calcination time of 5 hours. After cooling to room temperature, the product is pressed and calcined, with a calcination temperature of 1000°C, a heating rate of 1-5°C / min, a calcination atmosphere of nitrogen, and a calcination time of 8 hours. The calcined product is sodium chromate. The scanning electron microscope image of sodium chromate is shown in Figure 11 , which shows that the particle size is significantly reduced.

[0047] (2) Sodium formate and sodium chromate are ball-milled and mixed according to a mass ratio of 1:1 to obtain a mixture.

[0048] (3) The mixture is pressed and calcined in a nitrogen atmosphere for 5 hours to form a molten salt composite, with a calcination temperature of 250°C and a heating rate of 5°C / min. The calcined product is a molten salt sodium supplementing additive.

[0049] The sodium supplementing additive treated by molten salt, conductive agent Super P and binder PVDF were mixed in a mass ratio of 80:10:10, dissolved in NMP solvent, and stirred under vacuum to obtain a uniform positive electrode slurry. The sodium supplementing additive accounted for 5% of the mass of the positive electrode slurry. The positive electrode slurry was uniformly coated on an aluminum foil current collector, and dried at 80°C under vacuum to obtain a positive electrode sheet.

[0050] The positive electrode sheet obtained in Example 1 was assembled into a half-cell with a sodium sheet, with a loading of 1.5 mg·cm -2 at a current density of 10 mA·g -1 , and a voltage range of 2.0V-4.0V for constant current charge and discharge test. Figure 1 The first cycle charge and discharge curve of the positive electrode sheet half-cell prepared in Example 1 is shown in the figure. Electrochemical test showed that the charge curve of the half-cell containing the sodium supplementing additive exhibited three platforms (2.95V, 3.5V and 3.8V). The platforms at 2.95V and 3.8V corresponded to the decomposition of sodium chromate, while the platform at 3.5V was due to the decomposition of sodium formate, which was lower than its original decomposition platform potential (4.1V), indicating that the addition of sodium chromate significantly reduced the decomposition potential of the sodium formate supplementing agent. The first charge and discharge capacities were 328.17 mAh·g -1 and 24.36 mAh·g -1 , respectively. The reaction process was irreversible, indicating that the sodium formate supplementing agent was completely decomposed and released sodium ions during the first charge process, and would not participate in the reaction in subsequent cycles.

[0051] Example 2

[0052] The difference from Example 1 was that the positive electrode slurry included a positive electrode active material tunnel type oxide (NMO). The positive electrode active material tunnel type oxide (NMO), the molten salt sodium supplementing additive prepared in Example 1, the conductive agent Super P and the binder PVDF were mixed in a mass ratio of 70:10:10:10, dissolved in NMP solvent, and stirred under vacuum to obtain a uniform positive electrode slurry. The sodium supplementing additive accounted for 5% of the mass of the positive electrode slurry. The positive electrode slurry was uniformly coated on an aluminum foil current collector, and dried at 80°C under vacuum to obtain a positive electrode sheet.

[0053] The positive electrode sheet obtained in Example 2 was matched with a hard carbon negative electrode to assemble a full cell, with a positive electrode active material (NMO) loading of 1.5 mg·cm -2 , and a N / P ratio of 1.05-1.10. The full cell was tested by constant current charge and discharge at a current density of 10 mA·g -1 , and a voltage range of 1.9V-3.9V. Figure 2The first cycle charge-discharge curve of the positive electrode sheet full cell prepared in Example 2 is shown in the figure. Electrochemical test shows that the charge curve of the full cell containing inorganic sodium supplementing agent sodium chromate and organic sodium supplementing agent sodium formate exhibits two platforms (3.25 V and 3.60 V), which correspond to the charging of sodium chromate to Na 0.5 CrO2, Cr 4+ The decomposition process of sodium formate oxide shows that sodium chromate significantly reduces the decomposition potential of sodium formate sodium supplementing agent, and other short platforms are the phase change platforms of the tunnel type positive electrode material itself; at the same time, the charge platform of the NMO positive electrode / / hard carbon full cell is basically consistent with that of Example 2, which shows that it has universality in half cells and full cells, and can better overcome the poor conductivity problem, thereby realizing the full decomposition of the sodium formate sodium supplementing agent. The first charge-discharge capacity is 166.09 mAh·g -1 and 107.32 mAh·g -1 , which shows that sodium formate realizes good decomposition in the first charge process and compensates for the discharge capacity. The results show that the sodium ions released by sodium formate make up for the sodium loss capacity of SEI formed on the hard carbon negative electrode side.

[0054] Comparative Example 1

[0055] A preparation method of a sodium supplementing additive, which is only different from Example 1 in that step (3) is omitted to obtain a sodium supplementing additive without molten salt treatment.

[0056] The same method as in Example 1 was used to obtain a positive electrode sheet, and a half cell was assembled. The loading was 1.5 mg·cm -2 , and the constant current charge-discharge test was carried out at a current density of 10 mA·g -1 in the voltage range of 2.0 V-4.0 V. Figure 3 is the first cycle charge-discharge curve of the positive electrode sheet half cell prepared in Comparative Example 1. Electrochemical test shows that the charge curve exhibits four platforms (2.95 V, 3.5 V, 3.75 V and 3.9 V), 2.95 V and 3.75 V corresponding to the decomposition of sodium chromate, and 3.5 V and 3.9 V from the decomposition of sodium formate, which is lower than its original decomposition platform potential (4.1 V). Compared with the sodium supplementing additive treated by molten salt in Example 1, the decomposition potential is increased. The first charge-discharge capacity is 371.21 mAh·g -1 and 13.32 mAh·g -1 , which is an irreversible reaction process, indicating that sodium formate is completely decomposed and releases sodium ions in the first charge process, and will not participate in the reaction in subsequent cycles. It is shown that the addition of sodium chromate significantly reduces the decomposition potential of the sodium formate supplementing agent, but the decomposition potential is still too high without molten salt treatment.

[0057] Comparative Example 2

[0058] Preparation of composite sodium supplement agent:

[0059] (1) Synthesis of sodium chromate, sodium chromate material was synthesized by solid phase method. Sodium carbonate and chromium trioxide were ball milled for 4h to obtain a precursor, and the precursor was tabletized and calcined. The calcination temperature was 850-1000℃, the heating rate was 1-5℃ / min, the calcination atmosphere was one of nitrogen or argon, and the calcination time was 8-15 hours. The calcined product was sodium chromate, and the scanning electron microscope image is as shown in Figure 12 .

[0060] (2) Sodium formate and sodium chromate were ball milled and mixed according to a mass ratio of 1:1.

[0061] (3) The mixed sodium formate and sodium chromate were tabletized and calcined. The calcination temperature was 250-350℃, the heating rate was 1-5℃ / min, the calcination atmosphere was one of nitrogen or argon, and the calcination time was 2-5 hours. The calcined product was a solid-phase-synthesized molten salt composite sodium supplement agent

[0062] The solid-phase-synthesized composite sodium supplement agent, conductive agent Super P and binder PVDF were mixed uniformly according to a mass ratio of 80:10:10, dissolved in NMP solvent, and vacuum stirred to obtain a uniform positive electrode slurry. The positive electrode slurry was uniformly coated on an aluminum foil current collector, and vacuum dried at 80℃ to obtain a positive electrode sheet.

[0063] The above obtained positive electrode sheet and sodium sheet were assembled into a half battery, with a loading of 1.5 mg·cm -2 , and a constant current charge-discharge test was carried out at a current density of 10 mA·g -1 in a voltage range of 2.0V-4.0V. Figure 4 The first cycle charge-discharge curve of the positive electrode sheet half battery prepared for Comparative Example 2 is shown in the figure. Electrochemical test showed that the charge curve of the half battery containing the sodium supplement additive exhibited three platforms (2.95V, 3.75V and 3.9V). The 2.95V and 3.75V platforms corresponded to the decomposition of sodium chromate, while the 3.9V platform was from the decomposition of sodium formate, which was lower than its original decomposition platform potential (4.1V). This indicated that the addition of sodium chromate significantly reduced the decomposition potential of the sodium formate supplement agent. The first charge and discharge capacities were 328.17 mAh·g -1 and 24.36 mAh·g -1 , respectively. The reaction process was irreversible, indicating that the sodium formate supplement agent was completely decomposed and released sodium ions during the first charging process, and would not participate in the reaction in subsequent cycles. It is shown that the addition of sodium chromate significantly reduces the decomposition potential of the sodium formate supplement agent, but the particle size of the solid-phase-synthesized sodium chromate is large, and the catalytic effect is weak, so the decomposition potential of sodium formate is still at a high level.

[0064] Comparative Example 3

[0065] The difference between the comparative example 1 is that the positive electrode slurry includes a positive electrode active material tunnel type oxide (NMO), and the positive electrode sheet is obtained by the same method as the comparative example 1, and is assembled into a full cell with a hard carbon negative electrode. The positive electrode active material (NMO) loading is 1.5 mg·cm -2 , the N / P ratio is 1.05-1.10, and the constant current charge-discharge test is carried out in the voltage interval of 1.9V-3.9V at the current density of 10 mA·g -1 . Figure 5 The first cycle charge-discharge curve diagram of the full cell of the positive electrode sheet and the hard carbon negative electrode used in the comparative example 3. The electrochemical test shows that the full cell containing sodium chromate and the organic sodium supplementing agent sodium formate exhibits two platforms (3.5V and 3.80V), respectively corresponding to the charging of sodium chromate to Na 0.5 CrO2, Cr 4+ The decomposition process of sodium formate, indicating that the sodium chromate additive significantly reduces the decomposition potential of the sodium formate sodium supplementing agent, and the other short platforms are the phase change platforms of the tunnel type positive electrode material itself. But the first charge-discharge capacity is 108.09 mAh·g -1 and 49.32 mAh·g -1 , indicating that the sodium supplementing additive without molten salt treatment cannot compensate for the sodium loss capacity of the SEI formed on the hard carbon negative electrode side in the voltage window of 1.9V-3.9V.

[0066] Comparative example 4

[0067] The difference between the comparative example 2 is that the positive electrode slurry includes a positive electrode active material tunnel type oxide (NMO), and the positive electrode sheet is obtained by the same method as the comparative example 2, and the obtained positive electrode sheet is assembled into a full cell with a hard carbon negative electrode. The positive electrode active material (NMO) loading is 1.5 mg·cm -2 , the N / P ratio is 1.05-1.10, and the constant current charge-discharge test is carried out in the voltage interval of 1.9V-3.9V at the current density of 10 mA·g -1 . Figure 6 The first cycle charge-discharge curve diagram of the full cell of the positive electrode sheet of the comparative example 4, the electrochemical test shows that there is no obvious decomposition platform, and the short platform in the charge-discharge curve is the phase change platform of the tunnel type positive electrode material itself, and the first charge-discharge capacity is 97.09 mAh·g -1 and 35.32 mAh·g -1 , indicating that the sodium supplementing additive synthesized by solid phase cannot compensate for the sodium loss capacity of the SEI formed on the hard carbon negative electrode side in the voltage window of 1.9V-3.9V, and cannot perform normal charge-discharge behavior.

[0068] Comparative example 5

[0069] The difference from Example 2 is that the positive electrode active material tunnel type oxide (NMO), conductive agent Super P and binder PVDF are mixed uniformly in a mass ratio of 80:10:10, dissolved in NMP solvent, and vacuum stirring to obtain a uniform positive electrode slurry. The positive electrode slurry is uniformly coated on an aluminum foil current collector, dried at 80°C under vacuum to obtain a positive electrode sheet.

[0070] The above obtained positive electrode sheet is matched with a hard carbon negative electrode to assemble a full battery, with a positive electrode active material (NMO) loading of 1.5 mg·cm -2 , an N / P ratio of 1.05-1.10, and a constant current charge-discharge test at a current density of 10 mA·g -1 in a voltage range of 1.9V-3.9V. Figure 7 The first cycle charge-discharge curve of the full battery of Comparative Example 5 is shown in the figure. Electrochemical tests show that the first charge-discharge capacity of the NMO full battery is 49.55 mAh·g -1 and 12.73 mAh·g -1 , respectively, indicating that the tunnel type oxide without adding a sodium supplement cannot perform normal charging behavior.

[0071] Comparative Example 6

[0072] The difference from Example 1 is that the layered transition metal oxide sodium chromate, conductive agent Super P and binder PVDF are directly mixed uniformly in a mass ratio of 80:10:10, dissolved in NMP solvent, and vacuum stirring to obtain a uniform positive electrode slurry. The positive electrode slurry is uniformly coated on an aluminum foil current collector, dried at 80°C under vacuum to obtain a positive electrode sheet.

[0073] The above obtained positive electrode sheet is matched with a sodium sheet to assemble a half battery, with a sodium chromate loading of 1.5 mg·cm -2 , and a constant current charge-discharge test at a current density of 10 mA·g -1 in a voltage range of 2.0V-4.2V. Figure 8 The first cycle charge-discharge curve of the positive electrode sheet half battery of Comparative Example 6 is shown in the figure. Electrochemical tests show that the charging curve of the sodium chromate half battery shows two platforms (2.95V and 3.75V), corresponding to charging of sodium 0.5 CrO2and Na 0.5 CrO2continues to charge and decompose. The first charge-discharge capacity is 225 mAh·g -1 and 20.2 mAh·g -1 , respectively, but the second platform potential is too high and the specific capacity is too low, which is not conducive to practical application.

[0074] Comparative Example 7

[0075] The difference from Example 1 is only that the organic sodium supplementing agent sodium formate, the conductive agent Super P and the binder PVDF are directly mixed uniformly in a mass ratio of 40:40:20, dissolved in NMP solvent, and vacuum stirred to obtain a uniform positive electrode slurry. The positive electrode slurry is uniformly coated on an aluminum foil current collector, and vacuum dried at 80°C to obtain a positive electrode sheet.

[0076] The above-obtained positive electrode sheet is assembled into a half-cell with a sodium sheet, and the active material loading is 1.5 mg·cm -2 At a current density of 10 mA·g -1 , the constant current charge-discharge test is carried out in a voltage range of 2.0V-4.2V. Figure 9 The first cycle charge-discharge curve of the positive electrode sheet half-cell of Comparative Example 7 is shown in the figure. Electrochemical test shows that the charge curve plateau potential of the sodium formate half-cell is 4.2V, which is too high for decomposition, and the decomposition process is easy to cause irreversible structural evolution on the positive electrode side, which is higher than the voltage window for practical application, and is not suitable as a positive electrode additive sodium supplementing agent.

[0077] Comparative Example 8

[0078] The tunnel-type oxide positive electrode material (NMO), the conductive agent Super P and the binder PVDF are mixed uniformly in a mass ratio of 80:10:10, dissolved in NMP solvent, and vacuum stirred to obtain a uniform positive electrode slurry. The positive electrode slurry is uniformly coated on an aluminum foil current collector, and vacuum dried at 80°C to obtain a positive electrode sheet.

[0079] The above-obtained positive electrode sheet is assembled into a half-cell with a sodium sheet, and the active material loading is 1.5 mg·cm -2 At a current density of 10 mA·g -1 , the constant current charge-discharge test is carried out in a voltage range of 2.0V-4.2V. Figure 10 The first two cycles of charge-discharge curves of the positive electrode sheet half-cell of Comparative Example 8 are shown in the figure. Electrochemical test shows that the half-cell of the tunnel-type oxide positive electrode material exhibits six continuous platforms in the charge-discharge curve, corresponding to six continuous phase change processes. The first charge-discharge capacities are 58 mAh·g -1 and 108 mAh·g -1 , respectively, and the first cycle coulombic efficiency is only 53.7 %. The low first cycle specific charge capacity and insufficient coulombic efficiency will cause continuous consumption of active sodium ions due to the formation of irreversible SEI on the hard carbon negative electrode in full cell application, thereby causing a significant decrease in overall reversible capacity. This phenomenon seriously restricts the improvement of the energy density of sodium ion batteries and is not conducive to large-scale practical application.

[0080] In summary, the method of pre-sodium of sodium ion batteries using inorganic additive sodium chromate and organic sodium supplementing agent sodium formate has remarkable effect. Cr 4+The sodium formate is effectively catalyzed to decompose and reduce the decomposition potential. The NMO positive electrode material obtained by the method has high first cycle efficiency and stable cycle performance. In addition, the entire pre-sodium and exhaust process can be compatible with the existing process, without additional equipment or complex adjustment, and has broad research prospect and application value.

[0081] The above only describes the preferred embodiments of the present application, and it should be noted that those skilled in the art can make several modifications and improvements without departing from the inventive concept, and these all belong to the protection scope of the present application.

Claims

1. A method for the preparation of a sodium supplement additive, characterized by, The method comprises the following steps: (1) synthesizing sodium chromate by sol-gel method; continuously stirring a sodium source, a chromium source and a chelating agent until a wet gel is formed, then performing freeze-drying, grinding and calcination to obtain sodium chromate; the sodium source is sodium carbonate, sodium bicarbonate or sodium acetate; the chromium source is chromium trioxide or chromium dioxide, and the chelating agent is oxalic acid or citric acid; (2) ball-milling an organic sodium supplement agent and sodium chromate at a mass ratio of 1-3:1 to obtain a mixture; (3) tabletting the mixture and performing melt salt compounding by calcination under an inert atmosphere to obtain a sodium supplement additive, wherein the calcination time is 8-15 hours, the temperature is 250-350 DEG C, the temperature rising rate is 1-5 DEG C / min, and the organic sodium supplement agent is one of sodium oxalate, sodium formate and sodium squarate.

2. The production method according to claim 1, characterized by, The inert atmosphere is nitrogen or argon.

3. A sodium supplement additive prepared by the method of claim 1 or 2.

4. A positive electrode sheet characterized by comprising: The positive electrode slurry is coated on the surface of a current collector, and the positive electrode slurry comprises the sodium supplement additive of claim 3, a conductive agent, a binder and a solvent.

5. The cathode electrode of claim 4, wherein, The positive electrode tab further comprises a positive electrode active material, and the positive electrode active material is at least one of a transition metal layered oxide, a tunnel phase oxide and a polyanion.

6. A sodium-ion battery, characterized in that, The positive electrode tab of claim 4 or 5.

7. Use of the sodium supplement additive prepared by the method of claim 1 or 2 in a sodium ion battery.

Images