A double-layer coated layered oxide cathode material for sodium-ion batteries, its preparation method, and sodium-ion batteries.

Abstract

This invention discloses a double-layer coated layered oxide cathode material for sodium-ion batteries. The material comprises a layered oxide matrix and a double-layer coating layer covering the surface of the matrix. The layered oxide matrix is ​​doped with Ti. The double-layer coating layer comprises an inner coating layer and an outer coating layer. The inner coating layer comprises Na₂Ti₃O₇, and the outer coating layer comprises TiO₂. This sodium-ion battery layered oxide cathode material exhibits high rate capability and low damping coefficient (DCR) growth performance.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, and particularly relates to a layered oxide cathode material for sodium-ion batteries with high rate capability and low DCR growth performance, and its preparation method. Background Technology

[0002] With the rapid growth of global demand for renewable energy storage, sodium-ion batteries have become an important supplement to lithium-ion batteries due to their abundant resources and low cost. Especially in large-scale energy storage systems, sodium-ion batteries have shown great application potential. However, for sodium-ion batteries to truly achieve commercialization and meet high-performance requirements, their shortcomings in energy density still need to be overcome. Developing high-voltage cathode materials is one of the key ways to improve the energy density of sodium-ion batteries. Although layered oxide cathode materials have high theoretical capacity and their operating voltage can be increased by adjusting the proportion of elemental doping or transition metals, they also face a series of challenges. For example, the strong alkalinity of the material surface, a large amount of internal stress concentration, and poor particle roundness lead to structural collapse due to phase transitions at high voltages. At the same time, severe side reactions between the electrolyte and the interface cause problems such as poor cycle performance, rapid increase in DC internal resistance (DCR), and poor rate performance.

[0003] To address the aforementioned issues, traditional methods typically employ a two-stage sintering process. For example, introducing Ti elements during the second sintering process to form an inactive coating layer; or utilizing the residual alkali reaction on the surface of the cathode material substrate to generate a coating layer, thereby improving the material's electrochemical performance. However, these single-layer coating strategies have significant limitations. The former easily leads to substantial capacity loss and low ionic conductivity, while the latter, utilizing the residual alkali reaction to generate a coating layer, suffers from insufficient uniformity and poor structural density, making it difficult to effectively suppress interfacial side reactions. Furthermore, the coating layer formed by the second sintering process often has weak adhesion to the substrate, easily leading to discontinuities or detachment due to poor interfacial adhesion. Other literature reports using Ti-containing materials for liquid-phase coating followed by sintering to form a coating layer, which can improve the material's cycle performance and reduce surface residual alkali to some extent. However, this method is complex, and if water is used as the solvent in the liquid-phase process, it can easily cause Na+ in the first-sintered material to be affected. + Delamination from the interlayer affects capacity utilization and leads to insufficient ionic conductivity, thus affecting rate performance and causing rapid DCR growth. If organic solvents are used, problems such as poor process environmental friendliness, difficulty in solvent recovery, and potential pollution are faced, making it difficult to meet the requirements of large-scale industrial production and resulting in higher costs. Summary of the Invention

[0004] The present invention aims to provide a layered oxide cathode material for sodium-ion batteries with high rate capability and low DCR growth performance, and also provides a simple and low-cost method for preparing the layered oxide cathode material for sodium-ion batteries.

[0005] A double-layer coated layered oxide cathode material for sodium-ion batteries, comprising a layered oxide matrix and a double-layer coating layer on the surface of the layered oxide matrix, wherein the layered oxide matrix is ​​doped with Ti, and the double-layer coating layer comprises an inner coating layer and an outer coating layer, wherein the inner coating layer comprises Na2Ti3O7 and the outer coating layer comprises TiO2.

[0006] Preferably, the molar content of Ti element in the double-layer coating layer is x, where x is taken from 0.3% to 3% of the total molar number of the sodium-ion battery layered oxide cathode material.

[0007] Preferably, the Na2Ti3O7 content in the inner coating layer accounts for 60% to 85% of the total mass of the double coating layer.

[0008] This invention constructs a bilayer coating structure consisting of an inner Na2Ti3O7 layer and an outer TiO2 layer, exhibiting significant synergistic advantages compared to a single coating layer. While the TiO2 coating layer provides a physical barrier, its intrinsic ionic conductivity is low, leading to a continuous deterioration of ion transport kinetics at the interface during long-term cycling, resulting in increased capacity, rate capability, DCR growth, and high-voltage cycling performance. The Na2Ti3O7 coating layer, due to its layered structure, possesses excellent ionic conductivity, but its chemical barrier ability against the electrolyte is insufficient, resulting in limited suppression of side reactions at high voltages. By forming a sequential inner and outer coating on the substrate surface, both layers can achieve a balance between excellent capacity, rate capability, DCR growth, and high-voltage cycling performance.

[0009] Preferably, the layered oxide matrix is ​​composed of Na. α Ni a Fe b Mn c Ti d M1 e M2 f M3 g O 2-f Wherein, M1 is selected from one or more of Mg, Zn, Co, Cu, or Li; M2 is selected from one or more of Cl, F, or N; M3 is selected from one or more of B, P, Si, or S; 0.85≤α≤1.00, a≥0, b≥0, c≥0, 0 <d<0.1,0.01≤e≤0.05,0.001≤f≤0.02,0.001≤g≤0.02,a+b+c+d+e+g=1。

[0010] Preferably, c+d+xa≤0.12, that is, controlling the amount of Mn, Ti and Ni added in the cathode material can prevent the average oxidation state from being too high, ensure that there are enough sodium ions that can be inserted and removed in the crystal structure, and suppress structural relaxation or distortion during cycling.

[0011] Preferably, the full width at half maximum (FWHM) values ​​of peaks 104 and 101 in the XRD pattern satisfy the following:

[0012] 1.060≤FWHM(104) / FWHM(101)≤1.090.

[0013] A method for preparing a layered oxide cathode material for sodium-ion batteries includes the following steps: uniformly mixing a layered oxide precursor for sodium-ion batteries, a sodium source, a Ti source, a compound M1, a compound M2, and a compound M3, wherein the sodium source is added in excess relative to the layered oxide precursor for sodium-ion batteries, and the diffusion barrier of the dopant element M1 in the compound M1 is lower than that of Ti. 4+ In the M2 compound, the dopant element M2 is doped at the oxygen site, and the ionic bonding strength between M2 and the transition metal is greater than that between O and the transition metal. In the M3 compound, the dopant element M3 is a non-metallic element doped at the tetrahedral interstitial site, and can form a covalent bond with oxygen ions to enhance the electronegativity of oxygen ions. The mixture after being thoroughly mixed is heated to 900℃~1000℃ and sintered for 10~15h, then cooled to 500℃~650℃ and sintered for 3~6h to obtain the sodium-ion battery layered oxide cathode material.

[0014] The sodium-ion battery layered oxide contains one or more of Ni, Fe, and Mn, preferably a nickel-iron-manganese ternary precursor. Preferably, the excess addition of the sodium source relative to the sodium-ion battery layered oxide precursor means that the molar ratio of the sodium element in the sodium source to the sodium source is 1:1.05~1.15; the sodium source is one or more of sodium bicarbonate, sodium hydroxide, or sodium carbonate; the Ti source is one or more of titanium dioxide, titanium hydroxide, titanium chloride, or lithium titanate.

[0015] Preferably, M1 is selected from one or more of Mg, Zn, Co, Cu or Li, M2 is selected from one or more of Cl, F or N, and M3 is selected from one or more of B, P, Si or S.

[0016] More preferably, the M1 compound is one or more of ZnO, MgO, MgCO3, Mg3(PO4)2, CuO, Cu(OH)2, Co3(PO4)2, Co(OH)2, Li2CO3, or LiOH; the M2 compound is one or more of NaCl, PVDF, NaF, or BN; and the M3 compound is one or more of H3BO3, BN, Na3PO4, Mg3(PO4)2, Co3(PO4)2, Na2SiO3, or Na2S.

[0017] Preferably, a pre-sintering treatment is performed before the two-stage sintering, wherein the pre-sintering treatment conditions are sintering at 700℃~850℃ for 3~8 hours. This added pre-sintering stage facilitates the melting of the sodium source and the diffusion of M1, thereby enhancing the reaction with the precursor, reducing residual sodium that cannot enter the crystal lattice on the surface, and promoting uniform migration of M1 towards the grain boundaries and surface. Compared to two-stage sintering, the particle morphology is more complete, the irregularity of growth along different crystal planes is reduced, and the roundness is increased. This reduces side reactions during material use and in the electrolyte, while also helping to suppress excessive internal defects in the crystal caused by uneven M1 occupancy, further optimizing the material's capacity, cycle life, and rate performance.

[0018] When the cathode material matrix meets the composition range of the above chemical formula and c+d+xa≤0.12, the pre-sintering stage added on the basis of the two-stage sintering can make the full width at half maximum (FWHM) values ​​of the 104 peak and the 101 peak in the XRD pattern of the material meet the following: 1.060≤FWHM(104) / FWHM(101)≤1.090. Above this range, the proportion of lattice distortion along the a-axis or c-axis is too high, which can easily induce anisotropic stress accumulation, causing microcracks to be generated in the material under high voltage, resulting in capacity decay. Below this range, the disorder of cation occupancy is too high, and the development along the anisotropic crystal plane is insufficient, resulting in poor phase transition reversibility during cycling and decreased structural stability.

[0019] A sodium-ion battery, comprising a sodium-ion battery layered oxide cathode material.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The sodium-ion battery layered oxide cathode material provided by this invention has a double-layer coating structure. The inner coating layer Na2Ti3O7, which is close to the substrate material, can provide a fast ion channel and significantly reduce the interfacial charge transfer impedance as an ion conduction layer. The outer coating layer TiO2 is located outside the inner coating layer and can effectively play its chemical barrier function, effectively reducing the dissolution of transition metals and interfacial side reactions. In the bulk phase of the substrate, Ti is uniformly doped in the transition metal layer in ionic form, stabilizing the crystal structure of the material. In the coating layer, Ti exists in the form of Na2Ti3O7 and TiO2, respectively. The Ti element continuously runs through the substrate, the inner coating layer and the outer coating layer, realizing the orderly evolution of local content and coordination environment from the inside to the outside. By utilizing the Ti-O coordination environment, the degree of lattice matching is improved, so that the inner and outer interfaces have excellent chemical compatibility, reducing interface defects and forming a stable interface that is conducive to ion conduction. This structural design enhances the interfacial bonding between the core and the coating layer, as well as between the inner and outer coating layers, ensuring a tight bond between the core, inner layer, and outer layer. This effectively mitigates interlayer delamination caused by volume expansion and contraction during charging and discharging, thereby comprehensively improving the structural and interfacial stability of the material. Through the synergy of the inner and outer coating layers, intrinsic ion transport is enhanced while reducing interfacial side reactions, effectively suppressing DCR growth during cycling, and simultaneously achieving excellent rate performance and high voltage cycling stability.

[0021] (2) The method for preparing the layered oxide cathode material for sodium-ion batteries provided by this invention introduces the M1 element and an excess sodium source during the solid-phase preparation process of the cathode, thereby achieving synergistic modification of the bulk phase and interface. This utilizes the fact that the diffusion barrier of M1 is lower than that of Ti. 4+ Its characteristics effectively suppress bulk doping of Ti ions, and its low valence state introduces cation vacancies, promoting the doping of more Na. + The inclusion of M1, M2, and M3 in the lattice maintains electroneutrality, thus mitigating capacity loss that may be caused by doping with inactive components such as Ti. Furthermore, M2, an anion doped at oxygen sites, exhibits strong ionic bonding with transition metal elements, enhancing structural stability under high voltage. M3, a non-metallic element doped at tetrahedral interstitial sites, enhances the electronegativity of oxygen ions through covalent bonding, strengthens the TM-O (transition metal-oxygen) binding energy, suppresses O extraction under high voltage, and prevents cycle degradation caused by structural collapse due to phase transitions. The combined use of M1, M2, and M3 significantly suppresses the adverse effects of structural collapse caused by phase transitions on the high-voltage cycling process. Additionally, this preparation method utilizes the diffusion barrier and Ti distribution regulation of M1, along with the further stabilizing effect of M2 and M3 as anchoring elements within the lattice. M2, doped at oxygen sites, exhibits strong bonding with transition metal ions, while M3, doped at interstitial sites, stabilizes atomic arrangement and expands the c-axis, facilitating Na... +Diffusion within the bulk phase not only forms a titanium-containing coating with a bilayer structure on the material surface, but also achieves dual stabilization of charge and structure within the bulk phase, successfully confining the Ti particles to the surface region. 4+ and excess Na + The reaction produces Na4Ti5O 12 Metastable intermediate phase. The process initially forms Na₄Ti₅O at high temperatures. 12 The intermediate phase is subsequently decomposed by intermediate-temperature annealing, which eliminates residual lattice stress and allows some Na to decompose. + The material is re-embedded within the bulk phase, forming Na2Ti3O7 near the Na-rich inner layer. Excess Ti, driven by chemical potential, is oxidized to a more thermodynamically stable TiO2 outer layer, thus constructing a Na2Ti3O7@TiO2 double-layer coating structure. The method for preparing layered oxide cathode materials for sodium-ion batteries provided by this invention directly generates the double coating layer through a simple, easy-to-implement, and industrially applicable one-step sintering method, resulting in lower costs. More importantly, because the coating layer is generated in situ during material synthesis, rather than through physical-mechanical mixing followed by sintering, the inner and outer coating layers can uniformly and completely coat the substrate surface, thereby better utilizing its electrochemical performance. Attached Figure Description

[0022] Figure 1 This is a TEM image of the layered oxide cathode material for sodium-ion batteries in Example 1 of the present invention; Figure 2 These are FE-SEM images of the layered oxide cathode materials for sodium-ion batteries in Examples 1, 3, and Comparative Example 3 of this invention. Figure 3 The particle size distribution curves of the layered oxide cathode material of sodium-ion batteries in Examples 1, 3 and Comparative Example 3 of this invention are shown in image recognition software. Figure 4 This is a mapping diagram of the layered oxide cathode material for sodium-ion batteries in Example 1 of the present invention; Figure 5 The above are XPS images of the layered oxide cathode materials for sodium-ion batteries in Embodiment 1 and Comparative Example 1 of this invention. Specific implementation methods The present invention will be further described below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0023] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0024] Example 1 The double-layer coated sodium-ion battery layered oxide cathode material provided in this embodiment includes a layered oxide matrix and a double-layer coating layer covering the surface of the layered oxide matrix. The chemical formula of the layered oxide matrix is ​​Na. 0.919 Ni 0.3013 Fe 0.3112 Mn 0.3123 Ti 0.0552 Zn 0.01 F 0.01 B 0.01 O 1.99 The double-layer coating consists of an inner coating and an outer coating. The inner coating is composed of Na2Ti3O7, and the outer coating is composed of TiO2. The molar content of Ti in the double-layer coating is 0.48 mol%, and the mass content of Na2Ti3O7 in the double-layer coating is 75 wt%.

[0025] The preparation method in this embodiment is as follows: (1) Prepare a 1.0 mol / L sulfate solution containing Ni, Fe, and Mn in a molar ratio of 1:1:1. Coprecipitate the solution with NaOH as a precipitant and ammonia as a complexing agent by heating and stirring. After the reaction is completed, separate the solid and liquid components, wash and dry to obtain Ni. 0.333 Fe 0.333 Mn 0.333 (OH)2 precursor; (2) Weigh Ni according to the elemental molar ratio (Ni+Fe+Mn):Na:Ti:Zn:F:B=1:1.10:0.06:0.01:0.01:0.01. 0.333 Fe 0.333 Mn 0.333 (OH)2 precursor, Na2CO3, TiO2, ZnO, PVDF, and H3BO3 were dry-mixed. After uniform mixing, the mixture was heated to 800℃ in compressed air at a heating rate of 3℃ / min and sintered at that temperature for 5 hours. Then, the temperature was increased to 980℃ at a heating rate of 3℃ / min and held for 12 hours. Subsequently, the temperature was cooled to 620℃ at a cooling rate of 0.7℃ / min and sintered for 5 hours. Finally, the temperature was reduced to room temperature at a cooling rate of 0.7℃ / min, crushed, and sieved to obtain the layered oxide cathode material for sodium-ion batteries.

[0026] To confirm the titanium content in the cathode material coating, a powder classification process combined with ICP-OES detection was employed: the cathode material was first processed into powder, separating it into collected granules and fine powder. After digestion, the Ti content was measured. Stability was monitored through multiple rounds of processing until the RSD was <3%, confirming the Ti content in the coating. The fine powder was further washed with dilute hydrochloric acid for 30 seconds to remove surface Na2Ti3O7, and the residual Ti content corresponded to the stable TiO2 in the coating. The tests showed that the molar content of Ti in the double-layer coating was 0.48 mol%, and the mass content of Na2Ti3O7 in the double-layer coating was 75 wt%. The Ti coating content and Na2Ti3O7 percentage in other examples and comparative examples were tested according to Example 1. Figure 1 As shown, the TEM image reveals the matrix material and the double-layer coating on the surface, namely the inner layer of Na2Ti3O7 and the outer layer of rutile TiO2.

[0027] Example 2 The double-layer coated sodium-ion battery layered oxide cathode material provided in this embodiment includes a layered oxide matrix and a double-layer coating layer covering the surface of the layered oxide matrix. The chemical formula of the layered oxide matrix is ​​Na. 0.918 Ni 0.282 Fe 0.311 Mn 0.2824 Ti 0.0846 Mg 0.03 Cl 0.005 P 0.01 O 1.995 The double-layer coating consists of an inner coating and an outer coating. The inner coating is composed of Na2Ti3O7, and the outer coating is composed of TiO2. The molar content of Ti in the double-layer coating is 1.54 mol%, and the mass content of Na2Ti3O7 in the double-layer coating is 68 wt%.

[0028] The preparation method in this embodiment is as follows: (1) Prepare a 1.0 mol / L sulfate solution containing Ni, Fe, and Mn in a molar ratio of 0.34:0.35:0.31. Coprecipitate the solution with NaOH as a precipitant and ammonia as a complexing agent by heating and stirring. After the reaction is completed, separate the solid and liquid components, wash and dry to obtain Ni. 0.34 Fe 0.35 Mn 0.31 (OH)2 precursor; (2) Weigh Ni according to the elemental molar ratio (Ni+Fe+Mn):Na:Ti:Mg:Cl:P=1:1.135:0.10:0.015:0.005:0.005. 0.34 Fe 0.35 Mn 0.31The precursor (OH)2, NaHCO3, Ti(OH)2, MgO, NaCl, and Mg3(PO4)2 were dry-mixed. After uniform mixing, the mixture was heated to 780℃ in a compressed air atmosphere at a heating rate of 3℃ / min and sintered at that temperature for 5 hours. Then, the temperature was increased to 960℃ at a heating rate of 3℃ / min and held for 13 hours. Subsequently, the temperature was cooled to 600℃ at a cooling rate of 0.7℃ / min and sintered for 6 hours. Finally, the temperature was reduced to room temperature at a cooling rate of 0.7℃ / min, crushed, and sieved to obtain the layered oxide cathode material for sodium-ion batteries.

[0029] Example 3 The double-layer coated sodium-ion battery layered oxide cathode material provided in this embodiment includes a layered oxide matrix and a double-layer coating layer covering the surface of the layered oxide matrix. The chemical formula of the layered oxide matrix is ​​Na. 0.917 Ni 0.300 Fe 0.3112 Mn 0.3132 Ti 0.0556 Zn 0.01 F 0.01 B 0.01 O 1.99 The double-layer coating consists of an inner coating and an outer coating. The inner coating is composed of Na2Ti3O7, and the outer coating is composed of TiO2. The molar content of Ti in the double-layer coating is 0.44 mol%, and the mass content of Na2Ti3O7 in the double-layer coating is 78 wt%.

[0030] The difference from the preparation method in Example 1 is that this example does not include sintering at 800°C for 5 hours, but otherwise it is the same as Example 1.

[0031] Example 4 The double-layer coated sodium-ion battery layered oxide cathode material provided in this embodiment includes a layered oxide matrix and a double-layer coating layer covering the surface of the layered oxide matrix. The double-layer coating layer includes an inner coating layer and an outer coating layer. The inner coating layer is composed of Na₂Ti₃O₇, and the outer coating layer is composed of TiO₂. The chemical formula of the layered oxide matrix is ​​Na₂Ti₃O₇. 0.917 Ni 0.3119 Fe 0.3111 Mn 0.3096 Ti 0.0554 Zn 0.01 F 0.002 B 0.002 O 1.998 The molar content of Ti in the double-layer coating is 0.46 mol% in the entire cathode material, and the mass content of Na2Ti3O7 in the entire double-layer coating is 78 wt%.

[0032] The difference from the preparation method in Example 1 is that step (2) in this example involves dry mixing of nickel-iron-manganese precursor, Na2CO3, TiO2, ZnO, PVDF, and H3BO3 in an elemental molar ratio of 1:1.10:0.06:0.01:0.002:0.002. The rest is the same as in Example 1.

[0033] Example 5 The double-layer coated sodium-ion battery layered oxide cathode material provided in this embodiment includes a layered oxide matrix and a double-layer coating layer covering the surface of the layered oxide matrix. The double-layer coating layer includes an inner coating layer and an outer coating layer. The inner coating layer is composed of Na₂Ti₃O₇, and the outer coating layer is composed of TiO₂. The chemical formula of the layered oxide matrix is ​​Na₂Ti₃O₇. 0.919 Ni 0.305 Fe 0.310 Mn 0.3111 Ti 0.0539 Zn 0.01 F 0.01 B 0.01 O 1.99 The molar content of Ti in the double-layer coating is 0.61 mol% in the entire cathode material, and the mass content of Na2Ti3O7 in the entire double-layer coating is 92 wt%.

[0034] The difference from the preparation method in Example 1 is that step (2) in this example involves dry mixing of nickel-iron-manganese precursor, Na2CO3, TiO2, ZnO, PVDF, and H3BO3 in an elemental molar ratio of 1:1.12:0.06:0.01:0.01:0.01. The rest is the same as in Example 1.

[0035] Example 6 The double-layer coated sodium-ion battery layered oxide cathode material provided in this embodiment includes a layered oxide matrix and a double-layer coating layer covering the surface of the layered oxide matrix. The double-layer coating layer includes an inner coating layer and an outer coating layer. The inner coating layer is composed of Na₂Ti₃O₇, and the outer coating layer is composed of TiO₂. The chemical formula of the layered oxide matrix is ​​Na₂Ti₃O₇. 0.916 Ni 0.2981 Fe 0.3131 Mn 0.3121 Ti 0.0567 Zn 0.01 F 0.01 B 0.01 O 1.99 The molar content of Ti in the double-layer coating is 0.33 mol%, and the mass content of Na2Ti3O7 in the double-layer coating is 51 wt%.

[0036] The difference from the preparation method in Example 1 is that step (2) in this example involves dry mixing of nickel-iron-manganese precursor, Na2CO3, TiO2, ZnO, PVDF, and H3BO3 in an elemental molar ratio of 1:1.07:0.06:0.01:0.01:0.01. The rest is the same as in Example 1.

[0037] Example 7 The double-layer coated sodium-ion battery layered oxide cathode material provided in this embodiment includes a layered oxide matrix and a double-layer coating layer covering the surface of the layered oxide matrix. The chemical formula of the layered oxide matrix is ​​Na. 0.912 Ni 0.287 Fe 0.2682 Mn 0.3619 Ti 0.0629 Zn 0.01 F 0.01 B 0.01 O 1.99 The double-layer coating consists of an inner coating and an outer coating. The inner coating is composed of Na2Ti3O7, and the outer coating is composed of TiO2. The molar content of Ti in the double-layer coating is 1.71 mol%, and the mass content of Na2Ti3O7 in the double-layer coating is 81 wt%.

[0038] The preparation method in this embodiment is as follows: (1) Prepare a 1.0 mol / L sulfate solution containing Ni, Fe, and Mn in a molar ratio of 0.32:0.29:0.39. Coprecipitate the solution with NaOH as a precipitant and ammonia as a complexing agent by heating and stirring. After the reaction is completed, separate the solid and liquid components, wash and dry to obtain Ni. 0.32 Fe 0.29 Mn 0.39 (OH)2 precursor; (2) Weigh Ni according to the elemental molar ratio (Ni+Fe+Mn):Na:Ti:Zn:F:B=1:1.06:0.08:0.01:0.01:0.01. 0.32 Fe 0.29 Mn 0.39 (OH)2 precursor, Na2CO3, TiO2, ZnO, PVDF, and H3BO3 were dry-mixed. After uniform mixing, the mixture was heated to 770℃ in compressed air at a heating rate of 3℃ / min and sintered at that temperature for 5 hours. Then, the temperature was increased to 960℃ at a heating rate of 3℃ / min and held for 13 hours. Subsequently, the temperature was cooled to 600℃ at a cooling rate of 0.7℃ / min and sintered for 6 hours. Finally, the temperature was reduced to room temperature at a cooling rate of 0.7℃ / min, crushed, and sieved to obtain the layered oxide cathode material for sodium-ion batteries.

[0039] Example 8 The double-layer coated sodium-ion battery layered oxide cathode material provided in this embodiment includes a layered oxide matrix and a double-layer coating layer covering the surface of the layered oxide matrix. The double-layer coating layer includes an inner coating layer and an outer coating layer. The inner coating layer is composed of Na₂Ti₃O₇, and the outer coating layer is composed of TiO₂. The chemical formula of the layered oxide matrix is ​​Na₂Ti₃O₇. 0.921 Ni 0.279 Fe 0.2987 Mn 0.303 Ti 0.0693 Co 0.03 F 0.005 P 0.02 O 1.995 The molar content of Ti in the double-layer coating is 1.07 mol%, and the mass content of Na2Ti3O7 in the double-layer coating is 66 wt%.

[0040] The difference from the preparation method in Example 1 is that step (2) in this example involves dry mixing of nickel-iron-manganese precursor, Na2CO3, TiO2, Co3(PO4)2, and NaF in an elemental molar ratio of 1:1.135:0.08:0.01:0.005. After uniform mixing, the mixture is heated to 830°C at a heating rate of 3°C / min in a compressed air atmosphere and sintered at that temperature for 6 hours. Then, the mixture is heated to 970°C at a heating rate of 2.5°C / min and held at that temperature for 13 hours. Subsequently, the mixture is cooled to 640°C at a cooling rate of 0.5°C / min and sintered for 8 hours. Finally, the mixture is cooled to room temperature at a cooling rate of 0.5°C / min, crushed, and sieved to obtain the sodium-ion battery layered oxide cathode material.

[0041] Example 9 The double-layer coated sodium-ion battery layered oxide cathode material provided in this embodiment includes a layered oxide matrix and a double-layer coating layer covering the surface of the layered oxide matrix. The double-layer coating layer includes an inner coating layer and an outer coating layer. The inner coating layer is composed of Na₂Ti₃O₇, and the outer coating layer is composed of TiO₂. The chemical formula of the layered oxide matrix is ​​Na₂Ti₃O₇. 0.917 Ni 0.3049 Fe 0.307 Mn 0.307 Ti 0.0611 Cu 0.01 N 0.01 B 0.01 O 1.99 The molar content of Ti in the double-layer coating is 0.89 mol% in the entire cathode material, and the mass content of Na2Ti3O7 in the entire double-layer coating is 72 wt%.

[0042] The difference from the preparation method in Example 1 is that step (2) in this example involves dry mixing of nickel-iron-manganese precursor, NaHCO3, TiO2, CuO, and BN in an elemental molar ratio of 1:1.11:0.07:0.01:0.01. After uniform mixing, the mixture is heated to 720°C at a heating rate of 4°C / min in a compressed air atmosphere and sintered at that temperature for 7 hours. Then, the mixture is heated to 910°C at a heating rate of 4°C / min and held for 12 hours. Subsequently, the mixture is cooled to 520°C at a cooling rate of 0.6°C / min and sintered for 5 hours. Finally, the mixture is cooled to room temperature at a cooling rate of 0.6°C / min, crushed, and sieved to obtain the sodium-ion battery layered oxide cathode material.

[0043] Comparative Example 1 The preparation method provided in this comparative example differs from that in Example 1 in that ZnO was not added in step (2), while the rest of the preparation method is the same as in Example 1. The chemical formula of the prepared layered oxide matrix is ​​Na. 0.920 Ni 0.3109 Fe 0.3108 Mn 0.31 1Ti 0.0573 F 0.01 B 0.01 O 1.99 The substrate surface is coated with Na2Ti3O7, and the molar content of Ti in the coating layer in the entire cathode material is 0.27 mol.

[0044] Comparative Example 2 The difference between the preparation method provided in this comparative example and Example 2 is that the nickel-iron-manganese precursor, NaHCO3, Ti(OH)2, MgO, NaCl, and Mg3(PO4)2 are dry-mixed in an elemental molar ratio of 1:1.035:0.10:0.015:0.005:0.005. The other preparation methods are the same as in Example 2. The resulting layered oxide matrix has the chemical formula Na. 0.889 Ni 0.2826 Fe 0.3108 Mn 0.2817 Ti 0.0849 Mg 0.03 Cl 0.005 P 0.01 O 1.995 The substrate surface is coated with TiO2, and the molar content of Ti in the coating layer in the entire cathode material is 1.51 mol.

[0045] Comparative Example 3 The double-layer coated sodium-ion battery layered oxide cathode material provided in this embodiment includes a layered oxide matrix and a coating layer coated on the surface of the layered oxide matrix. The coating layer is composed of Na₄Ti₅O. 12The chemical formula of the layered oxide matrix is ​​Na. 0.911 Ni 0.302 Fe 0.3121 Mn 0.311 Ti 0.0549 Zn 0.01 F 0.01 B 0.01 O 1.99 The molar content of Ti in the double-layer coating is 0.51 mol% in the entire cathode material.

[0046] The difference from the preparation method in Example 1 is that the sintering process in step (2) of this comparative example does not include a medium-temperature annealing step at 620°C, but the rest is the same as in Example 1.

[0047] The layered positive electrode materials for sodium-ion batteries prepared in all examples and comparative examples were stirred and dispersed with conductive carbon black (SP), polyvinylidene fluoride (PVDF) and solvent NMP in a mass ratio of 93:5:2, coated onto an aluminum foil substrate, and rolled to prepare the positive electrode; sodium metal sheet was used as the negative electrode; a 1 mol / L NaPF6 solution was used as the electrolyte, with a mixed solvent of EC and PC in a volume ratio of 1:1, and 1% FEC was added as an additive. The above components were assembled into CR2032 coin cells for battery testing. After two 0.1C constant current charge-discharge cycles within a voltage range of 2.0~4.2V at room temperature, rate performance testing was performed. The charging process was uniformly carried out at a 0.2C rate, and the discharge rates were successively set to 0.2C, 0.5C, 1C, and 2C. Then, 50 cycles were performed under 1C / 1C conditions. XRD was performed on the sodium-ion battery layered oxide cathode materials in all examples and comparative examples, and Topas was used for refinement. The refinement results and electrical performance results are shown in Table 1, where: AtomOcc(Na) is the sodium ion occupancy in the lattice.

[0048] Table 1. Material properties prepared in each embodiment and comparative example.

[0049] As can be seen from the examples and comparative data in Table 1, the examples introduced elements M1, M2, and M3, as well as excess sodium carbonate, and used high-temperature sintering and medium-temperature annealing to achieve synergistic modification of the bulk phase and interface, such as... Figure 1 As shown, the special double-layer coating structure formed can take into account excellent capacity, rate capability, DCR growth and high voltage cycling performance, especially the DCR growth performance is significantly reduced.

[0050] Combination Figure 5 The XPS spectra show that the distribution of Ti changed due to other doping elements. Compared to Comparative Example 1, the distribution of Ni in Example 1... 2+The area ratio decreases, and the binding energy of Ni2p shifts to lower energies, corresponding to the charge compensation mechanism. In a highly alkaline sintering environment, some Ti will be converted into Ti... 3+ The form exists within the bulk phase, and the introduction of M1 simultaneously inhibits Ti. 3+ The formation of this layer drives the reaction of Ti with O to generate more stable TiO2 coating on the surface. Compared to the residual sodium free on the material surface, Na2Ti3O7 has good crystallinity and uniform distribution. However, the lattice environment of the double-layer coating is more complex than that of the single-layer coating. Moreover, the Na-O binding energy in the formed Na2Ti3O7 coating is weaker than that in the matrix material lattice. Therefore, on the one hand, the proportion of Na-O signal intensity increases, and on the other hand, the peak position shifts towards the direction of lower binding energy. Macroscopically, the residual alkali decreases, such as... Figure 5 As shown in (d) and 5(e).

[0051] Combination Figure 2 FE-SEM image, Figure 3 As can be seen from the particle size distribution curve, the addition of the pre-calcination platform in Example 1 facilitates the melting of the sodium source and the diffusion of M1, increases the volume, and makes the particles more uniform than in Example 3, while suppressing interfacial side reactions caused by preferred orientation growth. Figure 4 The mapping diagram shows that Zn and Ti are uniformly distributed inside the particles.

[0052] Examples 1, 5, and 6 illustrate that controlling the proportion of Na2Ti3O7 in the coating layer can balance high-voltage cycling performance and rate performance, and reduce DCR growth.

[0053] Examples 1 and 7 illustrate that proper control of c+d+xa is more conducive to balancing capacity and other electrical properties.

[0054] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A double-layer coated layered oxide cathode material for sodium-ion batteries, characterized in that: The sodium-ion battery layered oxide cathode material includes a layered oxide matrix and a double-layer coating layer covering the surface of the layered oxide matrix. The layered oxide matrix is ​​doped with Ti. The double-layer coating layer includes an inner coating layer and an outer coating layer. The inner coating layer is composed of Na2Ti3O7, and the outer coating layer is composed of TiO2.

2. The sodium-ion battery layered oxide cathode material according to claim 1, characterized in that: The molar content of Ti element in the double-layer coating is x, which is taken from 0.3% to 3% of the total molar number of the sodium-ion battery layered oxide cathode material.

3. The sodium-ion battery layered oxide cathode material according to claim 1, characterized in that: The inner coating layer contains 60% to 85% of the total mass of the double-layer coating layer.

4. The sodium-ion battery layered oxide cathode material according to any one of claims 1-3, characterized in that: The layered oxide matrix is ​​composed of Na. α Ni a Fe b Mn c Ti d M1 e M2 f M3 g O 2-f Wherein, M1 is selected from one or more of Mg, Zn, Co, Cu, or Li; M2 is selected from one or more of Cl, F, or N; M3 is selected from one or more of B, P, Si, or S; 0.85≤α≤1.00, a≥0, b≥0, c≥0, 0 <d<0.1,0.01≤e≤0.05,0.001≤f≤0.02,0.001≤g≤0.02,a+b+c+d+e+g=1。 5. The sodium-ion battery layered oxide cathode material according to claim 4, characterized in that: c+d+xa≤0.

12.

6. The sodium-ion battery layered oxide cathode material according to claim 1, characterized in that: The full width at half maximum (FWHM) ratios of peaks 104 and 101 in the XRD pattern satisfy the following: 1.060 ≤ FWHM(104) / FWHM(101) ≤ 1.

090.

7. The method for preparing the layered oxide cathode material for sodium-ion batteries according to any one of claims 1-6, characterized in that: Includes the following steps: A sodium-ion battery layered oxide precursor, a sodium source, a Ti source, compound M1, compound M2, and compound M3 are mixed uniformly. The sodium source is added in excess relative to the sodium-ion battery layered oxide precursor. The diffusion barrier of the dopant element M1 in compound M1 is lower than that of Ti. 4+ The dopant element M2 in the M2 compound is doped at oxygen sites, and the ionic bonding strength between M2 and the transition metal is greater than that between O and the transition metal. The dopant element M3 in the M3 compound is a non-metallic element doped at tetrahedral interstitial sites, and can form covalent bonds with oxygen ions to enhance the electronegativity of oxygen ions. The above-mentioned homogenized mixture is heated to 900℃~1000℃ and sintered for 10~15h, and then cooled to 500℃~650℃ and sintered for 3~6h to obtain the sodium-ion battery layered oxide cathode material.

8. The method for preparing the layered oxide cathode material for sodium-ion batteries according to claim 7, characterized in that: The excessive addition of sodium source relative to sodium-ion battery layered oxide precursor means that the molar ratio of sodium element in sodium source to sodium element in sodium-ion battery layered oxide precursor is 1:1.05~1.15; the sodium source is one or more of sodium bicarbonate, sodium hydroxide or sodium carbonate; the Ti source is one or more of titanium dioxide, titanium hydroxide, titanium chloride or lithium titanate.

9. The method for preparing the layered oxide cathode material for sodium-ion batteries according to claim 7, characterized in that: M1 is selected from one or more of Mg, Zn, Co, Cu or Li; M2 is selected from one or more of Cl, F or N; and M3 is selected from one or more of B, P, Si or S.

10. The method for preparing the layered oxide cathode material for sodium-ion batteries according to claim 9, characterized in that: The M1 compound is one or more of ZnO, MgO, MgCO3, Mg3(PO4)2, CuO, Cu(OH)2, Co3(PO4)2, Co(OH)2, Li2CO3, or LiOH; the M2 compound is one or more of NaCl, PVDF, NaF, or BN; and the M3 compound is one or more of H3BO3, BN, Na3PO4, Mg3(PO4)2, Co3(PO4)2, Na2SiO3, or Na2S.

11. The method for preparing the layered oxide cathode material for sodium-ion batteries according to claim 7, characterized in that: Before the two-stage sintering, a pre-sintering treatment is performed, and the pre-sintering treatment conditions are sintering at 700℃~850℃ for 3~8h.

12. A sodium-ion battery, characterized in that, This includes the sodium-ion battery layered oxide cathode material as described in any one of claims 1-6, or the sodium-ion battery layered oxide cathode material prepared by the preparation method described in any one of claims 7-11.

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