A sodium-ion battery layered-oxide cathode material with low residual alkali content and a preparation method thereof

Abstract

The application relates to the technical field of battery materials, and discloses a low-residual-alkali-content sodium-ion battery layered oxide positive electrode material and a preparation method thereof. a Ni x Fe y Mn z M 1‑x‑y‑z O 2‑δ F δ ; wherein, 0.8<=a<=1.1, 0

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of battery materials, in particular to a sodium-ion battery layered oxide positive electrode material with low residual alkali content and a preparation method thereof. BACKGROUND

[0002] With the rapid development of new energy vehicles, new energy batteries are also in continuous research and innovation. From the classification, new energy batteries include lithium-ion batteries, sodium-ion batteries, lead-acid batteries, hydrogen fuel cells and the like. Among them, lithium-ion batteries gradually become the main part of modern new energy batteries due to their great advantages in battery capacity, service life, safety performance, charging and discharging speed, green environmental protection and the like, but other new energy batteries still have certain market prospects.

[0003] Sodium-ion batteries mainly rely on the movement of sodium ions between the positive electrode and the negative electrode to work, which is similar to the working principle of lithium-ion batteries, but sodium-ion batteries have the advantages of excellent low-temperature performance, high safety, fast charging and low cost, therefore, the development of sodium-ion batteries is a beneficial supplement and replacement for lithium-ion batteries and ternary lithium-ion batteries. Due to the inherent advantages of sodium-ion batteries, they can be widely applied to electric tools, large energy storage, passenger cars and the like, and in the future, with the continuous reduction of the cost of sodium-ion batteries, it is an inevitable trend to replace lead-acid batteries used in two-wheeled vehicles and low-speed vehicles.

[0004] At present, due to the poor processing performance, low discharge capacity, high gas production and short cycle life of the positive electrode material of sodium-ion batteries, the market application of sodium-ion batteries is slow, one of the reasons is that the residual alkali and pH of the layered oxide positive electrode material used in sodium-ion batteries are too high. The main components of residual alkali are NaOH, Na2CO3 and NaHCO3, collectively referred to as free sodium (Na + ). The main reasons for the high free sodium are as follows: (1) the radius of sodium ions is larger than that of lithium ions, which leads to a slower diffusion speed of sodium ions in the crystal lattice than that of lithium ions, so the residual sodium content on the surface of the sodium-ion layered oxide positive electrode material is relatively high, generally more than Na + > 5%.(2) Na + of the sodium-ion positive electrode material is prone to ion exchange with H + of the H2O molecules in the air, which leads to the continuous release of Na in the crystal lattice of the sodium-ion positive electrode material, resulting in an increase in residual alkali. Therefore, it is a key technical problem to prepare a sodium-ion battery layered oxide positive electrode material with low residual alkali content. SUMMARY

[0005] The technical problem solved by the application is:

[0006] To solve the problem that the electrochemical performance of existing sodium-ion cathode materials is affected due to the presence of residual alkali.

[0007] The technical solution adopted by the present invention:

[0008] In view of the above technical problems, the purpose of the present invention is to provide a sodium-ion battery layered oxide cathode material with a low residual alkali content and a preparation method thereof.

[0009] In this solution, during the preparation process of the sodium-ion battery layered oxide cathode material, on the one hand, the high-entropy doping technology can form a superlattice structure, which can inhibit Na in the sodium metal layer + from reacting with H in the water molecules in the air + to undergo Na + / H + exchange. Therefore, the residual alkali on the surface of the sodium-ion material is reduced and the stability of the sodium-ion material is improved; at the same time, the high-entropy doping elements can expand the sodium metal layer spacing and reduce the transition metal layer spacing, thereby increasing the diffusion coefficient of Na + , and thus the residual alkali on the surface of the sodium-ion battery layered oxide cathode material can be reduced. On the other hand, during the secondary sintering process, a nano-coating substance that can neutralize excessive free sodium is added to form a conductive coating layer with ions and electrons. This coating layer reduces the surface residual alkali on the one hand, and enhances the interface stability and cycling performance of the sodium-ion battery layered oxide cathode material and reduces the generation of gas on the other hand. Through the bulk doping and surface nano-coating processes, not only the surface residual alkali of the sodium-ion battery layered oxide cathode material is reduced, but also the processing performance, capacity, cycling, and gas generation of the sodium-ion battery can be improved.

[0010] Based on this, the specific implementation scheme is as follows:

[0011] The present invention provides a sodium-ion battery layered oxide cathode material with a low residual alkali content, and its chemical formula is Na a Ni x Fe y Mn z M 1-x-y-z O 2-δ F δ ;

[0012] where 0.8 ≤ a ≤ 1.1, 0 < x ≤ 1, 0 ≤ y ≤ 1, 0 < z ≤ 1, 0 < δ ≤ 0.1, M = M1 + M2, M1 = at least one of Li, Zn, Ca, Cu, Sb, M2 = at least one of Ba, Mg, Zr, Y, Cs, Mo, Sn, Ce, La, F, and M1 + M2 includes at least four or more elements.

[0013] Furthermore, the particle size D of the cathode material 50=3.5~15μm.

[0014] This invention provides a method for preparing the aforementioned low residual alkali content layered oxide cathode material for sodium-ion batteries, comprising the following steps:

[0015] S1 will Ni x Fe y Mn z M 1-x-y-z (OH)2, sodium salt, and dopants are blended together; a high-speed mixer can be used for this process.

[0016] S2 involves sintering the blended mixture once, then treating it with a coating agent and sintering it a second time to obtain the finished product; the coating process is carried out in a coating machine.

[0017] In this invention, Ni x Fe y Mn z M 1-x-y-z The molar ratio of (OH)2 to sodium salt is 1:0.8 to 1.1. Preferably, it can also be 1:0.8, 1:0.9, 1:1.0, or 1:1.1.

[0018] In this invention, the sodium salt is selected from at least one of sodium carbonate, sodium bicarbonate, and sodium hydroxide.

[0019] In this invention, a single sintering process is performed at a temperature of 800–980°C for a time of 6–16 hours. Preferably, the temperatures can be 800°C, 850°C, 880°C, 900°C, 950°C, 980°C, or any of the aforementioned temperature ranges; similarly, the times can be 6 hours, 8 hours, 10 hours, 12 hours, 14 hours, 16 hours, or any of the aforementioned time ranges.

[0020] In this invention, the secondary sintering is performed at a temperature of 400–800°C for a time of 5–10 hours. Preferably, the temperature can be 400°C, 500°C, 650°C, 700°C, 750°C, 800°C, or any of the aforementioned temperature ranges; similarly, the time can be 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, or any of the aforementioned time ranges.

[0021] In this invention, during primary sintering and / or secondary sintering, the sintering atmosphere is air, oxygen, or a mixture of gases with an oxygen concentration of 10% or higher.

[0022] In this invention, the dopant includes at least one of features (1-1) to (1-4):

[0023] (1-1) The dopant includes compound M1 and compound M2, wherein M1 is selected from at least one of Li, Zn, Ca, Cu, and Sb; and M2 is selected from at least one of Ba, Mg, Zr, Y, Cs, Mo, Sn, Ce, La, and F.

[0024] (1-2) The dopant includes at least one of barium oxide, barium hydroxide, magnesium oxide, magnesium hydroxide, calcium oxide, calcium hydroxide, zinc oxide, zinc hydroxide, zirconium oxide, zirconium hydroxide, tellurium oxide, yttrium oxide, copper oxide, lithium hydroxide, lithium carbonate, cesium oxide, molybdenum oxide, ammonium molybdate, tin oxide, cerium oxide, lanthanum oxide, ammonium fluoride, and sodium fluoride;

[0025] (1-3) The total elemental mass ratio of M1+M2 in the dopant is 0.02-8%;

[0026] (1-4) Particle size D of the dopant 50 <1μm.

[0027] In this invention, the coating agent comprises at least one of features (2-1) to (2-4):

[0028] (2-1) The coating agent includes compound M3, wherein M3 is selected from at least one of B, Al, Zr, Ti, Ce, W, and P;

[0029] (2-2) The coating agent includes at least one of boron oxide, boric acid, aluminum oxide, aluminum hydroxide, zirconium oxide, zirconium hydroxide, titanium oxide, cerium oxide, ammonium tungstate, monoammonium phosphate, and diammonium phosphate;

[0030] (2-3) The total elemental mass ratio of M3 in the coating agent is 0.05-1%;

[0031] (2-4) Particle size D of the coating agent 50 <50nm.

[0032] The technical mechanism and beneficial effects of this invention are as follows:

[0033] On the one hand, residual alkali is typically an unstable compound with non-electrochemical activity and high impedance, which can degrade capacity, rate capability, and cycle performance. In the preparation of layered oxide cathode materials for sodium-ion batteries, sodium compounds are added to the precursor as a sodium source. Due to the large radius of sodium ions, they have difficulty diffusing into the crystal lattice of the layered oxide cathode material, resulting in the presence of residual alkali substances such as NaOH, Na₂CO₃, and NaHCO₃ on the cathode material surface. This residual alkali causes a high pH, ​​leading to the breakage of carbon-fluorine bonds in PVDF dissolved in NMP solvent, resulting in gelation of the slurry and making it unprocessable. On the other hand, residual alkali increases the surface impedance of the cathode material, causing a decrease in discharge capacity and cycle performance. Furthermore, under certain conditions, residual alkali can undergo side reactions with the electrolyte, leading to battery gas generation and safety hazards.

[0034] Based on the aforementioned phenomena, this invention incorporates multiple combined dopants to improve the sodium ion diffusion rate during the preparation of layered oxide cathode materials for sodium-ion batteries. This reduces residual alkali content at the source, and the dopants stabilize the crystal structure of the cathode material and suppress phase transitions. Simultaneously, by coating with multiple combinations of highly active nano-alkali-reducing coating agents, a uniform reaction with surface residual alkali substances can be achieved, forming a low-alkali-content nano-coating layer, thus improving the stability of the electrochemical interface. By using doping and coating with multiple combined alkali-reducing substances, the pH of the layered oxide cathode material for sodium-ion batteries can be reduced to below 12, and free sodium can be controlled below 5000 ppm, improving the slurry processing performance. Furthermore, the capacity, cycle life, and gas production of sodium-ion batteries are significantly improved, thereby facilitating the promotion and use of sodium-ion batteries.

[0035] High-entropy doping can form structurally stable solid solutions, thus reducing the residual alkali content of sodium-ion battery cathode materials and improving their thermal stability and electrochemical performance. For example, Li, Zn, and Ca atoms can suppress the mixing of sodium atoms with transition metal atoms and prevent sodium ions from migrating to the surface of material particles, thereby reducing the Na+ ion and H+ ion content in sodium-ion battery cathode materials. + Ion exchange reduces the residual alkali content on the surface of sodium-ion cathode materials. F ions have high electronegativity and can form stable compounds with sodium ions, thus preventing Na+ from reacting with sodium-ion cathodes. + Ions and H in water + Ion exchange reduces residual alkali content. Cu and Sb atoms can form a superlattice structure, preventing water molecules from penetrating the sodium electrode material, thus reducing the material's alkalinity and improving its processing and high-temperature storage performance. Therefore, the combination of five or more elements, including Li, Zn, Ca, Cu, and Sb, has the most significant synergistic effect in reducing residual alkali.

[0036] Furthermore, for example, Ba, Mg, and Ca atoms can form high bond energies with oxygen atoms, thus suppressing oxygen release and reducing interfacial side reactions at a high voltage of 4.3V, thereby reducing gas generation in sodium-ion battery cathode materials and improving their thermal stability. Zn atoms can expand the size of the unit cell along the C-axis, thus increasing the diffusion rate of sodium ions and improving the capacity and cycle performance of sodium-ion battery cathode materials. High-valence Mo and Sb atoms can increase the content of divalent nickel in sodium-ion battery cathode materials, thereby improving the discharge capacity of sodium-ion battery cathode materials. In particular, Mo atoms can improve the particle strength of sodium-ion battery cathode materials, thus improving the high-temperature cycle performance of sodium-ion battery cathode materials at high voltages. Zr and Y atoms can suppress the migration of transition metal ions and the shrinkage of unit cell volume during charge and discharge, while Sb, Cs, and Ce atoms can prevent the expansion of unit cell volume. Therefore, the synergistic effect of Zr, Y, Sb, Cs, and Ce atoms can suppress the O3→P3→P3→O3 phase transformation and suppress particle breakage, thereby improving the cycle performance of sodium-ion battery cathode materials. In particular, F and Sb ions in high-entropy doping can induce local disorder within the cell structure, leading to a redistribution of energy at electron overlap sites and the formation of new charged ions, thus improving the conductivity of sodium-electric cathodes. Cu, Sn, and La atoms can accelerate the crystal growth of sodium-electric cathodes and improve their crystal structure and structural stability. Attached Figure Description

[0037] Figure 1 The graphs show the capacity curves of the layered oxide cathode materials for sodium-ion batteries in Example 1 and Comparative Example 1.

[0038] Figure 2 Cycle curves of the sodium-ion battery layered oxide cathode materials in Example 1 and Comparative Example 1 are shown.

[0039] Figures 3-5 SEM images of the layered oxide cathode material for sodium-ion batteries with low residual alkali content in Example 1 (scale bars are 10 μm, 5 μm, and 2 μm respectively);

[0040] Figures 6-8 SEM images of the layered oxide cathode material for sodium-ion batteries with high residual alkali content in Comparative Example 1 (scale bars are 10 μm, 5 μm, and 2 μm, respectively). Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0042] Example 1

[0043] This embodiment provides a method for preparing a layered oxide cathode material for sodium-ion batteries with low residual alkali content, comprising the following steps:

[0044] (1) According to the molecular formula Na 0.8-1.1 Ni 0.25 Fe 0.35 Mn 0.40 O2, to the 3.5μm precursor Ni 0.25 Fe 0.35 Mn 0.40 (OH)₂ and Na₂CO₃ are weighed according to a 1:1 molar ratio of total metal atoms to sodium atoms. Simultaneously, five dopants for reducing residual alkali—BaO, MgO, CaO, ZnO, and NH₄F—are added in a 1% (Ba+Mg+Ca+Zn+F) mass ratio. The particle size D of the five dopants is... 50 <1μm, then mix evenly in a high-speed mixer.

[0045] (2) The above mixture was sintered at 880℃ in air for 16 hours, then pulverized and sieved. It was then coated with 1% (Zr+Ti+Ce) of ZrO2, TiO2, and CeO2 (three residual alkali reducing agents) in a coating machine. The size of all three coating agents was less than 50 nm. The mixture was then sintered again at 800℃ for 5 hours. After pulverization and sieving, a 3.5 μm layered oxide cathode material for sodium-ion batteries was finally obtained.

[0046] Example 2

[0047] This embodiment provides a method for preparing a layered oxide cathode material for sodium-ion batteries with low residual alkali content, comprising the following steps:

[0048] (1) According to the molecular formula Na 0.8-1.1 Ni 0.25 Fe 0.35 Mn 0.40 O2, to convert 15μm precursor Ni 0.25 Fe 0.35 Mn 0.40 (OH)₂ and NaOH are weighed according to a total metal atom to sodium atom molar ratio of 1:1.1. Simultaneously, five dopants for reducing residual alkali—ZnO, CaO, ZrO₂, CuO, and NH₄F—are added in a mass ratio of 2% (Zn+Ca+Zr+Cu+F). The particle size D of the five dopants is... 50 <1μm, then mix evenly in a high-speed mixer.

[0049] (2) The above mixture was sintered at 980℃ in an oxygen atmosphere for 6 hours. After crushing and sieving, it was coated with four residual alkali reducing agents, namely Al2O3, ZrO2, TiO2 and CeO2, with a mass ratio of 0.6% (Al+Zr+Ti+Ce). The size of the four coating agents was less than 50nm. Then, it was sintered again at 650℃ for 10 hours. After crushing and sieving, the final 15μm layered oxide cathode material for sodium-ion batteries was obtained.

[0050] Example 3

[0051] This embodiment provides a method for preparing a layered oxide cathode material for sodium-ion batteries with low residual alkali content, including the following steps:

[0052] (1) According to the molecular formula Na 0.8-1.1 Ni 0.33 Fe 0.33 Mn 0.34 O2, 7μm precursor Ni 0.33 Fe 0.33 Mn 0.34 (OH)₂ and Na₂CO₃ are weighed according to a 1:1 molar ratio of total metal atoms to sodium atoms. Simultaneously, four dopants for reducing residual alkali—Sb₂O₃, Y₂O₃, CuO, and LiOH·H₂O—are added in an 8% (Sb+Y+Cu+Li) mass ratio. The particle size D of the four dopants is... 50 <1μm, then mix evenly in a high-speed mixer.

[0053] (2) The above mixture was sintered at 950°C in a mixed atmosphere with an oxygen content of 10% for 14 hours, then crushed and sieved, and finally coated with H at a mass ratio of 1% (W+P) in a coating machine. 28 N6O 41 W 12 Two coating agents, NH4H2PO4 and NH4H2PO4, were used to reduce residual alkali, and both coating agents had a size of less than 50 nm. The mixture was then sintered at 700℃ for 5 hours, and after pulverization and sieving, a 7 μm layered oxide cathode material for sodium-ion batteries was finally obtained.

[0054] Example 4

[0055] This embodiment provides a method for preparing a layered oxide cathode material for sodium-ion batteries with low residual alkali content, including the following steps:

[0056] (1) According to the molecular formula Na 0.8-1.1 Ni 0.40 Fe 0.20 Mn 0.40 O2, 4μm precursor Ni 0.40 Fe 0.20 Mn 0.40(OH)2 and Na2CO3 are weighed according to a 1:1 molar ratio of total metal atoms to sodium atoms. At the same time, four dopants for reducing residual alkali, namely Cs2O, (NH4)2MoO4, SnO2 and CeO2, are added in a mass ratio of 4% (Cs+Mo+Sn+Ce). The particle size D50 of the four dopants is <1μm. Then, they are mixed evenly in a high-speed mixer.

[0057] (2) The above mixture was sintered at 800℃ in air for 16 hours, then crushed and sieved. It was then coated with a ZrO2 coating agent with a Zr mass ratio of 0.6% to reduce residual alkali in a coating machine. The ZrO2 size was less than 50nm. Then it was sintered again at 750℃ for 5 hours. After crushing and sieving, the final 4μm sodium-ion battery layered oxide cathode material was obtained.

[0058] Example 5

[0059] This embodiment provides a method for preparing a layered oxide cathode material for sodium-ion batteries with low residual alkali content, comprising the following steps:

[0060] (1) According to the molecular formula Na 0.8-1.1 Ni 0.25 Fe 0.35 Mn 0.40 O2, to convert the 10μm precursor Ni 0.25 Fe 0.35 Mn 0.40 (OH)2 and NaHCO3 are weighed at a total metal atom to sodium atom ratio of 1:0.8. At the same time, 8% (Ba+Mg+Ca+Zn+La+F) of six dopants, namely BaO, MgO, CaO, ZnO, La2O3 and NaF, are added. The particle size D50 of the six dopants is less than 1 μm. Then, they are mixed evenly in a high-speed mixer.

[0061] (2) The above mixture was sintered at 850℃ in a mixed atmosphere with 50% oxygen content for 15 hours. After crushing and sieving, it was coated with three residual alkali reducing agents, Zr(OH)4, TiO2 and CeO2, at a mass ratio of 1% (Zr+Ti+Ce), in a coating machine. The size of the three coating agents was less than 50 nm. Then, it was sintered again at 500℃ for 10 hours. After crushing and sieving, the final 10 μm layered oxide cathode material for sodium-ion batteries was obtained.

[0062] Example 6

[0063] This embodiment provides a method for preparing a layered oxide cathode material for sodium-ion batteries with low residual alkali content, including the following steps:

[0064] (1) According to the molecular formula Na 0.8-1.1 Ni 0.25 Fe 0.35Mn 0.40 O2, 5μm precursor Ni 0.25 Fe 0.35 Mn 0.40 (OH)2 and NaHCO3 are weighed at a total metal atom to sodium atom ratio of 1:0.9. At the same time, five dopants for reducing residual alkali, namely Ba(OH)2, Mg(OH)2, Ca(OH)2, Zn(OH)2 and Li2CO3, are added in a mass ratio of 8% (Ba+Mg+Ca+Zn+Li). The particle size D50 of the five dopants is <1μm. Then, they are mixed evenly in a high-speed mixer.

[0065] (2) The above mixture was sintered at 900°C in air for 8 hours, then crushed and sieved. It was then coated with a 1% B2O3 coating agent (B mass ratio) to reduce residual alkali, with the B2O3 size less than 50 nm. After a second sintering at 400°C for 5 hours, and after crushing and sieving, a 5 μm layered oxide cathode material for sodium-ion batteries was finally obtained.

[0066] Comparative Example 1

[0067] The difference between this comparative example and Example 1 is that no residual alkali reducing dopant and residual alkali reducing coating agent were added.

[0068] Comparative Example 2:

[0069] The difference between this comparative example and Example 1 is that no residual alkali dopant was added.

[0070] Test case

[0071] In embodiments of the present invention, the performance of the sodium ion layered oxide cathode material product obtained above is tested using the following method.

[0072] 1. Free sodium or residual alkali test

[0073] The free sodium or residual alkali test was performed using an automatic potentiometric titrator, model: METTLER TOLEDO G20. 5g of sample solvent was weighed into 40mL of aqueous solution, sonicated for 30min, filtered, and diluted to a 100mL volumetric flask. After shaking and standing, the supernatant was used for testing. The results are shown in Table 1.

[0074] 2. Discharge capacity and cycle performance

[0075] The discharge capacity and cycle performance were tested using the following method: Sodium-ion battery positive electrode material was mixed with conductive carbon black and PVDF binder at a mass ratio of 80:10:10. NMP was added to form a uniform slurry, which was then coated onto aluminum foil. After drying and rolling, the slurry was cut into positive electrode sheets with a diameter of 14 mm. A sodium-ion battery was assembled using a CR2032 button cell. The separator was glass fiber, the electrolyte was a 1 mol / L NaPF6 solution with EC / PC / DEC solvent, and the negative electrode was a sodium sheet. The results are shown in Table 1 and... Figures 1-2 As shown (the materials obtained in Example 1 and Comparative Example 1 were used as samples for measurement).

[0076] 3. Sodium-ion battery test

[0077] Sodium-ion battery test conditions: temperature 25℃±1℃, charge / discharge cycle voltage range 2.0V-4.0V, current 0.1C (150mAh / g), cycle test conducted at 0.5C charge 1C discharge. Results are shown in Table 1.

[0078] 4. Appearance and morphology

[0079] The morphology of the samples obtained in Example 1 and Comparative Example 1 was observed using a Hitachi SU5000 scanning electron microscope. The results are as follows: Figures 3-8 As shown.

[0080] Table 1 Test Results

[0081]

[0082] As shown in Table 1, the sodium electrochemical positive materials prepared in Examples 1-6 have excellent discharge capacity, cycle life, free sodium content, and pH value. For example, the retention rate after 50 cycles is above 93%, and the free sodium content and pH value are below 0.5% and 12, respectively.

[0083] according to Figure 1 and Figure 2 It can be seen that Na in Example 1 0.8-1.10 Ni 0.25 Fe 0.35 Mn 0.40The initial discharge capacity of O2 was 146 mAh / g, with a 50-cycle retention rate of 95.26%. In contrast, the initial capacity of Comparative Example 1 was only 137 mAh / g, with a 50-cycle retention rate of 88.72%. The higher discharge capacity indicates that the low-alkali-content sodium-ion battery cathode material has a high content of electrochemically active sodium in the cell and a low content of non-electrochemically active free sodium on the surface, resulting in a greater number of sodium ion insertions and extractions during electrochemical reactions. Simultaneously, the better cycle performance indicates that the low-alkali-content sodium-ion battery cathode material has lower interfacial electrochemical resistance, stronger stability, and fewer side reactions. Therefore, alkali-reducing dopants and coating agents can effectively reduce the content of non-electrochemically active free sodium in sodium-ion battery cathode materials, improve material interfacial stability, and enhance discharge capacity and cycle performance.

[0084] Figures 3-5 The sodium-ion battery cathode particles are well-dispersed and have smooth surfaces, with virtually no residual alkali on the particle surface. During the synthesis of the sodium-ion battery cathode, BaO, MgO, CaO, ZnO, and NH4F, which increase the interlayer spacing of sodium metals in the layered oxide cathode cells, are added, reducing the resistance to sodium ion diffusion into the cell interior. Therefore, under certain reaction temperatures and times, the diffusion rate of sodium ions is accelerated, thereby reducing the residual alkali on the matrix surface after the first sintering. On the other hand, during the second sintering process, nano-oxides such as ZrO2, TiO2, and CeO2, which can react with residual alkalis such as NaOH, Na2CO3, and NaHCO3, are added. At a certain temperature, these react to form an ionic and electronic conductor coating layer containing sodium ions. This coating layer not only reduces residual alkali but also effectively passivates and stabilizes the electrochemical interface of the sodium-ion battery cathode, thereby reducing the resistance of electrons and ions and side reactions during the electrochemical process. Therefore, it improves the capacity and cycle performance of the sodium-ion battery and reduces gas generation.

[0085] Figures 6-8 The SEM images show white sodium residue on the surface of the sodium-ion battery positive material particles. Because no dopant or coating agent to reduce residual alkali was added during the synthesis of the sodium-ion battery positive material, excessive sodium remains on the surface of the particles, resulting in high levels of free sodium (7.254%) and a high pH (12.37).

[0086] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a layered oxide cathode material for sodium-ion batteries with low residual alkali content, characterized in that, The method includes: S1 according to the molecular formula Na 0.8-1.1 Ni 0.25 Fe 0.35 Mn 0.40 O2, the precursor Ni 0.25 Fe 0.35 Mn 0.40 (OH)2, sodium salt, and dopant are co-treated; the dopant includes compound M1 and compound M2, where M1 is selected from at least one of Li, Zn, Ca, Cu, and Sb; and M2 is selected from at least one of Ba, Mg, Zr, Y, Cs, Mo, Sn, Ce, La, and F. S2 The blended mixture is sintered once, then treated with a nano-coating agent and sintered a second time to obtain the finished product; The dopants include BaO, MgO, CaO, and ZnO; the dopants also include NH4F or NaF; the coating agent includes an M3 compound, where M3 includes Zr, Ti, and Ce; the particle size D of the coating agent... 50 <50nm.

2. The method for preparing the low residual alkali content sodium-ion battery layered oxide cathode material according to claim 1, wherein the dopant further includes La2O3.

3. The method for preparing the low residual alkali content layered oxide cathode material for sodium-ion batteries according to claim 1, wherein the particle size D of the dopant is... 50 <1μm.

4. The method for preparing the low residual alkali content layered oxide cathode material for sodium-ion batteries according to claim 1, characterized in that, First sintering: temperature 800~980℃, time 6~16h.

5. The method for preparing the low residual alkali content layered oxide cathode material for sodium-ion batteries according to claim 4, characterized in that, Secondary sintering: temperature 400~800℃, time 5~10h.

6. The method for preparing the low residual alkali content layered oxide cathode material for sodium-ion batteries according to claim 5, characterized in that, In primary sintering and / or secondary sintering, the sintering atmosphere is air, oxygen, or a mixture of gases with an oxygen concentration of 10% or higher.

7. The method for preparing the low residual alkali content layered oxide cathode material for sodium-ion batteries according to claim 1, characterized in that, Cathode material particle size D 50 =3.5~15μm.

8. The method for preparing the low residual alkali content layered oxide cathode material for sodium-ion batteries according to claim 1, characterized in that, S1 includes: Na according to the molecular formula 0.8-1.1 Ni 0.25 Fe 0.35 Mn 0.40 O2, to the 3.5μm precursor Ni 0.25 Fe 0.35 Mn 0.40 (OH)₂ and Na₂CO₃ are weighed at a total metal atom to sodium atom molar ratio of 1:

1. Simultaneously, five dopants—BaO, MgO, CaO, ZnO, and NH₄F—are added at a mass ratio of 1% (Ba + Mg + Ca + Zn + F). The particle size D of these five dopants is... 50 <1μm, then mix thoroughly in a high-speed mixer; S2 includes: sintering the mixture obtained in step S1 at 880°C in air for 16 hours, pulverizing and sieving, and then coating it with a coating agent containing 1% (Zr+Ti+Ce) of three alkaline coating agents: ZrO2, TiO2, and CeO2, with the size of each of the three coating agents being less than 50 nm; then sintering again at 800°C for 5 hours, and finally obtaining a 3.5 μm sodium-ion battery layered oxide cathode material after pulverizing and sieving.

9. The method for preparing the low residual alkali content layered oxide cathode material for sodium-ion batteries according to claim 1, characterized in that, S1 includes: Na according to the molecular formula 0.8-1.1 Ni 0.25 Fe 0.35 Mn 0.40 O2, to convert the 10μm precursor Ni 0.25 Fe 0.35 Mn 0.40 (OH)2 and NaHCO3 are weighed at a total metal atom to sodium atom ratio of 1:0.8, and six dopants, namely BaO, MgO, CaO, ZnO, La2O3 and NaF, with a mass ratio of 8% (Ba+Mg+Ca+Zn+La+F), are added at the same time. The particle size D50 of the six dopants is less than 1 μm. The mixture is then mixed evenly in a high-speed mixer. S2 includes: sintering the above mixture at 850°C in a mixed atmosphere with 50% oxygen content for 15 hours, pulverizing and sieving, and then coating the mixture with 1% (Zr+Ti+Ce) of three alkaline coating agents: Zr(OH)4, TiO2, and CeO2, with the size of each coating agent being less than 50 nm. Then, sintering the mixture again at 500°C for 10 hours, and after pulverizing and sieving, finally obtaining a 10 μm sodium-ion battery layered oxide cathode material.

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