A layered oxide cathode material, a preparation method thereof, and a sodium ion battery

Abstract

The present invention provides a layered oxide cathode material, a preparation method thereof, and a sodium ion battery. The layered oxide cathode material includes a core and a shell coated on the surface of the core. The core includes Na x Mn a M 1‑a O2, and the shell includes Na x Ni b Mn c Fe d O2, where 0.7 < x ≤ 0.9, 0.8 ≤ a < 1, 0.2 ≤ b < 0.5, 0.2 ≤ c < 0.6, 0.2 ≤ d ≤ 0.5, and M includes any one or a combination of at least two of Ni, Ti, Fe, and Cu. Through the synergistic effect of the core and the shell, the present invention prevents the dissolution of Mn, reduces the side reaction between the material and the electrolyte during the reaction process, and improves the discharge capacity and cycling performance of the layered oxide cathode material.

Description

Technical Field

[0001] The invention belongs to the technical field of batteries and relates to a layered oxide positive electrode material and a preparation method thereof and a sodium ion battery. Background Art

[0002] Lithium-ion batteries (LIBs) have achieved initial success in large-scale electrochemical energy storage systems, and solid-state LIBs using metallic lithium as the anode have also been well developed. However, the sharp increase in demand and cost, as well as the limited reserves of lithium and cobalt, important metal elements in LIBs, have attracted attention for future development. Sodium-ion batteries (SIBs) equipped with advanced cobalt-free cathodes have shown great potential in solving the "lithium panic" and "cobalt panic" and have made significant progress in recent years.

[0003] As one of the core materials of sodium ion batteries, sodium positive electrode materials have become the focus of researchers. The positive electrode materials for sodium ion batteries currently studied are mainly crystalline materials, including transition metal oxides, polyanion compounds and Prussian blue compounds. Transition metal layered oxide positive electrode materials have great application potential in the field of low-cost, large-scale energy storage due to their advantages such as wide source of raw materials, good processing performance and high specific capacity. CN112563484A provides a sodium ion battery positive electrode material and a preparation method thereof, which mixes salt solutions of sodium salt, nickel salt and M salt, and reacts and sinters under high temperature and high pressure conditions to obtain a layered structure of Na x Ni y M 1-y O 2 Positive electrode material, improving the capacity and cycle performance of the material. CN108899538A provides a ternary sodium ion battery positive electrode material and a preparation method thereof, which mixes a salt solution containing a divalent nickel salt, a divalent cobalt salt and a divalent manganese salt with an alkaline solution, performs a coprecipitation reaction, and then pre-calcines. After sintering, a sodium source and a titanium source are added and calcined again to obtain a ternary sodium ion battery positive electrode material with good cycle stability and a discharge voltage platform. CN109607624B adds soluble carbonate, manganese salt, and cobalt salt to water for stirring reaction, adds sodium hydroxide to generate a precipitate and sinters to obtain a sodium ion battery positive electrode material with a layered-tunnel composite structure. The preparation process has few steps, simple operation, and improves the capacity of the positive electrode material.

[0004] The existing sodium ion battery transition metal layered oxides include single metal oxides, double metal oxides and multi-metal oxides. For the single metal oxide sodium manganate, although its capacity is high, the material internal resistance is large and polarization is serious during the cycle. 3+Octahedral compounds usually exhibit a strong Jahn-Teller effect, resulting in relatively rapid attenuation of the material capacity and unsatisfactory cycling performance; while the multi-metal nickel-iron-manganese layered oxide cathode material has good air stability and excellent cycling stability under normal pressure, but its specific capacity is low, which hinders its application in the field of high-energy density batteries.

[0005] In summary, the preparation of a sodium-ion battery cathode material with a high specific capacity and good cycling stability is of great significance for the research and development of sodium-ion batteries. Summary of the Invention

[0006] Aiming at the problems existing in the prior art, the purpose of the present invention is to provide a layered oxide cathode material, a preparation method thereof and a sodium-ion battery. The layered oxide cathode material of the present invention has a core-shell structure, and the inner core includes Na x Mn a M 1-a O 2 , and the outer shell includes Na x Ni b Mn c Fe d O 2 . The inner core and the outer shell act synergistically to prevent the dissolution of Mn, reduce the side reaction between the material and the electrolyte during the reaction, and improve the discharge capacity and cycling performance of the layered oxide cathode material.

[0007] To achieve the object of the present invention, the following technical solutions are adopted:

[0008] In the first aspect, the present invention provides a layered oxide cathode material, which includes an inner core and an outer shell coated on the surface of the inner core. The inner core includes Na x Mn a M 1-a O 2 , and the outer shell includes Na x Ni b Mn c Fe d O 2 , where 0.7 < x ≤ 0.9, 0.8 ≤ a < 1, 0.2 ≤ b < 0.5, 0.2 ≤ c < 0.6, 0.2 ≤ d ≤ 0.5, and M includes any one or at least two combinations of Ni, Ti, Fe, and Cu.

[0009] In the present invention, 0.7 < x ≤ 0.9, for example, it can be 0.71, 0.72, 0.75, 0.78, 0.8, 0.82, 0.85, 0.88 or 0.9, etc.; 0.8 ≤ a < 1, for example, it can be 0.8, 0.82, 0.84, 0.86, 0.88, 0.9, 0.92, 0.94, 0.96, 0.98 or 0.99, etc.; 0.2 ≤ b < 0.5, for example, it can be 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 or 0.49, etc.; 0.2 ≤ c < 0.6, for example, it can be 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55 or 0.59, etc.; 0.2 ≤ d ≤ 0.5, for example, it can be 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 or 0.5, etc.

[0010] The layered oxide cathode material prepared by the present invention has a core - shell structure. The inner core contains Na with a high manganese content x Mn a M 1-a O 2 , and the outer shell includes Na x Ni b Mn c Fe d O 2 . The inner - core material can provide a high specific capacity for the cathode, and the outer shell can effectively block the contact between Na x Mn a M 1-a O 2 and the electrolyte, avoiding the dissolution of Mn and reducing the occurrence of side reactions between the material and the electrolyte during the reaction. At the same time, the core - shell materials cooperate with each other, which can improve the comprehensive electrochemical performance of the material, especially the discharge capacity and cycle performance.

[0011] The cathode material provided by the present invention has a small Ni content and does not contain rare precious metals such as Co, and has advantages such as low price and simple preparation method, and has good application prospects in the fields of energy storage and so on.

[0012] Preferably, the D50 particle size of the layered oxide cathode material is 3 - 15 μm, for example, it can be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm or 15 μm, etc.

[0013] Preferably, the thickness of the outer shell is 5 - 10% of the D50 particle size of the layered oxide cathode material, for example, it can be 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 9.5% or 10%, etc.

[0014] It should be noted that in the present invention, the thickness of the outer shell refers to the average thickness of the outer shell in the layered oxide cathode material; for example, when the layered oxide layer is a sphere, if its D50 particle size is 10 μm and the D50 particle size of the inner core is 8 μm, then the total length occupied by the outer shell in the thickness direction of the cross-section is 2 μm, that is, the thickness of the outer shell is 1 μm.

[0015] In the present invention, by optimizing the size of the layered oxide cathode material and adjusting the thickness of the outer shell, the structural stability of the sodium cathode material can be improved, and the cycle stability and thermal stability of the material can be further improved.

[0016] In a second aspect, the present invention provides a method for preparing the layered oxide cathode material according to the first aspect, and the preparation method includes:

[0017] (1) Mixing a first salt solution, a precipitant, and a complexing agent to carry out a first coprecipitation reaction to obtain an inner core precursor, adding a second salt solution to carry out a second coprecipitation reaction, and generating an outer shell precursor on the surface of the inner core precursor to obtain a hydroxide precursor;

[0018] The inner core precursor includes Mn a M 1-a (OH) 2 , and the outer shell precursor includes Ni b Mn c Fe d (OH) 2 ;

[0019] (2) Mixing the hydroxide precursor and a sodium source and sintering to obtain the layered oxide cathode material.

[0020] The present invention adopts a two-step feeding and two-step coprecipitation method to generate a hydroxide precursor with a core-shell structure, and obtains the layered oxide cathode material by adding a sodium source and sintering. The preparation method is simple, the morphology and size of the material have strong controllability, and the prepared layered oxide cathode material has a high specific capacity and good cycle stability.

[0021] It should be noted that in the present invention, when carrying out the second coprecipitation, the second salt solution can be directly added and the coprecipitation reaction can be carried out under the action of the original precipitant and complexing agent, or a certain amount of precipitant and complexing agent can be added again to carry out the coprecipitation reaction.

[0022] Preferably, the D50 particle size of the inner core precursor is 80-90% of the D50 particle size of the hydroxide precursor, and for example, it can be 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89% or 90%, etc.

[0023] In the present invention, during coprecipitation, when the core precursor grows to 80-90% of the target size particle diameter, a second coprecipitation reaction is carried out by adding materials again, which can effectively control the composition and thickness of the shell.

[0024] Preferably, the first salt solution includes Mn and M with a molar ratio of a:(1-a).

[0025] Preferably, the second salt solution includes Ni, Mn and Fe with a molar ratio of b:c:d.

[0026] Preferably, the type of salt in the first salt solution and the second salt solution is independently any one or a combination of at least two of chloride salts, oxalate salts, sulfate salts and nitrate salts. For example, it can be a combination of chloride salts and nitrate salts, a combination of oxalate salts and sulfate salts, a combination of sulfate salts and nitrate salts, a combination of chloride salts, oxalate salts and sulfate salts, or a combination of chloride salts, oxalate salts, sulfate salts and nitrate salts, etc.

[0027] In the present invention, "independently" means that the selections of the two do not interfere with each other. For example, the type of salt in the first salt solution and the second salt solution is independently any one or a combination of at least two of chloride salts, oxalate salts, sulfate salts and nitrate salts, which means that when the type of salt in the first salt solution is chloride salt, the type of salt in the second salt solution can be chloride salt, can be oxalate salt, or can be a combination of sulfate salts and nitrate salts, etc., and the selections of the type of salt in the first salt solution and the second salt solution do not interfere with each other.

[0028] Preferably, the concentration of the first salt solution and the second salt solution is independently 80-120 g / L. For example, it can be 80 g / L, 85 g / L, 90 g / L, 95 g / L, 100 g / L, 105 g / L, 110 g / L, 115 g / L or 120 g / L, etc.

[0029] Preferably, the precipitant includes an aqueous sodium hydroxide solution.

[0030] Preferably, based on the mass of the precipitant being 100%, the mass fraction of the solute in the precipitant is 20-40%. For example, it can be 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38% or 40%, etc.

[0031] Preferably, the complexing agent includes any one or a combination of at least two of ammonia water, oxalic acid, lactic acid, sodium oxalate and EDTA solution. For example, it can be a combination of oxalic acid and lactic acid, a combination of sodium oxalate and EDTA solution, a combination of ammonia water and sodium oxalate, or a combination of oxalic acid, lactic acid, sodium oxalate and EDTA solution, etc.

[0032] Preferably, based on the volume of the complexing agent, the concentration of the complexing agent is 8 to 10 mol / L, for example, it can be 8 mol / L, 8.2 mol / L, 8.4 mol / L, 8.6 mol / L, 8.8 mol / L, 9 mol / L, 9.2 mol / L, 9.4 mol / L, 9.6 mol / L, 9.8 mol / L or 10 mol / L, etc.

[0033] As a preferred technical solution of the preparation method of the present invention, the temperatures of the first co-precipitation and the second co-precipitation are independently 40 to 70 °C, for example, it can be 40 °C, 45 °C, 50 °C, 55 °C, 60 °C, 65 °C or 70 °C, etc.

[0034] Preferably, the pH values of the first co-precipitation and the second co-precipitation are independently 9.5 to 11.5, for example, it can be 9.5, 9.8, 10, 10.2, 10.5, 10.8, 11, 11.2 or 11.5, etc.

[0035] In the present invention, the co-precipitation reaction is carried out at a suitable temperature and pH value, which can improve the sphericity and crystallinity of the precursor.

[0036] Preferably, based on the volumes of the solutions of the first co-precipitation and the second co-precipitation, the concentration of the complexing agent is independently 0.1 to 0.5 mol / L, for example, it can be 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L or 0.5 mol / L, etc. A co-precipitation solution containing a suitable concentration of the complexing agent is beneficial to controlling the growth rate of the precursor and improving the sphericity of the precursor.

[0037] Preferably, the rotation speeds of the first co-precipitation and the second co-precipitation are independently 320 to 380 rpm / min, for example, it can be 320 rpm / min, 330 rpm / min, 340 rpm / min, 350 rpm / min, 360 rpm / min, 370 rpm / min or 380 rpm / min, etc.

[0038] Preferably, after the second co-precipitation and before sintering, it further includes the steps of aging, filtering, washing and drying.

[0039] Preferably, the drying temperature is 100 to 120 °C, for example, it can be 100 °C, 102 °C, 104 °C, 106 °C, 108 °C, 110 °C, 112 °C, 114 °C, 116 °C, 118 °C or 120 °C, etc.

[0040] Exemplarily, the drying method includes any one or a mixed drying of at least two of rotary kiln drying, microwave drying, tray dryer drying and box furnace drying.

[0041] Preferably, the molar ratio of Na in the sodium source to the sum of Ni, Mn, Fe, and M in the hydroxide precursor is 0.7 to 0.9, and can be, for example, 0.7, 0.75, 0.8, 0.85, or 0.9, etc.

[0042] Preferably, the sodium source includes any one or a combination of at least two of sodium hydroxide, sodium carbonate, sodium oxalate, sodium chloride, and sodium nitrate. For example, it can be a combination of sodium hydroxide and sodium carbonate, a combination of sodium oxalate and sodium chloride, a combination of sodium chloride and sodium nitrate, or a combination of sodium hydroxide, sodium carbonate, sodium oxalate, sodium chloride, and sodium nitrate, etc.

[0043] As a preferred technical solution of the preparation method of the present invention, the sintering includes first sintering and second sintering.

[0044] Preferably, the temperature of the first sintering is 450 to 550 °C, and can be, for example, 450 °C, 460 °C, 470 °C, 480 °C, 490 °C, 500 °C, 510 °C, 520 °C, 530 °C, 540 °C, or 550 °C, etc.

[0045] Preferably, the time of the first sintering is 4 to 6 h, and can be, for example, 4 h, 4.5 h, 5 h, 5.5 h, or 6 h, etc.

[0046] Preferably, the heating rate of the first sintering is 3 to 5 °C / min, and can be, for example, 3 °C / min, 3.5 °C / min, 4 °C / min, 4.5 °C / min, or 5 °C / min, etc.

[0047] Preferably, the temperature of the second sintering is 600 to 800 °C, and can be, for example, 600 °C, 620 °C, 640 °C, 660 °C, 680 °C, 700 °C, 720 °C, 740 °C, 760 °C, 780 °C, or 800 °C, etc.

[0048] Preferably, the time of the second sintering is 10 to 16 h, and can be, for example, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, or 16 h, etc.

[0049] Preferably, the heating rate of the second sintering is 3 to 5 °C / min, and can be, for example, 3 °C / min, 3.5 °C / min, 4 °C / min, 4.5 °C / min, or 5 °C / min, etc.

[0050] In the present invention, by performing two-step sintering at appropriate temperature and time, it is beneficial to the diffusion of sodium ions during the high-temperature sintering process and reduces the residual sodium on the material surface.

[0051] As a preferred technical solution of the preparation method of the present invention, the preparation method includes:

[0052] (1) Mix the first salt solution, precipitating agent, and complexing agent to conduct the first coprecipitation reaction. The first salt solution contains Mn and M with a molar ratio of a:(1 - a), obtaining a core precursor. Then, add the second salt solution to conduct the second coprecipitation reaction. The second salt solution contains Ni, Mn, and Fe with a molar ratio of b:c:d, generating a shell precursor on the surface of the core precursor to obtain a hydroxide precursor;

[0053] The temperature of the first coprecipitation and the second coprecipitation is independently 40 - 70 °C, and the pH value of the first coprecipitation and the second coprecipitation is independently 9.5 - 11.5. Based on the volume of the solution of the first coprecipitation and the solution of the second coprecipitation, the concentration of the complexing agent is independently 0.1 - 0.5 mol / L. The core precursor contains Mn a M 1-a (OH) 2 , and the shell precursor contains Ni b Mn c Fe d (OH) 2 . The D50 particle size of the core precursor is 80 - 90% of the D50 particle size of the hydroxide precursor;

[0054] (2) Mix the hydroxide precursor and sodium source, sinter at 450 - 550 °C for 4 - 6 h, and then sinter again at 600 - 800 °C for 10 - 16 h to obtain a layered oxide cathode material.

[0055] In a third aspect, the present invention provides a sodium-ion battery, and the cathode of the sodium-ion battery contains the layered oxide cathode material according to the first aspect.

[0056] The sodium-ion battery prepared by using the layered oxide cathode material of the present invention has a high specific capacity and good cycle stability, showing excellent comprehensive electrochemical performance.

[0057] Compared with the prior art, the present invention has the following beneficial effects:

[0058] (1) The layered oxide cathode material prepared by the present invention has a core-shell structure. The core contains Na x Mn a M 1-a O 2 with a high manganese content, and the shell contains Na x Ni b Mn c Fe d O 2 . The core material can provide a high specific capacity for the cathode, and the shell can effectively block Na x Mna M 1-a O 2 Contacting with the electrolyte avoids the dissolution of Mn and reduces the occurrence of side reactions between the material and the electrolyte during the reaction. At the same time, the core-shell materials interact synergistically, which can improve the comprehensive electrochemical performance of the material, especially the discharge capacity and cycling performance.

[0059] (2) The cathode material provided by the present invention has a small Ni content and does not contain rare precious metals such as Co, and has advantages such as low price and simple preparation method, and has good application prospects in the fields of energy storage and the like. Description of the Drawings

[0060] Figure 1 It is a cross-sectional SEM image of the hydroxide precursor prepared in Example 1 of the present invention.

[0061] Figure 2 It is a sectional distribution diagram of Ni element in the hydroxide precursor prepared in Example 1 of the present invention.

[0062] Figure 3 It is an SEM image of the layered oxide cathode material prepared in Example 1 of the present invention. Detailed Embodiments

[0063] The technical solutions of the present invention will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention.

[0064] Example 1

[0065] This example provides a layered oxide cathode material, including a core of Na 0.8 Mn 0.85 Cu 0.15 O 2 and a shell of Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33 O 2 The thickness of the shell is 1 μm, and the D50 particle size of the layered oxide cathode material is 12 μm.

[0066] This example also provides a preparation method of the above-mentioned layered oxide cathode material, including:

[0067] (1) Weigh manganese sulfate and copper sulfate in sequence, such that the molar ratio of manganese to copper is 0.85:0.15, and prepare a first salt solution with a concentration of 98 g / L; weigh manganese sulfate, ferrous sulfate and nickel sulfate in sequence, such that the molar ratio of nickel, iron and manganese is 0.33:0.33:0.33, and prepare a second salt solution with a concentration of 98 g / L. Prepare a precipitant with a mass fraction of 25% from sodium hydroxide, and prepare a complexing agent with a concentration of 8.5 mol / L from ammonia water;

[0068] (2) Simultaneously and countercurrently add the first salt solution, the precipitant and the complexing agent into a reaction kettle for a first coprecipitation reaction. Control the reaction temperature at 60 °C, the pH at 10.5, the ammonia water concentration at 0.34 mol / L, and the rotation speed of the reaction kettle at 350 rmp. After the particle size reaches 10 μm, stop introducing the first salt solution, add the second salt solution for reaction. After the particle size reaches 12 μm, age and stand still for 12 h, filter, wash with deionized water twice, and dry at 100 °C for 10 h to obtain a hydroxide precursor with a core-shell structure, where the core precursor is Mn 0.85 Cu 0.15 (OH) 2 and the shell precursor is Ni 0.33 Fe 0.33 Mn 0.33 (OH) 2 ;

[0069] (3) Uniformly mix the hydroxide precursor obtained in step (2) with sodium carbonate according to a molar ratio of Na to transition metals (Ni, Fe, Mn and Cu) of 0.8:1. Under an air atmosphere, heat it to 500 °C at a heating rate of 3 °C / min and hold for 5 h, then heat it to 700 °C at a heating rate of 3 °C / min and hold for 15 h, and cool it naturally to obtain a layered oxide cathode material with a core-shell structure, where the core is Na 0.8 Mn 0.85 Cu 0.15 O 2 and the shell is Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33 O 2 ;

[0070] Example 2

[0071] This example provides a layered oxide cathode material, including a core of Na 0.8 Mn 0.8 Ni 0.2 O 2 and a shell of Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33 O 2, the thickness of the outer shell is 0.8 μm, and the D50 particle size of the layered oxide cathode material is 8 μm.

[0072] This embodiment also provides a preparation method of the above-mentioned layered oxide cathode material, including:

[0073] (1) Weigh manganese sulfate and nickel sulfate in sequence, so that the molar ratio of manganese to nickel is 0.8:0.2, and make a first salt solution with a concentration of 105 g / L; weigh manganese sulfate, ferrous sulfate and nickel sulfate in sequence, so that the molar ratio of nickel, iron and manganese is 0.33:0.33:0.33, and make a second salt solution with a concentration of 98 g / L. Prepare sodium hydroxide into a precipitant with a mass fraction of 30%, and prepare ammonia water into a complexing agent with a concentration of 9 mol / L;

[0074] (2) Simultaneously and co-currently add the first salt solution, the precipitant and the complexing agent into the reaction kettle for the first co-precipitation reaction. Control the reaction temperature at 70 °C, the pH at 10, the ammonia water concentration at 0.3 mol / L, and the rotation speed of the reaction kettle at 380 rmp. When the particle size reaches 6.4 μm, stop feeding the first salt solution, add the second salt solution for reaction. When the particle size reaches 8 μm, age and stand still for 12 h, filter, wash twice with deionized water, and dry at 110 °C for 10 h to obtain a core precursor of Mn 0.8 Ni 0.2 (OH) 2 and a shell precursor of Ni 0.33 Fe 0.33 Mn 0.33 (OH) 2 of a core-shell structured hydroxide precursor, and its cross-sectional SEM image and sectional Ni element distribution map are respectively as Figure 1 and Figure 2 shown;

[0075] (3) Uniformly mix the hydroxide precursor obtained in step (2) with sodium carbonate according to the molar ratio of Na to transition metals (Ni, Fe, Mn and Cu) of 0.85:1. Under an air atmosphere, heat it to 550 °C at a heating rate of 3 °C / min and hold for 4 h, then heat it to 650 °C at a heating rate of 3 °C / min and hold for 16 h, and cool it naturally to obtain a core of Na 0.8 Mn 0.85 Cu 0.15 O 2 and a shell of Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33 O 2 of a core-shell structured layered oxide cathode material, and its SEM image is as Figure 3 shown.

[0076] Example 3

[0077] This embodiment provides a layered oxide cathode material, including a core of Na 0.8 Mn 0.85 Cu 0.15 O 2 and a shell of Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33 O 2 coated on the surface of the core. The thickness of the shell is 1.5 μm, and the D50 particle size of the layered oxide cathode material is 15 μm.

[0078] This embodiment also provides a preparation method of the above-mentioned layered oxide cathode material, including:

[0079] (1) Weigh manganese sulfate and copper sulfate in sequence, with the molar ratio of manganese to copper being 0.85:0.15, to prepare a first salt solution with a concentration of 80 g / L; weigh manganese sulfate, ferrous sulfate and nickel sulfate in sequence, with the molar ratio of nickel, iron and manganese being 0.33:0.33:0.33, to prepare a second salt solution with a concentration of 80 g / L. Prepare a precipitant with a mass fraction of 35% from sodium hydroxide, and prepare a complexing agent with a concentration of 8.5 mol / L from ammonia water;

[0080] (2) Simultaneously and co-currently add the first salt solution, the precipitant and the complexing agent into a reaction kettle for a first co-precipitation reaction. Control the reaction temperature at 55 °C, the pH at 11, the ammonia water concentration at 0.4 mol / L, and the rotation speed of the reaction kettle at 380 rmp. After the particle size reaches 12 μm, stop introducing the first salt solution and add the second salt solution for reaction. After the particle size reaches 15 μm, age and stand still for 12 h, filter, wash twice with deionized water, and dry at 100 °C for 10 h to obtain a core precursor of Mn 0.85 Cu 0.15 (OH) 2 and a shell precursor of Ni 0.33 Fe 0.33 Mn 0.33 (OH) 2 in the form of a core-shell structured hydroxide precursor;

[0081] (3) Uniformly mix the hydroxide precursor obtained in step (2) with sodium carbonate according to the molar ratio of Na to transition metals (Ni, Fe, Mn and Cu) being 0.8:1. Under an air atmosphere, heat it to 500 °C at a heating rate of 3 °C / min and hold for 5 h, then heat it to 700 °C at a heating rate of 3 °C / min and hold for 15 h, and cool it naturally to obtain a core of Na 0.8 Mn 0.85 Cu 0.15 O 2 and a shell of Na 0.8 Ni0.33 Fe 0.33 Mn 0.33 O 2 Layered oxide cathode material with a core-shell structure

[0082] Example 4

[0083] Except that in step (2), when the particle size reaches 9 μm, the first salt solution is stopped from being introduced, and the second salt solution is added for reaction so that the thickness of the outer shell is 15% of the D50 particle size of the layered oxide cathode material, the rest is the same as in Example 1

[0084] Example 5

[0085] Except that in step (2), when the particle size reaches 11 μm, the first salt solution is stopped from being introduced, and the second salt solution is added for reaction so that the thickness of the outer shell is 3% of the D50 particle size of the layered oxide cathode material, the rest is the same as in Example 1

[0086] Example 6

[0087] Except that the pH of both the first coprecipitation and the second coprecipitation is replaced with 8.5, the rest is the same as in Example 1

[0088] Example 7

[0089] Except that the pH of both the first coprecipitation and the second coprecipitation is replaced with 12, the rest is the same as in Example 1

[0090] Example 8

[0091] Except that in step (2), the concentration of ammonia water (complexing agent) during the coprecipitation reaction is replaced with 0.09 mol / L, the rest is the same as in Example 1

[0092] Example 9

[0093] Except that in step (2), the concentration of ammonia water (complexing agent) during the coprecipitation reaction is replaced with 0.6 mol / L, the rest is the same as in Example 1

[0094] Example 10

[0095] Except that in step (3), the temperature of the first sintering is replaced from 500 °C to 400 °C, the rest is the same as in Example 1

[0096] Example 11

[0097] Except that in step (3), the temperature of the first sintering is replaced from 500 °C to 600 °C, the rest is the same as in Example 1

[0098] Example 12

[0099] Except that the temperature of the second sintering in step (3) is replaced from 700 °C to 550 °C, the rest is the same as in Example 1.

[0100] Example 13

[0101] Except that the temperature of the second sintering in step (3) is replaced from 700 °C to 850 °C, the rest is the same as in Example 1.

[0102] Comparative Example 1

[0103] Except that only the first salt solution is introduced in step (2) and the second salt solution is not introduced, the rest is the same as in Example 1;

[0104] In this comparative example, a Na 0.8 Mn 0.85 Cu 0.15 O 2 positive electrode material with a D50 particle size of 12 μm is prepared.

[0105] Comparative Example 2

[0106] Except that only the second salt solution is introduced in step (2) and the first salt solution is not introduced, the rest is the same as in Example 1;

[0107] In this comparative example, a Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33 O 2 positive electrode material with a D50 particle size of 12 μm is prepared.

[0108] Comparative Example 3

[0109] Except that the first salt solution and the second salt solution are not introduced, and only a manganese sulfate solution with a concentration of 98 g / L is introduced, the rest is the same as in Example 1;

[0110] In this comparative example, a Na 0.8 MnO 2 positive electrode material with a D50 particle size of 12 μm is prepared.

[0111] I. Preparation of Sodium-ion Battery

[0112] The cathode materials prepared in the above examples and comparative examples were mixed evenly with conductive carbon black and polyvinylidene fluoride in a mass ratio of 90:5:5, placed in a high-speed stirrer mold, and after adding an appropriate amount of N-methylpyrrolidone, stirred at a speed of 3000 rpm for 10 min to obtain a slurry with appropriate viscosity. Then the slurry was coated on a clean current collector and placed in a vacuum drying oven for drying for 24 h. After the dried electrode sheet was rolled to an appropriate thickness, it was punched into an electrode sheet with a diameter of 10 mm, and dried in a vacuum environment with a separator and a battery case for 12 h. Finally, in a glove box, sodium sheets were used as counter electrodes to assemble 2025-type button cells.

[0113] II. Electrochemical performance test

[0114] The charge-discharge test was carried out using a BlueTEC CT2001A type electrochemical tester, with a voltage range of 2.5 - 4.0 V, a test current density of 0.2 C, and 50 cycles. Record the initial specific capacity of the battery and the specific capacity after 50 cycles. Divide the specific capacity after 50 cycles by the initial specific capacity to obtain the capacity retention rate after 50 cycles. The experimental results are shown in Table 1.

[0115] Table 1

[0116] Serial number <![CDATA[Initial specific capacity (mAh g -1 )]]> Capacity retention rate after 50 cycles (%) Example 1 160.3 84.1 Example 2 159.3 84.5 Example 3 161.2 85.1 Example 4 158.1 82.2 Example 5 159.5 82.1 Example 6 158.1 80.2 Example 7 157.2 81.3 Example 8 157.3 80.4 Example 9 156.6 82.2 Example 10 158.2 80.1 Example 11 159.9 80.9 Example 12 157.2 75.8 Example 13 157.3 78.9 Comparative example 1 155.2 71.1 Comparative example 2 150.3 83.2 Comparative example 3 158.2 50.1

[0117] In summary, from Examples 1 - 13, it can be seen that the layered oxide cathode material of the present invention has a core-shell structure. The inner core includes Na x Mn a M 1-a O 2 , and the outer shell includes Na x Ni b Mn c Fe d O 2 . The inner core and the outer shell act synergistically to prevent the dissolution of Mn, reduce the side reactions between the material and the electrolyte during the reaction, and improve the discharge capacity and cycling performance of the layered oxide cathode material.

[0118] By comparing Example 1 with Examples 4 - 5, it can be seen that in the present invention, by controlling the particle sizes of the inner core and the outer shell, the synergistic effect between the core and the shell is fully exerted, improving the initial specific capacity and cycling performance of the material. In Example 4, the outer shell is too thick, resulting in low material capacity and decreased cycling performance; in Example 5, the outer shell is too thin, resulting in decreased cycling performance of the material. Therefore, Example 1 has a higher capacity and better cycling performance.

[0119] From the comparison between Example 1 and Examples 6 - 9, it can be seen that in the present invention, by controlling the reaction conditions of the two co - precipitation reactions and the concentration of the complexing agent, the structural stability of the material can be further optimized. If the pH value is too high or too low in Examples 6 - 7, or if the complexing agent is too high or too low in Examples 8 - 9, the cycling performance of the material will be unsatisfactory. Therefore, compared with Examples 6 - 9, Example 1 has a higher capacity and better cycling performance.

[0120] From the comparison between Example 1 and Examples 10 - 13, it can be seen that in the present invention, by two - step sintering and further optimizing the temperatures of the first sintering and the second sintering, the residual sodium in the material can be reduced; compared with Examples 10 - 13, Example 1 has better cycling performance.

[0121] From the comparison between Example 1 and Comparative Examples 1 - 3, it can be seen that in the present invention, when using only Na 0.8 Mn 0.85 Cu 0.15 O 2 as the cathode material, Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33 O 2 as the cathode material, or Na 0.8 MnO 2 as the cathode material, the effect of core - shell synergy to improve the specific capacity and cycling performance of the material in this application cannot be achieved; in Comparative Example 1, using Na 0.8 Mn 0.85 Cu 0.15 O 2 as the cathode material, manganese is easily dissolved in the electrolyte, affecting the specific capacity and cycling stability of the material. Therefore, both the initial specific capacity and the capacity retention rate after 50 cycles of the comparative example are significantly worse than those of Example 1; in Comparative Example 2, using Na 0.8 Ni 0.33 Fe 0.33 Mn 0.33 O 2 as the cathode material, its initial specific capacity is 10 mAh g lower than that of this application -1 , and the effect is significantly worse than that of this application; in Comparative Example 3, although the initial specific capacity of Na 0.8 MnO 2 as the cathode material is not much different from that of Example 1, its capacity retention rate after 50 cycles is only 50.1%, and the cycling performance is extremely poor; in summary, the comprehensive electrochemical performances of Comparative Examples 1 - 3 are all significantly worse than those of this application.

[0122] The above are only specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A layered oxide cathode material, characterized in that, The layered oxide cathode material includes a core and a shell coated on the surface of the core, and the core includes Na x Mn a M 1-a O 2 , and the shell includes Na x Ni b Mn c Fe d O 2 , where 0.7 < x ≤ 0.9, 0.8 ≤ a < 1, 0.2 ≤ b < 0.5, 0.2 ≤ c < 0.6, 0.2 ≤ d ≤ 0.5, and M includes any one or at least two combinations of Ni, Ti, Fe, and Cu; the D50 particle size of the layered oxide cathode material is 3 - 15 μm; the thickness of the outer shell is 5 - 10% of the D50 particle size of the layered oxide cathode material.

2. A preparation method of the layered oxide cathode material according to claim 1, characterized in that, the preparation method includes: (1) Mixing a first salt solution, a precipitating agent and a complexing agent to carry out a first co - precipitation reaction. The first salt solution includes Mn and M with a molar ratio of a:(1 - a), obtaining a core precursor. Adding a second salt solution to carry out a second co - precipitation reaction. The second salt solution includes Ni, Mn and Fe with a molar ratio of b:c:d, generating an outer shell precursor on the surface of the core precursor to obtain a hydroxide precursor; The temperature of the first coprecipitation and the second coprecipitation is independently 40 to 70 °C, the pH value of the first coprecipitation and the second coprecipitation is independently 9.5 to 11.5, based on the volume of the solution of the first coprecipitation and the solution of the second coprecipitation, the concentration of the complexing agent is independently 0.1 to 0.5 mol / L, and the core precursor includes Mn a M 1-a (OH) 2 , and the shell precursor includes Ni b Mn c Fe d (OH) 2 , and the D50 particle size of the core precursor is 80 to 90% of the D50 particle size of the hydroxide precursor; (2) Mixing the hydroxide precursor and a sodium source, sintering at 450 - 550 °C for the first time for 4 - 6 h, and then sintering at 600 - 800 °C for the second time for 10 - 16 h to obtain the layered oxide cathode material.

3. The preparation method according to claim 2, characterized in that, the types of salts in the first salt solution and the second salt solution are independently any one or a combination of at least two of chlorides, oxalates, sulfates and nitrates.

4. The preparation method according to claim 2, characterized in that, the concentrations of the first salt solution and the second salt solution are independently 80 - 120 g / L.

5. The preparation method according to claim 2, characterized in that, the precipitating agent includes an aqueous sodium hydroxide solution.

6. The preparation method according to claim 2, characterized in that, calculated based on the mass of the precipitating agent being 100%, the mass fraction of the solute in the precipitating agent is 20 - 40%.

7. The preparation method according to claim 2, characterized in that, the complexing agent includes any one or a combination of at least two of ammonia water, oxalic acid, lactic acid, sodium oxalate and EDTA solution.

8. The preparation method according to claim 2, characterized in that, calculated based on the volume of the complexing agent, the concentration of the complexing agent is 8 - 10 mol / L.

9. The preparation method according to claim 2, characterized in that, the rotation speeds of the first co - precipitation and the second co - precipitation are independently 320 - 380 rpm / min.

10. The preparation method according to claim 2, characterized in that, after the second co - precipitation and before sintering, it further includes the steps of aging, filtering, washing and drying.

11. The preparation method according to claim 10, characterized in that, the drying temperature is 100 - 120 °C.

12. The preparation method according to claim 2, characterized in that, the molar ratio of Na in the sodium source to the sum of Ni, Mn, Fe and M in the hydroxide precursor is 0.7 - 0.

9.

13. The preparation method according to claim 2, characterized in that, the sodium source includes any one or a combination of at least two of sodium hydroxide, sodium carbonate, sodium oxalate, sodium chloride and sodium nitrate.

14. The preparation method according to claim 2, characterized in that, The heating rate of the first sintering is 3 to 5 °C / min.

15. According to the preparation method described in claim 2, it is characterized in that the heating rate of the second sintering is 3 to 5 °C / min.

16. A sodium-ion battery, it is characterized in that the positive electrode of the sodium-ion battery includes the layered oxide positive electrode material according to claim 1.

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