A high-capacity, low-residual-sodium layered cathode material for sodium-ion batteries, its preparation method, and sodium-ion batteries.

Abstract

A high-capacity, low-residual-sodium layered cathode material for sodium-ion batteries, its preparation method, and a sodium-ion battery thereof, wherein the chemical formula of the layered cathode material for sodium-ion batteries is Na. a M1 b Ni w Fe x Mn y Cu z M2 1‑w‑x‑y‑z O2, wherein M1 includes one or more of Ca, Li, or K, and M2 includes one or more of Mg, Zn, Al, Zr, Ti, Nb, Mo, Y, Ta, W, Sr, Ba, B, or P, with 0.85≤a<0.98, 0<b≤0.05, and 0.90≤a+b≤0.98, w≥0.1, x≥0.1, y≥0.1, z≥0, and 0.9≤w+x+y+z≤1.0. This invention improves the residual alkali in the layered cathode material of sodium-ion batteries by simultaneously controlling the stoichiometry of sodium and M1 doping elements. Furthermore, by using M1 elements with similar radii or chemical properties to sodium ions for doping, the resulting layered cathode material exhibits low residual alkali and high capacity.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, and particularly relates to a sodium-ion battery cathode material, its preparation method, and its application. Background Technology

[0002] With the continuous expansion of the lithium-ion battery market in recent years, the demand for lithium resources has also been rising. However, due to the uneven geographical distribution of lithium resources, the price of lithium-ion batteries fluctuates greatly, making their application in some low-cost scenarios difficult. Therefore, developing a new type of energy storage system that is abundant in resources and inexpensive to replace lithium-ion batteries is currently an urgent task for battery development.

[0003] Sodium ions possess an electrochemical insertion / extraction mechanism similar to lithium ions. Although sodium-ion batteries lag behind lithium-ion batteries in energy density, their applications in the small-power market and future large-scale energy storage still hold immense potential. The cathode, as the most critical component of the battery system, determines the electrochemical performance of sodium-ion batteries. Among them, transition metal layered oxide cathodes offer advantages such as high energy density, high reversible capacity, and high operating potential. Furthermore, their synthesis process is simple and compatible with existing high-nickel ternary material production lines, making them considered the most likely sodium-ion cathode material for mass production.

[0004] However, due to the low solubility of transition metal ions in sodium-based materials, insufficient sodium source leads to the formation of oxide impurities in the sodium-based cathode material. Electrochemically active elements such as Ni and Cu precipitate, affecting the charge transfer number during charge and discharge, and reducing the material's reversible capacity. Simultaneously, oxides adhering to the material surface affect ion / electron conduction, impacting electrochemical performance. While sufficient sodium source allows transition metal ions to remain in the bulk structure and suppress oxide impurity formation, excessive sodium remains on the surface, forming residual alkali substances such as sodium carbonate / sodium hydroxide, which are ionic compounds. These cations and anions are densely packed in the lattice through ionic bonds, lacking conductivity, increasing interfacial resistance, and affecting ion / electron conduction, thus reducing capacity and hindering cathode material performance. The presence of residual alkali in the electrode also causes defluorination of the PVDF (polyvinylidene fluoride) adhesive, leading to particle agglomeration, affecting coating adhesion, causing slurry gelation, and making the electrode prone to powdering and peeling after drying. Alkaline slurry can also corrode aluminum current collectors, affecting electron transport and even generating gas under high voltage, posing certain safety hazards.

[0005] Therefore, how to avoid the formation of oxide impurities while keeping the residual alkali content of the material at a low level is a thorny problem that needs to be solved in the industrialization of high-performance sodium cathodes. Summary of the Invention

[0006] The technical problem to be solved by the present invention is how to avoid the formation of oxide impurity phases while controlling the residual alkali content of the material at a low level, overcoming the deficiencies and defects mentioned in the background art above, and providing a high-capacity, low-residual-sodium layered cathode material for sodium-ion batteries, its preparation method and sodium-ion batteries.

[0007] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows:

[0008] A high-capacity, low-residual-sodium layered cathode material for sodium-ion batteries, wherein the chemical formula of the sodium-ion battery layered cathode material is Na. a M1 b Ni w Fe x Mn y Cu z M2 1-w-x-y-z O2, wherein M1 is the first dopant element, including one or more of Ca, Li or K, and M2 is the second dopant element, including one or more of Mg, Zn, Al, Zr, Ti, Nb, Mo, Y, Ta, W, Sr, Ba, B or P, 0.85≤a<0.98, 0<b≤0.05, and 0.90≤a+b≤0.98, w≥0.1, x≥0.1, y≥0.1, z≥0, and 0.9≤w+x+y+z≤1.0.

[0009] This invention involves doping the layered cathode material of sodium-ion batteries with an element M1 that has a similar radius or chemical properties to sodium ions, allowing it to occupy sodium sites during high-temperature sintering. Simultaneously, the stoichiometry of sodium and M1 dopant is controlled to satisfy 0.90≤a+b≤0.98, ensuring that the material has a suitable amount of sodium while avoiding obvious oxide impurities, thereby guaranteeing low residual alkali and high capacity.

[0010] Preferably, the layered cathode material of the sodium-ion battery contains an oxide impurity phase, which includes one or more of NiO, CuO or ZnO, and the total mass of the oxide impurity phase in the layered cathode material of the sodium-ion battery does not exceed 2 wt%.

[0011] Preferably, the mass percentage of the oxide impurity phase is obtained by XRD diffraction and refined using Jade software, wherein the XRD scanning range is 10°≤2θ≤80° and the scanning rate is 5° / min.

[0012] Preferably, the residual sodium (Na2CO3) on the surface of the layered cathode material of the sodium-ion battery is no more than 0.15 wt%, and the residual NaOH is no more than 0.10 wt%.

[0013] Preferably, the method for testing residual sodium on the surface includes: using deionized water and an organic solvent as residual alkali test solvents, adding an acid solution for acid-base neutralization, using potentiometric titration to test the residual Na2CO3 in the sample in deionized water, and testing the NaOH content in the organic solvent.

[0014] More preferably, the organic solvent includes one or more of methanol, ethanol, isopropanol, ethylene glycol, benzyl alcohol, or glycerol; the acid used in the acid-base neutralization includes one or more of sulfuric acid, nitric acid, hydrochloric acid, acetic acid, or oxalic acid.

[0015] Under the same technical concept, the present invention also provides a method for preparing a high-capacity, low-residual-sodium layered cathode material for sodium-ion batteries, comprising the following steps:

[0016] (1) A salt solution containing Ni, Fe, Mn or a salt solution containing Ni, Fe, Mn, Cu is mixed in proportion to carry out a co-precipitation reaction to prepare nickel-iron-manganese ternary or nickel-iron-manganese-copper quaternary precursor materials.

[0017] (2) The precursor material obtained in step (1) is mixed with sodium source, compound containing element M1 and compound containing element M2 and sintered. After cooling to room temperature, it is crushed and sieved to obtain layered cathode material for sodium-ion battery.

[0018] Preferably, the sodium source in step (2) includes one or more of Na2CO3, NaHCO3, or NaOH; the compound containing element M1 includes, but is not limited to, one or more of Li2CO3, K2CO3, CaCO3, or CaO; the compound containing element M2 includes, but is not limited to, one or more of MgCO3, MgO, ZnO, Al2O3, ZrO2, TiO2, Nb2O5, MoO3, Y2O3, Ta2O5, WO3, BaO, BaCO3, SrCO3, SrO, H3BO3, B2O3, H3PO4, Na3PO4, or NaH2PO4.

[0019] Preferably, the sintering in step (2) is a two-stage sintering. The first stage of sintering is carried out at 450℃~850℃ and held for 3~8h. Then, the temperature is raised to 860℃~1000℃ for the second stage of sintering and held for 10~15h. The heating rate during the sintering process is 1-10℃ / min.

[0020] More preferably, the heating rate is 3°C / min.

[0021] Preferably, the sintering in step (2) is carried out in a compressed air atmosphere with a dew point of -50°C to -25°C. Controlling the dew point can further prevent sodium ions from precipitating from the interlayer during the cooling process.

[0022] Under the same technical concept, the present invention also provides a sodium-ion battery, wherein the positive electrode material in the sodium-ion battery is either a layered positive electrode material of the sodium-ion battery or a layered positive electrode material of the sodium-ion battery obtained by the preparation method described above.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] (1) This invention improves the residual alkali of the layered cathode material of sodium-ion battery by simultaneously controlling the stoichiometry of sodium and M1 doping elements to satisfy 0.90≤a+b≤0.98. At the same time, it uses M1 elements with similar radius or chemical properties to sodium ions for doping. After high-temperature calcination, M1 occupies the sodium layer and suppresses the generation of inactive oxide impurities when the sodium source is insufficient. This results in the sodium-ion battery layered cathode material without obvious oxide impurities, and has the characteristics of low residual alkali and high capacity.

[0025] (2) The present invention controls the amount of raw materials and additives fed, while constraining the molar amount of sodium and dopant element M1, so that the proportion of oxide impurity phase in the structure does not exceed 2wt%, the residual alkali Na2CO3 on the material surface is not higher than 0.15wt%, and the residual NaOH is not higher than 0.10wt%, thereby obtaining a high-capacity, low-residual sodium layered cathode material for sodium-ion batteries.

[0026] (3) The present invention adopts a two-step sintering process. In the first stage of sintering, the precursor is fully pre-oxidized to remove moisture and eliminate internal defects. After the defects are repaired, the precursor grains are continuously fused in the second stage of sintering at a higher temperature to form an internally ordered phase structure, which avoids the precipitation of oxides caused by local instability of the material. At the same time, the environmental dew point is controlled within a specific range throughout the material preparation process to prevent sodium carbonate and M1 element raw materials from absorbing environmental moisture and affecting the uniformity of mixing, thus preventing element segregation. It can also effectively protect the cathode material and prevent sodium ions from precipitating from the interlayer during the cooling process, which would cause the residual alkali in the material to rise. Attached Figure Description

[0027] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a SEM image of the layered cathode material for the sodium-ion battery corresponding to Example 1;

[0029] Figure 2 The image shows the SEM image of the layered cathode material of the sodium-ion battery corresponding to Comparative Example 1.

[0030] Figure 3 The image shows the XRD pattern of the layered cathode material for the sodium-ion battery corresponding to Comparative Example 1.

[0031] Figure 4 The image shows the XRD pattern of the layered cathode material for sodium-ion batteries corresponding to Comparative Example 3.

[0032] Figure 5 The image shows the XRD pattern of the layered cathode material for the sodium-ion battery corresponding to Example 1.

[0033] Figure 6 This is a comparison of the first-cycle electrochemical curves of the coin cells corresponding to Example 1 and Comparative Example 1. Detailed Implementation

[0034] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0035] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0036] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0037] In the following embodiments, the oxide impurity phase ratio of the layered cathode material for sodium-ion batteries was determined by XRD diffraction and refined using Jade software. The XRD scanning range was 10°≤2θ≤80°, the scan rate was 5° / min, and the cell parameter c was obtained using TOPAS software. The error values ​​Rp and Rwp of the fitted curves were both less than 5%.

[0038] In the following embodiments, the method for testing residual sodium on the surface is as follows: the material is dissolved in deionized water and 99% ethanol solution respectively, titrated with HCl solution, and the residual Na2CO3 in the sample is tested in deionized water and the NaOH content is tested in ethanol.

[0039] In the following embodiments, the assembly method and testing conditions of the sodium-ion battery are as follows: the positive electrode material, acetylene black and PVDF are mixed in a ratio of 90:5:5, NMP is added to make a slurry, and the sodium electrode is dried under vacuum at 80°C to make a sodium electrode sheet. The sodium electrode sheet, separator and electrolyte are then assembled into a sodium button cell. The capacity is tested on the Blue Battery Testing System. The test voltage range is 2.0-4.2V and the test current density is 20mA / g.

[0040] Example 1

[0041] The sodium-ion battery layered cathode material in this embodiment has the chemical formula Na. 0.96 Ca 0.01 Ni 0.33 Fe 0.33 Mn 0.33 O2, the oxide impurity phase is NiO, the proportion of the impurity phase in the layered cathode material of sodium-ion battery is 1.01 wt%, the residual sodium Na2CO3 is 0.11 wt%, and NaOH is 0.09 wt%.

[0042] The method for preparing the layered cathode material for sodium-ion batteries in this embodiment includes the following steps:

[0043] (1) Preparation of nickel-manganese-iron precursor: NiSO4, MnSO4 and FeSO4 were mixed in a metal molar ratio of 1:1:1 to prepare a 1.0 mol / L mixed metal salt solution. The prepared mixed metal salt solution was then added to the reactor in parallel with the precipitant NaOH solution and the complexing agent NH3·H2O solution. The pH of the system was controlled between 8.5 and 9.5. The mixture was heated and stirred to carry out a co-precipitation reaction. After the reaction was completed, the slurry was separated into solid and liquid. The solid was washed with deionized water and dried in an oven to obtain the nickel-iron-manganese precursor.

[0044] (2) The nickel-iron-manganese precursor prepared in step (1), Na2CO3, and CaCO3 were dry-mixed at an elemental molar ratio of 1.00:0.96:0.01. After uniform mixing, the mixture was heated to 450°C in a compressed air atmosphere (with the dew point of the compressed air controlled at -45°C to -40°C) at a heating rate of 5°C / min and sintered for 6 hours. Then, it was heated to 900°C at a constant heating rate of 3°C / min and sintered for 12 hours. After natural cooling to room temperature, the mixture was passed through a 400-mesh sieve to obtain the layered cathode material for sodium-ion batteries with the chemical formula Na. 0.96 Ca 0.0 1Ni 0.33 Fe 0.33 Mn 0.33 O2, its XRD pattern is as follows Figure 5 As shown, the material structure is a single O3 phase, without any oxide impurities. (SEM image shown below) Figure 1 As shown.

[0045] Example 2

[0046] The sodium-ion battery layered cathode material in this embodiment has the chemical formula Na. 0.92 Ca 0.01 Ni 0.33 Fe 0.33 Mn 0.33O2, the oxide impurity phase is NiO, the impurity phase accounts for 1.45 wt% in the layered cathode material of sodium-ion battery, the tested residual sodium Na2CO3 is 0.05 wt%, and NaOH is 0.04 wt%.

[0047] The method for preparing the cathode material in this embodiment includes the following steps:

[0048] The preparation method of the precursor is the same as in Example 1. Nickel-iron-manganese precursor, Na₂CO₃, and CaCO₃ are dry-mixed at an elemental molar ratio of 1.00:0.92:0.01. The preparation method of the cathode material is the same as in Example 1. The resulting layered cathode material for sodium-ion batteries has the chemical formula Na. 0.92 Ca 0.01 Ni 0.33 Fe 0.33 Mn 0.33 O2.

[0049] Example 3

[0050] The sodium-ion battery layered cathode material in this embodiment has the chemical formula Na. 0.88 Ca 0.02 Ni 0.33 Fe 0.33 Mn 0.33 O2, the oxide impurity phase is NiO, the impurity phase accounts for 1.78 wt% in the layered cathode material of sodium-ion battery, the residual sodium Na2CO3 is 0.03 wt%, and NaOH is 0.02 wt%.

[0051] The preparation method of the cathode material in this comparative example includes the following steps:

[0052] The preparation method of the precursor is the same as in Example 1. Nickel-iron-manganese precursor, Na₂CO₃, and CaCO₃ are dry-mixed at an elemental molar ratio of 1.00:0.88:0.02. The preparation method of the cathode material is the same as in Example 1. The resulting layered cathode material for sodium-ion batteries has the chemical formula Na. 0.88 Ca 0.02 Ni 0.33 Fe 0.33 Mn 0.33 O2.

[0053] Example 4

[0054] The sodium-ion battery layered cathode material in this embodiment has the chemical formula Na. 0.96 Li 0.01 Ni 0.33 Fe 0.33 Mn 0.33O2, the oxide impurity phase is NiO, the impurity phase accounts for 1.11 wt% in the layered cathode material of sodium-ion battery, the residual sodium Na2CO3 is 0.14 wt%, and NaOH is 0.08 wt%.

[0055] The preparation method of the cathode material in this comparative example includes the following steps:

[0056] The preparation method of the precursor is the same as in Example 1. Nickel-iron-manganese precursor, Na₂CO₃, and Li₂CO₃ are dry-mixed at an elemental molar ratio of 1.00:0.96:0.01. The preparation method of the cathode material is the same as in Example 1. The resulting layered cathode material for sodium-ion batteries has the chemical formula Na. 0.96 Li 0.01 Ni 0.33 Fe 0.33 Mn 0.33 O2.

[0057] Example 5

[0058] The sodium-ion battery layered cathode material in this embodiment has the chemical formula Na. 0.96 K 0.01 Ni 0.33 Fe 0.33 Mn 0.33 O2, the oxide impurity phase is NiO, the impurity phase accounts for 1.09 wt% in the layered cathode material of sodium-ion battery, the tested residual sodium Na2CO3 is 0.13 wt%, and NaOH is 0.06 wt%.

[0059] The method for preparing the cathode material in this embodiment includes the following steps:

[0060] The preparation method of the precursor is the same as in Example 1. Nickel-iron-manganese precursor, Na₂CO₃, and K₂CO₃ are dry-mixed at an elemental molar ratio of 1.00:0.96:0.01. The preparation method of the cathode material is the same as in Example 1. The resulting layered cathode material for sodium-ion batteries has the chemical formula Na. 0.96 K 0.01 Ni 0.33 Fe 0.33 Mn 0.33 O2.

[0061] Example 6

[0062] The sodium-ion battery layered cathode material in this embodiment has the chemical formula Na. 0.96 Li 0.01 K 0.01 Ni 0.33 Fe 0.33 Mn 0.33O2, the oxide impurity phase is NiO, the proportion of the impurity phase in the layered cathode material of sodium-ion battery is 0.92 wt%, the residual sodium Na2CO3 is 0.13 wt%, and NaOH is 0.06 wt%.

[0063] The preparation method of the cathode material in this comparative example includes the following steps:

[0064] The preparation method of the precursor is the same as in Example 1. Nickel-iron-manganese precursor, Na₂CO₃, Li₂CO₃, and K₂CO₃ are dry-mixed in an elemental molar ratio of 1.00:0.96:0.01:0.01. The preparation method of the cathode material is the same as in Example 1. The resulting sodium-ion battery layered cathode material has the chemical formula Na. 0.96 Li 0.01 K 0.01 Ni 0.33 Fe 0.33 Mn 0.33 O2.

[0065] Example 7

[0066] The sodium-ion battery layered cathode material in this embodiment has the chemical formula Na. 0.96 Ca 0.01 Ni 0.40 Fe 0.20 Mn 0.40 O2, the oxide impurity phase is NiO, the impurity phase accounts for 1.89 wt% in the layered cathode material of sodium-ion battery, the residual sodium Na2CO3 is 0.14 wt%, and NaOH is 0.09 wt%.

[0067] The method for preparing the cathode material in this embodiment includes the following steps:

[0068] (1) Preparation of nickel-manganese-iron precursor: NiSO4, MnSO4 and FeSO4 were mixed in a metal molar ratio of 4:2:4 to prepare a 1.0 mol / L mixed metal salt solution. The prepared mixed metal salt solution was then added to the reactor in parallel with the precipitant NaOH solution and the complexing agent NH3·H2O solution. The pH of the system was controlled between 8.5 and 9.5. The mixture was heated and stirred to carry out a co-precipitation reaction. After the reaction was completed, the slurry was separated into solid and liquid. The solid was washed with deionized water and dried in an oven to obtain the nickel-iron-manganese precursor.

[0069] (2) The nickel-iron-manganese precursor prepared in step (1) and Na2CO 33CaCO3 was dry-mixed in an elemental molar ratio of 1.00:0.96:0.01. After uniform mixing, the mixture was heated to 450℃ in a compressed air atmosphere (dew point controlled at -30℃ to -25℃) at a heating rate of 5℃ / min and sintered for 6 hours. Then, it was heated to 900℃ at a heating rate of 3℃ / min and sintered for 12 hours. After natural cooling to room temperature, the mixture was passed through a 400-mesh sieve to obtain a layered cathode material for sodium-ion batteries with the chemical formula Na. 0.96 Ca 0.01 Ni 0.40 Fe 0.20 Mn 0.40 O2.

[0070] Example 8

[0071] The sodium-ion battery layered cathode material in this embodiment has the chemical formula Na. 0.96 Ca 0.01 Ni 0.25 Fe 0.30 Mn 0.25 Cu 0.20 O2, oxide impurities are NiO and CuO, the proportion of impurities in the layered cathode material of sodium-ion battery is 1.56 wt%, the residual sodium Na2CO3 is 0.07 wt%, and NaOH is 0.05 wt%.

[0072] The method for preparing the cathode material in this embodiment includes the following steps:

[0073] NiSO4, MnSO4, FeSO4, and CuSO4 were prepared into a 1.0 mol / L solution at a metal molar ratio of 5:6:5:4. The remaining steps were the same as the precursor preparation method in Example 1. The nickel-iron-manganese-copper precursor, Na2CO3, and CaCO3 were dry-mixed at an elemental molar ratio of 1.00:0.96:0.01. The preparation method of the cathode material was the same as in Example 1. The resulting layered cathode material for sodium-ion batteries has the chemical formula Na. 0.96 Ca 0.01 Ni 0.25 Fe 0.30 Mn 0.25 Cu 0.20 O2.

[0074] Comparative Example 1

[0075] The sodium-ion battery layered cathode material in this comparative example has the chemical formula Na. 0.96 Ni 0.33 Fe 0.33 Mn 0.33O2, with NiO as the oxide impurity phase, accounts for 3.07 wt% of the layered cathode material for sodium-ion batteries. The residual sodium (Na2CO3) is 0.10 wt%, and the residual NaOH is 0.07 wt%. The XRD pattern of this comparative sodium-ion battery layered cathode material is shown below. Figure 3 As shown, in addition to the structural diffraction peaks of O3, the material structure also exhibits obvious NiO impurity phase peaks, as shown in the SEM image. Figure 2 As shown.

[0076] The preparation method of the cathode material in this comparative example includes the following steps:

[0077] The preparation method of the precursor is the same as in Example 1, wherein the nickel-iron-manganese precursor and Na2CO3 are dry-mixed at an elemental molar ratio of 1.00:0.96. The preparation method of the cathode material is the same as in Example 1, and the resulting layered cathode material for sodium-ion batteries has the chemical formula Na. 0.96 Ni 0.33 Fe 0.33 Mn 0.33 O2.

[0078] Comparative Example 2

[0079] The sodium-ion battery layered cathode material in this comparative example has the chemical formula Na. 0.96 Ni 0.25 Fe 0.30 Mn 0.25 Cu 0.20 O2, oxide impurities are NiO and CuO, the proportion of impurities in the layered cathode material of sodium-ion battery is 3.99 wt%, the residual sodium Na2CO3 is 0.13 wt%, and NaOH is 0.09 wt%.

[0080] The preparation method of the cathode material in this comparative example includes the following steps:

[0081] The preparation method of the precursor is the same as in Example 8. Nickel-iron-manganese-copper precursors and Na₂CO₃ are dry-mixed at an elemental molar ratio of 1.00:0.96. The preparation method of the cathode material is the same as in Example 1. The resulting layered cathode material for sodium-ion batteries has the chemical formula Na. 0.96 Ni 0.25 Fe 0.30 Mn 0.25 Cu 0.20 O2.

[0082] Comparative Example 3

[0083] The sodium-ion battery layered cathode material in this comparative example has the chemical formula Na. 0.96 Ni 0.31 Fe 0.31 Mn 0.31 Zn0.07 O2, oxide impurities are NiO and ZnO, the proportion of impurities in the layered cathode material of sodium-ion batteries is 3.98 wt%, the residual sodium Na2CO3 is 0.14 wt%, and NaOH is 0.12 wt%. The XRD pattern of this comparative sodium-ion battery layered cathode material is shown below. Figure 4 As shown, in addition to the structural diffraction peaks of O3, the material structure is also accompanied by obvious ZnO impurity phase peaks.

[0084] The preparation method of the cathode material in this comparative example includes the following steps:

[0085] Prepare a 1.0 mol / L solution of sulfate or nitrate containing Ni, Mn, Fe, and Zn according to the corresponding molar ratio. The remaining steps are the same as the preparation method of the precursor in Example 1. The nickel-iron-manganese-zinc precursor and Na2CO3 are dry-mixed at an elemental molar ratio of 1.00:0.96. The preparation method of the cathode material is the same as in Example 1. The resulting layered cathode material for sodium-ion batteries has the chemical formula Na. 0.96 Ni 0.31 Fe 0.31 Mn 0.31 Zn 0.07 O2.

[0086] Comparative Example 4

[0087] The sodium-ion battery layered cathode material in this comparative example has the chemical formula Na. 0.88 Ca 0.01 Ni 0.33 Fe 0.33 Mn 0.33 O2, the oxide impurity phase is NiO, the impurity phase accounts for 3.95 wt% in the layered cathode material of sodium-ion battery, the tested residual sodium Na2CO3 is 0.04 wt%, and NaOH is 0.03 wt%.

[0088] The preparation method of the cathode material in this comparative example includes the following steps:

[0089] The preparation method of the precursor is the same as in Example 1. Nickel-iron-manganese precursor, Na₂CO₃, and CaCO₃ are dry-mixed at an elemental molar ratio of 1.00:0.88:0.01. The preparation method of the cathode material is the same as in Example 1. The resulting layered cathode material for sodium-ion batteries has the chemical formula Na. 0.88 Ca 0.01 Ni 0.33 Fe 0.33 Mn 0.33 O2.

[0090] Comparative Example 5

[0091] The sodium-ion battery layered cathode material in this comparative example has the chemical formula Na.1.00 Ca 0.01 Ni 0.33 Fe 0.33 Mn 0.33 O2, the oxide impurity phase is NiO, the impurity phase accounts for 1.43 wt% in the layered cathode material of sodium-ion battery, the residual sodium Na2CO3 is 0.19 wt%, and NaOH is 0.12 wt%.

[0092] The preparation method of the cathode material in this comparative example includes the following steps:

[0093] The preparation method of the precursor is the same as in Example 1. Nickel-iron-manganese precursor, Na₂CO₃, and CaCO₃ are dry-mixed at an elemental molar ratio of 1.00:1.00:0.01. The preparation method of the cathode material is the same as in Example 1. The resulting layered cathode material for sodium-ion batteries has the chemical formula Na. 1.00 Ca 0.01 Ni 0.33 Fe 0.33 Mn 0.33 O2.

[0094] Table 1. Performance test data of sodium-ion battery layered cathode materials in Examples 1-8 and Comparative Examples 1-5

[0095]

[0096] As shown in the table above, after sintering, the proportion of NiO impurity phase in Comparative Example 1 was 3.07%, and the discharge specific capacity at 2.0-4.2V was 168.5 mAh / g. However, after doping with 0.01 mol of Ca (Example 1), the proportion of NiO impurity phase in the material decreased to 1.01%, and the discharge specific capacity increased to 176.8 mAh / g. This is because in the sodium-deficient O3 phase structure, elements with similar chemical properties or ionic radii can be doped into the octahedral sites occupied by sodium ions, ensuring structural stability and confining the transition metal in a layered structure to provide the oxidation / reduction charge required during charging and discharging. This results in a high-capacity and low-alkali-residue layered cathode material for sodium-ion batteries. The decrease in the cell parameter c indicates that Ca was successfully incorporated into the Na layer, reducing the electrostatic repulsion of adjacent oxygen layers. Comparative Examples 2 and 3 show that even when Cu and Zn from the transition metals are selected as doping elements, the material still cannot achieve a high specific capacity. As can be seen from Examples 2 / 3 / 4 / 5 / 6 / 7 / 8, by controlling 0.90≤a+b≤0.98 for different sodium formulations and different transition metal Ca / Li / K ratios, the doped samples significantly reduced the proportion of oxide impurities in the material, resulting in high-capacity layered cathode materials for sodium-ion batteries. As can be seen from Comparative Example 4, when the chemical formula Na... a M1 b Ni w Fex Mn y Cu z M2 1-w-x-y-z In O2, a+b < 0.9, resulting in insufficient sodium ion occupancy in the layer, leading to transition metal dissolution and a high proportion of inactive oxides, thus affecting capacity performance. In Comparative Example 5, when Ca doping is performed on a sodium-rich material (a+b > 0.98), Ca cannot be incorporated into the sodium layer, or the Na layer is occupied, causing Na ions to escape to the surface and form Na2CO3 / NaOH. This results in excessive residual alkali, affecting specific capacity. In conclusion, under appropriate process conditions, sodium doping of sodium-deficient O3 phase materials, satisfying 0.90 ≤ a+b ≤ 0.98, oxide impurity phase proportion ≤ 2 wt%, Na2CO3 ≤ 0.15 wt%, and NaOH ≤ 0.10 wt%, can produce materials exhibiting high discharge specific capacity.

Claims

1. A high-capacity, low-residual-sodium layered cathode material for sodium-ion batteries, characterized in that, The chemical formula of the layered positive electrode material of the sodium-ion battery is Na. a M1 b Ni w Fe x Mn y Cu z M2 1-w-x-y-z O2, wherein M1 is the first doping element, including one or more of Ca or K, and M2 is the second doping element, including one or more of Mg, Zn, Al, Zr, Ti, Nb, Mo, Y, Ta, W, Sr, Ba, B or P, 0.85≤a<0.98, 0<b≤0.05, and 0.90≤a+b≤0.98, w≥0.1, x≥0.1, y≥0.1, z≥0, 0.9≤w+x+y+z≤1.0; the sodium-ion battery layered cathode material contains oxide impurities, and the total mass of the oxide impurities in the sodium-ion battery layered cathode material does not exceed 2wt% of the mass percentage; the oxide impurities include one or more of NiO, CuO or ZnO, and the residual sodium Na2CO3 on the surface of the sodium-ion battery layered cathode material is not higher than 0.15wt%, and the residual NaOH is not higher than 0.10wt%.

2. The layered cathode material for sodium-ion batteries as described in claim 1, characterized in that, The mass percentage of the oxide impurity phase was obtained by XRD diffraction and refined using Jade software. The scanning range of the XRD diffraction test was 10°≤2θ≤80°, and the scanning rate was 5° / min.

3. A method for preparing the layered cathode material for sodium-ion batteries as described in any one of claims 1-2, characterized in that, Includes the following steps: (1) A salt solution containing Ni, Fe, Mn or a salt solution containing Ni, Fe, Mn, Cu is mixed in proportion to carry out a co-precipitation reaction to prepare nickel-iron-manganese ternary or nickel-iron-manganese-copper quaternary precursor materials; (2) The precursor material obtained in step (1) is mixed with sodium source, compound containing element M1 and compound containing element M2 and sintered to obtain a layered cathode material for sodium-ion battery. The sintering is a two-stage sintering. First, the first stage sintering is carried out at 450℃~850℃ and held for 3~8h. Then, the temperature is raised to 860℃~1000℃ for the second stage sintering and held for 10~15h. The heating rate during the sintering process is 1~10℃ / min. The sintering is carried out in compressed air with a dew point of -50℃~-25℃.

4. The method as described in claim 3, characterized in that, In step (2), the sodium source includes one or more of Na2CO3, NaHCO3, or NaOH; the compound containing element M1 includes one or more of K2CO3, CaCO3, or CaO; and the compound containing element M2 includes one or more of MgCO3, MgO, ZnO, Al2O3, ZrO2, TiO2, Nb2O5, MoO3, Y2O3, Ta2O5, WO3, BaO, BaCO3, SrCO3, SrO, H3BO3, B2O3, H3PO4, Na3PO4, or NaH2PO4.

5. A sodium-ion battery, characterized in that, The positive electrode material in the sodium-ion battery is the layered positive electrode material of sodium-ion battery according to any one of claims 1-2 or the layered positive electrode material of sodium-ion battery obtained by the preparation method according to any one of claims 3-4.

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