A positive electrode material with a gradient dual-phase structure, a preparation method and application thereof

Abstract

This invention belongs to the field of sodium-ion battery technology, specifically relating to a cathode material with a gradient dual-phase structure, its preparation method, and its application. The preparation method provided by this invention includes: 1) weighing sodium, nickel, manganese, iron, magnesium, and aluminum sources according to stoichiometric ratios, mixing them, and grinding them to obtain a blended powder; 2) subjecting the blended powder from step 1) to microwave treatment, and then calcining it in a mixed atmosphere with an oxygen volume content of 30%-50% to obtain a calcined material; 3) depositing a LiAlO2 coating layer on the surface of the calcined material obtained in step 2) to obtain the gradient dual-phase structure cathode material. The cathode material with a gradient dual-phase structure obtained by this invention has low synthesis energy consumption, uniform phase distribution, and can effectively suppress phase transformation cracks, alleviate volume expansion during the charging and discharging process of the cathode material, reduce the influence of air on the material, and simultaneously possess high cycle stability and high air stability.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, specifically relating to a cathode material with a gradient dual-phase structure, its preparation method, and its application. Background Technology

[0002] Sodium resources are widely distributed and inexpensive, and have electrochemical properties similar to lithium. Layered sodium-ion battery cathode materials have high energy density and compaction density, and have the potential to replace lithium-ion battery cathode materials. They are highly competitive in applications, and therefore sodium-ion battery cathode materials have attracted widespread market attention.

[0003] Unlike lithium-ion battery cathode materials, sodium-ion battery cathode materials exhibit multiple phase structures, including O3, P2, P3, and O2, with O3 and P2 being the most common. Existing sodium-ion battery cathode materials readily achieve a dual-phase coexistence of O3 and P2 phases. While the P2 / O3 dual-phase structure can balance capacity and cycle stability, its cycle stability still needs improvement. Furthermore, it suffers from uneven phase distribution, high synthesis energy consumption, and poor air stability. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is to overcome the shortcomings of existing sodium-ion battery cathode materials, such as the need to improve cycle stability, uneven phase distribution, high synthesis energy consumption, and poor air stability, so as to provide a cathode material with a gradient two-phase structure, its preparation method and application.

[0005] This invention provides a method for preparing a cathode material with a gradient dual-phase structure, comprising the following steps: 1) Weigh out sodium source, nickel source, manganese source, iron source, magnesium source and aluminum source according to stoichiometric ratio, mix them, and grind them to obtain a blended powder; 2) The powder mixture from step 1) is microwave-treated and then calcined in a mixed atmosphere with an oxygen volume content of 30%-50% to obtain a calcined material; 3) Deposit a LiAlO2 coating layer on the surface of the sinter obtained in step 2) to obtain the cathode material with gradient dual-phase structure.

[0006] Preferably, the mixed atmosphere in step 2) is selected from a mixed atmosphere formed by mixing nitrogen and oxygen; Optionally, the volume ratio of nitrogen to oxygen is (5-7):(3-5); The chemical formula of the burner is Na a Ni b Mn c Fe d Mg e Al fO2, 0.7 < a < 0.9, 0.2 < b < 0.4, 0.3 < c < 0.6, 0.01 < d < 0.15, 0.01 < e < 0.15, 0.01 < f < 0.15 and b + c + d + e + f = 1; The first fired material has a gradient distribution structure with a core O3 phase and an outer layer P2 phase.

[0007] Preferably, the sodium source in step 1) is selected from at least one of Na2CO3, NaOH, NaHCO3, sodium acetate, and sodium oxalate; The nickel source is selected from at least one of NiO, Ni(OH)2, NiCO3, and nickel acetate; The manganese source is selected from at least one of MnO2, Mn2O3, MnCO3, and manganese acetate; The iron source is selected from at least one of Fe2O3, Fe(OH)3, and iron acetate.

[0008] Preferably, the magnesium source in step 1) is selected from at least one of MgO, Mg(OH)2, and MgCO3; The aluminum source is selected from at least one of Al2O3 and Al(OH)3; The D50 particle size of the milled and blended powder is 0.5 - 2 μm; Optionally, the milling time is 1 - 5 h.

[0009] Preferably, the temperature of the microwave treatment in step 2) is 300 - 500 °C, and the microwave treatment time is 20 - 120 min; The calcination temperature in step 2) is 600 - 800 °C, and the calcination time is 3 - 5 h; The heating rate of the calcination is 2 - 5 °C / min; After the calcination step, there is also a cooling treatment step, and the cooling rate is 0.5 - 2 °C / min; Preferably, the cooling rate is 1 °C / min; Optionally, after the cooling treatment step, there are also steps of crushing and sieving.

[0010] Preferably, the average thickness of the LiAlO₂ coating in step 3) is 2 - 5 nm; The D50 particle size of the first fired material is 4 - 6 μm; The deposition method for depositing the LiAlO₂ coating on the surface of the first fired material includes atomic layer deposition.

[0011] Preferably, the preparation step of depositing the LiAlO₂ coating on the surface of the first fired material obtained in step 2) in step 3) includes: S1: The surface of the sintered material obtained in step 2) is subjected to a deposition treatment, including sequentially depositing an organic aluminum source and an oxygen source to form an Al-O layer, and then continuing to deposit an organic lithium source and an oxygen source to form a Li-O layer; S2: Repeat the deposition process of step S1; S3: After the repeated deposition process in step S2 is completed, annealing is performed to deposit a LiAlO2 coating layer on the surface of a sintered material, thereby obtaining the gradient dual-phase structure cathode material.

[0012] Preferably, in step S1, before the deposition treatment step, the material obtained in step 2) is further subjected to pre-annealing treatment. The pre-annealing temperature is 100-250℃, and the pre-annealing time is 0.5-2h; The pre-annealing is performed in a nitrogen atmosphere; Optionally, in step S1, the deposition temperature is 150-300℃; Optionally, the deposition process is performed under vacuum conditions; When the organic aluminum source and oxygen source are deposited to form the Al-O layer, the deposition time of the organic aluminum source is 0.1-1s, the deposition time of the oxygen source is 0.2-2s, the organic aluminum source is selected from trimethylaluminum, and the oxygen source is selected from at least one of water, oxygen, and ozone. The process after depositing the organoaluminum source also includes gas purging to remove excess organoaluminum source; Preferably, after the deposition of the organoaluminum source is completed, the process further includes purging with inert gas to remove excess organoaluminum source; When the organic lithium source and oxygen source are deposited to form the Li-O layer, the deposition time of the organic lithium source is 0.2-2s, the deposition time of the oxygen source is 0.2-2s, the organic lithium source is selected from tert-butyllithium, and the oxygen source is selected from at least one of water, oxygen, and ozone. The process of depositing the organic lithium source also includes gas purging to remove excess organic lithium source. Preferably, after the deposition of the organic lithium source is completed, the process further includes purging with inert gas to remove excess organic lithium source; Optionally, in step S2, the number of repetitions is 80-120 times; Optionally, in step S3, the annealing temperature is 250-400℃ and the annealing time is 3-6h.

[0013] This invention provides a cathode material with a gradient dual-phase structure, which is prepared by the above-described method for preparing a cathode material with a gradient dual-phase structure.

[0014] This invention also provides an application of the above-described cathode material with a gradient dual-phase structure in a sodium-ion battery. The technical solution of this invention has the following advantages: 1. The present invention provides a method for preparing a cathode material with a gradient dual-phase structure, comprising the following steps: 1) Weighing sodium source, nickel source, manganese source, iron source, magnesium source, and aluminum source according to stoichiometric ratio, mixing and grinding to obtain a blended powder; 2) Microwave-treating the blended powder in step 1), and then calcining it in a mixed atmosphere with an oxygen volume content of 30%-50% to obtain a calcined material; 3) Depositing a LiAlO2 coating layer on the surface of the calcined material obtained in step 2), thereby obtaining the cathode material with the gradient dual-phase structure. The present invention uses sodium source, nickel source, manganese source, iron source, magnesium source, and aluminum source as raw materials for mixing, and introduces an aluminum source to further reduce costs and improve structural stability; microwave selective heating of polar substances (such as sodium source, iron source, etc.) excites local atomic vibrations, reduces the reaction activation energy, and promotes Na+ reaction. + The pre-diffusion of transition metal ions forms a preliminary elemental concentration gradient, which can shorten the subsequent calcination time, reduce energy consumption, and avoid element homogenization at high temperatures. During calcination in a mixed atmosphere with an oxygen volume content of 30%-50% after microwave treatment, the diffusion rates of transition metal ions such as Ni, Mn, and Fe differ (e.g., the diffusion rate is Ni ion > Mn ion > Fe ion). 2+ / Ni 3+ Its high diffusivity causes it to accumulate on the particle surface, interacting with Na. + Formation of the P2 phase (larger interlayer spacing in Na layers facilitates rapid ion transport); Fe 3+ Diffusion is slow, and most of it remains inside the particle, interacting with Na. + O3 phase is formed (high sodium content, increasing capacity). Simultaneously, calcination in a mixed atmosphere with an oxygen volume content of 30%-50% results in a high oxygen partial pressure on the particle surface, promoting higher oxidation states (such as Ni). 3+ Mn 4+ Enrichment of Fe promotes the formation of the P2 phase on the particle surface; the internal oxygen partial pressure decreases, and Fe... 2+ / Fe 3+ The coexistence of these phases facilitates the formation of the O3 phase within the particles. The synergistic cooperation between the specific process steps of this invention results in a sintered material exhibiting a gradient distribution structure of an O3 core and an outer P2 phase. The interface between these two phases achieves a continuous lattice transition through the concentration gradients of Ni and Mn ions. This more uniform gradient distribution structure of the O3 and outer P2 phases provides a stable framework and fast ion channels, while the inner O3 phase provides high capacity, achieving a dynamic synergistic effect. This uniform gradient distribution structure alleviates volume expansion during the charging and discharging process of the cathode material, suppresses phase transformation cracks, and improves cycle stability and air stability. Furthermore, a LiAlO2 coating layer is formed on the particle surface to isolate it from the erosion of water and carbon dioxide in the air, synergistically improving the material's cycle stability and air stability. The LiAlO2 coating layer contains Li... + Partially replaces Na + Sites, enhancing surface structural rigidity, Al3+ By filling oxygen vacancies, isolating electrolyte corrosion, and reducing surface side reactions, this invention also inhibits the irreversible P2→OP4 phase transition of the P2 phase surface during cycling. The cathode material with a gradient dual-phase structure obtained through a specific preparation method exhibits low synthesis energy consumption, uniform phase distribution, and effectively suppresses phase transition cracks, mitigates volume expansion during charge and discharge, reduces the influence of air on the material, and simultaneously possesses high cycle stability and high air stability.

[0015] 2. The method for preparing a cathode material with a gradient dual-phase structure provided by the present invention, wherein the mixing atmosphere in step 2) is selected from a mixed atmosphere formed by mixing nitrogen and oxygen, and optionally, the volume ratio of nitrogen to oxygen is (5-7):(3-5); the present invention provides a mixed atmosphere formed by mixing nitrogen and oxygen in a specific volume ratio, which further ensures that the prepared cathode material has a uniform gradient distribution structure of core O3 phase and outer P2 phase, and further ensures that the cathode material with a gradient dual-phase structure obtained in conjunction with other steps has both high cycle stability and high air stability.

[0016] 3. The method for preparing the cathode material with a gradient dual-phase structure provided by this invention, wherein the calcination temperature in step 2) is 600-800℃, and the calcination time is 3-5 h; the heating rate of the calcination is 2-5℃ / min. During high-temperature calcination at 600-800℃, the Ni / Mn-rich region on the surface is further oxidized under a higher oxygen partial pressure, resulting in Ni... 2+ →Ni 3+ , and Na + A P2-type layered structure is formed. Simultaneously, the relatively short holding time of 3-5 hours restricts the diffusion of Ni / Mn into the interior, maintaining the dominance of the surface P2 phase. Furthermore, due to the slow diffusion of Fe / Mg / Al within the particles, an O3 phase is formed under relatively low oxygen partial pressure.

[0017] 4. The method for preparing a cathode material with a gradient dual-phase structure provided by the present invention further includes a cooling step after the calcination step, with a cooling rate of 0.5-2℃ / min; during the slow cooling process, the inner O3 phase is stabilized through atomic rearrangement, avoiding phase transformation stress concentration. Preferably, the cooling rate is 1℃ / min; 5. The method for preparing cathode materials with gradient dual-phase structure provided by the present invention has low energy consumption, less impurity phase formation, and uses Al to replace noble metals, thereby reducing costs.

[0018] 6. The method for preparing a cathode material with a gradient dual-phase structure provided by the present invention yields a cathode material with a gradient dual-phase structure that exhibits high air stability. After being exposed to an environment of 25°C and 60% humidity for 24 hours, the cycle capacity retention rate can still reach over 95%. Detailed Implementation

[0019] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0020] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0021] Example 1 This embodiment provides a method for preparing a cathode material with a gradient dual-phase structure, including the following steps: 1) Weigh out Na2CO3, NiO, MnO2, Fe2O3, MgO and Al2O3 according to the stoichiometric ratio, mix them, and ball mill for 2 hours to obtain a blended powder with a D50 particle size of 1.1 μm; 2) The powder from step 1) was microwaved at 400℃ for 30 minutes. Then, it was calcined at 700℃ for 3 hours under a nitrogen and oxygen mixture (7:3 volume ratio) at a heating rate of 5℃ / min. The calcined powder was then cooled to room temperature at a cooling rate of 1℃ / min, pulverized, and sieved to obtain the chemical formula Na. 0.85 Ni 0.3 Mn 0.5 Fe 0.1 Mg 0.05 Al 0.05 The first sintering material of O2 has a D50 particle size of 5.3 μm. TEM transmission electron microscopy revealed that the first sintering material exhibits a gradient distribution structure with an O3 core phase and an outer P2 phase. 3) The sinter obtained in step 2) is pre-annealed at 200°C for 1 hour in a nitrogen atmosphere, and then subjected to deposition treatment at 200°C under vacuum conditions, including sequential deposition of trimethylaluminum, nitrogen purging to remove excess trimethylaluminum, and water deposition to form an Al-O layer. The deposition time of trimethylaluminum is 0.5 s and the deposition time of water is 1 s. Then, tert-butyllithium is deposited, nitrogen purging to remove excess tert-butyllithium, and water deposition to form a Li-O layer. The deposition time of tert-butyllithium is 0.5 s and the deposition time of water is 1 s. The above deposition treatment steps are repeated 100 times. After the repeated deposition treatment is completed, the sinter is annealed at 400°C for 4 hours to deposit a LiAlO2 coating layer with an average thickness of 5 nm on the surface of the sinter, thus obtaining the gradient dual-phase structure cathode material.

[0022] Example 2 This embodiment provides a method for preparing a cathode material with a gradient dual-phase structure, including the following steps: 1) Weigh out Na2CO3, NiO, MnO2, Fe2O3, MgO and Al2O3 according to the stoichiometric ratio, mix them, and ball mill for 2 hours to obtain a blended powder with a D50 particle size of 0.9μm; 2) The powder from step 1) was microwaved at 300℃ for 30 minutes. Then, it was calcined at 800℃ for 2 hours under a nitrogen and oxygen mixture (7:3 volume ratio) at a heating rate of 5℃ / min. The calcined powder was then cooled to room temperature at a cooling rate of 1℃ / min, pulverized, and sieved to obtain the chemical formula Na. 0.85 Ni 0.3 Mn 0.5 Fe 0.1 Mg 0.05 Al 0.05 The first sintering material of O2 has a D50 particle size of 4.9 μm. TEM transmission electron microscopy revealed that the first sintering material exhibits a gradient distribution structure with an O3 core phase and an outer P2 phase. 3) The sinter obtained in step 2) is pre-annealed at 200°C for 1 hour in a nitrogen atmosphere, and then subjected to deposition treatment at 300°C under vacuum conditions, including sequential deposition of trimethylaluminum, nitrogen purging to remove excess trimethylaluminum, and water deposition to form an Al-O layer. The deposition time of trimethylaluminum is 0.9 s and the deposition time of water is 1.2 s. Then, tert-butyllithium is deposited, nitrogen purging to remove excess tert-butyllithium, and water deposition to form a Li-O layer. The deposition time of tert-butyllithium is 0.9 s and the deposition time of water is 1.2 s. The above deposition treatment steps are repeated 100 times. After the repeated deposition treatment is completed, the sinter is annealed at 250°C for 5 hours to deposit a LiAlO2 coating layer with an average thickness of 4.5 nm on the surface of the sinter, thus obtaining the gradient dual-phase structure cathode material.

[0023] Example 3 This embodiment provides a method for preparing a cathode material with a gradient dual-phase structure, including the following steps: 1) Weigh out Na2CO3, NiO, MnO2, Fe2O3, MgO and Al2O3 according to the stoichiometric ratio, mix them, and ball mill for 2 hours to obtain a blended powder with a D50 particle size of 1.3μm; 2) The powder from step 1) was microwaved at 500℃ for 30 minutes. Then, it was calcined at 600℃ for 5 hours under a nitrogen and oxygen mixture (6:4 volume ratio) at a heating rate of 5℃ / min. The calcined powder was then cooled to room temperature at a cooling rate of 1℃ / min, pulverized, and sieved to obtain the chemical formula Na. 0.85 Ni 0.3 Mn0.5 Fe 0.1 Mg 0.05 Al 0.05 The first sintering material of O2 has a D50 particle size of 5.5 μm. TEM transmission electron microscopy revealed that the first sintering material exhibits a gradient distribution structure with an O3 core phase and an outer P2 phase. 3) The sinter obtained in step 2) is pre-annealed at 100°C for 0.5 h in a nitrogen atmosphere, and then subjected to deposition treatment at 250°C under vacuum conditions, including sequential deposition of trimethylaluminum, nitrogen purging to remove excess trimethylaluminum, and water deposition to form an Al-O layer. The deposition time of trimethylaluminum is 0.7 s and the deposition time of water is 0.9 s. Then, tert-butyllithium is deposited, nitrogen purging to remove excess tert-butyllithium, and water deposition to form a Li-O layer. The deposition time of tert-butyllithium is 0.7 s and the deposition time of water is 0.9 s. The above deposition treatment steps are repeated 100 times. After the repeated deposition treatment is completed, the sinter is annealed at 250°C for 6 h to deposit a LiAlO2 coating layer with an average thickness of 4 nm on the surface of the sinter, thus obtaining the gradient dual-phase structure cathode material.

[0024] Example 4 This embodiment provides a method for preparing a cathode material with a gradient dual-phase structure. The only difference between this method and Embodiment 1 is that in step 3), the sinter obtained in step 2) is ball-milled with LiAlO2 at 3000 rpm for 2 hours, so that a LiAlO2 coating layer with an average thickness of 5 nm is coated on the surface of the sinter, thereby obtaining the gradient dual-phase structure cathode material.

[0025] Comparative Example 1 This comparative example provides a method for preparing a cathode material, which differs from Example 1 only in that in step 2), "then calcining at 700°C for 3 hours at a heating rate of 5°C / min in a mixed atmosphere of nitrogen and oxygen in a volume ratio of 7:3" is replaced with "then calcining at 900°C for 3 hours in an air atmosphere at a heating rate of 5°C / min".

[0026] Comparative Example 2 This comparative example provides a method for preparing a cathode material, which differs from Example 1 only in that in step 2), "then calcining at 700°C for 3 hours at a heating rate of 5°C / min in a mixed atmosphere of nitrogen and oxygen in a volume ratio of 7:3" is replaced with "then calcining at 700°C for 3 hours in an air atmosphere at a heating rate of 5°C / min".

[0027] Comparative Example 3 This comparative example provides a method for preparing a cathode material, including the following steps: 1) Weigh out Na2CO3, MnO2, Fe2O3, MgO and Al2O3 according to the stoichiometric ratio, mix them, and ball mill for 2 hours to obtain a blended powder with a D50 particle size of 1.1 μm; 2) The powder from step 1) was microwaved at 400℃ for 30 minutes. Then, it was calcined at 700℃ for 3 hours under a nitrogen and oxygen mixture (7:3 volume ratio) at a heating rate of 5℃ / min. The calcined powder was then cooled to room temperature at a cooling rate of 1℃ / min, pulverized, and sieved to obtain the chemical formula Na. 0.85 Mn 0.7144 Fe 0.1428 Mg 0.0714 Al 0.0714 O2-based feedstock; the D50 particle size of the feedstock is 5.3 μm; 3) The sinter obtained in step 2) is pre-annealed at 200°C for 1 hour in a nitrogen atmosphere, and then subjected to deposition treatment at 200°C under vacuum conditions, including sequential deposition of trimethylaluminum, nitrogen purging to remove excess trimethylaluminum, and water deposition to form an Al-O layer. The deposition time of trimethylaluminum is 0.5 s and the deposition time of water is 1 s. Then, tert-butyllithium is deposited, nitrogen purging to remove excess tert-butyllithium, and water deposition to form a Li-O layer. The deposition time of tert-butyllithium is 0.5 s and the deposition time of water is 1 s. The above deposition treatment steps are repeated 100 times. After the repeated deposition treatment is completed, the sinter is annealed at 400°C for 4 hours to deposit a LiAlO2 coating layer with an average thickness of 5 nm on the surface of the sinter, thus obtaining the cathode material.

[0028] Comparative Example 4 This comparative example provides a method for preparing a cathode material, including the following steps: 1) Weigh out Na2CO3, NiO, MnO2, MgO and Al2O3 according to the stoichiometric ratio, mix them, and ball mill for 2 hours to obtain a blended powder with a D50 particle size of 1.1 μm; 2) The powder from step 1) was microwaved at 400℃ for 30 minutes. Then, it was calcined at 700℃ for 3 hours under a nitrogen and oxygen mixture (7:3 volume ratio) at a heating rate of 5℃ / min. The calcined powder was then cooled to room temperature at a cooling rate of 1℃ / min, pulverized, and sieved to obtain the chemical formula Na. 0.85 Ni 0.3333 Mn 0.5557 Mg 0.0555 Al 0.0555 O2-based feedstock; the D50 particle size of the feedstock is 5.3 μm; 3) The sinter obtained in step 2) is pre-annealed at 200°C for 1 hour in a nitrogen atmosphere, and then subjected to deposition treatment at 200°C under vacuum conditions, including sequential deposition of trimethylaluminum, nitrogen purging to remove excess trimethylaluminum, and water deposition to form an Al-O layer. The deposition time of trimethylaluminum is 0.5 s and the deposition time of water is 1 s. Then, tert-butyllithium is deposited, nitrogen purging to remove excess tert-butyllithium, and water deposition to form a Li-O layer. The deposition time of tert-butyllithium is 0.5 s and the deposition time of water is 1 s. The above deposition treatment steps are repeated 100 times. After the repeated deposition treatment is completed, the sinter is annealed at 400°C for 4 hours to deposit a LiAlO2 coating layer with an average thickness of 5 nm on the surface of the sinter, thus obtaining the gradient dual-phase structure cathode material.

[0029] Comparative Example 5 This comparative example provides a method for preparing a cathode material, which differs from Example 1 only in step 2) by heating the mixed powder from step 1) to 700°C for 3 hours at a heating rate of 5°C / min under a mixed atmosphere of nitrogen and oxygen in a volume ratio of 7:3, then cooling it to room temperature at a cooling rate of 1°C / min, pulverizing, and sieving to obtain the material with the chemical formula Na. 0.85 Ni 0.3 Mn 0.5 Fe 0.1 Mg 0.05 Al 0.05 O2 is the primary feedstock; the D50 particle size of the primary feedstock is 5.3 μm.

[0030] Comparative Example 6 This comparative example provides a method for preparing a cathode material, including the following steps: 1) Weigh out Na2CO3, NiO, MnO2 and Fe2O3 according to the stoichiometric ratio, mix them, and ball mill for 2 hours to obtain a blended powder with a D50 particle size of 1.1 μm; 2) The powder from step 1) was microwaved at 400℃ for 30 minutes. Then, it was calcined at 700℃ for 3 hours under a nitrogen and oxygen mixture (7:3 volume ratio) at a heating rate of 5℃ / min. The calcined powder was then cooled to room temperature at a cooling rate of 1℃ / min, pulverized, and sieved to obtain the chemical formula Na. 0.85 Ni 0.3 Mn 0.6 Fe 0.1 O2-based feedstock; the D50 particle size of the feedstock is 5.3 μm; 3) The sinter obtained in step 2) is pre-annealed at 200°C for 1 hour in a nitrogen atmosphere, and then subjected to deposition treatment at 200°C under vacuum conditions, including sequential deposition of trimethylaluminum, nitrogen purging to remove excess trimethylaluminum, and water deposition to form an Al-O layer. The deposition time of trimethylaluminum is 0.5 s and the deposition time of water is 1 s. Then, tert-butyllithium is deposited, nitrogen purging to remove excess tert-butyllithium, and water deposition to form a Li-O layer. The deposition time of tert-butyllithium is 0.5 s and the deposition time of water is 1 s. The above deposition treatment steps are repeated 100 times. After the repeated deposition treatment is completed, the sinter is annealed at 400°C for 4 hours to deposit a LiAlO2 coating layer with an average thickness of 5 nm on the surface of the sinter, thus obtaining the gradient dual-phase structure cathode material.

[0031] Test case The sintered materials prepared in Examples 1-4 and Comparative Examples 1-6 were observed using TEM (Transmission Electron Microscopy) to determine whether they exhibited a gradient distribution structure of the core O3 phase and the outer P2 phase. The results are shown in Table 1. When a segment along the observation diameter showed a relatively uniform decrease or increase in the content of the O3 or P2 phase without a sharp drop, the gradient distribution of the core O3 phase and the outer P2 phase was considered uniform. When a segment along the observation diameter showed a relatively constant or small change in the content of the O3 or P2 phase, followed by a sharp drop or increase, the gradient distribution of the core O3 phase and the outer P2 phase was considered somewhat uneven. The results regarding the uniformity of the gradient distribution of the core O3 phase and the outer P2 phase are shown in Table 1.

[0032] The positive electrode materials obtained in Examples 1-4 and Comparative Examples 1-6 were exposed to an environment of 25°C and 60% humidity for 24 hours and then used as positive electrode materials to prepare button batteries. Specifically, the positive electrode materials were uniformly mixed in N-methylpyrrolidone (solvent) at a ratio of 90 (positive electrode material): 5 (PVDF): 5 (SP), and then homogenized, coated, dried, and cut to form positive electrode sheets (the areal density of the positive electrode material was 13.0 mg / cm³). 2 A Na-metal sheet was used as the counter electrode, and glass fiber was used as the separator. NaPF6 was mixed in a solution of ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) (volume ratio 1:1:0.5) as the electrolyte (the concentration of NaPF6 in the electrolyte was 1 mol / L). During testing, it was assembled into a CR2032 coin cell consisting of a positive electrode, a glass fiber separator, a Na-metal sheet, a gasket, and a spring.

[0033] The cathode materials obtained in Examples 1-4 and Comparative Examples 1-6 were used as cathode materials to prepare button batteries without exposure treatment. Specifically, they were uniformly mixed in N-methylpyrrolidone (solvent) at a ratio of 90 (cathode material): 5 (PVDF): 5 (SP), homogenized, coated, dried, and cut to form cathode sheets (cathode material areal density of 13.0 mg / cm³). 2 A Na-metal sheet was used as the counter electrode, and glass fiber was used as the separator. NaPF6 was mixed in a solution of ethylene carbonate (EC) / dimethyl carbonate (DMC) / ethyl methyl carbonate (EMC) (volume ratio 1:1:0.5) as the electrolyte (the concentration of NaPF6 in the electrolyte was 1 mol / L). During testing, it was assembled into a CR2032 coin cell consisting of a positive electrode, a glass fiber separator, a Na-metal sheet, a gasket, and a spring.

[0034] The prepared coin cells were placed in the Blue Electric testing system for cycle performance testing. Cycle performance testing: voltage range 2.0V-4.0V, 50 cycles of 1C / 1C charge / discharge were performed. Cycle capacity retention rate = (specific capacity at discharge cycle 50 / specific capacity at discharge cycle 1) * 100%. The results of the cycle capacity retention rate test are shown in Table 1.

[0035] Table 1

[0036] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a cathode material with a gradient dual-phase structure, characterized in that, Includes the following steps: 1) Weigh out sodium source, nickel source, manganese source, iron source, magnesium source and aluminum source according to stoichiometric ratio, mix them, and grind them to obtain a blended powder; 2) The powder mixture from step 1) is microwave-treated and then calcined in a mixed atmosphere with an oxygen volume content of 30%-50% to obtain a calcined material; 3) A LiAlO2 coating layer is deposited on the surface of the sintered material obtained in step 2) to obtain the cathode material with a gradient dual-phase structure; In step 2), the microwave treatment temperature is 300-500℃ and the microwave treatment time is 20-120min. The calcination temperature in step 2) is 600-800℃, and the calcination time is 3-5 hours. The heating rate for calcination is 2-5℃ / min; After the calcination step is completed, a cooling step is also included, with a cooling rate of 0.5-2℃ / min.

2. The method for preparing the cathode material with a gradient dual-phase structure according to claim 1, characterized in that, The mixed atmosphere mentioned in step 2) is selected from a mixed atmosphere formed by mixing nitrogen and oxygen. The volume ratio of nitrogen to oxygen is (5-7):(3-5); The chemical general formula of the first fired material is Na a Ni b Mn c Fe d Mg e Al f O2, where 0.7 < a < 0.9, 0.2 < b < 0.4, 0.3 < c < 0.6, 0.01 < d < 0.15, 0.01 < e < 0.15, 0.01 < f < 0.15 and b + c + d + e + f = 1; The sintered material has a gradient distribution structure with an O3 core phase and an P2 outer layer phase.

3. The method for preparing the cathode material with a gradient dual-phase structure according to claim 1, characterized in that, The sodium source mentioned in step 1) is selected from at least one of Na2CO3, NaOH, NaHCO3, sodium acetate, and sodium oxalate; The nickel source is selected from at least one of NiO, Ni(OH)2, NiCO3, and nickel acetate; The manganese source is selected from at least one of MnO2, Mn2O3, MnCO3, and manganese acetate; The iron source is selected from at least one of Fe2O3, Fe(OH)3, and ferric acetate.

4. The method for preparing the cathode material with a gradient dual-phase structure according to any one of claims 1-3, characterized in that, The magnesium source mentioned in step 1) is selected from at least one of MgO, Mg(OH)2, and MgCO3; The aluminum source is selected from at least one of Al2O3 and Al(OH)3; The D50 particle size of the ground and blended powder is 0.5-2μm; The grinding time is 1-5 hours.

5. The method for preparing the cathode material with a gradient dual-phase structure according to claim 1, characterized in that, Step 2) The cooling process is followed by crushing and sieving steps.

6. The method for preparing the cathode material with a gradient dual-phase structure according to claim 1, characterized in that, The average thickness of the LiAlO2 coating layer mentioned in step 3) is 2-5 nm; The D50 particle size of the sintered material is 4-6 μm; The deposition method for depositing a LiAlO2 coating layer on the surface of a burner includes atomic layer deposition.

7. The method for preparing the cathode material with a gradient dual-phase structure according to claim 1, characterized in that, Step 3) describes the preparation steps for depositing a LiAlO2 coating layer on the surface of the sintering material obtained in step 2), which includes: S1: The surface of the sintered material obtained in step 2) is subjected to a deposition treatment, including sequentially depositing an organic aluminum source and an oxygen source to form an Al-O layer, and then continuing to deposit an organic lithium source and an oxygen source to form a Li-O layer; S2: Repeat the deposition process of step S1; S3: After the repeated deposition process in step S2 is completed, annealing is performed to deposit a LiAlO2 coating layer on the surface of a sintered material, thereby obtaining the gradient dual-phase structure cathode material.

8. The method for preparing the cathode material with a gradient dual-phase structure according to claim 7, characterized in that, In step S1, before the deposition treatment step, the material obtained in step 2) is further subjected to pre-annealing treatment. The pre-annealing temperature is 100-250℃, and the pre-annealing time is 0.5-2h; The pre-annealing is performed in a nitrogen atmosphere; In step S1, the deposition temperature is 150-300℃; When the organic aluminum source and oxygen source are deposited to form the Al-O layer, the deposition time of the organic aluminum source is 0.1-1s, the deposition time of the oxygen source is 0.2-2s, the organic aluminum source is selected from trimethylaluminum, and the oxygen source is selected from at least one of water, oxygen, and ozone. The process after depositing the organoaluminum source also includes gas purging to remove excess organoaluminum source; When the organic lithium source and oxygen source are deposited to form the Li-O layer, the deposition time of the organic lithium source is 0.1-2s, the deposition time of the oxygen source is 0.2-2s, the organic lithium source is selected from tert-butyllithium, and the oxygen source is selected from at least one of water, oxygen, and ozone. The process of depositing the organic lithium source also includes gas purging to remove excess organic lithium source. In step S2, the repetition is performed 80-120 times. In step S3, the annealing temperature is 250-400℃ and the annealing time is 3-6h.

9. A cathode material with a gradient dual-phase structure, characterized in that, It is prepared by the method for preparing a cathode material with a gradient dual-phase structure as described in any one of claims 1-8.

10. The application of the cathode material with a gradient dual-phase structure as described in claim 9 in a sodium-ion battery.

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