A positive electrode material, a secondary battery, and an electric device

Abstract

The embodiment of the present application provides a positive electrode material, a secondary battery and an electric device, wherein the positive electrode material comprises a core and a shell layer arranged on the surface of the core, the core comprises a sodium ion layered oxide, and the shell layer comprises a polyanionic lithium salt and a polyanionic sodium salt. The positive electrode material provided by the embodiment of the present application improves the structural stability of the layered positive electrode material of the secondary battery and the rate performance of the secondary battery.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery manufacturing technology, and in particular to a positive electrode material, a secondary battery, and an electrical device. Background Technology

[0002] Currently, sodium-ion batteries are receiving widespread attention due to their advantages such as high theoretical capacity, abundant sodium resources, absence of internal water of crystallization, and compatibility with lithium-ion batteries in production methods.

[0003] However, the existing layered cathode materials for sodium-ion batteries have poor structural stability and poor rate performance. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a cathode material, a secondary battery and an electrical device to improve the poor structural stability and poor rate performance of the existing layered cathode material of sodium-ion batteries.

[0005] To address the aforementioned problems, the present invention is implemented through the following technical solution:

[0006] The present invention proposes a cathode material, wherein the cathode material includes a core and a shell disposed on the surface of the core, the core comprising a sodium ion layered oxide, and the shell comprising a polyanionic lithium salt and a polyanionic sodium salt.

[0007] Furthermore, in the cathode material, the shell layer includes a first shell layer disposed on the surface of the core and a second shell layer disposed on the surface of the first shell layer, wherein the first shell layer includes the polyanionic sodium salt and the second shell layer includes the polyanionic lithium salt.

[0008] Furthermore, in the cathode material, the polyanionic lithium salt includes a polyanionic iron-based lithium salt;

[0009] and / or

[0010] The polyanionic sodium salt includes polyanionic iron-based sodium salt.

[0011] Furthermore, in the aforementioned cathode material, the polyanionic iron-based sodium salt includes sodium iron phosphate, and the polyanionic iron-based lithium salt includes lithium iron phosphate.

[0012] Furthermore, in the positive electrode material, the molar ratio of polyanionic sodium salt to polyanionic lithium salt in the shell layer is 0.5:1 to 2:1.

[0013] Furthermore, in the positive electrode material, the thickness of the shell layer is 50–500 nm.

[0014] Furthermore, the pH value of the positive electrode material is 10 to 12.5.

[0015] Furthermore, in the cathode material, the crystal structure of the sodium ion layered oxide includes the O3 type;

[0016] And / or, the chemical formula of the sodium ion layered oxide is Na x MO2, wherein M comprises at least one of Fe, Ni, Li, Cu, Zn, Co, Ti, and Mn, and 1.1 ≥ x ≥ 1.

[0017] Furthermore, in the cathode material, the average particle size of the lithium iron phosphate is 400-1000 nm, and the average particle size of the sodium iron phosphate is 100-1000 nm.

[0018] And / or the particle size Dv50 of the cathode material is 1μm to 20μm.

[0019] The present invention also proposes a secondary battery, including a positive electrode sheet, wherein the positive electrode sheet includes a positive current collector and a positive active material layer disposed on the positive current collector, the positive active material layer including the positive electrode material as described above.

[0020] Furthermore, in the aforementioned secondary battery, the areal density of the positive electrode sheet is 10 mg / cm³. 2 ~20mg / cm 2 .

[0021] The present invention also proposes an electrical device, wherein the aforementioned secondary battery is provided as the power supply for the electrical device.

[0022] Compared with the prior art, the embodiments of the present invention have the following advantages:

[0023] In this embodiment of the invention, the provided cathode material includes a core and a shell disposed on the surface of the core. The core comprises a sodium-ion layered oxide, and the shell comprises a polyanionic lithium salt and a polyanionic sodium salt. When the shell comprising the polyanionic lithium salt and the polyanionic sodium salt is disposed on the surface of the sodium-ion layered oxide, it reduces the contact between the transition metal in the sodium-ion layered oxide and the electrolyte, lowering the failure probability and thus improving cycle stability. Simultaneously, it also reduces the contact between the sodium-ion layered oxide and carbon dioxide and water in the air, thereby improving the air stability of the sodium-ion layered oxide and increasing the reversible charge-discharge specific capacity of the sodium-ion battery. Furthermore, the presence of the shell enhances the lattice structure stability of the cathode material, reducing the risk of internal cracks and fissures caused by repeated cell contraction during long cycles, and providing a more convenient channel for the rapid transport of sodium ions within the lattice, thereby improving the rate performance of the cathode material. Therefore, the cathode material provided in this embodiment of the invention improves upon the problems of poor structural stability and poor rate performance of existing layered cathode materials for sodium-ion batteries.

[0024] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and do not limit this application. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the structure of the cathode material provided in an embodiment of the present invention.

[0026] Explanation of reference numerals in the attached figures:

[0027] 11-Kernel, 12-Shell, 121-First Shell, 122-Second Shell. Detailed Implementation

[0028] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0029] The applicant of this invention discovered that the diffusion channels of the layered cathode material in sodium-ion batteries are two-dimensional channels, which results in a large impedance at the cathode-electrolyte interface, which is not conducive to the rapid shuttle of sodium ions and leads to poor rate performance of the battery. At the same time, the layered cathode material fails after reacting with water and carbon dioxide in the air, resulting in a decrease in performance. In addition, the reversible capacity of sodium ions is lower than that of traditional lithium-ion batteries, and the long-term cycle stability of sodium-ion batteries is worse than that of lithium-ion batteries.

[0030] To address the aforementioned problems, this invention provides a cathode material, such as... Figure 1As shown, the cathode material includes a core 11 and a shell 12 disposed on the surface of the core 11. The core 11 includes a sodium ion layered oxide, and the shell 12 includes a polyanionic lithium salt and a polyanionic sodium salt.

[0031] Among them, polyanionic compounds refer to compounds composed of a series of anionic tetrahedra (XO4). n- or its derivative groups (X) m O 3m+1 ) n- (X = B, S, P, Si, As, Mo, W) and transition metal-oxygen polyhedra (MO) x Materials with an open framework structure, where M can be ions of elements such as chromium, manganese, iron, cobalt, nickel, copper, zinc, palladium, silver, titanium, and lithium. These materials offer the following advantages: 1) The strong XO covalent bond framework structure endows them with outstanding stability and high safety; 2) The 3D framework structure contains abundant lattice vacancies, mitigating the volume changes and complex phase transition reactions caused by repeated insertion and extraction of sodium ions; 3) The inductive effect of polyanionic groups weakens the MO bonds, thereby increasing the redox potential of transition metal ions.

[0032] Therefore, in this embodiment of the invention, a shell layer comprising polyanionic lithium salt and polyanionic sodium salt is disposed on the surface of the sodium-ion layered oxide. This not only matches the characteristics of sodium-ion batteries but also reduces the contact between the transition metal in the sodium-ion layered oxide and the electrolyte, lowering the probability of failure and thus improving cycle stability. Simultaneously, the shell layer also reduces the contact between the sodium-ion layered oxide and carbon dioxide and water in the air, thereby improving the air stability of the sodium-ion layered oxide and increasing the reversible charge-discharge specific capacity of the sodium-ion battery. Furthermore, the presence of the shell layer enhances the lattice structure stability of the cathode material, reducing the risk of internal cracks caused by repeated cell contraction during long cycles. This provides a more convenient channel for the rapid transport of sodium ions within the lattice, thereby improving the rate performance of the cathode material. Therefore, the cathode material provided by this embodiment of the invention solves the problems of high interfacial impedance, poor structural stability, and poor rate performance of existing layered cathode materials for sodium-ion batteries. Furthermore, during the first cycle of the battery, some lithium ions in the polyanionic lithium salt participate in the formation of the electrode passivation film. The lithium ions form complex compounds with other organic groups in the electrolyte, which improves the density and stability of the electrode protective film and further optimizes the cycle performance and rate performance of the battery.

[0033] In practical applications, TEM-SAED characterization of the cathode material provided in the embodiments of the present invention shows that its surface material has a crystal structure different from the internal layered oxide. At the same time, by using SEM EDS to characterize the surface material, the elemental composition and ratio of polyanionic lithium salt and polyanionic sodium salt can be determined. Combined with the crystal structure of the surface material characterized by TEM-SAED, the presence of polyanionic lithium salt and polyanionic sodium salt in the surface material can be determined.

[0034] In this embodiment of the invention, the pH value of the above-mentioned positive electrode material is 10-12.5. Because the polyanionic sodium salt and polyanionic lithium salt have low alkalinity, coating the sodium ion layered oxide with polyanionic sodium salt and polyanionic lithium salt can reduce the contact between the sodium ion layered oxide and carbon dioxide and water in the air, that is, lower the pH value of the system, thereby reducing the attack on the positive electrode binder and facilitating the processing to form the positive electrode sheet.

[0035] Optionally, in one embodiment, the shell layer 12 includes a first shell layer 121 disposed on the surface of the core 11 and a second shell layer 122 disposed on the surface of the first shell layer 121. The first shell layer 121 comprises a polyanionic sodium salt, and the second shell layer 122 comprises the polyanionic lithium salt. That is, from the inside out, the first shell layer 121 composed of polyanionic sodium salt and the second shell layer 122 composed of polyanionic lithium salt are sequentially disposed on the surface of the sodium ion layered oxide, thereby forming a double-shell structure. The double-shell structure can not only further enhance the lattice structure stability of the cathode material, but also reduce the contact between the transition metal in the sodium ion layered oxide and the electrolyte, and reduce the contact between the sodium ion layered oxide and carbon dioxide and water in the air. Therefore, it can significantly reduce the failure probability and improve the air stability of the sodium ion layered oxide, thereby improving the cycle stability and reversible charge-discharge specific capacity.

[0036] Optionally, in some embodiments, the above-mentioned polyanionic lithium salt includes polyanionic iron-based lithium salt; and / or the above-mentioned polyanionic sodium salt includes polyanionic iron-based sodium salt.

[0037] In the above embodiments, since the polyanionic iron-based lithium salt itself does not react with carbon dioxide and water, it is placed on the outermost layer of the cathode material through coating or other means, which can effectively reduce the contact between the layered cathode material and carbon dioxide and water in the air, thereby improving the air stability of the layered cathode material of sodium-ion batteries. Furthermore, placing the polyanionic iron-based sodium salt between the sodium ion layered oxide and the polyanionic iron-based lithium salt through coating or other means can not only play a good buffering role, reducing the risk of cracks forming inside the particles due to repeated shrinkage of the unit cell and improving the stability of the crystal structure, but also provide a more convenient channel for the rapid transport of sodium ions in the crystal lattice.

[0038] Optionally, in one specific embodiment, the above-mentioned polyanionic iron-based sodium salt includes sodium iron phosphate, and the above-mentioned polyanionic iron-based lithium salt includes lithium iron phosphate.

[0039] Among them, the reversible discharge specific capacity of lithium iron phosphate cathode material reaches 155 mAh / g, while that of sodium iron phosphate reaches 142 mAh / g, both of which are greater than those of traditional layered sodium ion cathode materials. By sequentially setting sodium iron phosphate and lithium iron phosphate on the surface of sodium ion layered oxide, not only can the problems of poor structural stability and poor rate performance of existing layered cathode materials in sodium ion batteries be solved, but the reversible charge and discharge specific capacity of sodium ion batteries can also be significantly improved.

[0040] In addition, the lithium iron phosphate material forming the second shell has an olivine structure with space group pnmb, while the sodium iron phosphate material forming the first shell has an olivine structure with space group Pnma. Both can form strong POM covalent bonds in the material, which greatly stabilizes the crystal structure of the material and makes the material have high thermal stability.

[0041] Optionally, the average particle size of the lithium iron phosphate is 400–1000 nm, and the average particle size of the sodium iron phosphate is 100–1000 nm.

[0042] In practical applications, after uniformly coating lithium iron phosphate onto the surface of sodium ion layered oxide, an ion exchange method based on organic solution is used to first delithiate the lithium iron phosphate to obtain iron phosphate. Then, the delithiated iron phosphate is charged with sodium, thereby obtaining high-purity sodium iron phosphate on the inner side of the shell layer near the sodium ion layered oxide particles, while the outer side of the shell layer away from the sodium ion layered oxide particles is still lithium iron phosphate material. Thus, a first shell layer including sodium iron phosphate and a second shell layer including lithium iron phosphate are formed on the surface of sodium ion layered oxide.

[0043] In the cathode material provided in this embodiment of the invention, the shell thickness is 50-500 nm, which can not only reduce the contact between the transition metal in the sodium ion layered oxide and the electrolyte and improve the stability of the crystal structure, but also effectively take into account the energy density of the battery.

[0044] Optionally, in one embodiment, the thickness of the shell is 200 to 300 nm, for example, any value of 200 nm, 220 nm, 250 nm, 280 nm, 300 nm or a range of both of the above.

[0045] Optionally, in one embodiment, the molar ratio of the sum of polyanionic sodium salt and polyanionic lithium salt in the shell to the sodium ion layered oxide is 1:20 to 2:20.

[0046] In practical applications, the above molar ratio and coating layer thickness can be achieved by adjusting the reaction time and coating substance concentration during each coating process. The longer the reaction time and the higher the coating substance concentration, the more and thicker the coating layers will be.

[0047] Optionally, the particle size Dv50 of the cathode material provided in the embodiments of the present invention is 1μm to 20μm.

[0048] In this embodiment of the invention, when the particle size Dv50 of the cathode material is within the above-mentioned range, the cathode material can have a suitable specific surface area, which can reduce the occurrence of side reactions and reduce gas production. In addition, it can also shorten the migration path of sodium ions, which is beneficial to improving the rate performance of the battery. Preferably, the Dv50 of the cathode material is 5μm to 12μm, for example, any value of 5μm, 7μm, 10μm, 12μm or any combination thereof.

[0049] The aforementioned cathode material has a spherical or near-spherical structure. In the actual preparation process, the Dv50 of the cathode material can be adjusted to 1–20 μm by changing the ball milling parameters. Specifically, the ball milling time and rotation speed can be controlled.

[0050] Optionally, in one embodiment, the molar ratio of polyanionic sodium salt to polyanionic lithium salt in the shell is 0.5:1 to 2:1.

[0051] In this embodiment of the invention, when the molar ratio of polyanionic sodium salt to polyanionic lithium salt in the shell is within the range of 0.5:1 to 2:1, the interfacial impedance of the formed cathode material can be effectively reduced and the structural stability and rate performance of the cathode material can be improved. In other embodiments, the molar ratio of polyanionic sodium salt to polyanionic lithium salt in the shell can be any value of 0.5:1, 0.8:1, 1:1, 1.5:1, 1.8:1, 2:1, or any range of both.

[0052] Optionally, in the cathode material provided in the embodiments of the present invention, the crystal structure of the above-mentioned sodium ion layered oxide includes the O3 type; and / or, the chemical formula of the above-mentioned sodium ion layered oxide is Na. x MO2, wherein M comprises at least one of Fe, Ni, Li, Cu, Zn, Co, Ti, and Mn, and 1.1 ≥ x ≥ 1. In this embodiment of the invention, an O3-type sodium ion layered oxide is used as the core of the positive electrode material, and a polyanionic lithium salt and a polyanionic sodium salt are coated on the surface of the core. This not only enhances the stability of the shell framework structure but also increases the number of cyclic sodium ions in the battery, thereby improving the specific capacity of the battery during the first charge.

[0053] In this embodiment of the invention, M includes at least two of Fe, Ni, Li, Cu, Zn, Co, Ti, and Mn.

[0054] In this embodiment of the invention, M comprises Fe and Ni, as well as at least one of Li, Cu, Zn, Co, Ti, and Mn. When M comprises the above-mentioned elements, the overall performance of the battery is improved.

[0055] The present invention also proposes a secondary battery, including a positive electrode sheet, wherein the positive electrode sheet includes a positive current collector and a positive active material layer disposed on the positive current collector, the positive active material layer including the positive electrode material as described above.

[0056] In some embodiments, the areal density of the positive electrode sheet is 10 mg / cm³. 2 ~20mg / cm 2 For example, it can be 10 mg / cm³ 2 12mg / cm 2 15mg / cm 2 18mg / cm 2 20mg / cm 2 The value can be any one of the values ​​in the range or any two of the above. When the areal density of the positive electrode is within the above range, it can effectively balance rate performance and energy density, and mitigate polarization.

[0057] Optionally, in one embodiment, the positive electrode sheet further includes a conductive agent and an adhesive, wherein the adhesive may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene-propylene terpolymer, ethylene-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0058] Optionally, in one embodiment, the conductive agent can be acetylene black, carbon fiber, carbon nanotubes, Ketjen black, artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, graphene, etc.

[0059] Optionally, in one embodiment, in the positive electrode sheet, based on the mass of the positive electrode active material layer, the mass ratio of the positive electrode material is 80-97%, the mass ratio of the binder is 0.1-10%, and the mass ratio of the conductive agent is 1-10 wt%, which can effectively balance rate performance and energy density.

[0060] Optionally, in one embodiment, in the positive electrode sheet, based on the mass of the positive electrode active material layer, the mass ratio of the positive electrode material is 80-92%, the mass ratio of the binder is 3-10%, and the mass ratio of the conductive agent is 1-10%. When the positive electrode active material layer meets the above characteristics, the rate performance and energy density of the battery can be further balanced.

[0061] In some embodiments, the positive electrode sheet is prepared as follows: the components used to prepare the positive electrode sheet, such as the positive active material including the above-mentioned positive electrode material, binder and any other components, are dispersed in a solvent such as N-methylpyrrolidone to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector; after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0062] The secondary battery provided in this embodiment of the invention also includes a negative electrode, a separator, and an electrolyte.

[0063] The negative electrode sheet includes a negative current collector and a negative active material layer disposed on the negative current collector. The negative active material layer can be a negative active material for batteries known in the art, such as artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, etc.

[0064] In this embodiment, the electrolyte acts as a conductor of ions between the positive and negative electrodes. In some implementations, the electrolyte comprises an electrolyte salt and a solvent, wherein the electrolyte salt is a sodium salt.

[0065] In the secondary battery provided by this invention, the concentrations of Mn and Fe ions in the electrolyte are both less than 100 ppm. The concentrations of manganese and iron ions in the electrolyte can be obtained by inductively coupled plasma atomic emission spectrometry (ICP-OFS). The concentrations of Mn and Fe ions in the electrolyte are less than 10 ppm, indicating that in this invention, the shell layer comprising polyanionic lithium salt and polyanionic sodium salt on the surface of the sodium ion layered oxide can reduce the contact between Mn and Fe in the layered positive electrode material and the electrolyte, thereby effectively reducing the concentrations of Mn and Fe ions in the electrolyte and on the negative electrode surface, and reducing the probability of failure.

[0066] This invention also proposes an electrical device including the aforementioned secondary battery, wherein the secondary battery is used to provide power.

[0067] For the above-described secondary battery embodiments and electrical device embodiments, the positive electrode sheet includes a positive electrode active material layer, which includes the above-described positive electrode material and can achieve the same technical effect. To avoid repetition, it will not be described again here. For relevant details, please refer to the description of the positive electrode material embodiments.

[0068] To make the inventive objectives, technical solutions, and beneficial effects of this invention clearer, the invention is further described below with reference to embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0069] The present invention will be described in detail below through embodiments.

[0070] (1) pH value test of positive electrode material:

[0071] Dissolve 5g of positive electrode material in 45ml of ethanol solution, stir at 200r / min for 10min, and then test the pH value of the solution.

[0072] (2) Discharge rate performance test:

[0073] At room temperature, the fully charged battery was discharged to 1.5V at a current density of 0.33C, and the capacity was recorded as C1.

[0074] Then, at room temperature, the fully charged battery was discharged to 1.5V at a current density of 5C, and the capacity was recorded as C2.

[0075] Calculate the capacity retention ratio C2 / C1 as a test indicator of discharge rate performance.

[0076] (3) Cyclic performance test: At room temperature, the battery is charged to 3.8V at a current density of 1C and then discharged to 1.5V at a current density of 1C. The capacity C1 during the discharge process is recorded.

[0077] At room temperature, the battery was charged and discharged at a 1C rate to perform a full charge-discharge cycle test, with a voltage range of 1.5V to 3.8V. The test was stopped when the discharge capacity gradually decreased to 80%*C1, and the number of cycles was recorded.

[0078] Example 1

[0079] (1) Preparation of cathode materials:

[0080] a. At room temperature, a certain mass of Na2CO3, NiO, Fe2O3 and MnO2 were added to a ball mill at a molar ratio of n(Na):n(Ni):n(Fe):n(Mn) = 1.1:1 / 3:1 / 3:1 / 3. The mixture was stirred at 300 rpm for 12 hours, and the powder precursor was pressed into a sheet to facilitate uniform heating. The sheet precursor was then sintered in air at 800℃ for 12 hours. After cooling, the cathode material was placed in a tube furnace, and a mixture of tetraisopropyl titanate and nitrogen was introduced into the tube furnace at a flow rate ratio of 1:1. The mixture was reacted at 100℃ for 12 hours, and then sintered in a muffle furnace at 800℃ in air for 12 hours to obtain a layered cathode material.

[0081] b. Place the above-mentioned layered cathode material into a ball milling device and add NaFePO4, with a molar ratio of 20:1 between the layered cathode material and NaFePO4. Then, after ball milling at 300 rpm for 12 hours, calcine at 350°C for 12 hours to coat the surface of the layered cathode material with NaFePO4, thereby obtaining a single-shell coated material.

[0082] c. Place the above single-shell coating material into a ball mill and add LiFePO4, with the molar ratio between NaFePO4 and the added LiFePO4 in the single-shell coating material being 1:1. Then, stir at 300 rpm for 12 hours and calcine at 350°C for 12 hours to coat the surface of NaFePO4 with LiFePO4, thus obtaining a double-shell coated cathode material.

[0083] (2) Preparation of positive electrode sheet

[0084] The positive electrode material prepared above was homogenized with PVDF and conductive carbon black at a mass ratio of 85:7.5:7.5, and then uniformly coated onto the positive electrode current collector, controlling the electrode surface density to be 15 mg / cm³. 2 After high-temperature drying, the positive electrode sheet is prepared by rolling, cutting, and slitting.

[0085] (3) Preparation of negative electrode sheet

[0086] The negative electrode materials—hard carbon, styrene-butadiene rubber (SBR), conductive carbon black, and sodium carboxymethyl cellulose (CMC)—were homogenized in a mass ratio of 85:5.5:5.5:4 and uniformly coated onto the negative electrode current collector, controlling the electrode surface density to be 8 mg / cm³. 2 After high-temperature drying, the negative electrode sheet is prepared by rolling, cutting, and slitting.

[0087] (4) Preparation of sodium-ion batteries

[0088] The positive electrode, negative electrode, and separator (PE separator) are wound into a shell and injected with electrolyte (the electrolyte is a mixture of carbonate solvent and 1M sodium hexafluorophosphate). After encapsulation, drying, and composition and capacity testing, a sodium-ion battery can be prepared.

[0089] Example 2

[0090] The difference between Example 2 and Example 1 is that in step (b), NaFePO4 is replaced with sodium iron sulfate; and in step (c), LiFePO4 is replaced with lithium iron sulfate.

[0091] Example 3

[0092] The difference between Example 3 and Example 1 is that in step (b), NaFePO4 is replaced with sodium iron silicate; and in step (c), LiFePO4 is replaced with lithium iron silicate.

[0093] Example 4

[0094] The difference between Example 4 and Example 1 is that in step (b), NaFePO4 is replaced with sodium vanadium phosphate; and in step (c), LiFePO4 is replaced with lithium vanadium phosphate.

[0095] Example 5

[0096] The difference between Example 5 and Example 1 is that in step (b), NaFePO4 is replaced with sodium vanadium phosphate.

[0097] Example 6

[0098] The difference between Example 6 and Example 1 is that in step (c), LiFePO4 is replaced with lithium vanadium phosphate.

[0099] Example 7

[0100] The difference between Example 7 and Example 1 is that in step (b), the molar ratio between the layered cathode material and NaFePO4 is adjusted to 20:0.5.

[0101] Example 8

[0102] The difference between Example 8 and Example 1 is that in step (b), the molar ratio between the layered cathode material and NaFePO4 is adjusted to 20:2.

[0103] Example 9

[0104] The difference between Example 9 and Example 1 is that in step (b), the molar ratio between the layered cathode material and NaFePO4 is adjusted to 20:3.

[0105] Example 10

[0106] The difference between Example 10 and Example 1 is that in step (c), the molar ratio between NaFePO4 and the added LiFePO4 in the single-shell coating material is adjusted to 0.5:1.

[0107] Example 11

[0108] The difference between Example 11 and Example 1 is that in step (c), the molar ratio between NaFePO4 and the added LiFePO4 in the single-shell coating material is adjusted to 1:2.

[0109] Example 12

[0110] The difference between Example 12 and Example 1 is that in step (c), the molar ratio between NaFePO4 and the added LiFePO4 in the single-shell coating material is adjusted to 1:3.

[0111] Example 13

[0112] The difference between Example 13 and Example 1 is that in step (b), the ball mill speed is adjusted to 400 rpm and the ball milling time is 18 hours.

[0113] Example 14

[0114] The difference between Example 14 and Example 1 is that in steps (b) and (c), the ball mill speed is adjusted to 300 rpm and the ball milling time is 18 hours.

[0115] Example 15

[0116] The difference between Example 15 and Example 1 is that in steps (b) and (c), the ball mill speed is adjusted to 200 rpm and the ball milling time is 12 hours.

[0117] Example 16

[0118] The difference between Example 16 and Example 1 is that in steps (b) and (c), the ball mill speed is adjusted to 100 rpm and the ball milling time is 6 hours.

[0119] Example 17

[0120] The difference between Example 17 and Example 1 is that in step (a), a certain mass of Na2CO3, NiO, Fe2O3 and MnO2 are added to the ball mill according to the molar ratio of n(Na):n(Ni):n(Fe):n(Mn) = 1:0.8:0.1:0.1.

[0121] Example 18

[0122] The difference between Example 18 and Example 1 is that in step (a), a certain mass of Na2CO3, NiO, Fe2O3 and MnO2 are added to the ball mill according to the molar ratio of n(Na):n(Ni):n(Fe):n(Mn) = 0.85:0.4:0.2:0.4.

[0123] Example 19

[0124] The difference between Example 19 and Example 1 is that in step (a), the ball mill speed is adjusted to 400 rpm and the ball milling time is 18 hours.

[0125] Example 20

[0126] The difference between Example 20 and Example 1 is that in step (a), the ball mill speed is adjusted to 300 rpm and the ball milling time is 18 hours.

[0127] Example 21

[0128] The difference between Example 21 and Example 1 is that in step (a), the ball mill speed is adjusted to 100 rpm and the ball milling time is 12 hours.

[0129] Example 22

[0130] The difference between Example 22 and Example 1 is that step (c) is omitted, and LiFePO4 and NaFePO4 are added simultaneously in step (b), with the molar ratio between the layered cathode material, NaFePO4, and LiFePO4 being 20:1:1. After ball milling at 300 rpm for 12 hours, the material is calcined at 350°C for 12 hours, so that NaFePO4 and LiFePO4 are coated on the surface of the layered cathode material to obtain the cathode material.

[0131] Comparative Example 1

[0132] The difference between Comparative Example 1 and Example 1 is that the layered cathode material prepared in step (a) is directly used as the cathode material.

[0133] The shell thickness, average particle size of sodium salt in the shell, average particle size of lithium salt in the shell, pH value, and particle Dv50 were tested in each embodiment and comparative example. The test data are shown in Table 1.

[0134] The batteries prepared in each embodiment and comparative example were subjected to electrolyte ion concentration testing and cycle performance testing. The test data are shown in Table 1.

[0135] Table 1

[0136]

[0137]

[0138]

[0139] In this application, the cathode material includes a core and a shell disposed on the surface of the core. The core comprises a sodium-ion layered oxide, and the shell comprises a polyanionic lithium salt and a polyanionic sodium salt. When the shell comprising the polyanionic lithium salt and the polyanionic sodium salt is disposed on the surface of the sodium-ion layered oxide, it reduces the contact between the transition metal in the sodium-ion layered oxide and the electrolyte, lowering the failure probability and thus improving cycle stability. Simultaneously, it also reduces the contact between the sodium-ion layered oxide and carbon dioxide and water in the air, thereby improving the air stability of the sodium-ion layered oxide and increasing the reversible charge-discharge specific capacity of the sodium-ion battery. Furthermore, the presence of the shell enhances the lattice structure stability of the cathode material, reducing the risk of internal cracks and fissures caused by repeated cell contraction during long cycles, and providing a more convenient channel for the rapid transport of sodium ions within the lattice, thereby improving the rate performance of the cathode material. Therefore, the cathode material provided by this invention solves the problems of poor structural stability and poor rate performance of existing layered cathode materials for sodium-ion batteries.

[0140] Although preferred embodiments of the present invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the embodiments of the present invention.

[0141] The present invention has provided a detailed description of a positive electrode material, a secondary battery, and an electrical device. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. At the same time, those skilled in the art will recognize that there will be changes in the specific implementation methods and application scope based on the ideas of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A positive electrode material, characterized by, The positive electrode material comprises an inner core and a shell layer arranged on the surface of the inner core, the inner core comprises a sodium ion layered oxide, and the shell layer comprises a polyanionic lithium salt and a polyanionic sodium salt; The shell layer comprises a first shell layer arranged on the surface of the inner core and a second shell layer arranged on the surface of the first shell layer, the first shell layer comprises the polyanionic sodium salt, and the second shell layer comprises the polyanionic lithium salt.

2. The positive electrode material of claim 1, wherein, The polyanionic lithium salt comprises a polyanionic iron-based lithium salt; And / or The polyanionic sodium salt comprises a polyanionic iron-based sodium salt.

3. The positive electrode material according to claim 2, characterized in that, The polyanionic iron-based sodium salt comprises sodium iron phosphate, and the polyanionic iron-based lithium salt comprises lithium iron phosphate.

4. The positive electrode material of claim 1, wherein, In the shell layer, the molar ratio of the polyanionic sodium salt to the polyanionic lithium salt is 0.5:1-2:

1.

5. The cathode material of claim 1, wherein, The thickness of the shell layer is 50-500 nm.

6. The cathode material of claim 1, wherein, The pH value of the positive electrode material is 10-12.

5.

7. The cathode material of claim 1, wherein, The crystal structure of the sodium ion layered oxide comprises O3 type; and / or the sodium-ion layered oxide has a chemical formula of Na x M02, wherein M comprises at least one of Fe, Ni, Li, Cu, Zn, Co, Ti, Mn, and 1.1 > x > 1.

8. The cathode material of claim 3, wherein, The average particle size of the lithium iron phosphate is 600-900 nm, and the average particle size of the sodium iron phosphate is 100-500 nm; And / or the particle size Dv50 of the positive electrode material is 1-20 μm.

9. A secondary battery characterized by comprising: The secondary battery comprises the positive electrode material as claimed in any one of claims 1-8.

10. An electric device, characterized by The secondary battery as claimed in claim 9 is used as a power supply for the electric device.

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