A sodium-ion cathode material and its preparation method

Abstract

The present application discloses a sodium-ion cathode material and a preparation method thereof, aiming to improve the stability of the sodium-ion cathode material. The sodium-ion cathode material includes an inner layer material and a coating layer covering the inner layer material. The molecular formula of the inner layer material is: NaNi x Fe y M 1‑x‑y O2, and the molecular formula of the coating layer is: Na p O2·nAl 2‑q Z q O k ; wherein, M is a first doping element, and M is selected from at least one of Mn, Al, Mg, Ti, Zr, Sr, Sb, Nb, Mo, Cu, Y, and W; Z is a second doping element, and Z is selected from at least one of Li, Mg, Ni, Ca, and Ba; when Z is Li, k is 3-q; when Z is Li and at least one of Mg, Ni, Ca, and Ba, k is 3; when Z is at least one of Mg, Ni, Ca, and Ba, k is 3-q / 2; x, y, p, q, and n independently satisfy 0.8 < x ≤ 0.96, 0 ≤ y < 0.2, 0 ≤ 1-x-y < 0.04, 1.5 ≤ p < 2, 0 ≤ q ≤ 0.05, 5 ≤ n ≤ 11, and n is an integer.

Description

Technical Field

[0001] This application relates to the technical field of energy storage battery materials, and particularly to a sodium-ion cathode material and a preparation method thereof. Background Art

[0002] Although lithium batteries have advantages such as high stability and high energy density, due to the low reserves and uneven distribution of the key component lithium element in lithium batteries, the large-scale application of lithium batteries is restricted. To solve the contradiction between the huge demand for power batteries and the scarce energy of lithium, sodium-ion batteries with rich raw material reserves (i.e., sodium element) are gradually realizing industrialization.

[0003] Currently, for sodium-ion batteries, especially sodium-ion cathode materials, there is a problem of low stability. This is because the active substance sodium in the sodium-ion cathode material is easily precipitated during the high-temperature sintering process and enriched on the surface of the sodium-ion cathode material. Specifically, whenever sodium ions are precipitated on the surface of the sodium-ion cathode material, these sodium ions can react with water and / or carbon dioxide (CO 2 ) to form sodium carbonate / sodium hydroxide. In this way, the cycle continues, causing sodium ions to continuously precipitate and continuously generating sodium carbonate / sodium hydroxide to be enriched on the surface of the sodium-ion cathode material. It can be seen that when the sodium ions inside the sodium-ion cathode material continuously precipitate to the surface in an irreversible form, the sodium-ion cathode material shows the problem of low stability. Summary of the Invention

[0004] This application provides a sodium-ion cathode material and a preparation method thereof to improve the stability of the sodium-ion cathode material.

[0005] In the first aspect, this application provides a sodium-ion cathode material, including:

[0006] An inner layer material and a coating layer covering the inner layer material, the molecular formula of the inner layer material is: NaNi x Fe y M 1-x-y O 2 , and the molecular formula of the coating layer is: Na p O 2 ·nAl 2-q Z q O k ; where

[0007] M is the first doping element, and M is selected from at least one of Mn, Al, Mg, Ti, Zr, Sr, Sb, Nb, Mo, Cu, Y, and W; Z is the second doping element, and Z is selected from at least one of Li, Mg, Ni, Ca, and Ba; when Z is Li, k is 3 - q; when Z is Li and at least one of Mg, Ni, Ca, and Ba, k is 3; when Z is at least one of Mg, Ni, Ca, and Ba, k is 3 - q / 2; x, y, p, q, and n each independently satisfy 0.8 < x ≤ 0.96, 0 ≤ y < 0.2, 0 ≤ 1 - x - y < 0.04, 1.5 ≤ p < 2, 0 ≤ q ≤ 0.05, 5 ≤ n ≤ 11, and n is an integer.

[0008] In the sodium ion cathode material of the embodiment of the present application, due to the presence of the coating layer, it can effectively isolate the inner layer material from contact with air or electrolyte, avoid the occurrence of side reactions, and thus improve the stability of the sodium ion cathode material. At the same time, since the sodium content in the coating layer of the sodium ion cathode material is between 1.5 and 2, it can not only provide sodium ions for the corresponding electrochemical reaction during the operation of the sodium ion cathode material, slow down the loss of sodium ions in the inner layer material, and further improve the stability of the sodium ion cathode material; it can also provide a stable three-dimensional channel for the extraction and insertion of sodium ions, so that the specific capacity of the sodium ion battery corresponding to the sodium ion cathode material is improved.

[0009] In a possible implementation manner, the thickness of the coating layer is 5 - 100 nm.

[0010] In a second aspect, the present application provides a method for preparing a sodium ion cathode material, including:

[0011] Under the condition of 400°C - 600°C, perform a first sintering treatment on the mixture of the inorganic oxide and the sodium ion cathode material to be coated to obtain a sodium ion cathode material with a coating layer; wherein, the inorganic oxide includes β - Al 2 O 3 , β″ - Al 2 O 3 , metal oxide ZO x - doped β - Al 2 O 3 , and at least one of the metal oxide ZO x - doped β″ - Al 2 O 3 .

[0012] In a possible implementation manner, the molecular formula of the sodium ion cathode material to be coated is: NaNi x Fe y M 1-x- y O 2; wherein, M is a first doping element, and M is selected from at least one of Mn, Al, Mg, Ti, Zr, Sr, Sb, Nb, Mo, Cu, Y, and W, and x and y each independently satisfy 0.8 < x ≤ 0.96, 0 ≤ y < 0.2, and 0 ≤ 1 - x - y < 0.04.

[0013] A possible implementation, the ZO x is selected from: Li 2 O, MgO, NiO, CaO, and at least one of BaO.

[0014] A possible implementation, before performing the first sintering treatment on the mixture of the inorganic oxide and the sodium ion cathode material to be coated under the conditions of 400°C - 600°C, it includes:

[0015] Mix the inorganic oxide and the sodium ion cathode material to be coated in an organic solvent to obtain a solid-liquid mixture;

[0016] Dry the solid-liquid mixture to obtain a mixture composed of the inorganic oxide and the sodium ion cathode material.

[0017] A possible implementation, the organic solvent is selected from at least one of ethanol, ethylene glycol, or propanol.

[0018] A possible implementation, the time of the first sintering treatment is 8 - 20 hours.

[0019] A possible implementation, the inorganic oxide includes β″-Al 2 O 3 When it is, then before performing the first sintering treatment on the mixture of the inorganic oxide and the sodium ion cathode material to be coated, it includes:

[0020] Mix the soluble sodium salt, soluble aluminum salt, and complexing agent in a solvent and evaporate to dryness to obtain a gel-like first mixture;

[0021] Perform a second sintering treatment and a third sintering treatment on the gel-like first mixture in sequence to obtain β″-Al 2 O 3 ; wherein, the sintering temperature of the second sintering treatment is 180°C - 300°C, and the sintering temperature of the third sintering treatment is 900 - 1200°C.

[0022] A possible implementation, the molecular formula of the β″-Al 2 O 3 is Na p O·nAl 2 O 3 , 1.5 ≤ p < 2, 5 ≤ n ≤ 7, and n is an integer.

[0023] A possible implementation, the molar ratio between the soluble sodium salt and the soluble aluminum is p:2n.

[0024] A possible implementation, the inorganic oxide includes the metal oxide ZO x Doped β″-Al 2 O 3 , then before the first sintering treatment of the mixture of the inorganic oxide and the sodium ion cathode material to be coated, it includes:

[0025] Mix the soluble salt corresponding to the second doping element Z, the soluble aluminum salt, the soluble sodium salt and the complexing agent in a solvent and evaporate to dryness to obtain a gel-like second mixture;

[0026] Perform a fourth sintering treatment and a fifth sintering treatment on the gel-like second mixture in sequence to obtain metal oxide ZO x Doped β″-Al 2 O 3 ; wherein, the sintering temperature of the fourth sintering treatment is 180°C - 300°C, and the sintering temperature of the fifth sintering treatment is 900 - 1200°C.

[0027] A possible implementation, the sintering time of the fourth sintering treatment is 1 - 3 hours, and the sintering time of the fifth sintering treatment is 8 - 20 hours.

[0028] A possible implementation, the molecular formula of the metal oxide ZO x Doped β″-Al 2 O 3 is: Na p O 2 ·nAl 2-q Z q O k ; wherein, Z is the second doping element, Z is selected from at least one of Li, Mg, Ni, Ca, and Ba. When Z is Li, k is 3 - q; when Z is Li and at least one of Mg, Ni, Ca, and Ba, k is 3; when Z is at least one of Mg, Ni, Ca, and Ba, k is 3 - q / 2; p, q, and n independently satisfy 1.5 ≤ p < 2, 0 < q ≤ 0.05, 5 ≤ n ≤ 7, and n is an integer.

[0029] A possible implementation, the salt anions of the soluble sodium salt, the soluble aluminum salt, and the soluble salt are each independently selected from NO 3 - , C 2 O 4 2- , CH 3 COO- , SO 4 2- Any one of them.

[0030] In a third aspect, the present application provides a sodium-ion battery, including:

[0031] The sodium-ion cathode material as described in the first aspect and any possible implementation manner, or the sodium-ion cathode material prepared by the method as described in the second aspect and any possible implementation manner. Description of the Drawings

[0032] Figure 1 SEM diagram of Synthesis Example 1 provided by an embodiment of the present application;

[0033] Figure 2 SEM diagram of Synthesis Example 2 provided by an embodiment of the present application;

[0034] Figure 3 SEM diagram of Synthesis Comparative Example 1 provided by an embodiment of the present application;

[0035] Figure 4 Charge and discharge performance test diagram of Device Example 1 provided by an embodiment of the present application;

[0036] Figure 5 Charge and discharge performance test diagram of Device Example 2 provided by an embodiment of the present application;

[0037] Figure 6 Charge and discharge performance test diagram of Device Example 3 provided by an embodiment of the present application;

[0038] Figure 7 Charge and discharge performance test diagram of Device Comparative Example 1 provided by an embodiment of the present application;

[0039] Figure 8 Cycle performance test diagram of Device Example 1 provided by an embodiment of the present application. Detailed Embodiments

[0040] Aiming at the problem of low stability of sodium-ion cathode materials in the prior art, the present application proposes a sodium-ion cathode material, including an inner layer material and a coating layer that coats the inner cathode material. Due to the stable coating layer Na p O 2 ·nAl 2-q Z q O k, avoiding the side reaction between the sodium-ion cathode material and the electrolyte, thus effectively improving the stability of the sodium-ion cathode material. Moreover, since the stoichiometric ratio of sodium ions in the coating layer is between 1.5 and 2, it can not only ensure the stable crystal phase structure of the coating layer, that is, the crystal phase structure of the coating layer does not collapse during charging and discharging, but also construct a three-dimensional sodium-ion transport channel to improve the transport efficiency of sodium ions, thereby effectively improving the conductivity of the sodium-ion cathode material and further improving the specific capacity of the corresponding sodium-ion battery.

[0041] To better understand the above technical solution, the technical solution of the present application will be described in detail below through the accompanying drawings and specific embodiments. It should be understood that the specific features in the embodiments of the present application and the embodiments are detailed descriptions of the technical solution of the present application, rather than limitations on the technical solution of the present application. Without conflict, the technical features in the embodiments of the present application and the embodiments can be combined with each other.

[0042] In a first aspect, the present application provides a sodium-ion cathode material. The sodium-ion cathode material includes an inner layer material and a coating layer covering the inner layer material.

[0043] The molecular formula of the inner layer material is: NaNi x Fe y M 1-x-y O 2 , and the molecular formula of the coating layer is: Na p O 2 ·nAl 2- q Z q O k .

[0044] Wherein, M is a first doping element, and M is selected from at least one of Mn, Al, Mg, Ti, Zr, Sr, Sb, Nb, Mo, Cu, Y, and W; Z is a second doping element, and Z is selected from at least one of Li, Mg, Ni, Ca, and Ba; when Z is Li, k is 3-q; when Z is Li and at least one of Mg, Ni, Ca, and Ba, k is 3; when Z is at least one of Mg, Ni, Ca, and Ba, k is 3-q / 2; x, y, p, and q each independently satisfy 0.8 < x ≤ 0.96, 0 ≤ y < 0.2, 0 ≤ 1 - x - y < 0.04, 1.5 ≤ p < 2, 0 ≤ q ≤ 0.05, 5 ≤ n ≤ 11, and n is an integer.

[0045] In the embodiment of the present application, the inner layer material has a layered structure. When the stoichiometric ratio of the doping element is greater than zero, the doping element is located between the layers of the inner layer material, effectively alleviating the collapse of the layered structure, thereby achieving the beneficial effect of improving the stability of the sodium-ion cathode material.

[0046] Further, the coating layer is Na p O 2 ·nAl 2-q Z q O k , which has the characteristic of stable performance. On the one hand, it can isolate the active material (i.e., the inner layer material) of the sodium ion cathode material from the electrolyte, effectively alleviating the side reaction between the sodium ion cathode material and the electrolyte, thereby improving the stability of the sodium ion cathode material. On the other hand, since the coating layer itself contains sodium and is located on the surface layer of the sodium ion cathode material, it can provide part of the sodium source during the charging and discharging processes, thus delaying the loss of the active material in the sodium ion cathode material, and further improving the stability of the sodium ion cathode material.

[0047] Meanwhile, the coating layer provided in the embodiment of the present application has the advantage of high conductivity. Compared with the sodium ion cathode material without the coating layer, it can not only absorb the sodium carbonate or sodium hydroxide enriched on the surface of the sodium ion material to improve the ionic conductivity of the sodium ion cathode material, but also further improve the ionic conductivity of the sodium ion cathode material due to its own high ionic conductivity (not less than 10 -2 S / cm), so as to improve the specific capacity of the sodium ion battery corresponding to the sodium ion cathode material.

[0048] In an embodiment of the present application, the thickness of the coating layer of the sodium ion cathode material is 5 - 100 nanometers. Preferably, the thickness of the coating layer is 5 - 20 nanometers.

[0049] In the second aspect, the embodiment of the present application also provides a preparation method of a sodium ion cathode material, which includes: performing a first sintering treatment on a mixture of an inorganic oxide and the sodium ion cathode material to be coated at 400°C - 600°C to obtain a sodium ion cathode material with a coating layer.

[0050] Among them, the time of the first sintering treatment can be 8 - 20 hours, and the inorganic oxide includes β - Al 2 O 3 , β″ - Al 2 O 3 , metal oxide ZO x - doped β - Al 2 O 3 , and the metal oxide ZO x - doped β″ - Al 2 O 3 of at least one.

[0051] The molecular formula of the sodium ion cathode material to be coated is: NaNi x Fe y M 1-x-y O 2; wherein, M is a first doping element, and M is selected from at least one of Mn, Al, Mg, Ti, Zr, Sr, Sb, Nb, Mo, Cu, Y, and W, and x and y each independently satisfy 0.8 < x ≤ 0.96, 0 ≤ y < 0.2, and 0 ≤ 1 - x - y < 0.04.

[0052] To ensure the coating effect, it is necessary to mix the inorganic oxide and the sodium ion cathode material to be coated evenly and then perform the first sintering treatment. Therefore, in an embodiment of the present application, before the first sintering treatment, the inorganic oxide and the sodium ion cathode material to be coated are first mixed in an organic solvent and stirred so that the mixture of the inorganic oxide and the sodium ion cathode material to be coated is evenly mixed in the liquid phase to obtain a solid-liquid mixture. Then, the solid-liquid mixture is dried to obtain a mixture composed of the inorganic oxide and the sodium ion cathode material.

[0053] It should be noted that the above-mentioned sodium ion cathode materials to be coated are all stored in a sealed manner to avoid the continuous precipitation of sodium ions in the sodium ion cathode materials to be coated, reacting with water and carbon dioxide in the air, resulting in the loss of active substances in the sodium ion cathode materials, and at the same time, it can also avoid the excessive enrichment of reaction products (i.e., sodium carbonate and sodium hydroxide) on the surface of the sodium ion cathode materials, causing the surface residual alkali amount of the sodium ion cathode materials to be coated to be too high.

[0054] The above-mentioned organic solvent is selected from one or more of ethanol, ethylene glycol, or propanol. Ethanol is preferred.

[0055] Before the first sintering treatment, the corresponding inorganic oxides can also be prepared separately for coating the above-mentioned sodium ion cathode materials to be coated.

[0056] It should be noted that the salt anions of all soluble salts (for example, soluble sodium salts and soluble aluminum salts) in the embodiments of the present application can be the same or different, and the salt anions of the soluble salts in the embodiments of the present application include but are not limited to nitrate, sulfate, and / or oxalate.

[0057] The following will separately describe the preparation of β-Al 2 O 3 、β″-Al 2 O 3 、metal oxide ZO x -doped β-Al 2 O 3 、and the metal oxide ZO x -doped β″-Al 2 O 3 .

[0058] In the preparation of β″-Al 2 O 3When preparing, soluble sodium salt, soluble aluminum salt, or complexing agent can be first mixed in a solvent and evaporated to dryness to obtain a gel-like first mixture. Then, the gel-like first mixture is subjected to a second sintering treatment and a third sintering treatment to obtain β″-Al p O·nAl 2 O 3 with the molecular formula of Na 2 O 3 . Wherein, 1.5 ≤ p < 2, 5 ≤ n ≤ 7, and n is an integer. The sintering temperature of the second sintering treatment is 180°C - 300°C, and the sintering temperature of the third sintering treatment is 900 - 1200°C.

[0059] The sintering time of the above-mentioned second sintering treatment is 1 - 3 hours, so that carbon and hydrogen elements in the gel-like first mixture are volatilized in the form of CO 2 , H 2 O respectively. In fact, the second sintering treatment, as a pretreatment step, corresponds to the self-combustion process of the gel-like first mixture at low temperature, making the mixture after the second sintering treatment change from a gel state to a fluffy solid. On the one hand, it is convenient to transfer from a glass container to a crucible for the third sintering treatment; on the other hand, it reduces the requirements for the equipment and settings of the third sintering treatment, and there is no need to pay attention to the influence of the evaporation rate of carbon dioxide or water vapor on the material, so that the third sintering treatment can be carried out in any sintering equipment. For example, a muffle furnace.

[0060] The time of the above-mentioned third sintering treatment is 8 - 20 hours.

[0061] In the above application examples, the addition ratio of soluble sodium salt and soluble aluminum salt is set based on the stoichiometric ratio of the product. Therefore, when preparing β″-Al 2 O 3 , the molar ratio between soluble sodium salt and soluble aluminum is p:2n.

[0062] Furthermore, according to "Research Progress of Sodium Ion Inorganic Solid Electrolytes", the difference between β-Al 2 O 3 and β″-Al 2 O 3 lies in the different crystal structures, specifically manifested as different ionic conductive layers and stacking sequences. Among them, β-Al 2 O 3 is a hexagonal crystal system, and its general molecular formula is: Na 2 O·nAl 2 O 3 , 5 ≤ n ≤ 7. β″-Al 2 O 3 is a rhombohedral crystal system, and its general molecular formula is Na 2 O·nAl 2 O3 , 8 ≤ n ≤ 11. It can be seen that β-Al 2 O 3 has the same preparation method as β″-Al 2 O 3 . It only needs to control the molar ratio between the soluble sodium salt and the soluble aluminum, so it will not be elaborated here.

[0063] Furthermore, the preparation of β″-Al x doped with the metal oxide ZO 2 O 3 is described as follows: First, the soluble salt corresponding to the second doping element Z, the soluble aluminum salt, the soluble sodium salt, and the complexing agent are mixed in a solvent and evaporated to dryness to obtain a gel-like second mixture. Then, the gel-like second mixture is subjected to a fourth sintering treatment and a fifth sintering treatment in sequence to obtain β″-Al x doped with the metal oxide ZO 2 O 3 .

[0064] Among them, the function of the fourth sintering treatment is the same as that of the second sintering treatment. Both are used as pretreatment to volatilize the carbon element and the hydrogen element in the gel-like second mixture in the forms of CO 2 , H 2 O respectively. And the molecular formula of β″-Al x doped with the metal oxide ZO 2 O 3 is: Na p O 2 ·nAl 2-q Z q O k ; where Z is the second doping element, and Z is selected from at least one of Li, Mg, Ni, Ca, and Ba. When Z is Li, k is 3 - q; when Z is Li and at least one of Mg, Ni, Ca, and Ba, k is 3; when Z is at least one of Mg, Ni, Ca, and Ba, k is 3 - q / 2; p, q, and n independently satisfy 1.5 ≤ p < 2, 0 < q ≤ 0.05, 5 ≤ k ≤ 7, 5 ≤ n ≤ 7, and n is an integer.

[0065] For example, when Z includes Li, the soluble salt corresponding to the second doping element Z can be lithium nitrate, lithium sulfate, lithium acetate, etc.

[0066] Furthermore, the sintering temperature of the fourth sintering treatment is 180°C - 300°C, and the sintering temperature of the fifth sintering treatment is 900 - 1200°C.

[0067] Furthermore, the sintering time of the fourth sintering treatment is 1 - 3 hours, and the sintering time of the fifth sintering treatment is 8 - 20 hours.

[0068] Similarly, prepare metal oxide ZO x Doped β-Al 2 O 3 Similar to the above preparation of metal oxide ZO x Doped β″-Al 2 O 3 The preparation method is the same.

[0069] In the above application examples, by doping metal oxides into the coating layer (β-Al 2 O 3 , and / or β″-Al 2 O 3 ), due to the doping of elements, extra electrons or holes can be generated, increasing the number of free electrons; and, the second doping element Z is doped into the coating layer, which can replace aluminum ions in the coating layer material, making the sodium-deficient structure of the coating layer more conducive to sodium conduction, thereby further improving the ionic conductivity of the sodium-ion cathode material.

[0070] In a third aspect, the embodiments of the present application further provide a sodium-ion battery, which includes the sodium-ion cathode material as described in the first aspect, or the sodium-ion cathode material prepared by the method as described in the second aspect.

[0071] Since the coating layer of the sodium-ion cathode material has high ionic conductivity and is a sodium-deficient structure, it can provide a transmission channel for sodium ions during the charging and discharging processes of the sodium-ion battery, thereby improving the efficiency of sodium ion extraction and insertion, and further enhancing the specific capacity of the sodium-ion battery. At the same time, since the coating layer of the sodium-ion cathode material can remove the residual alkali (sodium carbonate, and / or sodium bicarbonate) generated on the surface of the sodium-ion cathode material during a single high-temperature sintering process by absorbing sodium ions, the gas generation performance of the above sodium-ion battery is also improved. Further, the presence of the first doping element in the inner layer material of the sodium-ion cathode material, and the coating layer separating the inner layer active material from the electrolyte can effectively enhance the stability of the sodium-ion battery. At the same time, since the coating layer of the sodium-ion cathode material contains sodium ions, it can provide reactants (sodium ions) for the electrochemical reactions involving sodium ions occurring at the negative electrode during the charging and discharging processes of the sodium-ion battery, thereby slowing down the loss of sodium ions in the inner layer material of the sodium-ion cathode material, and further enhancing the stability of the sodium-ion cathode material.

[0072] The following is described in conjunction with Synthesis Examples 1-4, Synthesis Comparative Example 1, Device Examples 1-4, and Device Comparative Example 1.

[0073] Synthesis Example 1

[0074] S1. Take the precursor Ni 0.82 Fe 0.12 Mn0.06 (OH) 2 is mixed evenly with sodium carbonate in a plowshare mixer, then sintered at 1100 °C for 20 h, and then crushed, sieved, washed with water, and dried to obtain a layered oxide material NaNi 0.82 Fe 0.12 Mn 0.06 O 2 。

[0075] S2. Mix aluminum nitrate, sodium nitrate, and citric acid monohydrate and dissolve them in water. Heat and stir at 80 °C until the water evaporates completely to obtain a gel-like first mixture; the molar ratio of all metal ions (Al 3+ , Na + ) to the molar amount of citric acid monohydrate is 1:1.

[0076] S3. Place the gel-like first mixture in an oven at 250 °C for self-combustion for 2 h, and then grind it to obtain a second mixture in the form of a black powder.

[0077] S4. Calcinate the second mixture at 1100 °C for 10 h to obtain β″-Al 2 O 3 。

[0078] S5. Dissolve β″-Al 2 O 3 and NaNi 0.82 Fe 0.12 Mn 0.06 O 2 in ethanol and stir to obtain a mixed slurry.

[0079] S6. Spray-dry the mixed slurry to obtain a mixed powder, and then place it in a muffle furnace for heat treatment at 500 °C for 10 h.

[0080] S7. Grind the heat-treated product to obtain a sodium-ion battery cathode material with the molecular formula NaNi 0.82 Fe 0.12 Mn 0.06 O 2 @β″-Al 2 O 3 for reference Figure 1 。

[0081] Synthesis Example 2

[0082] S1. Use the precursor of the layered oxide Ni 0.83 Fe 0.12 Mn 0.05 (OH) 2Mix it evenly with sodium carbonate in a plowshare mixer, then sinter it at 1100 °C for 20 h, and then crush, sieve, wash with water, and dry to obtain the layered oxide material NaNi 0.82 Fe 0.12 Mn 0.05 O 2 。

[0083] Steps S2 - S7 are the same as those in Synthesis Example 1 to obtain the sodium-ion battery cathode material with the molecular formula NaNi 0.82 Fe 0.12 Mn 0.05 O 2 @β″-Al 2 O 3 Please refer to Figure 2 。

[0084] Synthesis Example 3

[0085] S1. Mix the precursor Ni 0.82 Fe 0.12 Mn 0.06 (OH) 2 of the layered oxide evenly with sodium carbonate in a plowshare mixer, then sinter it at 1100 °C for 20 h, and then crush, sieve, wash with water, and dry to obtain the layered oxide material NaNi 0.82 Fe 0.12 Mn 0.06 O 2 ;

[0086] S2. Mix aluminum nitrate, sodium nitrate, lithium oxalate, and citric acid monohydrate and dissolve them in water, and heat and stir at 80 °C until the water evaporates completely to obtain a gel-like first mixture; the total molar amount of all metal ions (Al 3+ 、Na + 、Li + ) is in a ratio of 1:1 to the molar amount of citric acid monohydrate.

[0087] Steps S3 - S7 are the same as those in Synthesis Example 1 to obtain the sodium-ion battery cathode material with the molecular formula NaNi 0.82 Fe 0.12 Mn 0.06 O 2 @Li 2 O-β″-Al 2 O 3 。

[0088] Synthesis Example 4

[0089] S1. Mix the precursor Ni 0.83 Fe 0.12 Mn 0.05 (OH) 2Mix it evenly with sodium carbonate in a plowshare mixer, then sinter it at 1100 °C for 20 h, and then crush, sieve, wash with water, and dry to obtain a layered oxide material NaNi 0.82 Fe 0.12 Mn 0.05 O 2 。

[0090] S2. Mix aluminum nitrate, sodium nitrate, magnesium nitrate, and citric acid monohydrate and dissolve them in water. Heat and stir at 80 °C until the water evaporates completely to obtain a gel-like first mixture; the total molar amount of all metal ions (Al 3+ 、Na + 、Mg 2+ ) is in a ratio of 1:1 to the molar amount of citric acid monohydrate.

[0091] Steps S3 - S7 are the same as those in Synthesis Example 1 to obtain a sodium-ion battery cathode material with the molecular formula NaNi 0.83 Fe 0.12 Mn 0.05 O 2 @MgO-β″-Al 2 O 3 。

[0092] Synthesis Comparative Example 1

[0093] Mix the precursor Ni 0.82 Fe 0.12 Mn 0.06 (OH) 2 of the layered oxide evenly with sodium carbonate in a plowshare mixer, and then sinter it at 1100 °C for 20 h; then crush, sieve, wash with water, and dry to obtain a layered oxide material NaNi 0.82 Fe 0.12 Mn 0.06 O 2 ,please refer to Figure 3 。

[0094] Dissolve the sodium-ion cathode materials prepared in Synthesis Examples 1 - 4 and Synthesis Comparative Example 1 in water / absolute ethanol to dissolve the sodium carbonate and / or sodium hydroxide on the surface of the sodium-ion cathode material. Test the carbonate (CO 3 2- ) in water and the hydroxide (OH - ) in absolute ethanol, and test the pH of the solution with a pH meter. At the same time, also test the impedance of the sodium-ion cathode material with a powder resistance meter at 19 MPa. For details, please refer to Table 1.

[0095] Table 1

[0096] Unit: ppm

[0097]

[0098] As can be seen from Table 1, the coating layers in Synthesis Examples 1-4 can effectively avoid the residual alkali amount on the surface of the sodium ion cathode material, and at the same time can effectively reduce the powder impedance, thereby improving the ionic conductivity of the sodium ion cathode material.

[0099] Device Examples 1-4, Device Comparative Example 1

[0100] The cathode materials in Device Examples 1-4 and Device Comparative Example 1 are the sodium ion cathode materials prepared in Synthesis Examples 1-4 and Synthesis Comparative Example 1 respectively. The preparation of Device Examples 1-4 and Device Comparative Example 1 is described as follows:

[0101] S1. Mix the sodium ion cathode material with acetylene black and polyvinylidene fluoride (PVDF), and then add an appropriate amount of N-methylpyrrolidone (NMP) solution, and stir to form a cathode slurry.

[0102] S2. Uniformly coat the cathode slurry on the aluminum foil, place it in a vacuum drying oven and dry for 24 h, and cut it into a pole piece with a diameter of 12 mm.

[0103] S3. Place the obtained sodium ion battery cathode pole piece and the sodium metal anode on both sides of the separator respectively, add an appropriate amount of sodium ion battery electrolyte (the solute is sodium hexafluorophosphate, and the solvent is ethylene carbonate (EC) and diethyl carbonate (DEC), and their volume ratio is EC:DEC = 1:1), and assemble them into a CR2032 sodium ion button battery in a glove box filled with argon.

[0104] Furthermore, the following tests are respectively carried out on Device Examples 1-4 and Device Comparative Example 1, and the test results are shown in Table 2:

[0105] At 25 °C, charge-discharge performance tests are carried out at 2.0-4.0 V with 0.1C charge / 0.1C discharge. According to the formula: first discharge efficiency = first cycle discharge specific capacity / first cycle charge specific capacity * 100%, the first discharge efficiency is determined.

[0106] At 25 °C, cycle performance tests are carried out at 2.0-4.0 V with 1C charge / 1C discharge. According to the formula: capacity retention rate = 100th cycle discharge specific capacity / first cycle discharge specific capacity * 100%, the capacity retention rate after 100 cycles is determined.

[0107] Table 2

[0108]

[0109]

[0110] Combined with Figures 4 - 8, and as can be seen from Table 2, the initial cycle discharge specific capacity, the first discharge efficiency, and the cycle retention rate of Synthesis Examples 1-4 are all superior to those of Synthesis Comparative Example 1.

[0111] Obviously, those skilled in the art can make various modifications and variations to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalent technologies, the present invention is also intended to include these modifications and variations.

Claims

1. A sodium-ion cathode material, characterized in that, it comprises: an inner layer material and a coating layer coating the inner layer material, wherein, The molecular formula of the inner layer material is: NaNi 0.82 Fe 0.12 Mn 0.06 O 2 , or, NaNi 0.82 Fe 0.12 Mn 0.05 O 2 ; The molecular formula of the coating layer is: β-Al 2 O 3 , or, β″-Al 2 O 3 .

2. The sodium-ion cathode material according to claim 1, characterized in that, the thickness of the coating layer is 5 - 100 nm.

3. A preparation method of the sodium-ion cathode material according to claim 1 or 2, characterized in that, it comprises: Under the condition of 400 °C - 600 °C, a first sintering treatment is carried out on a mixture of an inorganic oxide and a sodium ion cathode material to be coated, and a sodium ion cathode material with a coating layer is obtained; wherein, the inorganic oxide includes β-Al 2 O 3 , β″-Al 2 O 3 .

4. The preparation method according to claim 3, characterized in that, before performing the first sintering treatment on the mixture of the inorganic oxide and the sodium-ion cathode material to be coated under the condition of 400°C - 600°C, it includes: mixing the inorganic oxide and the sodium-ion cathode material to be coated in an organic solvent to obtain a solid-liquid mixture; drying the solid-liquid mixture to obtain a mixture composed of the inorganic oxide and the sodium-ion cathode material.

5. The preparation method according to claim 4, characterized in that, the organic solvent is selected from at least one of ethanol, ethylene glycol, or propanol.

6. The preparation method according to claim 3, characterized in that, The inorganic oxide includes β″-Al 2 O 3 When, before the first sintering treatment of the mixture of the inorganic oxide and the sodium ion cathode material to be coated, it includes: mixing a soluble sodium salt, a soluble aluminum salt, and a complexing agent in a solvent and evaporating to dryness to obtain a gel-like first mixture; The gel-like first mixture is successively subjected to a second sintering treatment and a third sintering treatment to obtain β″-Al 2 O 3 ; wherein, the sintering temperature of the second sintering treatment is 180°C - 300°C, and the sintering temperature of the third sintering treatment is 900 - 1200°C.

7. The preparation method according to claim 6, characterized in that, The said β″-Al 2 O 3 has a molecular formula of Na p O·nAl 2 O 3 , where 1.5 ≤ p < 2, 5 ≤ n ≤ 7, and n is an integer.

8. The preparation method according to claim 7, characterized in that, the molar ratio between the soluble sodium salt and the soluble aluminum is p:2n.

9. A sodium-ion battery, characterized in that, it comprises: the sodium-ion cathode material according to claim 1 or 2, or the sodium-ion cathode material prepared by the preparation method according to any one of claims 3 - 8.

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