Multifunctional cathode slurry additive based on lewis acid-base self-assembly and its application in sodium-ion batteries

Abstract

The present application relates to a kind of multifunctional positive electrode slurry additive based on Lewis acid-base self-assembly and its application in sodium-ion battery, belong to sodium-ion battery technical field.The additive is organic binary or polyacid sodium salt;Positive electrode slurry includes sodium-ion battery positive active material, conductive agent, binder and solvent, and the positive active material is O3 type layered oxide positive material;The mass of additive is 2%~8% of the total mass of sodium-ion battery positive active material, conductive agent and binder.Adding organic binary or polyacid sodium salt as additive in slurry, it can be used as pH buffer to neutralize the alkaline impurities on the surface of material, and the reaction product generated in situ is electrochemically active substance with sodium supplement effect, realize the stable cycle of high reversible capacity of sodium-ion battery.

Description

TECHNICAL FIELD

[0001] The application relates to a multifunctional positive electrode slurry additive based on Lewis acid-base self-assembly and application thereof in sodium ion batteries, and belongs to the technical field of sodium ion batteries. BACKGROUND

[0002] Due to large sodium resource reserves, sodium ion batteries as a promising low-cost and sustainable large-scale energy storage solution have attracted high attention. However, so far, there are still challenges in developing high-performance sodium ion full batteries (NIFCs).

[0003] Specifically, in terms of positive electrode materials, the air stability of the most concerned O3-type layered transition metal oxide (NaTMO2) which is easy to synthesize and has a high reversible capacity is relatively poor, and the residual alkali in the production process and the strong alkaline surface caused by air instability will cause the dehydrofluorination reaction of polyvinylidene fluoride (PVDF) binder in the slurry and lead to slurry gelation, which is one of the main challenges of sodium ion high-capacity layered oxide positive electrode materials (NaTMO2) in industrial large-scale production; in terms of negative electrode materials, hard carbon is considered to be the most suitable negative electrode material for sodium ion batteries due to its low working potential, medium capacity and good capacity retention rate. However, hard carbon has a lower initial coulombic efficiency due to its surface defects which consume more sodium ions during formation film forming, and the irreversible consumption of a large amount of sodium ions from the positive electrode will cause the serious deterioration of the electrochemical performance of the sodium ion full battery.

[0004] At present, the main technical solution to the high alkaline surface problem of the positive electrode material is to add an organic acid to the slurry to neutralize the alkalinity to avoid slurry gelation, but the problem of the product after the reaction of the organic acid and the alkaline impurities is not considered. In response to the problem that the hard carbon negative electrode material irreversibly consumes too much sodium ion in the first week, thereby leading to low reversible capacity, some positive electrode sodium supplementing agents are proposed to be applied, such as sodium carbonate, sodium oxalate, sodium azide, sodium oxide, etc., but the supplementing agent will leave large holes on the surface of the electrode after decomposition, which affects the conductivity and thus the rate performance of the battery, and leaves a large area of fresh interface which needs to be filmed again, which is not conducive to the cycle stability. SUMMARY

[0005] In light of this, the present invention aims to provide a multifunctional cathode slurry additive based on Lewis acid-base self-assembly and its application in sodium-ion batteries. By adding an organic dibasic or polybasic sodium salt to the slurry as an additive, it can act as a pH buffer to neutralize alkaline impurities on the material surface. The in-situ generated reaction product is an electrochemically active substance with a sodium-replenishing effect, achieving high reversible capacity and stable cycling for sodium-ion full batteries. Furthermore, the use of nanoscale additives enables Lewis acid-base self-assembly at the nanoscale, thereby regulating the surface crystal reconstruction of the layered oxide cathode material (NaTMO2) during the electrochemical process, achieving excellent rate performance.

[0006] To achieve the above objectives, the technical solutions of the present invention are as follows.

[0007] A multifunctional positive electrode slurry additive based on Lewis acid-base self-assembly, wherein the additive is an organic dibasic or polybasic acid sodium salt; the positive electrode slurry comprises a sodium ion battery positive electrode active material, a conductive agent, a binder and a solvent, wherein the sodium ion battery positive electrode active material is an O3-type layered oxide positive electrode material; the mass of the additive is 2% to 8% of the total mass of the sodium ion battery positive electrode active material, the conductive agent and the binder.

[0008] Preferably, the additive is one or more of sodium hydrogen oxalate, sodium hydrogen fumarate, sodium hydrogen maleate, sodium monohydrogen citrate and sodium dihydrogen citrate, more preferably sodium hydrogen oxalate.

[0009] Preferably, the mass of the additive is 4% to 6% of the total mass of the positive electrode active material, the conductive agent and the binder.

[0010] Preferably, the particle size of the additive is 300 nm to 800 nm.

[0011] A use of the multifunctional positive electrode slurry additive based on Lewis acid-base self-assembly of the present invention in a sodium ion battery, wherein the additive is added to the positive electrode slurry of the sodium ion battery, the positive electrode slurry comprising a positive electrode active material for the sodium ion battery, a conductive agent, a binder and a solvent. Based on the total mass of the positive electrode active material, the conductive agent and the binder for the sodium ion battery being 100%, the mass fraction of the positive electrode active material for the sodium ion battery is 80% to 96%, the mass fraction of the conductive agent is 2% to 10%, and the mass fraction of the binder is 2% to 10%.

[0012] Preferably, the solid content of the positive electrode slurry is 25% to 55%.

[0013] Preferably, the chemical formula of the sodium ion battery positive electrode active material is Na x Ni u Mn v M wO2, wherein, 0.9≤x≤1, 0.15≤u≤0.33, 0 + Mg 2+ Ca 2+ Al 3+ Ti 4+ V 3+ Fe 3 + Co 3+ Cu 2+ Zn 2+ Zr 4+ Nb 5+ W 5+ Y 3+ La 3+ Ce 4+ and Er 3+ , Ni, Mn and M elements together constitute a transition metal layer, and the transition metal layer and the Na layer are alternately arranged in the middle of the oxygen layer to form a layered oxide.

[0014] Preferably, the conductive agent is one or more of conductive carbon black, carbon nanotubes, ketjen black, graphene, graphene oxide, porous carbon, graphite carbon and activated carbon. More preferably, it is ketjen black.

[0015] Preferably, the binder is polyvinylidene fluoride (PVDF).

[0016] Preferably, the solvent is N-methyl pyrrolidone (NMP) or N,N-dimethylformamide (DFM). More preferably, it is N-methyl pyrrolidone.

[0017] A sodium ion battery includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, the positive electrode sheet is prepared by the following method: a slurry obtained by mixing a sodium ion battery positive electrode active material, a conductive agent, a binder, a multifunctional positive electrode slurry additive based on Lewis acid-base self-assembly of the application and a solvent is coated on a current collector, and a positive electrode sheet is obtained by drying; 4.0V

[0018] Preferably, the current collector is one or more of aluminum foil, copper foil, steel wire mesh and foamed nickel.

[0019] Advantages

[0020] The application provides a multifunctional positive electrode slurry additive based on Lewis acid-base self-assembly and application thereof in sodium ion batteries. By adding an organic binary or multi-acid sodium salt as an additive into the positive electrode slurry, on the one hand, the shortcomings of O3-type sodium ion battery layered oxides that cannot be pickled and washed with water are overcome, the residual alkali on the surface of the material and impurities generated by exposure to air during slurry preparation are neutralized in situ, and the slurry is stabilized; on the other hand, after the residual alkali is removed, the generated reaction product is an electrochemically active substance, which can release the fixed sodium ions to supplement the excess irreversible consumption of hard carbon by adjusting the voltage window, and the sodium ions are released at the same time, and only carbon dioxide gas is additionally released, which is compatible with the formation stage; further, the use of nanoscale additives can achieve the removal of residual alkali at the nanoscale, thereby regulating the surface crystal reconstruction of the layered oxide positive electrode material (NaTMO2) during the electrochemical process, and finally achieving excellent rate performance. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 Rate performance of half-cells of Example 1 (a) and Comparative Example 3 (b).

[0022] Figure 2 TEM images of half-cells of Example 1 (a) and Comparative Example 3 (b) after rate cycling.

[0023] Figure 3 Cycle performance of full cells of Example 1 and Comparative Example 3. DETAILED DESCRIPTION

[0024] To make the purpose, technical scheme and advantages of the embodiments of the application clearer, the sodium ion battery positive electrode material of the multifunctional slurry additive described in the application will be further described below in combination with specific embodiments and the drawings of the specification. It should be understood that the protection scope of the application is not limited to the following embodiments.

[0025] The specific conditions not mentioned in the embodiments are carried out according to the conventional conditions or the conditions recommended by the manufacturer. The reagents or instruments used are not marked with the manufacturer, and are all conventional products that can be purchased on the market.

[0026] Example 1

[0027] The embodiment provides a sodium ion battery positive electrode material slurry additive and an application method thereof. Before being applied to the slurry additive, anhydrous sodium hydrogen oxalate is subjected to two-step nanocrystallization treatment, including the following steps: a certain amount of commercial anhydrous sodium hydrogen oxalate is added to a zirconia ball mill jar, a planetary ball mill is used for ball milling treatment, in the first ball milling, a certain amount of zirconia balls with a diameter of Φ5-Φ10 mm is used, a ball-to-material ratio of 30:1, a rotating speed of 300 rmp, and 10 h of ball milling in an isopropyl alcohol dispersant environment, and after suction filtration and drying, the second ball milling is performed; in the second ball milling, a certain amount of zirconia balls with a diameter of Φ0.6 mm is used, a ball-to-material ratio of 30:1, a rotating speed of 300 rmp, and 10 h of ball milling in an isopropyl alcohol dispersant environment, and suction filtration and drying are performed; and after the two steps, the particle size of the slurry additive is 300 nm-800 nm.

[0028] The positive electrode active material is a layered oxide NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 (NFM), a conductive additive Ketjen black, a binder PVDF, and a slurry additive sodium hydrogen oxalate are uniformly mixed in a proportion of 90:5:5:5 by weight, and then added into a solvent N-methyl pyrrolidone to obtain a slurry, the solid content of the slurry is controlled to be between 40%, the slurry is coated on an aluminum foil with a thickness of 0.1 mm and a size of 50 mm*200 mm, the coating thickness is 0.025 mm, and then the slurry is dried in a vacuum environment at 120 DEG C for 12 h, and a positive electrode sheet is obtained by rolling.

[0029] Half battery assembly: the obtained electrode sheet is used as a positive electrode, 1 mol / L NaClO4(PC+5 vt%FEC) is used as an electrolyte, a glass fiber separator is used, and a metal sodium sheet is used as a negative electrode, and a button-type half battery is assembled in an argon glove box with O2<0.1 ppm and H2O<0.1 ppm.

[0030] Full battery assembly: the obtained electrode sheet is used as a positive electrode, 1.0 mol / L NaPF6(PC / DEC3:7+5 vt%FEC) is used as an electrolyte, a PE ceramic is used as a separator, and a hard carbon electrode sheet is used as a negative electrode, and a button-type full battery is assembled in an argon glove box with O2<0.1 ppm and H2O<0.1 ppm.

[0031] Electrochemical performance test: in the process of constant current charge and discharge cycle test of the half battery, the first cycle charge cut-off voltage is controlled to be 4.3 V, the discharge cut-off voltage is controlled to be 2 V, the first cycle charge and discharge rate is 0.1C, and the voltage is in the voltage range of 2.0-4.0 V in the subsequent charge and discharge process, and the charge and discharge rate is 1C.

[0032] During the full battery constant current charge-discharge cycle test process, the first circle charging cut-off voltage is controlled to be 4.3 V, the discharging cut-off voltage is 1.5 V, and the first circle charge-discharge rate is 0.1C; the voltage in the subsequent charge-discharge process is in the voltage interval of 1.5-4.0 V, and the charge-discharge rate is 1C.

[0033] Example 2

[0034] The difference from Example 1 is only that sodium hydrogen oxalate is replaced by sodium hydrogen fumarate of equal mass ratio.

[0035] Example 3

[0036] The difference from Example 1 is only that sodium hydrogen oxalate is replaced by sodium hydrogen maleate of equal mass ratio.

[0037] Example 4

[0038] The difference from Example 1 is only that sodium hydrogen oxalate is replaced by sodium hydrogen citrate of equal mass ratio.

[0039] Example 5

[0040] The difference from Example 1 is only that sodium hydrogen oxalate is replaced by sodium hydrogen citrate of equal mass ratio.

[0041] Example 6

[0042] The difference from Example 1 is only that the positive active material layered oxide NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 (NFM), conductive additive Ketjen black, binder PVDF, slurry additive sodium hydrogen oxalate, the weight ratio is 90:5:5:2.

[0043] Example 7

[0044] The difference from Example 1 is only that the positive active material layered oxide NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 (NFM), conductive additive Ketjen black, binder PVDF, slurry additive sodium hydrogen oxalate, the weight ratio is 90:5:5:8.

[0045] Comparative Example 1

[0046] The difference from Example 1 is only that sodium hydrogen oxalate is not added.

[0047] Comparative Example 2

[0048] The difference from Example 1 is only that the first circle charging cut-off voltage is 4.0 V.

[0049] Comparative Example 3

[0050] The difference from Example 1 is only that the positive active material is a layered oxide NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 (NFM), a conductive additive Ketjen black, a binder PVDF, and a slurry additive sodium hydrogen oxalate in a weight ratio of 90:5:5:0.5.

[0051] Comparative Example 4

[0052] The difference from Comparative Example 3 is only that the first cycle charging cutoff voltage is 4.0 V.

[0053] Comparative Example 5

[0054] The difference from Example 1 is only that the positive active material is a layered oxide NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 (NFM), a conductive additive Ketjen black, a binder PVDF, and a slurry additive sodium hydrogen oxalate in a weight ratio of 90:5:5:10.

[0055] Table 1. Results of electrochemical performance tests of half-cells

[0056]

[0057] Table 2. Results of electrochemical performance tests of full-cells

[0058]

[0059] From the half-cell electrochemical performance test results of each positive electrode containing slurry additive in Table 1, it can be seen that after adding an appropriate amount of additive to the slurry of the sodium-ion battery positive electrode material, the first charge capacity of the battery assembled by Examples 1-7 is obviously improved compared with Comparative Example 3, and the subsequent cycle capacity retention rate has no obvious difference compared with Comparative Example 3, indicating that the sodium supplementing agent generated in situ by Lewis acid-base self-assembly in the slurry is fully released after decomposition, and its by-product does not affect the cycle stability of the battery. As can be seen from Comparative Example 1, for O3-type oxide materials with high residual alkali, when the industrial adhesive PVDF commonly used as an electrode adhesive is used as the adhesive, the slurry will gel and fail without acidic slurry additive. As can be seen from the half-cell electrochemical performance test and results of Comparative Example 3, when the mass ratio of the slurry additive is low, the slurry can be stable, but there is no improvement in the first charge capacity. By comparing the cycle stability of Comparative Example 3 and Comparative Example 4, it can be seen that adjusting the voltage in the first circle can also release additional capacity without affecting the structural stability of the material. As can be seen from the test results of Comparative Example 5, when the mass ratio of the sodium supplementing additive is too high, although the first charge capacity is further improved, the discharge retention rate of the battery is not good with the increase of the cycle number, which may be due to the fact that the slurry additive does not participate in the Lewis acid-base self-assembly on the surface of the material and is dispersed in the conductive agent. When it decomposes, it may leave micropores affecting the conductive network. Finally, by comparing Example 1 with Comparative Example 2, it can be found that after treating the surface residual alkali with an additive amount of 5% and releasing additional sodium ions, the rate performance of the battery is very excellent, and the 5C rate capacity is about 70% of the 1C reversible specific capacity, as shown in Figure 1 As can be seen from the transmission electron microscopy analysis, the nano-scale Lewis acid-base self-assembly realizes the regulation of the nano-scale surface lattice reconstruction, as shown in Figure 2 As can be seen from Table 2, the full battery assembled by the electrode sheet prepared with a slurry additive amount of 5% can be stably operated at a high reversible capacity by the in-situ generated positive electrode sodium supplementing agent, as shown in Figure 3

[0060] In summary, the present application includes but is not limited to the above examples, and any equivalent replacement or partial improvement made within the spirit and principles of the present application shall be considered within the protection scope of the present application.​

Claims

1. A multifunctional cathode slurry additive based on Lewis acid-base self-assembly, characterized by: The additive is one or more of sodium hydrogen oxalate, sodium hydrogen fumarate, sodium hydrogen maleate, sodium monohydrogen citrate and sodium dihydrogen citrate, and the particle size of the additive is 300 nm to 800 nm; the positive electrode slurry includes a sodium ion battery positive electrode active material, a conductive agent, a binder and a solvent, and the sodium ion battery positive electrode active material is an O3-type layered oxide positive electrode material; the mass of the additive is 2% to 8% of the total mass of the sodium ion battery positive electrode active material, the conductive agent and the binder.

2. The multifunctional cathode slurry additive based on Lewis acid-base self-assembly according to claim 1, characterized in that: The mass of the additive is 4% to 6% of the total mass of the positive electrode active material, the conductive agent and the binder.

3. A use of the multifunctional positive electrode slurry additive based on Lewis acid-base self-assembly in a sodium ion battery according to claim 1 or 2, characterized in that: The additive is added to the positive electrode slurry of the sodium ion battery, wherein the positive electrode slurry includes a sodium ion battery positive electrode active material, a conductive agent, a binder and a solvent. Based on the total mass of the sodium ion battery positive electrode active material, the conductive agent and the binder as 100%, the mass fraction of the sodium ion battery positive electrode active material is 80% to 96%, the mass fraction of the conductive agent is 2% to 10%, and the mass fraction of the binder is 2% to 10%.

4. The use of the multifunctional positive electrode slurry additive based on Lewis acid-base self-assembly in a sodium ion battery according to claim 3, characterized in that: The solid content of the positive electrode slurry is 25% to 55%.

5. The use of the multifunctional cathode slurry additive based on Lewis acid-base self-assembly in a sodium ion battery according to claim 3, characterized in that: The general chemical formula of the positive electrode active material of the sodium ion battery is Na x Ni u Mn v M w O2, where 0.9≤x≤1, 0.15≤u≤0.33, 0<v<0.67, 0<w<1, u+v+w=1, and M is Li + Mg 2+ , Ca 2+ 、Al 3+ 、Ti 4+ 、V 3+ 、Fe 3+ 、Co 3+ 、Cu 2+ 、Zn 2+ 、Zr 4+ 、Nb 5+ 、W 5+ 、Y 3+ 、La 3+ 、Ce 4+ and Er 3+ More than one of the above, Ni, Mn and M elements together constitute a transition metal layer, and the transition metal layer and the Na layer are alternately arranged in the middle of the oxygen layer to form a layered oxide.

6. The use of the multifunctional cathode slurry additive based on Lewis acid-base self-assembly in a sodium ion battery according to claim 3, characterized in that: The conductive agent is one or more of carbon nanotubes, Ketjen black, graphene, graphene oxide and activated carbon; The binder is polyvinylidene fluoride; The solvent is N-methylpyrrolidone or N,N-dimethylformamide.

7. The use of the multifunctional cathode slurry additive based on Lewis acid-base self-assembly in sodium ion batteries according to claim 3, characterized in that: The conductive agent is Ketjen black.

8. A sodium ion battery, characterized in that: The invention comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the positive electrode sheet is prepared by the following method: a slurry obtained by mixing a positive electrode active material of a sodium ion battery, a conductive agent, a binder, the additive according to claim 1 or 2 and a solvent is coated on a current collector, and dried to obtain a positive electrode sheet; 4.0V<the first cycle charging cut-off voltage of the sodium ion battery<4.5V.

9. A sodium ion battery according to claim 8, characterized in that: The current collector is one or more of aluminum foil, copper foil, steel wire mesh and nickel foam.

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