Co-coated gradient-doped sodium battery positive electrode material, preparation method therefor, and use thereof

Abstract

A Co-coated gradient-doped sodium battery positive electrode material, a preparation method therefor, and the use thereof. The sodium battery positive electrode material comprises a Co coating layer and a gradient-doped sodium battery inner core; the sodium battery inner core is a layered transition metal oxide having the chemical formula: NaxNiaFebCucZndMneO2, wherein 0.5 ≤ x ≤ 1, 0.1 ≤ a ≤ 0.4, 0.2 ≤ b ≤ 0.4, 0.01 ≤ c ≤ 0.2, 0.01 ≤ d ≤ 0.2, 0.2 ≤ e ≤ 0.5, and a + b + c + d + e = 1; Ni and Fe content gradually reduces from the center of the sodium battery inner core to the surface, while Cu, Zn, and Mn content gradually increases. Gradient doping is carried out on the sodium battery inner core of the sodium battery positive electrode material, and a Co coating layer is arranged on the surface thereof, thus significantly improving the stability of the layered oxide sodium battery positive electrode material, and facilitating widespread popularization and use.

Description

Co-coated gradient-doped sodium cathode material and its preparation method and application Technical Field

[0001] The present application belongs to the field of battery manufacturing technology and relates to a sodium battery positive electrode material, and in particular to a Co-coated gradient-doped sodium battery positive electrode material and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries, with their high energy density and long cycle life, are expected to be widely used in energy storage systems to promote the transformation of humanity's energy structure. However, the scarcity, uneven distribution, and high price of lithium resources have limited their large-scale application.

[0003] As a suitable alternative, sodium-ion batteries (SIBs) have become a key development direction in the energy storage field due to their abundant resource reserves and similar energy storage mechanisms to lithium-ion batteries. Like lithium-ion batteries, the energy storage performance of SIBs is primarily influenced by the cathode material. Therefore, those skilled in the art are urgently seeking a cathode material with high energy density, low cost, and long cycle life.

[0004] Currently, sodium-ion battery cathode materials primarily include layered oxides, Prussian blue / white compounds, and polyanionic compounds. Layered oxides, with their preparation processes similar to those for lithium-ion battery ternary materials, offer high production line reusability, high energy density, excellent cycle performance, and high rate capability, making them poised to become the mainstream cathode material for sodium-ion batteries.

[0005] However, the transition metal oxide material Na x MO2 is highly hygroscopic and will absorb moisture from the air even for a very short time, thus affecting its electrochemical performance. Therefore, the structure of transition metal oxide materials is extremely sensitive to conditions such as synthesis conditions and sodium content, and their stability is slightly inferior to other cathode materials.

[0006] It can be seen that how to provide a sodium cathode material, especially to improve the stability of layered oxide sodium cathode materials, has become an urgent problem that technicians in this field need to solve.

[0007] Summary of the Invention

[0008] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0009] In response to the shortcomings of the existing technology, the purpose of this application is to provide a Co-coated gradient-doped sodium battery positive electrode material and its preparation method and application. By gradient-doping the sodium battery core of the sodium battery positive electrode material and providing a Co coating layer on the surface, the stability of the layered oxide sodium battery positive electrode material is significantly improved, which is conducive to large-scale promotion and application.

[0010] To achieve this goal, this application adopts the following technical solutions:

[0011] In a first aspect, the present application provides a Co-coated gradient-doped sodium battery cathode material, wherein the sodium battery cathode material comprises a Co coating layer and a gradient-doped sodium battery core.

[0012] The sodium core is a layered transition metal oxide with the chemical formula: Na x Ni a Fe b Cu c Zn d Mn e O2.

[0013] Wherein: 0.5≤x≤1, 0.1≤a≤0.4, 0.2≤b≤0.4, 0.01≤c≤0.2, 0.01≤d≤0.2, 0.2≤e≤0.5, and a+b+c+d+e=1, for example, x=0.5, 0.6, 0.7, 0.8, 0.9 or 1, a=0.1, 0.15, 0.2, 0.25, 0.3, 0.35 or 0.4, b=0.2, 0.22, 0.24, 0.26, 0.28, 0.3, 0.32, 0.34, 0.36, 0.38 or 0.4, c = 0.01, 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18 or 0.2, d = 0.01, 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.16, 0.18 or 0.2, e = 0.2, 0.25, 0.3, 0.35, 0.4, 0.45 or 0.5, but is not limited to the listed values, other values ​​not listed within the numerical range are also applicable.

[0014] From the center to the surface of the sodium battery core, the contents of Ni and Fe gradually decrease, and the contents of Cu, Zn and Mn gradually increase.

[0015] In the sodium battery positive electrode material provided in the present application, Ni and Fe provide high capacity for the battery, and their content gradually decreases from the inside to the outside; the Mn content gradually increases from the inside to the outside, reducing its side reactions with the electrolyte, thereby improving the cycle stability of the battery; the Cu and Zn content gradually increases from the inside to the outside, reducing the water absorption of the layered transition metal oxide, thereby improving the stability of the positive electrode material; the outermost layer coated with Co is beneficial to increasing the operating voltage of the battery, thereby further increasing the battery capacity.

[0016] In one embodiment, the composition of the Co coating layer includes any one or a combination of at least two of CoO, Co2O3 or Co3O4, typical but non-limiting combinations include a combination of CoO and Co2O3, a combination of Co2O3 and Co3O4, a combination of CoO and Co3O4, or a combination of CoO, Co2O3 and Co3O4.

[0017] In one embodiment, the mass of the Co coating layer is 1%-2% of the total mass of the sodium cathode material, for example, it can be 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9% or 2%, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0018] In one embodiment, the average particle size of the sodium positive electrode material is 3-20 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, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm or 20 μm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0019] In a second aspect, the present application provides a method for preparing the sodium cathode material as described in the first aspect, the preparation method comprising the following steps:

[0020] (1) mixing nickel salt, ferrous salt and deionized water to obtain a first salt solution;

[0021] (2) mixing a copper salt, a zinc salt, a manganese salt, and deionized water to obtain a second salt solution;

[0022] (3) mixing the first salt solution, the second salt solution, the precipitant solution, and the complexing agent solution to perform a coprecipitation reaction, wherein during the reaction, the flow rate of the first salt solution decreases at a uniform rate, the flow rate of the second salt solution increases at a uniform rate, and the sum of the flow rates of the first salt solution and the second salt solution remains unchanged, thereby obtaining an intermediate solution containing a sodium electrode precursor;

[0023] (4) mixing the intermediate solution, the cobalt salt solution, the precipitant solution, and the complexing agent solution to continue the coprecipitation reaction, and obtaining the Co-coated gradient-doped sodium electrode precursor after solid-liquid separation;

[0024] (5) Mixing the sodium source and the sodium battery precursor described in step (4) and sintering them to obtain a Co-coated gradient-doped sodium battery positive electrode material.

[0025] Among them, step (1) and step (2) are performed in no particular order.

[0026] In one embodiment, the nickel salt and the ferrous salt in step (1) are independently any one of sulfate, nitrate, acetate or chloride salts of the corresponding metal ions, or a combination of at least two thereof. Typical but non-limiting combinations include a combination of sulfate and nitrate, a combination of nitrate and acetate, a combination of acetate and chloride, a combination of sulfate, nitrate and acetate, or a combination of nitrate, acetate and chloride.

[0027] In one embodiment, the total concentration of metal ions in the first salt solution in step (1) is 1-2 mol / L, for example, 1 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L or 2 mol / L, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0028] In one embodiment, the first salt solution in step (1) is also mixed with ascorbic acid to prevent Fe 2+ Oxidized.

[0029] In one embodiment, the concentration of ascorbic acid in the first salt solution is 0.2-0.5 g / L, for example, 0.2 g / L, 0.25 g / L, 0.3 g / L, 0.35 g / L, 0.4 g / L, 0.45 g / L or 0.5 g / L, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0030] In one embodiment, the copper salt, zinc salt and manganese salt in step (2) are independently any one of sulfate, nitrate, acetate or chloride salts of the corresponding metal ions, or a combination of at least two thereof. Typical but non-limiting combinations include a combination of sulfate and nitrate, a combination of nitrate and acetate, a combination of acetate and chloride, a combination of sulfate, nitrate and acetate, or a combination of nitrate, acetate and chloride.

[0031] In one embodiment, the total metal ion concentration in the second salt solution in step (2) is 1-2 mol / L, for example, 1 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L or 2 mol / L, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0032] In one embodiment, the precipitant in the precipitant solution in step (3) comprises any one or a combination of at least two of sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate or ammonium bicarbonate. Typical but non-limiting combinations include a combination of sodium hydroxide and potassium hydroxide, a combination of potassium hydroxide and sodium carbonate, a combination of sodium carbonate and sodium bicarbonate, a combination of sodium bicarbonate and ammonium bicarbonate, a combination of sodium hydroxide, potassium hydroxide and sodium carbonate, a combination of potassium hydroxide, sodium carbonate and sodium bicarbonate, or a combination of sodium carbonate, sodium bicarbonate and ammonium bicarbonate. Sodium hydroxide may further be selected.

[0033] In one embodiment, the complexing agent in the complexing agent solution of step (3) includes any one or a combination of at least two of ammonia water, EDTA, citric acid, sodium citrate, oxalic acid, sodium oxalate, acetic acid, ammonium sulfate or sodium pyrophosphate. Typical but non-limiting combinations include a combination of ammonia water and EDTA, a combination of EDTA and citric acid, a combination of citric acid and sodium citrate, a combination of sodium citrate and oxalic acid, a combination of oxalic acid and sodium oxalate, a combination of sodium oxalate and acetic acid, a combination of acetic acid and ammonium sulfate, or a combination of ammonium sulfate and sodium pyrophosphate. Ammonia water can further be selected.

[0034] In one embodiment, the coprecipitation reaction in step (3) is carried out in a protective atmosphere, and the protective atmosphere includes any one or a combination of at least two of nitrogen, helium or argon. Typical but non-limiting combinations include a combination of nitrogen and helium, a combination of helium and argon, a combination of nitrogen and argon, or a combination of nitrogen, helium and argon, and nitrogen can further be selected.

[0035] In one embodiment, the concentration of the complexing agent in the coprecipitation reaction solution in step (3) is 0.1-1 mol / L, for example, it can be 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L or 1 mol / L, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0036] In one embodiment, the pH value of the solution in the coprecipitation reaction in step (3) is 11-13, for example, it can be 11, 11.2, 11.4, 11.6, 11.8, 12, 12.2, 12.4, 12.6, 12.8 or 13, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0037] In one embodiment, the temperature of the coprecipitation reaction in step (3) is 40-80°C, for example, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C or 80°C, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0038] In one embodiment, the coprecipitation reaction time in step (3) is 40-100h, for example, it can be 40h, 45h, 50h, 55h, 60h, 65h, 70h, 75h, 80h, 85h, 90h, 95h or 100h, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0039] In one embodiment, the coprecipitation reaction in step (3) is accompanied by stirring, and the stirring rate is 200-500 rpm, for example, it can be 200 rpm, 250 rpm, 300 rpm, 350 rpm, 400 rpm, 450 rpm or 500 rpm, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0040] In one embodiment, the cobalt salt in the cobalt salt solution of step (4) comprises any one or a combination of at least two of cobalt sulfate, cobalt nitrate, cobalt acetate or cobalt chloride. Typical but non-limiting combinations include a combination of cobalt sulfate and cobalt nitrate, a combination of cobalt nitrate and cobalt acetate, a combination of cobalt acetate and cobalt chloride, a combination of cobalt sulfate, cobalt nitrate and cobalt acetate, or a combination of cobalt nitrate, cobalt acetate and cobalt chloride.

[0041] In one embodiment, the concentration of the cobalt salt solution in step (4) is 0.3-1 mol / L, for example, 0.3 mol / L, 0.35 mol / L, 0.4 mol / L, 0.45 mol / L, 0.5 mol / L, 0.55 mol / L, 0.6 mol / L, 0.65 mol / L, 0.7 mol / L, 0.75 mol / L, 0.8 mol / L, 0.85 mol / L, 0.9 mol / L, 0.95 mol / L or 1 mol / L, but is not limited to the listed values, and other values ​​not listed within this numerical range are also applicable.

[0042] In one embodiment, the coprecipitation reaction time in step (4) is 3-5 hours, and the other conditions are the same as those in step (3), for example, it can be 3 hours, 3.2 hours, 3.4 hours, 3.6 hours, 3.8 hours, 4 hours, 4.2 hours, 4.4 hours, 4.6 hours, 4.8 hours or 5 hours, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0043] In one embodiment, the solid-liquid separation in step (4) comprises filtration and / or centrifugation.

[0044] In one embodiment, after the solid-liquid separation in step (4), the sodium electrolyte precursor is washed and dried in sequence.

[0045] In one embodiment, the washing is performed until the sodium precursor is neutral.

[0046] In one embodiment, the drying temperature is 50-150°C, for example, it can be 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, 130°C, 135°C, 140°C, 145°C or 150°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0047] In one embodiment, the sodium source in step (5) includes sodium carbonate and / or sodium bicarbonate, and can further be sodium carbonate.

[0048] In one embodiment, the sintering process in step (5) is performed in an oxygen atmosphere.

[0049] In one embodiment, the temperature of the sintering treatment in step (5) is 800-1300°C, for example, it can be 800°C, 850°C, 900°C, 950°C, 1000°C, 1050°C, 1100°C, 1150°C, 1200°C, 1250°C or 1300°C, but is not limited to the listed values, and other unlisted values ​​within this numerical range are also applicable.

[0050] In one embodiment, the sintering treatment time in step (5) is 10-25 hours, for example, it can be 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, 19 hours, 20 hours, 21 hours, 22 hours, 23 hours, 24 hours or 25 hours, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0051] In one embodiment, after the sintering treatment in step (5), the sodium cathode material is crushed and screened in sequence.

[0052] As an optional technical solution of the second aspect of the present application, the preparation method comprises the following steps:

[0053] (1) mixing a nickel salt, a ferrous salt, and deionized water to obtain a first salt solution having a total metal ion concentration of 1-2 mol / L; the nickel salt and the ferrous salt are independently any one of sulfate, nitrate, acetate, or chloride salts of the corresponding metal ions, or a combination of at least two thereof; and the first salt solution is further mixed with ascorbic acid having a concentration of 0.2-0.5 g / L;

[0054] (2) mixing a copper salt, a zinc salt, a manganese salt, and deionized water to obtain a second salt solution having a total metal ion concentration of 1-2 mol / L; wherein the copper salt, zinc salt, and manganese salt are independently any one of sulfate, nitrate, acetate, or chloride salts of the corresponding metal ions, or a combination of at least two thereof;

[0055] (3) Under nitrogen protection, the first salt solution, the second salt solution, the sodium hydroxide solution and the ammonia solution are added to the reactor in parallel, and a coprecipitation reaction is carried out at 40-80° C. for 40-100 hours, accompanied by stirring at a rate of 200-500 rpm. During the reaction, the ammonia concentration in the solution is maintained at 0.1-1 mol / L, the pH value of the solution is 11-13, and the flow rate of the first salt solution is controlled to decrease at a uniform rate, the flow rate of the second salt solution is controlled to increase at a uniform rate, and the sum of the flow rates of the first salt solution and the second salt solution remains unchanged, to obtain an intermediate solution containing a sodium electrode precursor;

[0056] (4) Under nitrogen protection, 0.3-1 mol / L cobalt salt solution, sodium hydroxide solution and ammonia water are added to the reactor in parallel to continue the coprecipitation reaction for 3-5 hours, and the reaction conditions are kept unchanged, filtered and / or centrifuged, washed to neutrality, and dried at 50-150° C. to obtain a Co-coated gradient-doped sodium electrode precursor; the cobalt salt in the cobalt salt solution includes any one of cobalt sulfate, cobalt nitrate, cobalt acetate or cobalt chloride, or a combination of at least two thereof;

[0057] (5) mixing sodium carbonate and the sodium battery precursor described in step (4), sintering the mixture at 800-1300° C. for 10-25 h in an oxygen atmosphere, crushing and sieving to obtain a Co-coated gradient-doped sodium battery positive electrode material.

[0058] Among them, step (1) and step (2) are performed in no particular order.

[0059] In a third aspect, the present application provides a sodium ion battery, which comprises the sodium cathode material as described in the first aspect.

[0060] The numerical range described in this application includes not only the point values ​​listed above, but also any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, this application no longer exhaustively lists the specific point values ​​included in the range.

[0061] Compared with the prior art, the present invention has the following advantages:

[0062] In the sodium battery positive electrode material provided in the present application, Ni and Fe provide high capacity for the battery, and their content gradually decreases from the inside to the outside; the Mn content gradually increases from the inside to the outside, reducing its side reactions with the electrolyte, thereby improving the cycle stability of the battery; the Cu and Zn content gradually increases from the inside to the outside, reducing the water absorption of the layered transition metal oxide, thereby improving the stability of the positive electrode material; the outermost layer coated with Co is beneficial to increasing the operating voltage of the battery, thereby further increasing the battery capacity.

[0063] Still other aspects will become apparent upon reading and understanding the detailed description. DETAILED DESCRIPTION

[0064] The technical solution of this application is further explained below through specific implementation methods.

[0065] Example 1

[0066] This embodiment provides a Co-coated gradient-doped sodium cathode material and a preparation method thereof, the preparation method comprising the following steps:

[0067] (1) mixing nickel sulfate, ferrous sulfate, and deionized water to obtain a first salt solution having a total metal ion concentration of 2 mol / L; the first salt solution is further mixed with ascorbic acid having a concentration of 0.5 g / L;

[0068] (2) mixing copper sulfate, zinc sulfate, manganese sulfate, and deionized water to obtain a second salt solution having a total metal ion concentration of 2 mol / L;

[0069] (3) Under nitrogen protection, the first salt solution, the second salt solution, the sodium hydroxide solution and the ammonia solution are added to the reactor in parallel, and a coprecipitation reaction is carried out at 80°C for 100 hours, accompanied by stirring at a rate of 500 rpm. During the reaction, the ammonia concentration in the solution is maintained at 1 mol / L and the pH value of the solution is 13. At the same time, the flow rate of the first salt solution is controlled to decrease at a uniform rate, the flow rate of the second salt solution is controlled to increase at a uniform rate, and the sum of the flow rates of the first salt solution and the second salt solution remains unchanged, thereby obtaining an intermediate solution containing a sodium electrode precursor;

[0070] (4) Under nitrogen protection, 0.5 mol / L cobalt sulfate solution, sodium hydroxide solution and ammonia water were added to the reactor in parallel to continue the coprecipitation reaction for 5 hours, and the reaction conditions were kept unchanged. After centrifugation, the mixture was washed to neutrality and dried at 100°C to obtain a Co-coated gradient-doped sodium electrode precursor;

[0071] (5) Sodium carbonate and the sodium battery precursor described in step (4) were mixed in a molar ratio of 1:1, sintered at 1300° C. for 10 h in an oxygen atmosphere, and crushed and sieved to obtain a Co-coated gradient-doped sodium battery positive electrode material with an average particle size of 10 μm.

[0072] The sodium cathode material obtained in this embodiment includes a Co coating layer and a gradient-doped sodium core. The sodium core is a layered transition metal oxide with the chemical formula: NaNi 0.3 Fe 0.2 Cu 0.05 Zn 0.05 Mn 0.4 O2; from the center to the surface of the sodium battery core, the content of Ni and Fe gradually decreases, and the content of Cu, Zn and Mn gradually increases.

[0073] In this embodiment, the composition of the Co coating layer includes CoO, Co2O3 or Co3O4; the mass of the Co coating layer is 1.5% of the total mass of the sodium battery positive electrode material.

[0074] Example 2

[0075] This embodiment provides a Co-coated gradient-doped sodium cathode material and a preparation method thereof, the preparation method comprising the following steps:

[0076] (1) mixing nickel nitrate, ferrous nitrate, and deionized water to obtain a first salt solution having a total metal ion concentration of 1.5 mol / L; the first salt solution is further mixed with ascorbic acid having a concentration of 0.3 g / L;

[0077] (2) mixing copper nitrate, zinc nitrate, manganese nitrate, and deionized water to obtain a second salt solution having a total metal ion concentration of 1.5 mol / L;

[0078] (3) Under nitrogen protection, the first salt solution, the second salt solution, the sodium hydroxide solution and the ammonia solution were added to the reactor in parallel, and a coprecipitation reaction was carried out at 70°C for 80 hours, accompanied by stirring at a rate of 400 rpm. During the reaction, the ammonia concentration in the solution was maintained at 0.5 mol / L and the pH value of the solution was 12. At the same time, the flow rate of the first salt solution was controlled to decrease at a uniform rate, the flow rate of the second salt solution was controlled to increase at a uniform rate, and the sum of the flow rates of the first salt solution and the second salt solution remained unchanged, thereby obtaining an intermediate solution containing a sodium electrode precursor;

[0079] (4) Under nitrogen protection, 1 mol / L cobalt nitrate solution, sodium hydroxide solution and ammonia water were added to the reactor in parallel and the coprecipitation reaction was continued for 4 hours while keeping the reaction conditions unchanged. After centrifugation, the mixture was washed to neutrality and dried at 50°C to obtain a Co-coated gradient-doped sodium electrode precursor.

[0080] (5) Sodium carbonate and the sodium battery precursor described in step (4) were mixed in a molar ratio of 1.02:1, sintered at 1000° C. for 15 h in an oxygen atmosphere, and crushed and sieved to obtain a Co-coated gradient-doped sodium battery positive electrode material with an average particle size of 3 μm.

[0081] The sodium cathode material obtained in this embodiment includes a Co coating layer and a gradient-doped sodium core. The sodium core is a layered transition metal oxide with the chemical formula: Na 0.9 Ni 0.2 Fe 0.3 Cu 0.08 Zn 0.02 Mn 0.4 O2; from the center to the surface of the sodium battery core, the content of Ni and Fe gradually decreases, and the content of Cu, Zn and Mn gradually increases.

[0082] In this embodiment, the composition of the Co coating layer includes CoO, Co2O3 or Co3O4; the mass of the Co coating layer is 2% of the total mass of the sodium battery positive electrode material.

[0083] Example 3

[0084] This embodiment provides a Co-coated gradient-doped sodium cathode material and a preparation method thereof, the preparation method comprising the following steps:

[0085] (1) mixing nickel chloride, ferrous chloride, and deionized water to obtain a first salt solution having a total metal ion concentration of 1 mol / L; the first salt solution is further mixed with ascorbic acid having a concentration of 0.2 g / L;

[0086] (2) mixing copper chloride, zinc chloride, manganese chloride, and deionized water to obtain a second salt solution having a total metal ion concentration of 1 mol / L;

[0087] (3) Under nitrogen protection, the first salt solution, the second salt solution, the sodium hydroxide solution and the ammonia solution were added to the reactor in parallel, and a coprecipitation reaction was carried out at 60°C for 60 hours, accompanied by stirring at a rate of 200 rpm. During the reaction, the ammonia concentration in the solution was maintained at 0.1 mol / L and the pH value of the solution was 11. At the same time, the flow rate of the first salt solution was controlled to decrease at a uniform rate, the flow rate of the second salt solution was controlled to increase at a uniform rate, and the sum of the flow rates of the first salt solution and the second salt solution remained unchanged, thereby obtaining an intermediate solution containing a sodium electrode precursor;

[0088] (4) Under nitrogen protection, 0.3 mol / L cobalt chloride solution, sodium hydroxide solution and ammonia water were added to the reactor in parallel and the coprecipitation reaction was continued for 3 h, while the reaction conditions were kept unchanged. After centrifugation, the mixture was washed to neutrality and dried at 150 ° C to obtain a Co-coated gradient-doped sodium electrode precursor;

[0089] (5) Sodium carbonate and the sodium battery precursor described in step (4) were mixed in a molar ratio of 1.05:1, sintered at 800° C. for 20 h in an oxygen atmosphere, and crushed and sieved to obtain a Co-coated gradient-doped sodium battery positive electrode material with an average particle size of 20 μm.

[0090] The sodium cathode material obtained in this embodiment includes a Co coating layer and a gradient-doped sodium core. The sodium core is a layered transition metal oxide with the chemical formula: NaNi 0.1 Fe 0.4 Cu 0.1 Zn 0.1 Mn 0.3 O2; from the center to the surface of the sodium battery core, the content of Ni and Fe gradually decreases, and the content of Cu, Zn and Mn gradually increases.

[0091] In this embodiment, the composition of the Co coating layer includes CoO, Co2O3 or Co3O4; the mass of the Co coating layer is 1% of the total mass of the sodium battery positive electrode material.

[0092] Example 4

[0093] This embodiment provides a Co-coated gradient-doped sodium cathode material and a preparation method thereof. The preparation method is the same as that of Example 1 except that ascorbic acid is not mixed into the first salt solution in step (1), and therefore will not be described in detail here.

[0094] Example 5

[0095] This embodiment provides a Co-coated gradient-doped sodium cathode material and a preparation method thereof. The preparation method is the same as that of Example 1 except that the sodium hydroxide solution in step (3) and step (4) is replaced by a sodium carbonate solution. Therefore, the remaining steps and conditions are the same as those of Example 1 and are not described in detail here.

[0096] Example 6

[0097] This embodiment provides a Co-coated gradient-doped sodium cathode material and a preparation method thereof. The preparation method is the same as that of Example 1 except that the ammonia water in step (3) and step (4) is replaced by EDTA solution. Therefore, the remaining steps and conditions are the same as those of Example 1 and are not described in detail here.

[0098] Comparative Example 1

[0099] This comparative example provides a sodium cathode material and a preparation method thereof. The preparation method is the same as Example 1 except that the flow rates of the first salt solution and the second salt solution are kept constant in step (3), and therefore will not be described in detail here.

[0100] Compared with Example 1, the sodium battery positive electrode material obtained in this comparative example has a uniform distribution of Ni, Fe, Cu, Zn and Mn contents from the center to the surface of the sodium battery core.

[0101] Comparative Example 2

[0102] This comparative example provides a sodium cathode material and a preparation method thereof. The preparation method does not perform the Co coating in step (4), but directly centrifuges and washes the intermediate solution containing the sodium cathode precursor to neutrality, and then dries it at 100°C to obtain a gradient-doped sodium cathode precursor. The remaining steps and conditions are the same as those in Example 1 and are not described here.

[0103] Compared with Example 1, the sodium battery positive electrode material obtained in this comparative example only contains a sodium battery core and no Co coating layer.

[0104] Comparative Example 3

[0105] This comparative example provides a sodium cathode material and a preparation method thereof. In the preparation method, except that the flow rates of the first salt solution and the second salt solution are kept constant in step (3), and the Co coating in step (4) is not performed, the intermediate solution containing the sodium cathode precursor is directly centrifuged and washed to neutrality, and dried at 100°C to obtain a gradient-doped sodium cathode precursor. The remaining steps and conditions are the same as those in Example 1 and are not described here.

[0106] Compared with Example 1, the sodium battery positive electrode material obtained in this comparative example only contains a sodium battery core without a Co coating layer, and the contents of Ni, Fe, Cu, Zn and Mn are evenly distributed from the center to the surface of the sodium battery core.

[0107] Performance Testing

[0108] At 25°C, the sodium cathode materials obtained in Examples 1-6 and Comparative Examples 1-3 were used as the main positive electrode materials, and hard carbon was used as the negative electrode. They were assembled into CR2032 button batteries, and then electrochemical performance tests were carried out in the voltage range of 2.0-4.2 V and a discharge current density of 20 mA / g. The battery discharge specific capacity test results after 500 cycles are shown in Table 1 below.

[0109] Table 1

[0110] It can be seen from Table 1 that the structure with element gradient distribution can greatly improve the cycle performance of the material and also greatly improve the capacity; cobalt coating effectively improves the capacity and cycle performance of the material at high voltage.

[0111] In addition, it can be seen from Example 4 that the addition of ascorbic acid is necessary during the precursor synthesis process to prevent the deterioration of material properties caused by oxidation of the raw materials.

[0112] It can be seen that in the sodium battery positive electrode material provided by the present application, Ni and Fe provide high capacity for the battery, and their content gradually decreases from the inside to the outside; the Mn content gradually increases from the inside to the outside, reducing its side reactions with the electrolyte, thereby improving the cycle stability of the battery; the Cu and Zn content gradually increases from the inside to the outside, reducing the water absorption of the layered transition metal oxide, thereby improving the stability of the positive electrode material; the outermost layer coated with Co is beneficial to increasing the operating voltage of the battery, thereby further increasing the battery capacity.

[0113] The specific embodiments described above further illustrate the purpose, technical solutions and beneficial effects of this application. It should be understood that the above is only a specific embodiment of this application and is not intended to limit this application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of this application should be included in the scope of protection of this application.

Claims

1. A Co-coated gradient-doped sodium battery cathode material, comprising a Co coating layer and a gradient-doped sodium battery core; The sodium battery core is a layered transition metal oxide with the chemical formula: Na x Ni a Fe b Cu c Zn d Mn e O 2 ; Wherein: 0.5 ≤ x ≤ 1, 0.1 ≤ a ≤ 0.4, 0.2 ≤ b ≤ 0.4, 0.01 ≤ c ≤ 0.2, 0.01 ≤ d ≤ 0.2, 0.2 ≤ e ≤ 0.5, and a + b + c + d + e = 1; From the center to the surface of the sodium battery core, the contents of Ni and Fe gradually decrease, and the contents of Cu, Zn, and Mn gradually increase.

2. The sodium battery cathode material according to claim 1, Wherein, The composition of the Co coating layer includes CoO, Co 2 O 3 or Co 3 O 4 or a combination of any one or at least two of them; Optionally, the mass of the Co coating layer is 1% - 2% of the total mass of the sodium battery cathode material; Optionally, the average particle size of the sodium battery cathode material is 3 - 20 μm.

3. A preparation method of the sodium battery cathode material according to claim 1 or 2, Comprising the following steps: (1) Mix a nickel salt, an iron(II) salt, and deionized water to obtain a first salt solution; (2) Mix a copper salt, a zinc salt, a manganese salt, and deionized water to obtain a second salt solution; (3) Mix the first salt solution, the second salt solution, a precipitant solution, and a complexing agent solution for coprecipitation reaction. During the reaction, the flow rate of the first salt solution decreases uniformly, the flow rate of the second salt solution increases uniformly, and the sum of the flow rates of the first salt solution and the second salt solution remains unchanged, to obtain an intermediate solution containing a sodium battery precursor; (4) Mix the intermediate solution, a cobalt salt solution, a precipitant solution, and a complexing agent solution to continue the coprecipitation reaction, and after solid-liquid separation, obtain a Co-coated gradient-doped sodium battery precursor; (5) Mix a sodium source and the sodium battery precursor described in step (4) for sintering treatment to obtain a Co-coated gradient-doped sodium battery cathode material; Wherein, steps (1) and (2) have no sequential order.

4. The preparation method according to claim 3, Wherein, The nickel salt and the iron(II) salt in step (1) are each independently any one or a combination of at least two of sulfates, nitrates, acetates, or chlorides of the corresponding metal ions; Optionally, the total metal ion concentration in the first salt solution in step (1) is 1 - 2 mol / L; Optionally, ascorbic acid is also mixed in the first salt solution in step (1); Optionally, the concentration of ascorbic acid in the first salt solution is 0.2 - 0.5 g / L.

5. The preparation method according to claim 3 or 4, Wherein, The copper salt, the zinc salt, and the manganese salt in step (2) are each independently any one or a combination of at least two of sulfates, nitrates, acetates, or chlorides of the corresponding metal ions; Optionally, the total metal ion concentration in the second salt solution in step (2) is 1 - 2 mol / L.

6. The preparation method according to any one of claims 3 - 5, Wherein, The precipitant in the precipitant solution in step (3) includes any one or a combination of at least two of sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, or ammonium bicarbonate, and further optionally sodium hydroxide; Optionally, the complexing agent in the complexing agent solution in step (3) includes any one or a combination of at least two of ammonia water, EDTA, citric acid, sodium citrate, oxalic acid, sodium oxalate, acetic acid, ammonium sulfate or sodium pyrophosphate, and further optionally ammonia water; Optionally, the coprecipitation reaction in step (3) is carried out in a protective atmosphere, and the protective atmosphere includes any one or a combination of at least two of nitrogen, helium or argon, and further optionally nitrogen; Optionally, in step (3), the concentration of the complexing agent in the coprecipitation reaction solution is 0.1 - 1 mol / L; Optionally, the pH value of the coprecipitation reaction solution in step (3) is 11 - 13; Optionally, the temperature of the coprecipitation reaction in step (3) is 40 - 80 °C; Optionally, the time of the coprecipitation reaction in step (3) is 40 - 100 h; Optionally, the coprecipitation reaction in step (3) is accompanied by stirring, and the stirring rate is 200 - 500 rpm.

7. The preparation method according to any one of claims 3 - 6, wherein, the cobalt salt in the cobalt salt solution in step (4) includes any one or a combination of at least two of cobalt sulfate, cobalt nitrate, cobalt acetate or cobalt chloride; Optionally, the concentration of the cobalt salt solution in step (4) is 0.3 - 1 mol / L; Optionally, the time of the coprecipitation reaction in step (4) is 3 - 5 h, and the other conditions are the same as those in step (3); Optionally, the solid - liquid separation in step (4) includes filtration and / or centrifugation; Optionally, after the solid - liquid separation in step (4), the sodium - ion battery precursor is successively washed and dried; Optionally, the washing is carried out until the sodium - ion battery precursor is neutral; Optionally, the drying temperature is 50 - 150 °C.

8. The preparation method according to any one of claims 3 - 7, wherein, the sodium source in step (5) includes sodium carbonate and / or sodium bicarbonate, and further optionally sodium carbonate; Optionally, the sintering treatment in step (5) is carried out in an oxygen atmosphere; Optionally, the temperature of the sintering treatment in step (5) is 800 - 1300 °C; Optionally, the time of the sintering treatment in step (5) is 10 - 25 h; Optionally, after the sintering treatment in step (5), the sodium - ion battery cathode material is successively crushed and sieved.

9. The preparation method according to any one of claims 3 - 8, wherein, the preparation method includes the following steps: (1) Mix a nickel salt, an iron(II) salt and deionized water to obtain a first salt solution with a total metal ion concentration of 1 - 2 mol / L; the nickel salt and the iron(II) salt are each independently any one or a combination of at least two of the sulfates, nitrates, acetates or chlorides of the corresponding metal ions; an ascorbic acid with a concentration of 0.2 - 0.5 g / L is also mixed in the first salt solution; (2) Mix a copper salt, a zinc salt, a manganese salt and deionized water to obtain a second salt solution with a total metal ion concentration of 1 - 2 mol / L ; the copper salt, the zinc salt and the manganese salt are each independently any one or a combination of at least two of the sulfates, nitrates, acetates or chlorides of the corresponding metal ions; (3) Under nitrogen protection, the first salt solution, the second salt solution, the sodium hydroxide solution and ammonia water are respectively added into the reaction kettle in a concurrent flow manner, and a coprecipitation reaction is carried out at 40 - 80 °C for 40 - 100 h, while stirring at a rate of 200 - 500 rpm. During the reaction, the ammonia concentration in the solution is maintained at 0.1 - 1 mol / L, the pH value of the solution is 11 - 13, and at the same time, the flow rate of the first salt solution is controlled to decrease uniformly, the flow rate of the second salt solution is controlled to increase uniformly, and the sum of the flow rates of the first salt solution and the second salt solution remains unchanged, obtaining an intermediate solution containing a sodium battery precursor; (4) Under nitrogen protection, a cobalt salt solution with a concentration of 0.3 - 1 mol / L, the sodium hydroxide solution and ammonia water are respectively added into the reaction kettle in a concurrent flow manner to continue the coprecipitation reaction for 3 - 5 h, and the reaction conditions are kept unchanged. After filtration and / or centrifugation, it is washed to neutrality and dried at 50 - 150 °C to obtain a Co-coated gradient-doped sodium battery precursor; the cobalt salt in the cobalt salt solution includes any one or at least two combinations of cobalt sulfate, cobalt nitrate, cobalt acetate or cobalt chloride; (5) Mix sodium carbonate and the sodium battery precursor described in step (4), and carry out sintering treatment at 800 - 1300 °C for 10 - 25 h in an oxygen atmosphere. After crushing and screening, a Co-coated gradient-doped sodium battery cathode material is obtained; Among them, steps (1) and (2) have no sequential order.

10. A sodium ion battery comprising the sodium battery cathode material according to claim 1 or 2.

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