A modified sodium-ion layered cathode material and preparation method thereof, and sodium-ion battery

Abstract

The present invention provides a modified sodium-based layered cathode material, comprising a substrate and a coating layer on the surface of the substrate; the coating layer comprises calcium and a first element; the modified sodium-based layered cathode material has a total surface residual sodium content of ≤3000 ppm, and the surface residual alkali and pH value are low; after the modified sodium-based layered cathode material is exposed to constant temperature and humidity conditions, the total surface residual sodium content of the material increases by less than 20% compared to before exposure, and the material moisture content is less than 1000 ppm. By first coating the first element and then coating calcium, the present invention can simultaneously improve the material's rate discharge, reduce the material's impedance, or further improve the cycle, while improving the crystal structure stability, air stability, and water resistance.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium ion battery materials, and in particular, relates to the modification of a layered positive electrode material for a sodium ion battery. Background Art

[0002] Layered sodium-ion cathode materials have problems such as strong hygroscopicity, poor air stability, and easy structural collapse after long cycles. To address these problems, coating and doping are the most common modification methods.

[0003] The current synthesis process, such as the patent application with publication number CN115548308A, discloses mixing a sodium salt with a precursor, a dopant or a coating agent, and preparing a positive electrode material through high-temperature solid-phase sintering; the patent application with publication number CN116190632A discloses first synthesizing the positive electrode material, then mixing it with a coating agent, and synthesizing the positive electrode material through high-temperature sintering.

[0004] In the above-mentioned synthesis process, the doping elements used are mainly one or more of Na, Mg, Ni, Ca, B, Fe, Al, Li, K, Ag, Zr, Ti, W, Mo, Cr, Sr, Y, Cd, Sn, Sb, and Ce; and the coating elements are mainly Al, Ti, Zr, and F. Among them, the doping elements are mostly added to the mixed materials at one time. After high-temperature sintering, the doping elements are evenly distributed in the material. Although doping can improve the air stability of the material, it often causes an increase in the material impedance or a decrease in the material discharge capacity. When Al, Ti, Zr, F and other elements are coated on the material, there is a Na ion exchange reaction during the coating process, which is used to form metal oxides on the surface of the material, which is bound to cause a decrease in the material capacity.

[0005] Therefore, it is imperative to develop new modification methods to ensure that the material does not reduce its capacity, significantly improve the air stability of the material, reduce the hygroscopicity of the sodium ion material, and improve the cyclic stability of the material. Summary of the Invention

[0006] In response to the above technical problems, the present invention provides a coated and modified layered positive electrode material, a preparation method thereof, and a sodium ion battery.

[0007] To achieve the above objectives, the present invention proposes the following solutions:

[0008] The present invention provides a modified sodium-electric layered positive electrode material, comprising a sodium-electric layered positive electrode material substrate and a coating layer on the substrate surface, wherein the coating layer contains calcium and a first element; in the modified sodium-electric layered positive electrode material, the total amount of residual sodium on the surface is ≤3000 ppm, wherein the mass percentage of residual Na2CO3 on the surface is ≤1.0%, the mass percentage of residual NaOH on the surface is ≤0.05%, and the pH value is 11.5<pH<13;

[0009] After the modified sodium-based layered cathode material is exposed to constant temperature and humidity conditions of 40° C. and 60% humidity for 4 hours, the total amount of residual sodium on the surface of the material increases by less than 20% compared with before exposure, and the moisture content of the material is less than 1000 ppm.

[0010] Preferably, the first element is selected from one or more of Mg, Al, Ti, Zr, W, Nb, B, Co, Cu, Zn, Sr, Ba, K, Li, Mn, Fe, Y, Sn, Si, Mo, Te, Bi, and Sb.

[0011] Preferably, the mass ratio of the matrix to the calcium element is 1:0.0005-0.02.

[0012] Preferably, the mass ratio of the matrix to the first element is 1:0.0001-0.02.

[0013] Preferably, the matrix is ​​an O3 type sodium-ion layered positive electrode material; the matrix is ​​one or more of a layered single crystal and a layered polycrystal.

[0014] Preferably, the chemical formula of the matrix is ​​Na 1+x Ni a Fe b Mn c Cu d Zn e O2, where -0.1≤x≤0.1, 0.2≤a≤0.5, 0.2≤b≤0.5, 0.2≤c≤0.5, 0≤d≤0.15, 0≤e≤0.15, and a+b+c+d+e=1.

[0015] As a general inventive concept, the present invention also provides a method for preparing a modified sodium layered cathode material, comprising:

[0016] S1, solid-phase mixing a sodium-based layered cathode material matrix with a first element source, and performing a first sintering on the resulting mixture to obtain a sintered material;

[0017] S2. Solid-phase mixing a sintered material with a calcium source, and performing a second sintering on the resulting mixture to obtain a modified sodium electrolyte layered positive electrode material.

[0018] Preferably, the first element source is selected from one or more compounds of Mg, Al, Ti, Zr, W, Nb, B, Co, Cu, Zn, Sr, Ba, K, Li, Mn, Fe, Y, Sn, Si, Mo, Te, Bi, and Sb.

[0019] Preferably, the first element source is selected from one or more of magnesium oxide, magnesium hydroxide, aluminum oxide, aluminum hydroxide, titanium oxide, titanium hydroxide, zirconium oxide, zirconium hydroxide, tungsten oxide, niobium pentoxide, niobium hydroxide, boron oxide, boric acid, cobaltous hydroxide, cobalt oxyhydroxy, copper oxide, zinc oxide, strontium oxide, strontium carbonate, barium hydroxide, barium hydroxide octahydrate, barium carbonate, potassium carbonate, potassium hydroxide, lithium hydroxide, lithium carbonate, manganese dioxide, manganese trioxide, manganese tetraoxide, ferrous oxide, iron oxide, ferrous oxide, ferrous oxalate, yttrium oxide, tin oxide, silicon dioxide, molybdenum trioxide, tellurium trioxide, bismuth oxide and antimony pentoxide.

[0020] Preferably, the chemical formula of the sodium layered cathode material matrix is ​​Na 1+x Ni a Fe b Mn c Cu d Zn e O2, where -0.1≤x≤0.1, 0.2≤a≤0.5, 0.2≤b≤0.5, 0.2≤c≤0.5, 0≤d≤0.15, 0≤e≤0.15, and a+b+c+d+e=1.

[0021] Preferably, the mass ratio of the first element in the first element source to the sodium-electrolyte layered cathode material matrix is ​​0.0001-0.02:1.

[0022] Preferably, the calcium source is selected from one or more of calcium oxide, calcium hydroxide, calcium carbonate, calcium acetate, calcium phosphate and calcium nitrate.

[0023] Preferably, the mass ratio of calcium in the calcium source to the sodium-electrolyte layered cathode material matrix is ​​0.0005-0.02:1.

[0024] Preferably, in step S1, the temperature of the first sintering is 250-800°C; the time of the first sintering is 3-20 hours; and the first sintering is performed in oxygen and / or dry air.

[0025] Preferably, in step S2, the temperature of the second sintering is 500-800°C; the time of the second sintering is 3-20 hours; and the second sintering is performed under oxygen and / or dry air conditions.

[0026] Preferably, in step S1 and step S2, the solid phase mixing is performed under a protective atmosphere; the protective atmosphere is selected from at least one of nitrogen, argon, dry air and oxygen.

[0027] As a general inventive concept, the present invention also provides a sodium ion battery, comprising the aforementioned modified sodium-ion layered cathode material or the modified sodium-ion layered cathode material prepared by the aforementioned preparation method.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] The present invention adopts Ca and the first element for composite coating modification, which can not only reduce the residual sodium on the surface and effectively improve the crystal structure stability, air stability and water resistance, but also improve the rate performance of the material, reduce the material impedance and improve the electrochemical performance.

[0030] In the present invention, the Ca coating has a fluxing effect on the material. During the coating process, some Ca ions occupy the sodium sites on the surface of the material, promote further reaction of residual sodium on the surface with the matrix, reduce residual sodium on the surface, and stabilize the surface crystal structure. Therefore, in the subsequent charge and discharge process, more surface sodium can participate in the deintercalation reaction, so that the capacity of the coated material will not be significantly reduced; and Ca coating on the surface of the material can also effectively reduce the pH value of the material, so that the surface of the particle is in a Ca-enriched state, which can lock the surface lattice Na to the greatest extent, thereby improving the air stability and water absorption of the material from the outside to the inside; however, in addition to improving the crystal structure stability, air stability and water resistance of the material, the other improvement effects of Ca coating are not significant. Therefore, by first coating the first metal element and then coating calcium, on the basis of improving the crystal structure stability, air stability and water resistance, it can also simultaneously play a role in improving the material rate discharge, reducing the material impedance or further improving the cycle.

[0031] In the present invention, by first coating the first element and then coating with Ca, the synergistic effect of the coating modification of the first element and Ca can be ensured, and the reaction between the first element and Ca that reduces the composite coating effect is avoided. The subsequent calcium coating can effectively promote the further reaction of residual sodium on the surface, and part of the Ca enters the sodium site of the surface crystal, which helps to repair the capacity reduction caused by the sodium ion exchange generated during the coating process of the first element, thereby ensuring that the modification effect of each coating element is exerted to the optimal level.

[0032] The present invention adopts the method of solid-phase mixing and solid-phase sintering under a protective atmosphere to achieve coating of layered sodium battery materials, which not only avoids the structural degradation reaction of the layered sodium battery positive electrode material due to absorption of water in the air, but also has a simple preparation process and is easy to industrialize. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0034] Figure 1 This is the SEM image of the modified layered sodium electrolyte polycrystalline material obtained in Example 1.

[0035] Figure 2 EPMA scanning images of a single particle cross section of the modified layered sodium polycrystalline material obtained in Example 1, where (a) is the single particle cross section morphology and (b) is the EPMA image of the Ca element.

[0036] Figure 3 This is a cycle performance curve of button batteries assembled with the materials obtained in Example 1 and Comparative Example 1 at 45°C.

[0037] Figure 4 These are EIS curves of the button-type batteries of Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 4 before and after cycling, where (a) is the EIS curve before cycling and (b) is the EIS curve after cycling. DETAILED DESCRIPTION

[0038] Some embodiments of the present invention provide a modified sodium-based layered cathode material, comprising a sodium-based layered cathode material substrate and a coating layer on the surface of the substrate, wherein the coating layer comprises calcium and a first element; wherein the modified sodium-based layered cathode material has a total surface residual sodium content of ≤3000 ppm, more preferably ≤2800 ppm, for example, 200 ppm, 300 ppm, 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 1100 ppm, 1200 ppm, 1300 ppm, 1400 ppm, 1500 ppm, 1600 ppm, 1700 ppm, 1800 ppm, 1900 ppm, 20 00ppm, 2100ppm, 2200ppm, 2300ppm, 2400ppm, 2500ppm, 2600ppm, 2700ppm, 2800ppm, etc., wherein the mass percentage of surface residual Na2CO3 is ≤1.0%, for example, 0.9%, 0.8%, 0.7%, 0.6%, 0.5%, 0.4%, 0.3%, 0.2%, 0.1%, etc., the mass percentage of surface residual NaOH is ≤0.05%, 11.5<pH<13, for example, 11.6, 11.7, 11.8, 11.9, 12.0, 12.1, 12.2, 12.3, 12.4, 12.5, 12.6, 12.7, 12.8, 12.9, etc.;

[0039] After the modified sodium-based layered cathode material is exposed to constant temperature and humidity conditions of 40° C. and 60% humidity for 4 hours, the total amount of residual sodium on the surface of the material increases by less than 20% compared with before exposure, and the moisture content of the material is less than 1000 ppm.

[0040] The study found that when calcium and the first element are used for composite coating modification, the introduction of the first element can play a role in improving circulation, enhancing air stability, reducing material interface impedance, and improving rate performance; calcium coating can effectively promote the further reaction of residual sodium on the surface, and part of the Ca enters the sodium site of the surface crystal, which helps to repair the capacity reduction caused by the sodium ion exchange generated during the coating process of the first element, ensuring that the modification effect of each coating element is exerted to the best level. The air stability of the material can be improved without obvious loss of capacity through calcium coating, thereby improving the cycle performance of the material, and the calcium coating is half-doped and half-wrapped on the surface of the sodium-electrolyte layered positive electrode material (the half-doped part of the calcium occupies the sodium site in the crystal structure, which can stabilize the crystal structure, facilitate the release of more Na, and improve the charge and discharge capacity), which can effectively lock the lattice Na on the surface of the material and prevent Na from occurring on the surface of the material. + / H + The exchange reaction reduces the water absorption of the material while preventing the degradation of the crystal structure of the material from the outside to the inside, and can effectively inhibit the generation of residual sodium on the surface of the material which is detrimental to the material.

[0041] In some preferred embodiments, in the modified sodium-ion layered cathode material, the mass percentage of residual Na2CO3 on the surface is 0.05-0.6%, the mass percentage of residual NaOH on the surface is ≤0.03%, and the pH value is 12-13.

[0042] In some preferred embodiments, the mass ratio of the matrix and the calcium element is 1: 0.0005 ~ 0.02, such as 1: 0.0005, 1: 0.0008, 1: 0.001, 1: 0.002, 1: 0.005, 1: 0.008, 1: 0.01, 1: 0.012, 1: 0.015, 1: 0.018, 1: 0.02, etc. When the calcium content is too low, no obvious modification effect is achieved. When the calcium content is too high, it is easy to block the Na ion transmission channel and reduce the discharge capacity. Further preferably, the mass ratio of the matrix and the calcium element is 1: 0.005 ~ 0.01. The inventors found in the research process that, in this preferred case, a modified sodium electric layered cathode material with better comprehensive performance can be obtained.

[0043] In some preferred embodiments, the first element is selected from one or more of Mg, Al, Ti, Zr, W, Nb, B, Co, Cu, Zn, Sr, Ba, K, Li, Mn, Fe, Y, Sn, Si, Mo, Te, Bi, and Sb, and further preferred elements are: Mg, Al, Ti, Zr, B, Co, Cu, Zn, K, and Mn.

[0044] In a particularly preferred embodiment of the present invention, the first element is Mn. The inventors have found that in this preferred embodiment, a cathode material with lower residual sodium and residual alkali contents can be obtained.

[0045] In another particularly preferred embodiment of the present invention, the first element is Mg and B. The inventors have found that in this preferred embodiment, a positive electrode material with higher charge and discharge capacity can be obtained.

[0046] In some preferred embodiments, the mass ratio of the matrix to the first element is 1:0.0001-0.02, for example, 1:0.0001, 1:0.0002, 1:0.0005, 1:0.0008, 1:0.001, 1:0.002, 1:0.005, 1:0.008, 1:0.01, 1:0.012, 1:0.015, 1:0.018, 1:0.02, etc. Optimizing the content of the first element can ensure a significant modification effect on the one hand, and on the other hand, avoid excessive consumption of sodium and capacity degradation. More preferably, the mass ratio of the matrix to the first element is 1:0.001-0.02.

[0047] In some preferred embodiments, the matrix is ​​an O3-type sodium-ion layered positive electrode material; the matrix is ​​one or more of a layered single crystal and a layered polycrystal.

[0048] In some preferred embodiments, the chemical formula of the sodium layered cathode material is Na 1+ x Ni a Fe b Mn c Cu d Zn e O2, wherein -0.1≤x≤0.1, 0.2≤a≤0.5, 0.2≤b≤0.5, 0.2≤c≤0.5, 0≤d≤0.15, 0≤e≤0.15, and a+b+c+d+e=1. The sodium-ion layered positive electrode material is selected from at least one of a layered sodium-ion polycrystalline NFM material, a layered sodium-ion single crystal NFM material, a layered single crystal NCFM material, a layered single crystal NZFM material, and a layered single crystal NZCFM material. Among them, the NFM111 material is a material whose main elements are Ni, Fe, and Mn, the NCFM material is a material whose main elements are Ni, Cu, Fe, and Mn, the NZFM material is a material whose main elements are Ni, Zn, Fe, and Mn, and the NZCFM material is a material whose main elements are Ni, Zn, Cu, Fe, and Mn.

[0049] Some embodiments of the present invention provide a method for preparing a modified sodium-ion layered cathode material, comprising:

[0050] S1, solid-phase mixing the sodium layered cathode material and the first element source, and performing a first sintering on the obtained mixture to obtain a sintered material;

[0051] S2. Solid-phase mixing a sintered material with a calcium source, and performing a second sintering on the resulting mixture to obtain a modified sodium electrolyte layered positive electrode material.

[0052] In the present invention, by first coating the first element and then coating with Ca, it is possible to ensure that the synergistic effect of the first element and Ca coating modification is optimal, avoiding the reaction between the first element and Ca to reduce the composite coating effect, and the subsequent calcium coating can effectively promote the further reaction of the residual sodium on the surface, and some Ca enters the sodium position of the surface crystal, which helps to repair the capacity reduction caused by the sodium ion exchange generated during the coating process of the first element, ensuring that the modification effect of each coating element is brought to the optimal level. Composite coating can not only reduce the residual sodium on the surface, effectively improve the crystal structure stability, air stability and water resistance, but also improve the rate performance of the material, reduce the material impedance or further improve the cycle performance without significantly reducing the capacity.

[0053] In some preferred embodiments, the first element source is selected from one or more compounds of Mg, Al, Ti, Zr, W, Nb, B, Co, Cu, Zn, Sr, Ba, K, Li, Mn, Fe, Y, Sn, Si, Mo, Te, Bi, and Sb.

[0054] In some preferred embodiments, the first element source is selected from one or more of magnesium oxide, magnesium hydroxide, aluminum oxide, aluminum hydroxide, titanium oxide, titanium hydroxide, zirconium oxide, zirconium hydroxide, tungsten oxide, niobium pentoxide, niobium hydroxide, boron oxide, boric acid, cobaltous hydroxide, cobalt oxyhydroxide, copper oxide, zinc oxide, strontium oxide, strontium carbonate, barium hydroxide, barium hydroxide octahydrate, barium carbonate, potassium carbonate, potassium hydroxide, lithium hydroxide, lithium carbonate, manganese dioxide, manganese trioxide, manganese tetraoxide, ferrous oxide, iron oxide, ferrous oxide, ferrous oxalate, yttrium oxide, tin oxide, silicon dioxide, molybdenum trioxide, tellurium trioxide, bismuth oxide and antimony pentoxide.

[0055] In some preferred embodiments, the mass ratio of the first element in the first element source to the sodium layered cathode material is 0.0001-0.02:1.

[0056] In some preferred embodiments, the calcium source is selected from one or more of calcium oxide, calcium hydroxide, calcium carbonate, calcium acetate, calcium phosphate, and calcium nitrate.

[0057] In some preferred embodiments, the mass ratio of calcium in the calcium source to the sodium layered positive electrode material is 0.0005~0.02:1, for example, 1:0.0005, 1:0.0008, 1:0.001, 1:0.002, 1:0.005, 1:0.008, 1:0.01, 1:0.012, 1:0.015, 1:0.018, 1:0.02, etc.

[0058] It should be noted that, in the present invention, the mass ratio of calcium in the calcium source to the sodium-based layered cathode material refers to the mass ratio of calcium in the calcium source to the sodium-based layered cathode material coated with the first element.

[0059] In some preferred embodiments, the matrix is ​​an O3-type sodium-ion layered positive electrode material; the matrix is ​​one or more of a layered single crystal and a layered polycrystal.

[0060] In some preferred embodiments, the chemical formula of the sodium layered cathode material is Na 1+ x Ni a Fe b Mn c Cu d Zn eO2, wherein -0.1≤x≤0.1, 0.2≤a≤0.5, 0.2≤b≤0.5, 0.2≤c≤0.5, 0≤d<0.15 (more preferably 0.05≤d≤0.15), 0≤e<0.15 (more preferably 0.02≤e<0.15), and a+b+c+d+e=1. The sodium-ion layered positive electrode material is selected from at least one of a layered sodium-ion polycrystalline NFM material, a layered sodium-ion single crystal NFM material, a layered single crystal NCFM material, a layered single crystal NZFM material, and a layered single crystal NZCFM material. Among them, the NFM111 material is a material whose main elements are Ni, Fe, and Mn, the NCFM material is a material whose main elements are Ni, Cu, Fe, and Mn, the NZFM material is a material whose main elements are Ni, Zn, Fe, and Mn, and the NZCFM material is a material whose main elements are Ni, Zn, Cu, Fe, and Mn.

[0061] In some preferred embodiments, in step S1, the temperature of the first sintering is 250-800°C, for example, 300°C, 350°C, 400°C, 450°C, 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, 800°C, etc.

[0062] In some preferred embodiments, in step S1, the first sintering time is 3 to 20 hours, more preferably 5 to 12 hours, for example, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, etc.

[0063] In some preferred embodiments, in step S1, the first sintering is performed under oxygen and / or air conditions.

[0064] In some preferred embodiments, in step S2, the temperature of the second sintering is 500°C to 800°C, more preferably 550°C to 700°C, for example, 550°C, 560°C, 570°C, 580°C, 590°C, 600°C, 610°C, 620°C, 630°C, 640°C, 650°C, 660°C, 670°C, 680°C, 690°C, 700°C, etc. When the temperature is too high, Ca will be doped into the crystal structure of the material, tending to form a uniform distribution within the particles, and no Ca will be enriched and coated on the surface; when the temperature is too low, the temperature at which Ca reacts with the material surface cannot be reached, and the effects of reducing residual sodium, improving crystal structure stability, air stability, and water resistance cannot be achieved.

[0065] In some preferred embodiments, in step S2, the second sintering time is 3 to 20 hours, more preferably 5 to 12 hours, for example, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, etc.

[0066] In some preferred embodiments, in step S2, the second sintering is performed under oxygen and / or air conditions.

[0067] In some preferred embodiments, in step S1 and step S2, the solid phase mixing is performed under a protective atmosphere.

[0068] In some preferred embodiments, the protective atmosphere is selected from at least one of nitrogen, argon, dry air and oxygen. More preferably, the dry air is air with a dew point of less than -40°C.

[0069] In some preferred embodiments, step S1 further includes passing the sintered material through a 200-400 mesh sieve.

[0070] In some preferred embodiments, step S1 further includes passing the product obtained from the second sintering through a 200-400 mesh sieve.

[0071] Some embodiments of the present invention provide sodium ion batteries, comprising the aforementioned modified sodium-ion layered cathode material or the modified sodium-ion layered cathode material prepared by the aforementioned preparation method.

[0072] To facilitate understanding of the present invention, the present invention will be described in more comprehensive and detailed form below in conjunction with the accompanying drawings and preferred embodiments. However, the scope of protection of the present invention is not limited to the following specific embodiments.

[0073] Example 1:

[0074] Take 500g of layered sodium polycrystalline NFM111 (NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 To this material, 1.89 g of Al2O3 (equivalent to 0.2% Al) was added. The mixture was mixed uniformly under a nitrogen atmosphere to obtain a primary mixture. After sintering at 600°C for 8 hours under an oxygen atmosphere, the mixture was cooled and sieved to produce Al-coated layered sodium electrolyte polycrystalline NFM111 material.

[0075] 500g of Al-coated layered sodium polycrystalline NFM111 material was then added with 3.56g of CaO coating agent, equivalent to a Ca addition of 0.5%. The mixture was mixed uniformly under a nitrogen atmosphere to obtain a mixture. The mixture was then sintered at 650°C in air for 8 hours, cooled, and sieved to produce a Ca-Al composite-coated layered sodium polycrystalline NFM111 material.

[0076] Figure 1 The surface morphology of the Ca and Al composite-coated layered sodium electrolyte polycrystalline NFM111 material in Example 1 was characterized. Figure 2 This is the EPMA scanning image of the Ca element in a single particle cross section. Figure 3Comparison of the button-type battery cycle performance curves of the Ca and Al composite-coated layered sodium battery polycrystalline NFM111 material and comparative example 1 at 45°C.

[0077] from Figure 2 It can be seen that the EPMA test of Ca element on the single particle cross-section of the coated and modified sodium-based layered positive electrode material shows obvious surface Ca enrichment, and part of the calcium enters the interior of the material to achieve doping.

[0078] Example 2:

[0079] Take 500g of layered sodium electric single crystal NFM111 material (NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 The mixture was sintered at 800°C for 10 hours in an oxygen atmosphere, cooled, and sieved to obtain the Mn-coated layered sodium electrolyte single crystal NFM111.

[0080] Then take 500g of Mn-coated layered sodium electric single crystal NFM111 material, add 19.41g of CaCO3 coating agent, which is equivalent to 1.55% of Ca added; mix evenly under an oxygen protective atmosphere to obtain a mixture, sinter the mixture at 700℃ under an oxygen atmosphere for 5h, cool and sieve to obtain Ca and Mn composite-coated layered sodium electric single crystal NFM111 material.

[0081] Example 3:

[0082] Take 500g of layered sodium polycrystalline NFM424 material (NaNi 0.4 Fe 0.2 Mn 0.4 O2), 0.84g of MgO (equivalent to 0.10% Mg addition); then 5.73g of H3BO3 (equivalent to 0.20% B addition); and mixing thoroughly under an oxygen atmosphere to obtain a primary mixture. After sintering at 300°C for 6 hours in a dry air atmosphere, the mixture was cooled and sieved to obtain Mg- and B-coated layered sodium electrolyte polycrystalline NFM424 material.

[0083] Take 500g of Mg and B coated layered sodium battery polycrystalline NFM424 material, add 19.93g of Ca(CH3COO)2 coating agent, which is equivalent to an amount of Ca added of about 1.0%; then mix evenly under a protective atmosphere of dry air to obtain a mixture, sinter the mixture at 550℃ in an air atmosphere for 12h, cool and sieve to obtain Ca, Mg, and B composite coated layered sodium battery polycrystalline NFM424 material.

[0084] Comparative Example 1:

[0085] The only difference between this comparative example and Example 1 is that the subsequent Al coating and Ca coating steps are omitted, and the material is unmodified layered sodium-ion polycrystalline NFM111 material.

[0086] Comparative Example 2:

[0087] This comparative example differs from Example 1 only in that the subsequent Ca coating step is omitted. Specifically, 500 g of layered sodium-ion polycrystalline NFM111 material was added with 1.89 g of Al₂O₃, equivalent to an Al content of 0.2%. The mixture was mixed uniformly under a nitrogen atmosphere to obtain a primary mixture. After sintering at 600°C for 8 h under an oxygen atmosphere, the mixture was cooled and sieved to produce Al-coated layered sodium-ion polycrystalline NFM111 material.

[0088] Comparative Example 3:

[0089] Take 500 g of the same layered sodium battery polycrystalline NFM111 material as in Example 1, mix 1.89 g of Al2O3 and 3.56 g of CaO coating agent evenly under a nitrogen protective atmosphere to obtain a mixture, place the mixture in a dry air atmosphere (air with a dew point of <-40°C) and sinter at 650°C for 8 hours, then cool and sieve to obtain a one-step coated Ca, Al composite coated layered sodium battery polycrystalline NFM111 material.

[0090] Comparative Example 4:

[0091] Take 500 g of the same layered sodium polycrystalline NFM111 material as in Example 1, and mix 3.56 g of CaO coating agent evenly under a nitrogen protective atmosphere to obtain a mixture. Place the mixture in a dry air atmosphere and sinter at 650°C for 8 hours, then cool and sieve to obtain Ca-coated layered sodium polycrystalline NFM111 material.

[0092] The surface residual sodium and pH value of the above embodiments and comparative examples were tested, and the test results are shown in Table 1.

[0093] After the above materials were exposed to constant temperature and humidity conditions of 40°C and 60% humidity for 4 hours, the residual sodium and moisture content of the materials were tested. The test results are shown in Table 2.

[0094] The above materials were made into button batteries and subjected to charge and discharge tests. The test conditions were: charge and discharge voltage range 2.0~4.0V, capacity test 0.1C, cycle performance test at 1C rate at 45℃, and capacity retention rate data after 80 cycles. The performance data of the button battery are shown in Table 3.

[0095] Test method for residual alkali on the surface: refer to the existing conventional test methods, and test the residual sodium carbonate and sodium hydroxide with deionized water and anhydrous ethanol respectively. The residual sodium carbonate is titrated using the equivalence point difference method (the total amount minus the interference of the hydroxide equivalence point that appears first), and the residual sodium hydroxide is titrated using the conventional titration method. Specifically, the potentiometric titration method is used to test the residual sodium content of the substance.

[0096] pH value test method: add 4 g sample to 40 g water, stir for 15 minutes, filter, and test with Mettler S220 multi-parameter tester at 10℃~30℃ and ≤80% humidity.

[0097] Moisture testing method: Use 831 Coulometric Moisture Meter in conjunction with 885 Heating Furnace for testing. Weigh 1g of sample and place it in a vial. Dry at 170℃ to measure the moisture content.

[0098] Table 1 Surface residual sodium and pH value test data

[0099]

[0100] The residual sodium carbonate and residual sodium hydroxide contents of the materials coated with Ca and other metal elements are both lower than those before coating, and the pH value decreases more significantly, indicating that the Ca coating improves the water stability of the material. In Comparative Example 3, the Ca and Al composite coated layered sodium polycrystalline NFM111 material coated in one step, because Ca and Al are mixed and sintered in one step, Ca and Al react with each other during the coating process, the residual sodium decreases to a small extent, and the water stability is not significantly improved, and the pH value is still greater than 13. Comparative Example 4 is a Ca-coated layered sodium polycrystalline NFM111 material. From the results, when the coating element is only Ca, the residual Na2CO3 content of the sample is significantly reduced, and the residual total sodium is significantly reduced. This shows that Ca coating has the effect of promoting the entry of residual sodium into the material lattice. The residual Na2CO3 content of Example 1 is higher than that of Comparative Example 4, because part of the residual sodium and the coating element Al generate a coating layer material, and the coating layer material also locks a certain amount of Na ions. Therefore, after Ca is coated, there is still more residual sodium retained on the surface relative to Comparative Example 4 and does not enter the lattice.

[0101] Table 2 Characterization of residual total sodium and moisture content of the materials after exposure to a constant temperature and humidity chamber at 40°C and 60% for 4 h

[0102]

[0103] Analysis of the data in Table 2 shows that Ca coating effectively reduces the water absorption of the material and inhibits the Na + / H + exchange reaction, effectively preventing Na +The water absorption of the layered sodium polycrystalline NFM111 material coated with Ca and Al in one step in Comparative Example 3 is basically the same as that in Comparative Example 2, indicating that the one-step coating of Ca and Al cannot achieve the best effect of the synergistic coating of Al and Ca.

[0104] Table 3 Sample button cell performance data

[0105]

[0106] Compared with the capacity of the material before coating, the charge-discharge capacity and the first efficiency of the Ca-coated material are not significantly reduced. However, when simply coated with non-Ca metal ions such as Al elements, the charging capacity of the material shows a downward trend. This is mainly due to the Na ion exchange reaction between the coated metal elements and the material. For the Ca and Al composite-coated samples prepared by the one-step coating method, due to the interactive reaction between Ca and Al during the reaction, the generated coating layer actually reduces the charge-discharge capacity of the material, and the cycle performance is not significantly improved. For materials coated with Ca alone, the charging capacity shows an increasing trend, and the discharge capacity does not decrease, indicating that part of the Ca coated with Ca enters the lattice of the surface layer of the material, promoting more surface lattice Na to participate in the deintercalation reaction. Compared with the uncoated material, the material coated with Ca and other metal elements prepared by the two-step coating method has no significant decrease in capacity and significantly improved cycle performance. The specific cycle performance improvements are as follows. Figure 3 shown.

[0107] Figure 4 The EIS curves of button batteries of Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 4 before and after cycling are shown. As can be seen from the figure, the R(ct) of the uncoated Comparative Example 1 and the Ca-coated Comparative Example 4 samples before and after cycling are larger, and the R(ct) of the sample coated with Ca alone is even larger. After Al coating, the R(ct) of the sample before and after cycling is significantly reduced compared to the uncoated and single-coated Ca samples, indicating that Al coating can effectively reduce the impedance of the material before and after cycling. Reducing the impedance can reduce the cycle DCR growth of the material in the finished battery. Reducing the battery DCR growth is beneficial to alleviate the heating phenomenon of long-cycle batteries and improve high-power charge and discharge performance. Combined with Figure 3 、 Figure 4 As shown in Table 3, the Ca and Al composite coated materials prepared by two-step coating basically maintain the R(ct) level of the aluminum-coated samples before and after cycling, and significantly improve the cycling performance of the materials.

[0108] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A modified sodium layered cathode material, characterized in that: The modified sodium-based layered cathode material comprises a substrate and a coating layer on the surface of the substrate, wherein the coating layer comprises calcium and a first element, and the first element is coated first and then the Ca coating is performed; in the modified sodium-based layered cathode material, the total amount of residual sodium on the surface is ≤3000 ppm, wherein the mass percentage of residual Na2CO3 on the surface is ≤1.0%, the mass percentage of residual NaOH on the surface is ≤0.05%, and the pH is 11.5<13; and the first element is selected from one or more of Mg, Al, Ti, Zr, W, Nb, B, Co, Cu, Zn, Sr, Ba, K, Li, Mn, Fe, Y, Sn, Si, Mo, Te, Bi, and Sb; After the modified sodium-based layered cathode material is exposed to constant temperature and humidity conditions of 40° C. and 60% humidity for 4 hours, the total amount of residual sodium on the surface of the material increases by less than 20% compared with before exposure, and the moisture content of the material is less than 1000 ppm.

2. The modified sodium layered cathode material according to claim 1, wherein The mass ratio of the matrix to the calcium element is 1:0.0005-0.

02.

3. The modified sodium-ion layered cathode material according to claim 1, wherein: The mass ratio of the matrix to the first element is 1:0.0001-0.

02.

4. The modified sodium-ion layered cathode material according to any one of claims 1 to 3, wherein: The matrix is ​​an O3 type sodium-based layered positive electrode material; the matrix is ​​one or more of a layered single crystal and a layered polycrystal; The chemical formula of the sodium layered cathode material matrix is ​​Na 1+x Ni a Fe b Mn c Cu d Zn e O2, where -0.1≤x≤0.1, 0.2≤a≤0.5, 0.2≤b≤0.5, 0.2≤c≤0.5, 0≤d≤0.15, 0≤e≤0.15, and a+b+c+d+e=1.

5. A method for preparing a modified sodium layered cathode material, characterized in that: include: S1. Solid-phase mixing a sodium-based layered cathode material matrix with a first element source, and performing a first sintering on the resulting mixture to obtain a sintered material; the first element source is selected from one or more compounds of Mg, Al, Ti, Zr, W, Nb, B, Co, Cu, Zn, Sr, Ba, K, Li, Mn, Fe, Y, Sn, Si, Mo, Te, Bi, and Sb; S2. Solid-phase mixing a sintered material with a calcium source, and performing a second sintering on the resulting mixture to obtain a modified sodium electrolyte layered positive electrode material.

6. The method for preparing the modified sodium layered cathode material according to claim 5, wherein: The first element source is selected from one or more of magnesium oxide, magnesium hydroxide, aluminum oxide, aluminum hydroxide, titanium oxide, titanium hydroxide, zirconium oxide, zirconium hydroxide, tungsten oxide, niobium pentoxide, niobium hydroxide, boron oxide, boric acid, cobaltous hydroxide, cobalt oxyhydroxide, copper oxide, zinc oxide, strontium oxide, strontium carbonate, barium hydroxide, barium hydroxide octahydrate, barium carbonate, potassium carbonate, potassium hydroxide, lithium hydroxide, lithium carbonate, manganese dioxide, manganese trioxide, manganese tetraoxide, ferrous oxide, iron oxide, ferrous oxide, ferrous oxalate, yttrium oxide, tin oxide, silicon dioxide, molybdenum trioxide, tellurium trioxide, bismuth oxide and antimony pentoxide.

7. The method for preparing the modified sodium layered cathode material according to claim 5, wherein: The chemical formula of the sodium layered cathode material matrix is ​​Na 1+x Ni a Fe b Mn c Cu d Zn e O2, where -0.1≤x≤0.1, 0.2≤a≤0.5, 0.2≤b≤0.5, 0.2≤c≤0.5, 0≤d≤0.15, 0≤e≤0.15, and a+b+c+d+e=1.

8. The method for preparing the modified sodium layered cathode material according to claim 5, wherein: The mass ratio of the first element in the first element source to the sodium-electrolyte layered cathode material matrix is ​​0.0001-0.02:

1.

9. The method for preparing the modified sodium layered cathode material according to claim 5, wherein: The calcium source is selected from one or more of calcium oxide, calcium hydroxide, calcium carbonate, calcium acetate, calcium phosphate and calcium nitrate.

10. The method for preparing the modified sodium layered cathode material according to claim 5, wherein: The mass ratio of calcium in the calcium source to the sodium layered positive electrode material is 0.0005-0.02:

1.

11. The method for preparing the modified sodium layered cathode material according to any one of claims 5 to 10, wherein: In step S1 , the temperature of the first sintering is 250° C. to 800° C.; the time of the first sintering is 3 to 20 hours; and the first sintering is performed under oxygen and / or dry air conditions.

12. The method for preparing the modified sodium layered cathode material according to any one of claims 5 to 10, wherein: In step S2, the temperature of the second sintering is 500°C to 800°C; the time of the second sintering is 3 to 20 hours; and the second sintering is performed under oxygen and / or dry air conditions.

13. The method for preparing the modified sodium layered cathode material according to any one of claims 5 to 10, wherein: In step S1 and step S2, the solid phase mixing is performed under a protective atmosphere; the protective atmosphere is selected from at least one of nitrogen, argon, oxygen and dry air.

14. A sodium ion battery, characterized in that It includes the modified sodium-ion layered positive electrode material according to any one of claims 1 to 4 or the modified sodium-ion layered positive electrode material prepared by the preparation method according to any one of claims 5 to 13.

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