Method for reducing residual alkali on surface of layered oxide positive electrode material and sodium ion battery application thereof

Abstract

The invention belongs to the technical field of electrochemical energy storage, and discloses a method for reducing residual alkali on the surface of a layered oxide positive electrode material and application of the layered oxide positive electrode material in a sodium ion battery, the chemical general formula of the layered oxide positive electrode material is NaxNiaMnbMcO2, wherein M is a mixture of more than one of Li < + >, K < + >, Mg < 2 + >, Ca < 2 + >, Ti < 4 + >, V < 3 + >, V < 4 + >, Cr < 3 + >, Fe < 3 + >, Co < 3 + >, Cu < 2 + > and Zn < 2 + >. X, a, b and c are molar ratios of corresponding elements respectively, and x is more than or equal to 0.6 and less than or equal to 1.0, 0lt; a is less than or equal to 0.5, 0.2 lt; b < = 0.7, 0 < = clt; 0.3 and a + b + c = 1; all the components meet charge conservation and stoichiometric conservation. The preparation method comprises the step of carrying out secondary sintering on the layered oxide positive electrode material synthesized by primary sintering in a specific atmosphere. The preparation process is simple, and a simple, convenient and easy-to-implement large-scale industrial production solution is provided.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrochemical energy storage, and relates to a method for reducing surface residual alkali of layered oxide cathode materials and its application in sodium-ion batteries. Background Art

[0002] The development of renewable energy has promoted the increasing demand for large-scale energy storage devices. Sodium-ion batteries have become a research hotspot in electrochemical energy storage technology due to their abundant sodium reserves and low cost. The energy density of sodium-ion batteries largely depends on the performance of the cathode material. Among many sodium-ion battery cathode materials, layered oxide cathode materials have attracted extensive attention due to their high theoretical specific capacity, high working voltage, and easy synthesis. However, layered oxides generally face the problem of poor air stability. In addition, the addition of excessive sodium sources during the synthesis process and the inevitable contact with air will cause a large amount of residual alkali, such as sodium carbonate, to be generated on the surface of the oxide. These electrochemically inert residual alkali substances are not conducive to electrochemical performance. Therefore, it is crucial to develop and explore technologies that can effectively reduce surface residual alkali.

[0003] Currently, the main improvement strategies for reducing surface residual alkali and improving air stability of layered oxide cathode materials include methods such as specific element doping, solvent washing, and surface reaction coating. The invention patent (publication number: CN116826032A) can effectively reduce the surface residual alkali content of the cathode material through doping with some specific elements. For some air-sensitive cathode materials, the invention patent (publication number: CN116154341A) can wash away surface residual alkaline substances through washing with anhydrous glycerol and anhydrous ethanol solutions. In addition, the invention patent (publication number: CN118062908A) uses boric acid that can react with residual alkali to eliminate them, and coats the reaction product on the surface of the oxide as a protective layer through high-temperature heat treatment. Although the above methods can effectively reduce surface residual alkali, they usually face problems such as complex synthesis processes and the selection of additional additive or coating materials. At the same time, it is difficult to effectively compensate for the bulk sodium vacancies caused by the loss of surface sodium ions, resulting in irreversible capacity decay.

[0004] Based on the deficiencies of the existing processes, it is necessary to develop a process for reducing surface residual alkali without introducing foreign substances. At the same time, part of the sodium ions in the residual alkali can be retained to the greatest extent. Inspired by the ion migration mechanism during the formation of solid solutions in high-temperature solid-state reactions, the surface reconstruction characteristics of the intrinsic components of the material at a specific temperature can be utilized to promote the directional migration of sodium ions in surface residual alkali to the bulk phase, achieving the conversion of residual alkali while compensating for sodium vacancies. Summary of the Invention

[0005] Based on the above background, the present invention provides a method for reducing the residual alkali on the surface of layered oxide cathode materials and its application in sodium-ion batteries. Through a low-cost, simple, feasible, safe and reliable secondary sintering process, this method not only simplifies the preparation process, but also can reduce the surface residual alkali without the need for additional additives or coating agents, and improve the structure and electrochemical performance of the materials. This method conducts secondary sintering on the synthesized primary sintering product in a specific atmosphere to decompose substances such as sodium carbonate on its surface, prompting the Na in the residual alkali + to re-embed into the lattice, compensating for the sodium loss caused by the formation of surface residual alkali, which can improve the crystal morphology and increase the crystallinity of the material. At the same time, the reduction of residual alkali can improve the ionic and electronic conductivities of the material, improve the sodium-ion diffusion kinetics, and ultimately improve the electrochemical performance such as the capacity and rate of the material.

[0006] The technical solution of the present invention:

[0007] A method for reducing the residual alkali on the surface of layered oxide cathode materials, comprising the following steps:

[0008] (1) Use primary sintering to synthesize layered oxide cathode materials;

[0009] (2) Perform secondary sintering treatment on the layered oxide cathode materials synthesized by primary sintering in a specific atmosphere.

[0010] The chemical general formula of the layered oxide cathode material is Na x Ni a Mn b M c O2; where Ni and Mn are transition metal elements, and M is Li + , K + , Mg 2+ , Ca 2+ , Ti 4+ , V 3+ , V 4+ , Cr 3+ , Fe 3+ , Co 3+ , Cu 2+ , Zn 2+ or a mixture of two or more of them. x, a, b, and c are the molar ratios of the corresponding elements, 0.6 ≤ x ≤ 1.0, 0 < a ≤ 0.5, 0.2 < b ≤ 0.7, 0 ≤ c < 0.3 and a + b + c = 1; each component satisfies charge conservation and chemical stoichiometry conservation.

[0011] The secondary sintering is carried out in a tubular furnace, and the sintering atmosphere is one or a mixture of two or more of helium, neon, argon, oxygen, and nitrogen.

[0012] The gas flow rate in the tubular furnace is 20 - 80 mL min -1。

[0013] The secondary sintering temperature is 300 - 900 °C, and the heating rate is 2 - 10 °C / min -1 , and the sintering time is 0.5 - 6 h.

[0014] The present invention provides a positive electrode sheet for a sodium-ion secondary battery, including the layered oxide positive electrode material. More specifically, the positive electrode sheet includes a current collector, a conductive agent coated on the current collector, a binder, and the layered oxide positive electrode material.

[0015] The present invention also provides a sodium-ion secondary battery, including the positive electrode sheet.

[0016] Advantages of the present invention:

[0017] 1. The present invention provides a method for effectively reducing the residual alkali on the surface of the layered oxide positive electrode material. This method can effectively reduce the surface residual alkali without additional additives or coating agents. By secondary heating to decompose the residual alkali, it promotes the re-embedding of Na in the residual alkali + back into the lattice main body, compensating for the sodium loss caused by the surface residual alkali, and significantly improving the capacity and rate performance of the material.

[0018] 2. The secondary sintering process described in the present invention can also improve the crystal morphology of the layered oxide material and increase the crystallinity.

[0019] 3. The layered oxide prepared by the preparation method provided by the present invention as a positive electrode material for sodium-ion batteries can equal or even exceed the existing modification processes in terms of electrochemical performance improvement. Moreover, this method has a simple process, is green and environmentally friendly, and has low costs, making it very suitable for large-scale production and practical applications, and is of great significance for the future commercial preparation of positive electrodes for sodium-ion batteries. Description of the Drawings

[0020] Figure 1 It is an XRD test result diagram of the layered oxide positive electrode materials prepared in Example 4b and Comparative Examples 1 - 2 of the present invention.

[0021] Figure 2 It is the rate performance of the layered oxide positive electrode materials prepared in Example 4b and Comparative Examples 1 - 2 of the present invention at a current density of 0.02 - 0.5 A / g -1 current density.

[0022] Figure 3 It is the charge-discharge curves of the first two cycles of the layered oxide positive electrode material prepared in Example 4b of the present invention at a current density of 0.02 A / g -1 current density.

[0023] Figure 4 It is the layered oxide positive electrode material prepared in Comparative Example 1 of the present invention at a current density of 0.02 A / g-1 Charge-discharge curves of the first two cycles at the current density.

[0024] Figure 5 Charge-discharge curves of the first two cycles at the current density of 0.02Ag for the layered oxide cathode material prepared in Comparative Example 2 of the present invention. -1 Charge-discharge curves of the first two cycles at the current density.

[0025] Figure 6 SEM images of the layered oxide cathode material, where (a) is the SEM image of the layered oxide cathode material prepared in Example 4b, and (b) is the SEM image of the layered oxide cathode material prepared in Comparative Example 1.

[0026] Figure 7 FT-IR result diagrams of the layered oxide cathode materials prepared in Example 4b and Comparative Example 1 of the present invention. Detailed implementation manners

[0027] The following further illustrates the detailed implementation manners of the present invention in combination with the accompanying drawings and technical solutions.

[0028] Example 1

[0029] This example provides a method for reducing the residual alkali on the surface of the layered oxide cathode material, and the specific operation steps are as follows:

[0030] Mix anhydrous sodium carbonate, nickel oxide, and manganese dioxide in a molar ratio of Na:Ni:Mn of 1:0.5:0.5, grind them in an agate mortar for 15 minutes. To further mix the materials, place them in a planetary ball mill and continue ball milling for 5 hours at a rotation speed of 400 r / min -1 , to obtain a precursor; transfer the obtained precursor to a corundum boat after pressing and mixing under a pressure of 10 Mpa, heat it in a muffle furnace at a heating rate of 5 °C / min -1 to 800 °C, and maintain it at this temperature for 12 hours, and then cool it naturally. The obtained black powder is the layered oxide material NaNi 0.5 Mn 0.5 O2.

[0031] Transfer the NaNi 0.5 Mn 0.5 O2 obtained from the first calcination to a corundum boat. The atmosphere in the tube furnace is argon, and the gas flow rate is 30 ml / min -1 , heat it in the tube furnace at a heating rate of 3 °C / min -1 to different temperatures as shown in Table 1, and maintain it at this temperature for 1 hour, and then cool it naturally to obtain the layered oxide cathode material.

[0032] The layered oxide prepared in Example 1 was used as the cathode active material for the preparation of a sodium-ion battery. The specific steps were as follows: The prepared layered oxide cathode material was mixed with the conductive agent Super-P and the binder polyvinylidene fluoride (PVDF) in a mass ratio of 8:1.5:0.5 in the N-methyl-2-pyrrolidone (NMP) solvent. Then the uniformly mixed slurry was evenly coated on the aluminum foil, vacuum-dried at 100 °C for 12 h, and then cut into circular pieces with a diameter of 12 mm to obtain the cathode electrode sheet. The loading amount of the cathode electrode sheet was 3 - 4 mg cm -2 . Using metallic sodium as the counter electrode, a glass fiber membrane GF / F was used as the separator in the middle, and the electrolyte was NaPF6 / (PC:FEC = 95:5). A CR2032 type button battery was assembled in an argon glove box with the water and oxygen content less than 0.1 ppm. Using a BlueTEC battery test system, the constant current charge and discharge test of the battery was carried out at a constant temperature of 30 °C and a current density of 0.02 A g -1 . The rate performance test was carried out at a current density of 0.02 - 0.5 A g -1 . The charge and discharge voltage range was 2.0 - 4.0 V. The test results are shown in Table 1.

[0033] Table 1 Comparison of the electrochemical performance of the materials obtained at different sintering temperatures

[0034]

[0035] Observing the results in Table 1, it can be seen that the electrochemical performance is the best at 700 °C. This is because when the temperature is relatively low, substances such as residual alkali on the surface are not completely decomposed, and when the temperature is too high, the crystal structure of the layered oxide phase will be affected.

[0036] Example 2

[0037] This example provides a method for reducing the residual alkali on the surface of the layered oxide cathode material. The specific operation steps are as follows:

[0038] The first calcination process is the same as that in Example 1.

[0039] Transfer the NaNi 0.5 Mn 0.5 O2 obtained from the first calcination into a corundum boat. The tubular furnace is connected to an argon atmosphere. Select different gas velocities as shown in Table 2. Heat the tubular furnace to 700 °C at a heating rate of 3 °C min -1 , and maintain it at this temperature for 1 h, and then cool it naturally to obtain the layered oxide cathode material.

[0040] The battery assembly and electrochemical performance test are the same as those in Example 1. The test results are shown in Table 2.

[0041] Table 2 Comparison of the electrochemical performance of the materials obtained at different gas velocities

[0042]

[0043] It can be seen from the results in Table 2 that the electrochemical performance is optimal when the gas velocity is 20 ml / min. -1 The reason may be that when the gas velocity is too fast, the residual alkali is purged away from the material surface before it has time to react at high temperature. At the same time, the bombardment of gas molecules on the material surface may cause the loss of oxygen atoms, resulting in bulk oxygen defects, which is not conducive to the integrity of the crystal structure.

[0044] Example 3

[0045] This example provides a method for reducing the residual alkali on the surface of the layered oxide cathode material. The specific operation steps are as follows:

[0046] The first calcination process is the same as that in Example 1.

[0047] Transfer the NaNi 0.5 Mn 0.5 O2 obtained from the first calcination into a corundum boat. The tubular furnace is connected to an argon atmosphere with a gas velocity of 20 ml / min -1 , and heat it to 700 °C at different heating rates shown in Table 3 in the tubular furnace, and hold it at this temperature for 1 h, then cool it naturally to obtain the layered oxide cathode material.

[0048] The battery assembly and electrochemical performance test are the same as those in Example 1. The test results are shown in Table 3.

[0049] Table 3 Comparison of the electrochemical performance of materials obtained at different heating rates

[0050]

[0051] Observing Table 3, the electrochemical performance is optimal when the heating rate is 2 °C / min -1 . The reason is that the slow heating process is conducive to the solid solution reaction of sodium ions on the material surface to re-embed into the bulk lattice, which helps to form a better crystal structure and improve the crystal morphology.

[0052] Example 4

[0053] This example provides a method for reducing the residual alkali on the surface of the layered oxide cathode material. The specific operation steps are as follows:

[0054] The first calcination process is the same as that in Example 1.

[0055] Transfer the NaNi 0.5 Mn 0.5 O2 obtained from the first calcination into a corundum boat. The tubular furnace is connected to an argon atmosphere with a gas velocity of 20 ml / min -1 , and in the tubular furnace at 2 °C / min -1Heat it to 700 °C at a heating rate, hold it at this temperature for different times shown in Table 4 respectively, and then cool it naturally to obtain the layered oxide cathode material.

[0056] The battery assembly and electrochemical performance test are the same as in Example 1. The test results are shown in Table 4.

[0057] Table 4 Comparison of the electrochemical performance of the materials obtained at different holding times

[0058]

[0059] Observing Table 4, it is found that the performance is optimal when the holding time is 2 h. It is speculated that the reason is that too short a holding time is not conducive to the complete conversion and decomposition of the residual alkali on the surface, while too long a time will affect the crystal structure.

[0060] Comparative Example 1

[0061] Mix anhydrous sodium carbonate, nickel oxide, and manganese dioxide in a molar ratio of Na:Ni:Mn of 1:0.5:0.5, grind them in an agate mortar for 15 min. To further mix, place the material in a planetary ball mill and continue ball milling for 5 h at a rotation speed of 400 r / min -1 , to obtain a precursor; transfer the obtained precursor tablets after mixing to a corundum boat, and heat it in a muffle furnace at a heating rate of 5 °C / min -1 to 800 °C, and hold it at this temperature for 12 h, and then cool it naturally to obtain a black powder, which is the layered oxide material NaNi 0.5 Mn 0.5 O2.

[0062] Comparative Example 2

[0063] The first calcination process is the same as in Comparative Example 1.

[0064] Heat the NaNi 0.5 Mn 0.5 O2 obtained from the first sintering in a muffle furnace at a heating rate of 2 °C / min -1 to 700 °C for the second time, and hold it at this temperature for 2 h, and then cool it naturally to obtain the layered oxide cathode material.

[0065] Comparative Example 3

[0066] The first sintering process is the same as in Comparative Example 1.

[0067] Transfer the NaNi 0.5 Mn 0.5 O2 obtained from the first calcination to a corundum boat, connect the tubular furnace with an argon atmosphere and a gas velocity of 100 ml / min -1 , and in the tubular furnace at a heating rate of 2 °C / min -1Heat it to 700 °C at a heating rate, hold it at this temperature for 2 h, and then cool it naturally to obtain the layered oxide cathode material.

[0068] Comparative Example 4

[0069] The primary sintering process is the same as that of Comparative Example 1.

[0070] Transfer the NaNi 0.5 Mn 0.5 O2 obtained from the primary calcination into a corundum boat. The tubular furnace is connected to an argon atmosphere with a gas flow rate of 20 ml min -1 , and heat it to 1000 °C in the tubular furnace at a heating rate of 2 °C min -1 respectively, hold it at this temperature for 2 h, and then cool it naturally to obtain the layered oxide cathode material.

[0071] Comparative Example 5

[0072] The primary sintering process is the same as that of Comparative Example 1.

[0073] Transfer the NaNi 0.5 Mn 0.5 O2 obtained from the primary calcination into a corundum boat. The tubular furnace is connected to an argon atmosphere with a gas flow rate of 20 ml min -1 , and heat it to 700 °C in the tubular furnace at a heating rate of 20 °C min -1 respectively, hold it at this temperature for 2 h, and then cool it naturally to obtain the layered oxide cathode material.

[0074] Comparative Example 6

[0075] The primary sintering process is the same as that of Comparative Example 1.

[0076] Transfer the NaNi 0.5 Mn 0.5 O2 obtained from the primary calcination into a corundum boat. The tubular furnace is connected to an argon atmosphere with a gas flow rate of 20 ml min -1 , and heat it to 700 °C in the tubular furnace at a heating rate of 2 °C min -1 respectively, hold it at this temperature for 12 h, and then cool it naturally to obtain the layered oxide cathode material.

[0077] The battery assembly and electrochemical performance test of the cathode materials obtained in Comparative Examples 1-6 are the same as those in Example 1. The test results are shown in Table 5.

[0078] Table 5 Comparison of the secondary sintering process and electrochemical performance between Example 4b and Comparative Examples 1-6

[0079]

[0080] Analyzing Table 5 and comparing the electrochemical performance of Example 4b under the optimal process conditions with that of Comparative Examples 1-6, it can be seen that under suitable secondary sintering process conditions (atmosphere, gas velocity, heating rate, temperature, time, etc.), secondary sintering can effectively increase the discharge specific capacity of the material. This is because during the secondary sintering process, the Na + rediffuses into the lattice, compensating for the sodium loss caused by the precipitation of sodium to form surface residual alkali. At the same time, the rate performance of the oxide material is improved after the second sintering. This is because the reduction of electrochemically inert residual alkali substances on the material surface improves the ionic and electronic conductivity on the material surface and the transport kinetics.

[0081] XRD tests were carried out on the layered oxide samples obtained in Example 4b and the layered oxide samples obtained in Comparative Examples 1-2. The results are as Figure 1 shown. After secondary sintering in argon, the layered oxide material still shows the O3 phase structure, and secondary sintering in an argon atmosphere does not affect the phase structure of the layered oxide. Combining the XRD test results of Comparative Example 1 and Comparative Example 2, it was found that secondary sintering in air would change the original crystal structure. It is proved that argon plays a role as a protective atmosphere.

[0082] The rate performance and charge-discharge curves of the layered oxide samples in Example 4b and the samples in Comparative Examples 1-2 are as Figures 2 - 5 shown. The second sintering can significantly improve the electrochemical performance of the material.

[0083] SEM tests were carried out on the layered oxide samples obtained in Example 4b and the layered oxide samples obtained in Comparative Example 1. The test results are as Figure 6 shown. By comparison, it can be seen that the material in Example 4b has a better crystal morphology, and the lamellar crystal structure is more uniform, proving that secondary sintering can improve the crystal morphology and increase the crystallinity of the layered oxide material.

[0084] FT-IR tests were carried out on the layered oxide samples obtained in Example 4b and the layered oxide samples obtained in Comparative Example 1. The test results are as Figure 7 shown. By comparison, it can be seen that the characteristic peak intensity representing Na2CO3 in the layered oxide material obtained in Example 4b is significantly weakened or even disappears, proving that the second sintering can effectively reduce the surface residual alkali of the layered oxide material.

Claims

1. A method for reducing residual alkali on the surface of a layered oxide cathode material, characterized in that, It includes the following steps: (1) Using one-time sintering to synthesize a layered oxide cathode material; (2) Subjecting the layered oxide cathode material synthesized by one-time sintering to secondary sintering treatment under a specific atmosphere.

2. The method for reducing the residual alkali on the surface of the layered oxide cathode material according to claim 1, wherein The chemical general formula of the layered oxide cathode material is Na x Ni a Mn b M c O2; where Ni and Mn are transition metal elements, and M is Li + , K + , Mg 2+ , Ca 2+ , Ti 4+ , V 3+ , V 4+ , Cr 3+ , Fe 3+ , Co 3+ , Cu 2+ , Zn 2+ or a mixture of two or more of them; x, a, b, and c are the molar ratios of the corresponding elements, 0.6 ≤ x ≤ 1.0, 0 < a ≤ 0.5, 0.2 < b ≤ 0.7, 0 ≤ c < 0.3 and a + b + c = 1; each component satisfies charge conservation and chemical stoichiometry conservation.

3. The method for reducing residual alkali on the surface of the layered oxide cathode material according to claim 1, characterized in that, The temperature of the secondary sintering is 300 - 900 °C, and the heating rate is 2 - 10 °C / min -1 , and the sintering time is 0.5 - 6 h.

4. The method for reducing residual alkali on the surface of the layered oxide cathode material according to claim 1, wherein The specific atmosphere is one or a mixture of two or more of helium, neon, argon, oxygen, and nitrogen.

5. The method for reducing residual alkali on the surface of the layered oxide cathode material according to claim 4, wherein The gas flow rate of the specific atmosphere is 20 - 80 mL / min -1 .

6. The positive electrode sheet of a sodium-ion secondary battery, characterized in that, It includes the layered oxide cathode material obtained by the method described in claims 1-5.

7. The positive electrode sheet according to claim 6, wherein, The positive electrode sheet includes a current collector, a conductive agent coated on the current collector, a binder, and a layered oxide cathode material.

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