A sodium layered oxide and its preparation method and application

Abstract

The present invention relates to a sodium layered oxide and a preparation method and application thereof. The present invention cools the sodium layered oxide obtained by calcination in stages while passing a mixed gas (acidic gas and carrier gas): first cooling, heat preservation, and second cooling, to obtain the sodium layered oxide with a coating layer. The present invention can reduce the total alkali content on the surface of the positive electrode material sintered once and also reduce the surface impurity content through a new cooling process. The sodium layered oxide with a coating layer can be directly obtained without secondary post-treatment coating sintering. The cost is low, the operation is easy, the process is controllable, and the air stability and cycle performance of the obtained material are improved.

Description

Technical Field

[0001] The invention belongs to the field of sodium ion battery positive electrode materials, and specifically relates to a sodium-ion battery layered oxide and a preparation method and application thereof, in particular to a method for reducing surface impurities of the sodium-ion battery layered oxide. Background Art

[0002] The synthesis of sodium layered oxide positive electrode materials is generally a process in which a transition metal hydroxide precursor and a Na2CO3 solid phase are evenly mixed, calcined at a high temperature, and then naturally cooled to room temperature to obtain the corresponding positive electrode material.

[0003] The positive electrode material obtained by conventional sintering method has no coating layer on the surface and contains more impurities, such as nickel oxide, sodium manganate, sodium carbonate, etc., which leads to high impedance and hinders the Na + The embedding and extraction of O2 and CO2 eventually lead to poor electrochemical performance. Under air conditions (O2, H2O, CO2 coexist), Na in the bulk phase of the cathode material is very easy to be extracted and reacted with it to form sodium carbonate, sodium bicarbonate, sodium hydroxide, etc. The total alkali content (sodium carbonate + sodium bicarbonate + sodium hydroxide) can reach more than 3%. Layered cathode materials with high impurity content and high residual alkali will affect the slurry coating of the material, and the resulting battery has poor cycle performance, is easy to produce gas, and has high safety risks.

[0004] Therefore, post-processing of the materials is usually required. Common methods include coating (TiO2, ZrO2, Al2O3, phosphate, etc.) or washing (water washing, acid washing, alcohol washing, etc.). These post-processing methods can reduce the impurities on the surface of the material and reduce the total surface alkali content to less than 1%, thereby improving the air stability, long cycle performance and safety performance of the positive electrode material. However, the general post-processing process is more complicated and usually requires secondary sintering (coating to reduce residual alkali through secondary sintering, thereby improving cycle stability), which not only increases the process cost, but also has insignificant effects, and usually reduces the electrochemical performance.

[0005] In order to reduce the release of Na in the bulk phase of layered cathode materials during sintering, reduce surface impurities, and improve the air stability and long cycle performance of cathode materials, a new calcination process was proposed. This process can obtain sodium-based layered cathode materials with high air stability and less surface impurities in one sintering. Summary of the invention

[0006] The purpose of the present invention is to provide a sodium layered oxide and a preparation method and application thereof in view of the problems existing in the prior art. The method can reduce the total alkali content on the surface of the sodium layered oxide by sintering once, and also reduce the surface impurities. The sodium layered oxide with a coating layer is directly obtained without the need for secondary post-treatment coating sintering, thereby improving its air stability and cycle performance.

[0007] The purpose of the present invention can be achieved by the following scheme:

[0008] The present invention provides a method for preparing a sodium layered oxide, the method comprising the following steps:

[0009] The sodium electric layered oxide obtained by calcination is cooled in stages to obtain the sodium electric layered oxide with a coating layer;

[0010] The segmented cooling includes the first cooling, heat preservation, and the second cooling (the first cooling is to the heat preservation temperature, and the second cooling is performed after the heat preservation);

[0011] As an embodiment of the present invention, the staged cooling (first cooling, heat preservation, second cooling) is performed under the condition of passing a mixed gas, wherein the mixed gas includes an acid gas and a carrier gas.

[0012] As an embodiment of the present invention, the flow rate of the mixed gas is 10-100 L / min, and can be 30-50 L / min. The ratio of the flow rate of the mixed gas to the amount of the layered oxide obtained by calcination is 10-20 L / min: 1 kg.

[0013] Calcination and cooling can be carried out in a calcination container such as a sintering furnace. During the cooling process, the main purpose of introducing the mixed gas is to gradually reduce the air in the furnace and reduce the content of H2O and CO2 in the furnace atmosphere. At the same time, the acidic gas can react with the water in the calcination atmosphere when entering the furnace. When the mixed gas is continuously introduced, it can further react with the water in the residual air in the sintering furnace to reduce the water content. After the acid is generated, it reacts with the residual alkali (Na2CO3, NaHCO 3、 NaOH, etc.) to generate a corresponding sodium salt (NaNO3, NaClO4, Na2SO3, etc.) coating layer, thereby reducing the surface residual alkali of the layered oxide and inhibiting the formation of surface impurities. The whole process occurs slowly and continuously. During the cooling process, the water content in the furnace is continuously reduced from the initial 15000ppm to less than 500ppm. In this process, not only can a coating layer with high ionic conductivity be formed on the surface of the material to stabilize the bulk structure, but the residual alkali content can also be further reduced, and finally a sodium-based layered oxide positive electrode material with high air stability and less surface impurities is obtained.

[0014] As an embodiment of the present invention, the acidic gas is one or more (more than two) of NO2, Cl2, and SO2.

[0015] The acidic gas (NO2, Cl2, SO2, etc.) of the present invention can react with H2O in the furnace cavity to generate corresponding acid (HNO3, HClO, H2SO3, etc.) during the cooling process, thereby reducing the H2O content in the air in the furnace and avoiding the escape of Na ions in the bulk phase.

[0016] As an embodiment of the present invention, the carrier gas includes one or more (more than two) of O2, N2, and Ar.

[0017] The carrier gas can replace the air in the furnace and reduce the content of H2O and CO2 in the air in the furnace; at the same time, as a carrier of acidic gas, it transports the acidic gas to the surface of the powder material to react with the residual alkali on the surface.

[0018] As an embodiment of the present invention, in the mixed gas, the volume proportion of the acidic gas is 10-50%. In the mixed gas, the volume proportion of the carrier gas is 50-90%. The volume ratio of the acidic gas to the carrier gas can be 10%:90%, 15%:85%, 25%:75%, 50%:50%. Although the acidic gas can reduce the residual alkali and the surface impurities, excessive acidity will corrode the material structure, which will deteriorate the electrochemical performance and stability.

[0019] As an embodiment of the present invention, the first temperature reduction rate is 5-10°C / min.

[0020] As an embodiment of the present invention, the holding temperature is 600-700° C., and / or the holding time is 4-12 h.

[0021] As an embodiment of the present invention, the second temperature reduction rate is 2-5°C / min. The second temperature reduction is to 25-50°C.

[0022] As an embodiment of the present invention, during the staged cooling process (first cooling, heat preservation, second cooling), the internal and external pressure difference is above 10Pa, and the internal and external pressure difference can be 10-30 Pa. The internal and external pressure difference refers to the pressure difference between the inside and outside of the equipment (sintering furnace). Since voids are generated inside the accumulated powder after calcination, the pressure in the furnace is increased in the cooling stage to maintain a certain pressure difference (the furnace pressure difference is above 10Pa). On the one hand, the mixed gas can be fully in contact with the inside of the powder, and on the other hand, the rapid flow of gas can take away the by-products of the surface reaction, so that the coating layer generated by the reaction is more evenly covered on the surface of the material.

[0023] As an embodiment of the present invention, the coating layer thickness of the obtained sodium electric layered oxide with a coating layer is 2-5 nm. The coating layer is the acid generated by the reaction of the acid gas with the water in the sintering furnace, and / or directly further reacts with the residual alkali Na2CO3, NaHCO 3、It is obtained by fully reacting NaOH and other substances at high temperature to generate corresponding Na salts (NaNO3, NaClO4, Na2SO3, etc.).

[0024] The preparation method prepared by the present invention makes the total alkali mass of the obtained positive electrode material less than 0.4%, the surface is coated with a uniform coating layer, and has excellent air stability and long cycle performance. The obtained coating layer is mainly formed during the first cooling and heat preservation. Long-term heat preservation allows the reaction of the surface residual alkali to proceed fully, which is conducive to the coating layer covering the material surface more uniformly.

[0025] As one embodiment of the present invention, the obtained sodium layered oxide includes Na x TMO2, 0.7≤x≤1; wherein TM includes transition metal elements, and the transition metal elements include one or more (more than two) of Ni, Fe, and Mn. The TM is preferably Ni, Fe, and Mn (molar ratio is 1:1:1).

[0026] As one embodiment of the present invention, the obtained sodium layered oxide includes Na x TMO2, 0.7≤x≤1; wherein TM includes transition metal elements and doping metal elements. The transition metal elements include one or more of Ni, Fe, and Mn, and the doping metal elements include one or more of Cu, Ti, Ca, Nb, Zn, and Mg.

[0027] As an embodiment of the present invention, the sodium layered oxide obtained by calcination is obtained by calcining a solid phase mixture including a metal hydroxide precursor (TM(OH)2) and Na2CO3. Calcination refers only to the calcination process of generating the sodium layered oxide (what is obtained is the original product of calcination). Conventional calcination processes of sodium layered oxides are all applicable to the staged cooling method of the present invention.

[0028] When TM includes a transition metal element, the sodium layered oxide obtained by calcination is obtained by calcining a solid phase mixture including a transition metal hydroxide precursor and Na2CO3.

[0029] When TM includes a transition metal element and a doping metal element, the sodium layered oxide obtained by calcination is obtained by calcining a solid phase mixture including a transition metal hydroxide precursor, a doping metal hydroxide precursor and Na2CO3.

[0030] As an embodiment of the present invention, during calcination, the water content in the calcination atmosphere is 10000-20000ppm. The appropriate water content in the atmosphere can ensure the generation of an appropriate amount of acid, which can react to remove the residual alkali on the surface without affecting the positive electrode material, and further obtain a suitable coating layer. The calcination atmosphere used in the heating and insulation stages is preferably compressed air (flow rate 30-50L / min).

[0031] The calcination process can be one of the following processes:

[0032] (1) Raise the temperature from room temperature (25°C) to 600-700°C, keep it warm for 2-6 hours, and the heating rate is 1-5°C / min. Then raise the temperature to 800-1000°C and keep it warm for 10-16 hours.

[0033] (2) Directly heat from room temperature (25°C) to 800-1000°C at a heating rate of 1-5°C / min and keep warm for 10-16 hours;

[0034] (3) Directly heat the room temperature (25°C) to 1050-1100°C at a heating rate of 1-5°C / min, keep warm for 1-3 hours, then cool to 800-1000°C and keep warm for 10-12 hours.

[0035] The invention also provides a sodium electric layered oxide obtained by the preparation method.

[0036] The present invention also provides an application of the sodium-ion layered oxide in the preparation of a sodium-ion battery.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] Through the new cooling process, the total alkali content on the surface of the positive electrode material sintered once can be reduced, and the surface impurities can also be reduced. There is no need for secondary post-processing coating and sintering to directly obtain a sodium electrode layered oxide with a uniform coating layer. This process is low in cost, easy to operate, and the process is controllable. The obtained material has excellent air stability and cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Other features, objects and advantages of the present invention will become more apparent from the detailed description of non-limiting embodiments made with reference to the following drawings:

[0040] Figure 1 is a field emission scanning electron microscope image of Examples 1-8 of the present invention;

[0041] Figure 2 Elemental energy spectrum analysis diagram of Examples 1-3 of the present invention;

[0042] Figure 3 It is a field emission scanning electron microscope image of comparative example 1 of the present invention. DETAILED DESCRIPTION

[0043] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. The following examples are implemented on the premise of the technical solution of the present invention, provide detailed implementation methods and specific operation processes, and will help those skilled in the art to further understand the present invention. It should be pointed out that the protection scope of the present invention is not limited to the following embodiments, and several adjustments and improvements made under the premise of the concept of the present invention all belong to the protection scope of the present invention.

[0044] Example 1

[0045] 1. High temperature calcination process:

[0046] (1) Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 and sodium carbonate are mixed uniformly in a certain molar ratio and placed in a sintering furnace, wherein the molar ratio of Na / TM(OH)2 is 1.03:1;

[0047] (2) The temperature was raised from room temperature (25°C) to 600°C at a rate of 2°C / min, and kept at 600°C for 6 hours, and then raised to 1000°C and kept for 10 hours. The atmosphere used was compressed air (flow rate 50L / min, unless otherwise specified, the water content of the compressed air used in the experiments of the present invention was about 15000ppm);

[0048] 2. Cooling process:

[0049] (1) A mixture of NO2 and O2 (volume ratio 10%:90%, flow rate 50 L / min, corresponding to 3 kg of material) is introduced;

[0050] (2) Maintain the pressure difference in the furnace at 10Pa by adjusting the gas outlet control valve;

[0051] (3) Program cooling: When the temperature drops from 1000℃ to 600℃, the cooling rate is 5℃ / min, and the temperature is kept at 600℃ for 4h. Then, the temperature is dropped from 600℃ to room temperature (25℃) at a cooling rate of 2℃ / min. Finally, the NaNO3-coated sodium-electrode layered cathode material, namely N-NaNi 0.33 Fe 0.33 Mn 0.33 O2.

[0052] The field emission scanning electron microscope image of the obtained positive electrode material is as follows Figure 1 As shown. Figure 2 As shown, through the surface energy spectrum EDS test, it was found that Na, N, and O elements were evenly covered on the surface of the material, proving that a NaNO3 coating layer (thickness 2-5nm) was evenly formed.

[0053] Example 2

[0054] 1. High temperature calcination process:

[0055] (1) Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 and sodium carbonate are uniformly mixed in a certain molar ratio and placed in a sintering furnace, wherein the molar ratio of Na / TM(OH)2 is 1.03:1.

[0056] (2) Heating from room temperature (25°C) to 600°C at a rate of 2°C / min, keeping at 600°C for 6 hours, then heating to 1000°C and keeping at that temperature for 10 hours. The atmosphere used is compressed air (flow rate 50L / min);

[0057] 2. Cooling process:

[0058] (1) Pass a mixture of Cl2 and O2 (volume ratio 10%:90%, flow rate 50 L / min, corresponding to 3 kg of material);

[0059] (2) Maintain the pressure difference in the furnace at 10Pa by adjusting the gas outlet control valve;

[0060] (3) Programmed cooling: When the temperature drops from 1000°C to 600°C, the cooling rate is 5°C / min, and the temperature is kept at 600°C for 4 hours. The temperature is then dropped from 600°C to room temperature (25°C) at a cooling rate of 2°C / min. Finally, a sodium-electrolyte layered cathode material coated with NaClO4, namely C-NaNi 0.33 Fe 0.33 Mn 0.33 O2.

[0061] The field emission scanning electron microscope image of the obtained positive electrode material is as follows Figure 1 As shown. Figure 2 As shown, through the surface energy spectrum EDS test, it was found that Na, Cl, and O elements were evenly covered on the surface of the material, proving that a NaClO4 coating layer was evenly formed.

[0062] Example 3

[0063] 1. High temperature calcination process:

[0064] (1) Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 and sodium carbonate are uniformly mixed in a certain molar ratio and placed in a sintering furnace, wherein the molar ratio of Na / TM(OH)2 is 1.03:1.

[0065] (2) Heating from room temperature (25°C) to 600°C at a rate of 2°C / min, keeping at 600°C for 6 hours, then heating to 1000°C and keeping at that temperature for 10 hours. The atmosphere used is compressed air (flow rate 50L / min);

[0066] 2. Cooling process:

[0067] (1) A mixture of SO2 and O2 (volume ratio 10%:90%, flow rate 50 L / min, corresponding to 3 kg of material) is introduced;

[0068] (2) Maintain the pressure difference in the furnace at 10Pa by adjusting the gas outlet control valve;

[0069] (3) Program cooling: When the temperature drops from 1000°C to 600°C, the cooling rate is 5°C / min, and the temperature is kept at 600°C for 4 hours. The temperature is then dropped from 600°C to room temperature (25°C) at a cooling rate of 2°C / min. Finally, a sodium-coated layered cathode material, namely S-NaNi, is obtained. 0.33 Fe 0.33 Mn 0.33 O2.

[0070] The field emission scanning electron microscope image of the obtained positive electrode material is as follows Figure 1 As shown. Figure 2 As shown, through the surface energy spectrum EDS test, it was found that Na, S, and O elements were evenly covered on the surface of the material, proving that a Na2SO3 coating layer was evenly formed.

[0071] Example 4

[0072] 1. High temperature calcination process:

[0073] (1) Ni 0.4 Fe 0.2 Mn 0.4 (OH)2 and sodium carbonate are uniformly mixed in a certain molar ratio and placed in a sintering furnace, wherein the molar ratio of Na / TM(OH)2 is 1.03:1.

[0074] (2) Heating from room temperature (25°C) to 600°C at a rate of 2°C / min, keeping at 600°C for 6 hours, then heating to 1000°C and keeping at that temperature for 10 hours. The atmosphere used is compressed air (flow rate 50L / min);

[0075] 2. Cooling process:

[0076] (1) A mixture of NO2 and N2 (volume ratio 30%:70%, flow rate 50 L / min, corresponding to 3 kg of material) is introduced;

[0077] (2) Maintain the pressure difference in the furnace at 20Pa by adjusting the gas outlet control valve;

[0078] (3) Program cooling: When the temperature drops from 1000°C to 650°C, the cooling rate is 8°C / min, and the temperature is kept at 650°C for 5 h. Then, the temperature is dropped from 650°C to room temperature (25°C) at a cooling rate of 3°C / min. Finally, NaNO3-coated sodium-electrode layered cathode material, namely N-NaNi 0.4 Fe 0.2 Mn 0.4 O2.

[0079] The field emission scanning electron microscope image of the obtained positive electrode material is as follows Figure 3 shown.

[0080] Example 5

[0081] 1. High temperature calcination process:

[0082] (1) Ni 0.5 Mn 0.5 (OH)2 and sodium carbonate are mixed uniformly in a certain molar ratio and placed in a sintering furnace, wherein the molar ratio of Na / TM(OH)2 is 1.03:1;

[0083] (2) Heating from room temperature (25°C) to 600°C at a rate of 2°C / min, keeping at 600°C for 6 hours, then heating to 1000°C and keeping at that temperature for 10 hours. The atmosphere used is compressed air (flow rate 50L / min);

[0084] 2. Cooling process:

[0085] (1) A mixture of NO2 and O2 (volume ratio 50%:50%, flow rate 50 L / min, corresponding to 3 kg of material) is introduced;

[0086] (2) Maintain the pressure difference in the furnace at 30 Pa by adjusting the gas outlet control valve;

[0087] (3) Program cooling: When the temperature drops from 1000°C to 700°C, the cooling rate is 10°C / min, and the temperature is kept at 700°C for 6 hours. The temperature is then dropped from 700°C to room temperature (25°C) at a cooling rate of 5°C / min. Finally, a sodium-coated layered cathode material, namely N-NaNi, is obtained. 0.5 Mn 0.5 O2.

[0088] The field emission scanning electron microscope image of the obtained positive electrode material is as follows Figure 3 shown.

[0089] Example 6

[0090] 1. High temperature calcination process:

[0091] (1) Ni 0.33 Fe 0.33 Mn0.33 (OH)2 and sodium carbonate are uniformly mixed in a certain molar ratio and placed in a sintering furnace, wherein the molar ratio of Na / TM(OH)2 is 1.03:1.

[0092] (2) Heating from room temperature (25°C) to 900°C at a rate of 2°C / min, and keeping warm for 14 hours. The atmosphere used is compressed air (flow rate 50L / min);

[0093] 2. Cooling process:

[0094] (1) Pass a mixture of Cl2 and O2 (volume ratio 10%:90%, flow rate 100 L / min, corresponding to 6 kg of material);

[0095] (2) Maintain the pressure difference in the furnace at 10Pa by adjusting the gas outlet control valve;

[0096] (3) Programmed cooling: When the temperature drops from 900°C to 600°C, the cooling rate is 5°C / min, and the temperature is kept at 600°C for 4 hours. The temperature is then dropped from 600°C to room temperature (25°C) at a cooling rate of 2°C / min. Finally, a sodium-electrolyte layered cathode material coated with NaClO4 is obtained, namely C2-NaNi 0.33 Fe 0.33 Mn 0.33 O2.

[0097] The field emission scanning electron microscope image of the obtained positive electrode material is as follows Figure 3 shown.

[0098] Example 7

[0099] 1. High temperature calcination process:

[0100] (1) Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 and sodium carbonate are uniformly mixed in a certain molar ratio and placed in a sintering furnace, wherein the molar ratio of Na / TM(OH)2 is 1.03:1.

[0101] (2) Heating from 50°C to 1100°C at a rate of 4°C / min, keeping the temperature for 2 hours, then cooling to 900°C and keeping the temperature for 10 hours. The atmosphere used is compressed air (flow rate 30L / min);

[0102] 2. Cooling process:

[0103] (1) Pass a mixture of Cl2, SO2 and Ar (volume ratio 10%:10%:80%, flow rate 10 L / min, corresponding to 1 kg of material);

[0104] (2) Maintain the pressure difference in the furnace at 10Pa by adjusting the gas outlet control valve;

[0105] (3) Programmed cooling: When the temperature drops from 900°C to 600°C, the cooling rate is 5°C / min, and the temperature is kept at 600°C for 4 hours. The temperature drops from 600°C to 50°C at a cooling rate of 2°C / min. Finally, a sodium-based layered cathode material co-coated with NaClO4 and Na2SO3 is obtained, namely X-NaNi 0.33 Fe 0.33 Mn 0.33 O2.

[0106] The field emission scanning electron microscope image of the obtained positive electrode material is as follows Figure 3 shown.

[0107] Example 8

[0108] 1. High temperature calcination process:

[0109] (1) Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 and sodium carbonate are mixed uniformly in a certain molar ratio and placed in a sintering furnace, wherein the molar ratio of Na / TM(OH)2 is 1.03:1;

[0110] (2) The temperature was raised from room temperature (25°C) to 600°C at a rate of 2°C / min, and kept at 600°C for 6 h, and then raised to 1000°C and kept for 10 h. The atmosphere used was compressed air (flow rate 50 L / min, the water content of the compressed air used in the experiment of the present invention was about 15000 ppm);

[0111] 2. Cooling process:

[0112] (1) A mixture of NO2 and O2 (volume ratio 10%:90%, flow rate 50 L / min, corresponding to 3 kg of material) is introduced;

[0113] (2) Maintain the pressure difference in the furnace at 10Pa by adjusting the gas outlet control valve;

[0114] (3) Programmed cooling: When the temperature drops from 1000°C to 600°C, the cooling rate is 5°C / min, and the temperature is kept at 600°C for 12 h. Then, the temperature is dropped from 600°C to room temperature (25°C) at a cooling rate of 2°C / min. Finally, a sodium-coated layered cathode material, namely N-NaNi, is obtained. 0.33 Fe 0.33 Mn 0.33 O2.

[0115] Comparative Example 1

[0116] 1. High temperature calcination process:

[0117] (1) Ni 0.33 Fe 0.33Mn 0.33 (OH)2 and sodium carbonate are uniformly mixed in a certain molar ratio and placed in a sintering furnace, wherein the molar ratio of Na / TM(OH)2 is 1.03:1.

[0118] (2) The temperature was raised from room temperature (25°C) to 600°C at a rate of 2°C / min, and kept at 600°C for 6 hours, then raised to 1000°C and kept at that temperature for 10 hours. The atmosphere used was compressed air (flow rate 50L / min);

[0119] 2. Cooling process:

[0120] There is no pressure difference between the inside and outside of the furnace during the cooling process. The atmosphere is compressed air, which is consistent with the heating process (flow rate 50L / min, corresponding to 3 kg of material). When the temperature drops from 1000℃ to 600℃, the cooling rate is 5℃ / min, and it is kept at 600℃ for 4h. When it drops from 600℃ to room temperature (25℃), the cooling rate is 2℃ / min.

[0121] Finally, a sodium electrode layered positive electrode material is obtained.

[0122] Comparative Example 2

[0123] 1. High temperature calcination process:

[0124] (1) Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 and sodium carbonate are uniformly mixed in a certain molar ratio and placed in a sintering furnace, wherein the molar ratio of Na / TM(OH)2 is 1.03:1.

[0125] (2) Heating from room temperature (25°C) to 600°C at a rate of 2°C / min, keeping at 600°C for 6 hours, then heating to 1000°C and keeping at that temperature for 10 hours. The atmosphere used was compressed air (flow rate 50L / min);

[0126] 2. Cooling process:

[0127] (1) Pass pure O2 (flow rate 50 L / min, corresponding to 3 kg of material);

[0128] (2) Maintain the pressure difference in the furnace at 10Pa by adjusting the gas outlet control valve;

[0129] (3) Programmed cooling: When the temperature drops from 1000°C to 600°C, the cooling rate is 5°C / min, and the temperature is kept at 600°C for 4 hours. The temperature is then dropped from 600°C to room temperature (25°C) at a cooling rate of 2°C / min, and finally a sodium-based layered positive electrode material is obtained.

[0130] Comparative Example 3

[0131] 1. High temperature calcination process:

[0132] (1) Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 and sodium carbonate are uniformly mixed in a certain molar ratio and placed in a sintering furnace, wherein the molar ratio of Na / TM(OH)2 is 1.03:1.

[0133] (2) Heating from room temperature (25°C) to 600°C at a rate of 2°C / min, keeping at 600°C for 6 hours, then heating to 1000°C and keeping at that temperature for 10 hours. The atmosphere used is compressed air (flow rate 50L / min);

[0134] 2. Cooling process:

[0135] (1) Introduce NO2 (flow rate 50 L / min, corresponding to 3 kg of material);

[0136] (2) Maintain the pressure difference in the furnace at 10Pa by adjusting the gas outlet control valve;

[0137] (3) Programmed cooling: When the temperature drops from 1000°C to 600°C, the cooling rate is 5°C / min, and the temperature is kept at 600°C for 4 hours. The temperature is then dropped from 600°C to room temperature (25°C) at a cooling rate of 2°C / min, and finally a sodium-based layered positive electrode material is obtained.

[0138] Comparative Example 4

[0139] 1. High temperature calcination process:

[0140] (1) Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 and sodium carbonate are uniformly mixed in a certain molar ratio and placed in a sintering furnace, wherein the molar ratio of Na / TM(OH)2 is 1.03:1.

[0141] (2) Heating from room temperature (25°C) to 600°C at a rate of 2°C / min, keeping at 600°C for 6 hours, then heating to 1000°C and keeping at that temperature for 10 hours. The atmosphere used is compressed air (flow rate 50L / min);

[0142] 2. Cooling process:

[0143] (1) A mixture of NO2 and O2 (volume ratio 10%:90%, flow rate 50 L / min, corresponding to 3 kg of material) is introduced;

[0144] (2) Maintain the pressure difference in the furnace at 10Pa by adjusting the gas outlet control valve;

[0145] (3) Programmed cooling: When the temperature drops from 1000°C to 600°C, the cooling rate is 5°C / min; when the temperature drops from 600°C to room temperature (25°C), the cooling rate is 2°C / min, and finally a sodium-based layered cathode material is obtained.

[0146] Comparative Example 5

[0147] 1. High temperature calcination process:

[0148] (1) Ni 0.33 Fe 0.33 Mn 0.33 (OH)2 and sodium carbonate are uniformly mixed in a certain molar ratio and placed in a sintering furnace, wherein the molar ratio of Na / TM(OH)2 is 1.03:1.

[0149] (2) Heating from room temperature (25°C) to 600°C at a rate of 2°C / min, keeping at 600°C for 6 hours, then heating to 1000°C and keeping at that temperature for 10 hours. The atmosphere used is compressed air (flow rate 50L / min);

[0150] 2. Cooling process:

[0151] (1) Introduce compressed air (flow rate 50 L / min, corresponding to 3 kg of material);

[0152] (2) Maintain the pressure difference in the furnace at 10Pa by adjusting the gas outlet control valve;

[0153] (3) Programmed cooling: When the temperature drops from 1000°C to 600°C, the cooling rate is 5°C / min, and the temperature is kept at 600°C for 4 hours. The temperature is then dropped from 600°C to room temperature (25°C) at a cooling rate of 2°C / min, and finally a sodium-based layered positive electrode material is obtained.

[0154] Detection Methods

[0155] 1. Residual alkali content detection: acid-base titration test

[0156] 1) The test method for residual alkali (sodium carbonate) content in the sample is:

[0157] Weigh the sample in a beaker, record the sample mass y1 g, add V1 ml of ethylene glycol with a pipette, seal the cup mouth with plastic wrap and stir for more than 10 minutes, filter with a funnel after stirring, take V2 ml of the filtrate and stir and mix with 2 to 6 times V2 ml of pure water to obtain a mixed solution. Add 2 to 4 drops of methyl red-bromocresol green indicator solution to the mixed solution. Then titrate with a standard hydrochloric acid solution with a concentration of c mol / L until the solution changes from green to slightly pink, and record the consumption of hydrochloric acid at this time V3. Calculate the residual alkali (sodium carbonate) content of the sample according to the following formula:

[0158]

[0159] 2) The test method for residual alkali (sodium hydroxide) content in the sample is:

[0160] Weigh the sample in a beaker, record the sample mass y2 grams, add V4 ml of ethanol with a pipette, seal the cup mouth with plastic wrap and stir for more than 10 minutes, filter with a funnel after stirring, take V2 ml of the filtrate and stir and mix with 2-6V2 ml of pure water to obtain a mixed solution. Add 2-4 drops of methyl red-bromocresol green to the mixed solution.

[0161] 3) Calculate the total residual alkali content of the sample according to the test results of steps 1) and 2) of the above method. The formula is as follows:

[0162]

[0163] The increase of residual alkali in the room after 7 days, that is, (total alkali content - initial total alkali content) / initial total alkali content after 7 days (indoor environment 50-60% humidity, temperature 25±2℃)

[0164] 2. Electrochemical performance test:

[0165] The button cell (assembled with reference to the 2032 type button cell) was charged and discharged at 25±2℃, with a voltage range of 2.0~4.1V. The first discharge capacity at 0.1C rate and the 100-cycle performance at 1C were tested respectively.

[0166] The button battery is composed of: a positive electrode shell, a spring sheet, a gasket, a positive electrode sheet, a diaphragm, a negative electrode sheet, and a negative electrode shell. The positive electrode sheet is prepared from the prepared positive electrode material, the diaphragm is a glass fiber diaphragm, and the negative electrode is a metal sodium sheet.

[0167] The positive electrode material pole piece is prepared as follows: the positive electrode slurry is prepared by mixing the positive electrode powder, PVDF, and conductive carbon black in N-methylpyrrolidone in a mass ratio of (92:4:4), coating, drying, and cutting to obtain a disc with a diameter of 15 mm.

[0168] The installation sequence of button batteries is to place the positive electrode shell flat on an insulating table, clamp the positive electrode sheet and place it in the center of the positive electrode shell, add an appropriate amount of electrolyte, and then add the diaphragm, negative electrode sheet, gasket, spring sheet, and negative electrode shell in sequence. Then place it in a sealing machine to seal it to obtain a button battery. The whole process is carried out in an argon glove box.

[0169] The test results are shown in Table 1:

[0170] Table 1

[0171]

[0172] like Figure 1 , as shown in Figure 3, by comparing the scanning electron microscope images of Examples 1-8 and Comparative Example 1, it can be seen that in Comparative Example 1, air is passed through the entire cooling process, and the surface of the obtained material is relatively rough, containing more impurities such as Na2CO3 and NaOH, resulting in increased residual alkali, poor electrochemical performance and cycle stability. After the mixed gas is passed through Examples 1-8, the surface is smooth and has a uniform coating layer.

[0173] The coating layer is composed of Figure 2 The element analysis of the material surface shows that after the mixed gas is passed through and cooled in Examples 1-3, the material surface is evenly covered with elements such as Na, O, N, Cl, and S, indicating that this cooling process can evenly form coating layers such as NaNO3, NaClO4, and Na2SO3 on the material surface.

[0174] It can be seen from Table 1 that the initial residual alkali of Examples 1-8 is relatively low, which is beneficial to the cooling of the mixed gas, which can reduce the water content in the air in the furnace and form a coating layer to protect the surface of the positive electrode material. The increase in total alkali after 7 days is also small. However, the total alkali content of Comparative Example 1, Comparative Example 2, and Comparative Example 5 increases significantly after 7 days under the same conditions, and the air stability is poor.

[0175] In addition, the electrochemical performance test shows that the cycle retention of Examples 1-8 after 100 cycles at 1C rate is greater than 90%, while the cycle retention rates of Comparative Examples 1, 2 and 5 under the same conditions are relatively low, only 58.4%, 61.2% and 63.1%, which are obviously worse than the cycle retention rates of the examples. It can be concluded that cooling by mixed gas can effectively reduce impurities and residual alkali on the surface of the material, which not only improves air stability, but also improves long-cycle performance.

[0176] In contrast to Example 1, in the cooling stage, the heat preservation time was increased from 4 hours to 12 hours, and the residual alkali reduction effect was similar in Example 8. In addition, the embodiment 8 had good air stability, and the 1C 100-cycle cycle retention rate was 91.5%, which was also close to that in Example 1. Figure 1 As shown in Figure 2, with the increase of holding time, the coating thickness increases and the surface becomes rough, resulting in a decrease in the first capacity at 0.1C.

[0177] In comparative example 2, when only carrier gas O2 is present, the cooling process can also slowly reduce the water content in the air in the furnace, resulting in a lower initial residual alkali. However, the air stability is poor. When exposed to humid air, Na in the bulk phase is easily released, resulting in an increase in residual alkali.

[0178] In Comparative Example 1, air is passed throughout the entire process, the water content in the furnace is maintained at a high level, the initial residual alkali is high, and the surface contains more Na2CO3 impurities. When there is more Na2CO3, a thicker film will also be formed on the surface of the material, which in disguise protects the material from erosion by water in the air, but the electrochemical performance and cycle stability are poor.

[0179] In Comparative Example 3, when there is only acidic gas NO2, although the residual alkali reduction effect is obvious and a coating layer can be formed, excessive acidic gas not only reacts with residual alkali impurities on the surface, but also reacts with Na in the bulk phase to destroy the crystal structure of the material. At the same time, the formed coating layer is not uniform, resulting in a decrease in the 0.1C first capacity and a decrease in the 100-cycle retention rate of 1C.

[0180] In comparative example 4, the mixed gas is introduced during the cooling process without a heat preservation step, and the acidic gas does not react fully with the residual alkali and impurities on the surface. The residual alkali can be reduced, but the coating layer is uneven, and the 1C 100-cycle cycle retention rate is only 80.5%.

[0181] Comparative Example 5 Compared with Comparative Example 1, the total amount of initial residual alkali can be significantly reduced by adjusting the gas outlet control valve to keep the pressure difference in the furnace at 10Pa. In the presence of pressure difference, the gas circulates quickly, and the impurities on the surface and the impurities inside the deposited material can be taken away with the gas flow. However, no coating layer is formed, resulting in poor air stability and a low 1C 100-cycle cycle retention rate.

[0182] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for preparing a sodium layered oxide, characterized in that: The preparation method comprises the following steps: The sodium electric layered oxide obtained by calcination is cooled in stages to obtain the sodium electric layered oxide with a coating layer; The sodium layered oxide obtained by calcination is obtained by calcining a solid phase mixture including a metal hydroxide precursor and Na2CO3; The segmented cooling includes first cooling, heat preservation, and second cooling in sequence; The insulation temperature is 600-700°C, and the insulation time is 4-12 h; the second cooling rate is 2-5°C / min; The segmented cooling is carried out under the condition of passing a mixed gas, wherein the mixed gas includes an acid gas and a carrier gas; the acid gas includes one or more of NO2, Cl2, and SO2; and the volume proportion of the acid gas in the mixed gas is 10-50%; During the stepwise cooling process, the internal and external pressure difference is above 10Pa.

2. The preparation method according to claim 1, characterized in that: The carrier gas includes one or more of O2, N2, and Ar; And / or, in the mixed gas, the volume proportion of the carrier gas is 50-90%; And / or, the ratio of the flow rate of the mixed gas to the amount of the layered oxide obtained by calcination is 10-20 L / min: 1 kg.

3. The preparation method according to claim 1, characterized in that: The first cooling rate is 5-10 °C / min; And / or, the second cooling is to cool to 25-50°C.

4. The preparation method according to claim 1, characterized in that: During the stepwise cooling process, the internal and external pressure difference is 10-50 Pa.

5. The preparation method according to claim 1, characterized in that: The obtained sodium layered oxide with a coating layer includes Na x TMO2, 0.7≤x≤1; wherein the TM includes transition metal elements, and the transition metal elements include one or more of Ni, Fe, and Mn.

6. The preparation method according to claim 1, characterized in that: The obtained sodium layered oxide with a coating layer includes Na x TMO2, 0.7≤x≤1; wherein the TM includes transition metal elements and doping metal elements; the transition metal elements include one or more of Ni, Fe, and Mn, and the doping metal elements include one or more of Cu, Ti, Ca, Nb, Zn, and Mg.

7. A sodium layered oxide obtained by the preparation method according to any one of claims 1 to 6.

8. Use of the sodium layered oxide as claimed in claim 7 in the preparation of sodium ion batteries.

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