A cathode material with a coating layer, a preparation method thereof, and a sodium-ion battery

Abstract

The present application provides a cathode material with a coating layer, a preparation method thereof, and a sodium-ion battery, which are used to solve the problem of high alkali content on the surface of sodium-ion cathode materials in the prior art, so as to improve the gas generation performance of sodium-ion cathode batteries. The cathode material includes: the molecular formula of the inner layer material of the cathode material is: Na e Ni x Fe y M 1‑x‑y O2, and the coating layer is Na 3‑2p PS 4‑p and / or Na 3‑ q PS 4‑q H q ; M is a first doping element, and M is selected from at least one of Cu, Mn, Al, Mg, Zn, Sn, Ti, Zr, Sr, Sb, Nb, Mo, Y, and W; H is a second doping element, and H is selected from at least one of F, Cl, and Br; e, x, y, p, and q are each selected from: 0.8 ≤ e ≤ 1.1, 0.2 < x ≤ 0.96, 0 ≤ y < 0.2, 0 ≤ 1 - x - y ≤ 0.04, 0 < p ≤ 0.1, 0 < q ≤ 0.1.

Description

Technical Field

[0001] The present application relates to the technical field of sodium-ion batteries, and in particular, to a cathode material with a coating layer, a preparation method thereof, and a sodium-ion battery. Background Art

[0002] In the face of the non-renewability of fossil fuels and the environmental damage caused by fossil fuels, renewable energy sources (e.g., secondary batteries) have emerged as the times require. Among secondary batteries, lithium batteries were the first to be put into application due to their advantages such as good cycle life and high energy density. However, due to a series of factors such as the scarcity of lithium element resources, uneven distribution on the earth's surface, and difficult extraction, lithium batteries are gradually in a dilemma of insufficient raw materials.

[0003] Due to the advantages of abundant reserves and low cost of the active substance sodium in sodium-ion batteries, more and more attention has been paid to sodium-ion batteries. The working principle of sodium-ion batteries is similar to that of lithium-ion batteries: by using the process of sodium ions escaping and embedding between the positive and negative electrodes to achieve the discharge or charging of sodium-ion batteries. During charging, Na + escapes from the positive electrode and embeds into the negative electrode through the electrolyte. At the same time, the compensating charge of electrons is supplied to the negative electrode through the external circuit to ensure the charge balance between the positive and negative electrodes. During discharge, Na + escapes from the negative electrode and embeds into the positive electrode through the electrolyte. However, in the sodium-ion cathode material, since sodium ions are extremely easy to react with water or dioxide to form sodium carbonate and sodium hydroxide, the problem of high alkali content on the surface of the sodium-ion cathode material appears, and then the problem of poor gas production performance of the corresponding sodium-ion battery occurs. Summary of the Invention

[0004] The present application provides a cathode material with a coating layer, a preparation method thereof, and a sodium-ion battery to solve the problem of high alkali content on the surface of the sodium-ion cathode material in the prior art, so as to improve the gas production performance of the sodium-ion cathode battery.

[0005] In a first aspect, the present application provides a cathode material with a coating layer, including:

[0006] The molecular formula of the inner layer material of the cathode material is: Na e Ni x Fe y M 1-x-y O 2 , and the coating layer is Na 3-2p PS 4-p and / or Na 3-q PS 4-q H q, where M is the first doping element, and M is selected from at least one of Cu, Mn, Al, Mg, Zn, Sn, Ti, Zr, Sr, Sb, Nb, Mo, Y, and W; H is the second doping element, and H is selected from at least one of F, Cl, and Br; e, x, y, p, and q are each selected from: 0.8 ≤ e ≤ 1.1, 0.2 < x ≤ 0.96, 0 ≤ y < 0.2, 0 ≤ 1 - x - y ≤ 0.04, 0 < p ≤ 0.1, 0 < q ≤ 0.1.

[0007] In the embodiment of the present application, the coating layer of the positive electrode material has a sodium-deficient structure, and can absorb sodium ions (Na + ) on the surface of the inner layer material, removing the residual alkali (such as sodium carbonate, sodium hydroxide) on the surface of the inner layer material, thereby avoiding the problem that the residual alkali adheres to the surface of the sodium-ion positive electrode material, resulting in low ionic conductivity of the sodium-ion positive electrode material. Further, the sodium-ion positive electrode material provided in the embodiment of the present application can effectively avoid the problem that the gas production performance of the sodium-ion battery decreases due to the decomposition of the surface residual alkali to generate gas during the charge-discharge process. At the same time, the coating layer has the characteristic of stable properties and can isolate the inner layer material from the electrolyte, thereby effectively suppressing the gas released due to the decomposition of the electrolyte by side reactions, so the gas production performance of the sodium-ion battery can be further improved. In addition, the sodium-deficient structure in the coating layer can provide a three-dimensional transmission channel for the extraction and insertion of sodium ions during the charge and discharge processes, so the ionic conductivity of the positive electrode material can be improved by effectively increasing the sodium ion deintercalation rate, thereby further improving the specific capacity and rate performance of the sodium-ion battery.

[0008] In a possible implementation manner, the structure of the inner layer material is a layered structure.

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

[0010] In a possible implementation manner, the value of the impedance of the positive electrode material powder is not greater than 2.2×10 4 Ohm / cm.

[0011] In a second aspect, the present application provides a method for preparing a positive electrode material with a coating layer as described in the first aspect and any possible implementation manner, including:

[0012] Sintering a mixture of a sulfide and the positive electrode material to be coated at 350°C - 500°C for 4 - 20 hours to obtain the positive electrode material with the coating layer; where the sulfide is Na 3-2p PS 4-p and / or Na 3-q PS 4-q H q; H is the second doping element, and H is selected from at least one of F, Cl, or Br, and p and q each independently selected from: 0 < p ≤ 0.1, 0 < q ≤ 0.1.

[0013] In a possible implementation manner, before subjecting the mixture of the sulfide and the cathode material to be coated to the first sintering treatment to obtain the cathode material with the sulfide as the coating layer, it further includes:

[0014] Sintering the mixture of the precursor and the sodium source at 800 °C - 1100 °C for 12 - 24 hours to obtain the cathode material to be coated;

[0015] Mixing the cathode material to be coated and the sulfide in an organic solvent, stirring, and drying to obtain the mixture of the sulfide and the cathode material to be coated.

[0016] In a possible implementation manner, the precursor is selected from: Ni x Fe y M 1-x-y (OH) 2 , or Ni x Fe y M 1-x-y CO 3 ; wherein, M is the first doping element, and M is selected from at least one of Cu, Mn, Al, Mg, Zn, Sn, Ti, Zr, Sr, Sb, Nb, Mo, Y, and W, and each of x and y independently selected from: 0.2 < x ≤ 0.96, 0 ≤ y < 0.2, 0 ≤ 1 - x - y ≤ 0.04.

[0017] In a possible implementation manner, when the coating layer includes Na 3-2p PS 4-p , then before subjecting the mixture of the sulfide and the cathode material to be coated to the first sintering treatment to obtain the cathode material with the sulfide as the coating layer, it further includes:

[0018] Performing a low-temperature annealing treatment on the mixture of Na 2 S and P 2 S 5 at 150 °C - 300 °C to obtain Na 3-q PS 4 ; wherein, the molar ratio between the Na 2 S and P 2 S 5 is (3 - 2p):1.

[0019] In a possible implementation manner, the coating layer includes Na 3-q PS 4-q H qWhen, before obtaining the positive electrode material with the coating layer being the sulfide by performing the first sintering treatment on the mixture of the sulfide and the positive electrode material to be coated, it further includes:

[0020] In an inert atmosphere or in vacuum, sinter a mixture of Na 2 S, P 2 S 5 and NaH at 650°C - 900°C to obtain Na 3-q PS 4-q H q ; wherein, the molar ratio among the Na 2 S, P 2 S 5 and NaH is (3 - 2q):1:2q.

[0021] In a third aspect, the present application provides a sodium-ion battery, including:

[0022] The ternary positive electrode material with a coating layer as described in the first aspect and any possible implementation manner, or the ternary positive electrode material with a coating layer prepared by the method as described in the second aspect and any possible implementation manner. Description of the Drawings

[0023] Figure 1 It is the electron microscope scanning image of Synthesis Example 1 provided by the embodiment of the present application;

[0024] Figure 2 It is the electron microscope scanning image of Synthesis Example 2 provided by the embodiment of the present application;

[0025] Figure 3 It is the electron microscope scanning image of Synthesis Example 3 provided by the embodiment of the present application;

[0026] Figure 4 It is the electron microscope scanning image of Synthesis Example 4 provided by the embodiment of the present application;

[0027] Figure 5 It is the electron microscope scanning image of Synthesis Comparative Example 1 provided by the embodiment of the present application;

[0028] Figure 6 It is the charge-discharge test graph of Device Example 1 provided by the embodiment of the present application;

[0029] Figure 7 It is the charge-discharge test graph of Device Example 2 provided by the embodiment of the present application;

[0030] Figure 8 It is the charge-discharge test graph of Device Comparative Example 1 provided by the embodiment of the present application. Detailed Embodiments

[0031] In view of the problem of high alkali content on the surface of sodium-ion cathode materials in the prior art, the present application proposes a sodium-ion cathode material with a specific coating layer. The molecular formula of the inner layer material of this cathode material is: Na e Ni x Fe y M 1-x-y O 2 , and the coating layer is Na 3-2p PS 4-p and / or Na 3-q PS 4-q H q . Among them, M is the first doping element, and M is selected from at least one of Cu, Mn, Al, Mg, Zn, Sn, Ti, Zr, Sr, Sb, Nb, Mo, Y, and W; H is the second doping element, and H is selected from at least one of F, Cl, and Br; e, x, y, p, q are each selected from: 0.8 ≤ e ≤ 1.1, 0.2 < x ≤ 0.96, 0 ≤ y < 0.2, 0 ≤ 1 - x - y ≤ 0.04, 0 < p ≤ 0.1, 0 < q ≤ 0.1.

[0032] On the one hand, since the coating layer of the sodium-ion cathode material proposed in the embodiment of the present application is a sodium-deficient structure, this sodium-deficient structure can absorb the sodium ions formed on the surface of the inner layer material before being coated, removing the residual alkali (mainly sodium carbonate and sodium hydroxide) on the surface of the inner layer material, thereby improving the ionic conductivity of the sodium-ion cathode material, avoiding the problem of low ionic conductivity of the sodium-ion cathode material caused by residual alkali in the prior art, and further avoiding the problem of low ionic conductivity of the sodium-ion cathode material. At the same time, due to the presence of the coating layer of the sodium-ion cathode material in the embodiment of the present application, it is avoided that gas is generated by the decomposition of residual alkali during the charge-discharge process of the sodium-ion cathode material. Therefore, the gas production performance of the sodium-ion cathode material can be improved. And because the coating layer has stable performance, it can effectively isolate the contact between the electrolyte and the inner layer material in the sodium-ion battery, so as to avoid the gas generated by the decomposition of the electrolyte caused by side reactions between the two, and thus can further improve the gas production performance and stability of the sodium-ion battery. Correspondingly, the specific capacity is also improved.

[0033] On the other hand, the above coating layer is a sulfide and / or its derivative. Since sulfides and their derivatives have the advantages of high ionic conductivity and low grain boundary resistance, and at the same time because the sulfur atom has a large radius and the electronegativity of sulfur is less than that of oxygen, the electrostatic force between the sulfur ions and sodium ions in the coating layer is small. Therefore, compared with other coating layers, this coating layer can more effectively improve the ionic conductivity of the sodium-ion cathode material, making the sodium ions more efficient in deintercalation and intercalation during the charge-discharge process of the sodium-ion battery, and thus more significantly improving the specific capacity and rate performance of the sodium-ion cathode material.

[0034] Furthermore, the structure of the inner layer material is a layered structure. Therefore, when the inner layer material contains doping elements, the doping elements can be distributed between the "layers" of the layered structure, which can play a role in supporting the layered structure, thereby alleviating the structural collapse phenomenon that occurs during the charging and discharging processes of the sodium-ion cathode material due to phase changes, and then effectively improving the stability of the sodium-ion cathode material.

[0035] Furthermore, the thickness of the coating layer is 5 - 100 nm. Preferably, it is 5 - 20 nm.

[0036] Furthermore, since the cathode material provided in this application contains a sodium-deficient structure coating layer, and the presence of this coating layer can effectively inhibit the precipitation (generation) of residual alkali on the surface of the cathode material, thereby effectively avoiding the problem of high impedance of the cathode material powder caused by surface residual alkali. Therefore, in an embodiment of this application, the impedance value of the cathode material powder is not greater than 2.2×10 4 Ohm / cm.

[0037] Based on the same inventive concept, this application provides a preparation method for the cathode material with a coating layer as described above. The method includes: sintering a mixture of a sulfide and the cathode material to be coated at 350°C - 500°C for 4 - 20 hours to obtain a cathode material with a coating layer.

[0038] Among them, the sulfide is Na 3-2p PS 4-p and / or Na 3-q PS 4-q H q , where H is a second doping element, and H is selected from at least one of F, Cl, or Br, and p and q each independently selected from: 0 < p ≤ 0.1, 0 < q ≤ 0.1.

[0039] The addition amount of the above sulfide can be determined according to the sodium content on the surface of the cathode material to be coated.

[0040] The cathode material to be coated can be obtained by sintering a mixture of a precursor and a sodium source at 800°C - 1100°C for 12 - 24 hours.

[0041] Among them, the sodium source can be sodium hydroxide. The precursor is selected from: Ni x Fe y M 1-x-y (OH) 2 , or Ni x Fe y M 1-x-y CO 3; wherein, M is a first doping element, and M is selected from at least one of Cu, Mn, Al, Mg, Zn, Sn, Ti, Zr, Sr, Sb, Nb, Mo, Y, and W. Each of x and y independently selects from: 0.2 < x ≤ 0.96, 0 ≤ y < 0.2, 0 ≤ 1 - x - y ≤ 0.04.

[0042] Further, before performing the first sintering treatment, the cathode material Na e Ni x Fe y M 1-x-y O 2 ; wherein, e, x, and y each select from: 0.8 ≤ e ≤ 1.1, 0.2 < x ≤ 0.96, 0 ≤ y < 0.2, 0 ≤ 1 - x - y ≤ 0.04. Then, the cathode material to be coated and the aforementioned sulfide are mixed and stirred in an organic solvent and dried to obtain a mixture of the sulfide and the cathode material to be coated.

[0043] Wherein, the organic solvent can be anhydrous ethanol.

[0044] Further, before performing the first sintering treatment, the corresponding sulfide can also be prepared, and the following is a specific description.

[0045] When the coating layer includes Na 3-2p PS 4-p , then a mixture of Na 2 S and P 2 S 5 is subjected to a low-temperature annealing treatment at 150°C - 300°C to obtain Na 3-2p PS 4-p .

[0046] Wherein, the molar ratio between the Na 2 S and P 2 S 5 is (3 - 2p):1. The time for low-temperature annealing does not exceed 4 hours. For example, 1 hour, 2 hours.

[0047] The above-mentioned mixture of Na 2 S and P 2 S 5 can be mixed by using a ball mill to make the grinding balls and abrasives collide with high energy under the action of centrifugal force, so as to obtain a well-mixed mixture.

[0048] When the coating layer includes Na 3-q PS 4-q H q , then in an inert atmosphere or vacuum, Na 2 S, P 2 S 5A mixture of [substance] and NaH is sintered at 650 °C - 900 °C to obtain Na 3-q PS 4-q H q .

[0049] Among them, the molar ratio between the said Na 2 S, P 2 S 5 and NaH is (3 - 2q):1:2q. Among them, the sintering time can be 2 - 10 hours.

[0050] It should be noted that the ratio of the above mixture before sintering is determined according to the stoichiometric ratio of the coating layer molecular formula, so that the prepared sulfide has a sodium-deficient structure, ensuring that the residual alkali on the surface of the cathode material to be coated can be removed during the mixed sintering process with the cathode material to be coated.

[0051] Furthermore, the above mixtures can all be ensured by the grinding balls in the ball mill, so that the abrasives (for example, Na 2 S, P 2 S 5 and NaH) are fully mixed under the action of the grinding balls and centrifugal force. Specifically, the rotation speed of the ball mill can be 100 - 800 r / min, and the ball milling time can be 8 - 24 hours.

[0052] Based on the same inventive concept, the embodiment of the present application also provides a sodium-ion battery, including the sodium-ion cathode material as described above.

[0053] Since the sodium-ion battery includes the above sodium-ion cathode material, the problems of low ionic conductivity and poor gas generation performance caused by the existence of residual alkali on the surface of the sodium-ion cathode material are avoided, thereby effectively improving the specific capacity and stability of the sodium-ion battery, and further improving the safety performance of the sodium-ion battery.

[0054] The following is illustrated by Synthesis Examples 1 - 4, Synthesis Comparative Example 1, Device Examples 1 - 8, Device Comparative Example 1 and Device Comparative Example 5.

[0055] Synthesis Example 1

[0056] S1. Mix and grind Na 2 S and P 2 S 5 in a molar ratio of 73:25, and transfer the pre-ground material into a ball mill, and ball mill at a rotation speed of 400 r / min for 10 h to obtain a mixed material.

[0057] S2. Perform low-temperature annealing treatment on the mixed material at 200 °C to obtain Na 2.92 PS 3.96 .

[0058] S3. Mix the precursor material Ni 0.82 Fe 0.12 Mn 0.06 (OH) 2 uniformly with sodium carbonate in a plowshare mixer, and sinter at 1100 °C for 20 h to obtain NaNi 0.82 Fe 0.12 Mn 0.06 O 2 .

[0059] S4. Dissolve Na 2.92 PS 3.96 and NaNi 0.82 Fe 0.12 Mn 0.06 O 2 in ethanol and stir to obtain a mixed slurry;

[0060] S5. Spray-dry the mixed slurry to obtain mixed powder; and place the mixed powder in a muffle furnace and heat-treat at 450 °C for 10 h.

[0061] S6. Grind the heat-treated powder to obtain a cathode material: NaNi 0.82 Fe 0.12 Mn 0.06 O 2 @Na 2.92 PS 3.96 . For the morphology of this cathode material, please refer to Figure 1 .

[0062] Synthesis Example 2

[0063] S1. Mix Na 2 S and P 2 S 5 in a molar ratio of 71.5:25, grind them uniformly, and transfer the pre-ground material to a ball mill and ball-mill at a rotation speed of 400 r / min for 10 h to obtain a mixed material.

[0064] S2. Perform low-temperature annealing treatment on the mixed material at 200 °C to obtain Na 2.86 PS 3.93 .

[0065] S3. Mix the precursor material NaNi 0.83 Fe 0.12 Mn 0.05 (OH) 2 uniformly with sodium carbonate in a plowshare mixer, and sinter at 1100 °C for 20 h to obtain NaNi 0.83 Fe 0.12 Mn 0.05 O 2 .

[0066] S4. Dissolve Na 2.86 PS 3.93 and NaNi 0.83 Fe 0.12 Mn 0.05 O 2 in ethanol and stir to obtain a mixed slurry.

[0067] S5. Spray-dry the mixed slurry to obtain mixed powder; and place the mixed powder in a muffle furnace and heat-treat it at 450 °C for 10 h.

[0068] S6. Grind the heat-treated powder to obtain a cathode material: NaNi 0.83 Fe 0.12 Mn 0.05 O 2 @Na 2.86 PS 3.93 . For the morphology of this cathode material, please refer to Figure 2 .

[0069] Synthesis Example 3

[0070] S1. Mix and grind Na 2 S, P 2 S 5 and NaBr in a molar ratio of 28.7:10:1.3, and transfer the pre-ground material to a ball mill and ball-mill it at a rotation speed of 800 r / min for 10 h to obtain a mixed material.

[0071] S2. Calcinate the mixed material in vacuum at 800 °C for 6 h, cool and grind to obtain Na 2.935 PS 3.935 Br 0.065 .

[0072] S3. Mix the precursor material NaNi 0.83 Fe 0.12 Mn 0.05 (OH) 2 and sodium carbonate evenly in a plowshare mixer and sinter at 1100 °C for 20 h to obtain NaNi 0.83 Fe 0.12 Mn 0.05 O 2 .

[0073] S4. Dissolve Na 2.935 PS 3.935 Br 0.065 and NaNi 0.83 Fe 0.12 Mn 0.05 O 2 in ethanol and stir to obtain a mixed slurry.

[0074] S5. Spray-dry the mixed slurry to obtain mixed powder; and place the mixed powder in a muffle furnace and heat-treat it at 450 °C for 10 h.

[0075] S6. Grind the heat-treated powder to obtain the cathode material: NaNi 0.83 Fe 0.12 Mn 0.05 O 2 @Na 2.935 PS 3.935 Br 0.065 。For the morphology of this cathode material, please refer to Figure 3 。

[0076] Synthesis Example 4

[0077] S1. Mix and grind Na 2 S, P 2 S 5 and NaCl in a molar ratio of 29.1:10:0.9, and transfer the pre-ground material into a ball mill and ball-mill it at a rotation speed of 800 r / min for 10 h to obtain a mixed material.

[0078] S2. Calcinate the mixed material in vacuum at 800 °C for 6 h, cool and grind it to obtain Na 2.955 PS 3.955 Cl 0.045 。

[0079] S3. Mix the precursor material Ni 0.82 Fe 0.12 Mn 0.06 (OH) 2 and sodium carbonate evenly in a plowshare mixer and sinter it at 1100 °C for 20 h to obtain NaNi 0.82 Fe 0.12 Mn 0.06 O 2 。

[0080] S4. Dissolve Na 2.955 PS 3.955 Cl 0.045 and NaNi 0.82 Fe 0.12 Mn 0.06 O 2 in ethanol and stir to obtain a mixed slurry.

[0081] S5. Spray-dry the mixed slurry to obtain mixed powder; and place the mixed powder in a muffle furnace and heat-treat it at 450 °C for 10 h.

[0082] S6. Grind the heat-treated powder to obtain the cathode material: NaNi 0.82 Fe 0.12 Mn 0.06 O 2 @Na 2.955 PS 3.955 Cl 0.045 . For the morphology of this cathode material, please refer to Figure 4 .

[0083] Synthesis Comparative Example 1

[0084] Mix the precursor material Ni 0.82 Fe 0.12 Mn 0.06 (OH) 2 and sodium carbonate evenly in a plowshare mixer, sinter at 1100 °C for 20 h, crush to obtain NaNi 0.82 Fe 0.12 Mn 0.06 O 2 . For the morphology of this cathode material, please refer to Figure 5 .

[0085] Dissolve the materials obtained in step S3 (inner layer) of Synthesis Examples 1-4, the cathode materials obtained in step S6, and the cathode materials obtained in Synthesis Comparative Example 1 in water and absolute ethanol respectively, so that the residual alkali on their surfaces is dissolved in water / absolute ethanol. And test the content of carbonate (CO 3 2- ) in water and the content of hydroxide (OH-) in absolute ethanol, and test the pH value of the aqueous solution with a pH meter. For the specific test results, please refer to Table 1

[0086] Table 1

[0087] Unit: ppm

[0088]

[0089] As can be seen from Table 1, for the cathode materials in Synthesis Examples 1-4, due to the presence of the coating layer, the amount of residual alkali on their surfaces is significantly reduced, and the powder impedance is also significantly reduced

[0090] Device Examples 1 - Device Example 8, Device Comparative Example 1, Device Comparative Example 5

[0091] Device Examples 1 - Device Example 4 and Device Comparative Example 1 are coin cells with Synthesis Examples 1-4 and Synthesis Comparative Example 1 as the cathodes respectively. First, the preparation of Device Examples 1 - Device Example 4 and Device Comparative Example 1 will be described as follows

[0092] S1. Mix the cathode material with acetylene black and polyvinylidene fluoride (PVDF), then add an appropriate amount of N-methylpyrrolidone (NMP) solution and stir to form a cathode slurry.

[0093] S2. Uniformly coat the cathode slurry on the aluminum foil, place it in a vacuum drying oven and dry for 24 h, then cut it into circular electrodes with a diameter of 12 mm.

[0094] S3. Place the circular electrode 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 solvents are ethylene carbonate (EC) and diethyl carbonate (DEC), and their volume ratio is EC:DEC = 1:1), and assemble a CR2032 coin cell in a glove box filled with argon.

[0095] Further, at 25 °C, charge / discharge at 0.1C from 2.0 - 4.0V, and perform charge-discharge performance tests on Device Examples 1-4 and Device Comparative Example 1. Calculate their first discharge efficiency, and the formula is as follows: First discharge efficiency = First cycle discharge specific capacity / First cycle charge specific capacity * 100%.

[0096] At 25 °C, charge / discharge at 1C from 2.0 - 4.0V, perform cycle performance tests on Device Examples 1-4 and Device Comparative Example 1, and calculate their capacity retention rate after 100 cycles. The formula is as follows: Capacity retention rate = 100th cycle discharge specific capacity / First cycle discharge specific capacity * 100%.

[0097] Please refer to Table 2 for specific test data.

[0098] Table 2

[0099]

[0100] Combined with Figure 6 、 Figure 7 、 Figure 8 , and Table 2 shows that the first cycle discharge specific capacity, first discharge efficiency, and stability (i.e., capacity retention rate) of Device Examples 1-4 are all better than those of Device Comparative Example 1.

[0101] At 25 °C, charge / discharge at 0.5C, 1C, 2C, 3C, 5C, 10C from 2.0 - 4.0V, and perform rate performance tests on Device Examples 1-4 and Device Comparative Example 1. Please refer to Table 3 for specific test data.

[0102] Table 3

[0103]

[0104]

[0105] As can be seen from the test results in Table 3, compared with Device Comparative Example 1, the rate performance of Device Examples 1-4 has been significantly improved.

[0106] Device Example 5 - Device Example 8, Device Comparative Example 5

[0107] Device Examples 5-8, and Device Comparative Example 5 are soft-pack batteries with Synthesis Examples 1-4 and Synthesis Comparative Example 1 as the positive electrode respectively. The preparation of Device Examples 5-8 and Device Comparative Example 5 will be described as follows:

[0108] Using the sodium-ion battery cathode material as the active substance, Super P as the conductive agent, and PVDF as the binder, the active substance: conductive agent: binder are mixed based on a mass ratio of 8:1:1 and then coated on the positive electrode sheet. Hard carbon: conductive carbon: CMC (sodium carboxymethyl cellulose) are mixed based on a mass ratio of 7:2:1 and then coated on the negative electrode sheet. Using PE as the separator and 1mol / L NaPF 6 solution (the solvent includes EC and DMC with a volume ratio of 1:1) as the electrolyte to assemble a soft-pack battery.

[0109] Fully charge Device Examples 5-8 and Device Comparative Example to 4.2V, then fully charge and store them at 70°C for 7 days. Immediately after taking them out of the high temperature, test the battery volume by the drainage method and record the measured battery volume as the hot test volume. After that, let the soft-pack battery taken out of the high temperature stand still and cool to 25°C, then test the battery volume by the drainage method and record the measured battery volume as the cold test volume.

[0110] Based on the above hot test volume and cold test volume, determine the volume expansion of the soft-pack batteries corresponding to Device Examples 5-8 and Device Comparative Example 1. Please refer to Table 4.

[0111] Table 4

[0112]

[0113]

[0114] As can be seen from Table 4, the volume growth rate corresponding to the hot test volume and the volume growth rate corresponding to the cold test volume of Device Examples 5-8 are both lower than the corresponding volume growth rate of Device Comparative Example 5. Therefore, compared with Device Comparative Example 5, the gas generation performance of Device Examples 5-8 has been significantly improved.

[0115] Obviously, those skilled in the art can make various changes and modifications to this application without departing from the spirit and scope of this application. Thus, if these modifications and variations of this application fall within the scope of the claims of this application and their equivalent technologies, this application is also intended to include these changes and modifications.

Claims

1. A cathode material with a coating layer, characterized in that, it includes: The molecular formula of the inner layer material of the positive electrode material is: Na e Ni x Fe y M 1-x-y O 2 , and the coating layer is Na 3-2p PS 4-p and / or Na 3-q PS 4-q H q , where M is a first doping element, and M is selected from at least one of Cu, Mn, Al, Mg, Zn, Sn, Ti, Zr, Sr, Sb, Nb, Mo, Y and W; H is a second doping element, and H is selected from at least one of F, Cl and Br; e, x, y, p, q are each selected from: 0.8 ≤ e ≤ 1.1, 0.2 < x ≤ 0.96, 0 ≤ y < 0.2, 0 ≤ 1 - x - y ≤ 0.04, 0 < p ≤ 0.1, 0 < q ≤ 0.

1.

2. The cathode material according to claim 1, characterized in that, the structure of the inner layer material is a layered structure.

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

4. The cathode material according to claim 3, characterized in that, The impedance value of the positive electrode material powder is not greater than 2.2×10 4 Ohm / cm.

5. A preparation method of a cathode material with a coating layer according to any one of claims 1 - 4, characterized in that, it includes: The mixture of the sulfide and the cathode material to be coated is sintered at 350 °C - 500 °C for 4 - 20 hours to obtain the cathode material with the coating layer; wherein, the sulfide is Na 3-2p PS 4-p and / or Na 3-q PS 4-q H q , where H is selected from at least one of F, Cl, or Br, and p and q are each independently selected from: 0 < p ≤ 0.1, 0 < q ≤ 0.

1.

6. The method according to claim 5, characterized in that, before the mixture of the sulfide and the cathode material to be coated is subjected to a first sintering treatment to obtain a cathode material with the sulfide as the coating layer, it further includes: sintering the mixture of the precursor and the sodium source at 800°C - 1100°C for 12 - 24 hours to obtain the cathode material to be coated; mixing the cathode material to be coated and the sulfide in an organic solvent, stirring and drying to obtain the mixture of the sulfide and the cathode material to be coated.

7. The method according to claim 6, characterized in that, The precursor is selected from: Ni x Fe y M 1-x-y (OH) 2 , or Ni x Fe y M 1-x-y CO 3 ; wherein, M is a first doping element, and M is selected from at least one of Cu, Mn, Al, Mg, Zn, Sn, Ti, Zr, Sr, Sb, Nb, Mo, Y, and W. Each of x and y independently selects: 0.2 < x ≤ 0.96, 0 ≤ y < 0.2, 0 ≤ 1 - x - y ≤ 0.

04.

8. The method according to claim 5, characterized in that, The coating layer includes Na 3-2p PS 4-p When, before the mixture of the sulfide and the cathode material to be coated is subjected to the first sintering treatment to obtain the cathode material with the coating layer being the sulfide, it further includes: For Na 2 S and P 2 S 5 The mixture is subjected to low-temperature annealing treatment at 150 °C - 300 °C to obtain Na 3-2p PS 4-p ; wherein, the molar ratio between the Na 2 S and P 2 S 5 is (3 - 2p):

1.

9. The method according to claim 5, characterized in that, The coating layer includes Na 3-q PS 4-q H q When, before subjecting the mixture of the sulfide and the cathode material to be coated to the first sintering treatment to obtain the cathode material with the coating layer being the sulfide, further comprising: In an inert atmosphere or in vacuo, sinter a mixture of Na 2 S, P 2 S 5 and NaH at 650 °C - 900 °C to obtain Na 3-q PS 4-q H q ; wherein the molar ratio between the said Na 2 S, P 2 S 5 and NaH is (3 - 2q):1:2q.

10. A sodium ion battery, characterized in that, it includes: a ternary cathode material with a coating layer according to any one of claims 1 - 4, or a ternary cathode material with a coating layer prepared by the method according to any one of claims 5 - 9.

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