A single-crystal-like sodium ion layered positive electrode material, a preparation method and application thereof

Abstract

This invention discloses a bulk-phase, single-crystal sodium-ion layered cathode material containing a secondary phase and its preparation method, belonging to the technical field of sodium-ion battery cathode materials. The main phase of the material is an O3-type layered structure, with at least one nanoscale secondary phase selected from nickel oxide, zinc oxide, copper oxide, or magnesium oxide embedded inside, and no obvious random grain boundaries within the particles. First, the precursor is mixed with a sodium source and initially sintered to induce the local segregation of transition metal elements or dopants on the particle surface and the precipitation of oxide particles. Subsequently, sodium is added for secondary sintering, driving a topological transformation of the main phase and embedding oxide nanoparticles in situ, constructing an embedded heterostructure. This invention utilizes the lattice mismatch generated at the heterostructure interface to guide the generation of controlled transgranular microcracks, which act as electrolyte penetration channels and shorten the ion solid-phase diffusion path. While maintaining the mechanical stability of the single crystal, it solves the bottleneck of limited rate performance of large-size single crystals, improving the energy density, rate characteristics, and long-cycle stability of the material.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery cathode material technology, and specifically relates to a bulk single-crystal layered oxide cathode material containing secondary phase embedding, its preparation method and application. Background Technology

[0002] With the advancement of global carbon neutrality goals, sodium-ion batteries, due to their abundant sodium resources, low cost, and excellent adaptability to high and low temperatures, have shown great commercial potential in large-scale grid-side energy storage and low-speed electrification. Among the three major cathode systems—layered, polyanionic, and Prussian blue—O3-type layered transition metal oxides (Na₂O₃) are the most promising. x TMO2 is considered the most promising technology for industrialization due to its high energy density, good conductivity, high compaction density, and high compatibility with existing lithium-ion battery production processes.

[0003] However, polycrystalline layered oxides widely used in industry currently suffer from severe chemical-mechanical failure mechanisms. Polycrystalline secondary spheres are composed of hundreds or thousands of anisotropically arranged primary particles stacked together. During frequent sodium ion insertion / extraction processes, due to the intense volume breathing effect along the c-axis of the crystal lattice, the uneven stress among the primary particles leads to the accumulation of huge phase transition strain energy at random grain boundaries. With further cycling, this microscopic internal stress eventually evolves into macroscopic intergranular cracking, causing the secondary particles to pulverize and disintegrate. The disintegration of particles not only cuts off electron / ion transport paths, generating a large amount of electrochemical "dead mass," but also exposes highly active fresh surfaces, exacerbating electrolyte oxidation and decomposition, gas generation, and transition metal dissolution, severely limiting the cycle stability and thermal safety of the battery.

[0004] To address the defects of polycrystalline materials, single-crystallization strategies have attracted attention due to their ability to completely eliminate internal grain boundaries and improve mechanical integrity. However, conventional large-size single crystals (typically with a particle size of 3-10 μm or larger) suffer from severe kinetic hysteresis. According to Fick's diffusion law, the diffusion time of ions is proportional to the square of the diffusion distance. The long-range solid-state diffusion path of single crystals leads to a sharp increase in polarization at high current densities, resulting in severe capacity decay. In existing technologies, simply mitigating this through nano-sized single crystals or surface coating often results in severe gas generation due to increased specific surface area, or problems such as sodium source volatilization and excessive residual alkali during synthesis.

[0005] Therefore, how to break through the rate limitation of ion transport inside large particles from the perspective of internal structure design while maintaining the mechanical stability of single crystals is a core issue that is extremely challenging in this field. Summary of the Invention

[0006] This invention addresses the technical problem of limited ion transport rate within existing single-crystal materials by proposing a single-crystal-like sodium ion layered cathode material, its preparation method, and its applications.

[0007] To achieve the above objectives, the technical solution of the present invention is implemented as follows:

[0008] This invention provides a quasi-single-crystal sodium ion layered cathode material, wherein the quasi-single-crystal sodium ion layered cathode material is a composite quasi-single-crystal particle with an embedded nano-heterogeneous structure, an average particle size D50 of 1-20 μm, and no obvious random grain boundaries inside the particle; the composite quasi-single-crystal particle is composed of a layered main phase and an oxide secondary phase embedded inside the main phase.

[0009] The main phase is an O3-type layered transition metal oxide with space group R-3m and the general chemical formula Na. x Ni a Fe b Mn c M d O2, where 0.66≤x≤1.0, 0.1≤a≤0.5, 0.1≤b≤0.5, 0.1≤c≤0.5, 0≤d≤0.2, a+b+c+d=1, and M is selected from at least one of Li, Mg, Ca, Zn, Cu, Al, Ti, Zr, V, Nb, Ta, and Cr; x, a, b, c, and d are the molar numbers of the corresponding ions, and the components in the general chemical formula satisfy charge conservation and stoichiometry conservation. By introducing multi-component doping, the local chemical potential of the transition metal layer can be adjusted by utilizing the radius differences and charge shielding effects of different cations, thereby precisely controlling the precipitation rate and spatial distribution density of secondary phases (such as NiO) in the sodium-deficient state.

[0010] The secondary phase is selected from at least one of nickel oxide, zinc oxide, copper oxide and magnesium oxide, and is embedded inside the main phase particles. The secondary phase nanoparticles are dispersed or arranged along specific crystal planes inside the main phase single crystal.

[0011] The secondary phase appears as short rectangular nanorods or near-spherical nanoparticles within the quasi-single crystal phase; a coherent or semi-coherent heterostructure interface is formed between the secondary phase and the layered main phase.

[0012] The aforementioned single-crystal sodium ion layered cathode material consists of well-developed regular polyhedral single-crystal particles. Through a two-step sintering method, the random orientation of primary grain boundaries within the particles is eliminated.

[0013] The aforementioned monocrystalline sodium ion layered cathode material exhibits stress-adaptive characteristics during the charge-discharge cycle of sodium ion insertion and extraction: controlled transgranular microcracks are generated by utilizing the lattice mismatch between the internal secondary phase and the main phase during the first charge. These microcracks develop along the heterogeneous phase boundary and serve as electrolyte permeation channels.

[0014] This invention provides a method for preparing the aforementioned single-crystal sodium-ion layered cathode material, employing a two-stage sintering process based on a "precipitation-embedding" mechanism, including the following steps:

[0015] (1) The transition metal precursor, the first sodium source and the flux are mixed and ground evenly to obtain the first precursor;

[0016] (2) The first precursor was sintered for the first time, cooled down and then ground to obtain a single crystal intermediate;

[0017] (3) Add a second part of sodium source to the single crystal intermediate and grind and mix evenly. Then, perform a second sintering in an air or oxygen atmosphere. Use the liquid phase environment provided by the flux to drive grain growth and embed the oxide particles on the surface in situ. After cooling to room temperature, grind to obtain the bulk phase containing the embedded secondary phase of the quasi-single crystal cathode material. After cooling, crush and sieve to obtain the quasi-single crystal sodium ion layered cathode material.

[0018] In step (1), the transition metal precursor is the transition metal hydroxide precursor Ni. a Fe b Mn c M d (OH)2 or carbonate precursor Ni a Fe b Mn c M d CO3, wherein the first portion of sodium source is selected from at least one of sodium carbonate, sodium nitrate, sodium hydroxide and sodium oxalate; the amount of the first portion of sodium source and transition metal precursor added is 0.4-0.9 based on the molar ratio of sodium to total transition metal, expressed as Na / TM value, which is in a substoichiometric state, and is intended to induce local segregation of transition metal elements or dopants on the particle surface and precipitate oxide particles; the Na / TM value is preferably 0.5-0.7.

[0019] The flux is selected from at least one of sodium nitrate, sodium borohydride, sodium borate, boron oxide, borohydride, borane, boric acid and borax; the amount of flux added is 0.1%-3% of the total mass of the precursor, preferably 1%.

[0020] The temperature of the first sintering in step (2) is 600℃-950℃, preferably 800-900℃, and the holding time is 5-20 h, preferably 15-18 h; the single crystal intermediate is a single crystal intermediate containing surface precipitated oxide particles and a P2 phase or a mixed P2 and O3 phase as the main phase, and the surface precipitated oxide particles are selected from at least one of nickel oxide, zinc oxide, copper oxide and magnesium oxide.

[0021] During sintering at 600℃-950℃, the liquid phase environment induces the initial growth of grains. Due to the large number of vacancies in the sodium layer, the lattice cannot accommodate excessive transition metal elements or dopants (such as Ni and Zn), forcing them to migrate to the particle surface and precipitate in the form of oxides, serving as secondary phase particles that are subsequently embedded in single crystals.

[0022] In step (3), the amount of sodium source added in the second part is to supplement the total sodium amount until the Na / TM molar ratio is 0.66-1.0. The preferred dispersion medium for grinding is anhydrous ethanol. The temperature of the second sintering is 800-950℃, preferably 800-900℃, and the holding time is 10-20 h, preferably 10-18 h. During the second sintering process, the added sodium ions drive the main phase to undergo a topological transformation from P2 to O3 phase, and the single crystal particles further grow in the flux-induced liquid phase environment and in situ embed the oxide particles on the surface.

[0023] Adding a second sodium source brings the total Na / TM ratio to 0.66-1.0, followed by calcination at 800℃-950℃. Within this thermodynamic window, sodium ions are re-intercalated and drive the main phase to undergo a topological transformation from P2 to O3. Single crystal particles, propelled by the melt, surround the oxide particles on the surface, deeply embedding them into the single crystal phase.

[0024] The aforementioned preparation process, through precise control of the flux component ratio, ensures efficient liquid-phase mass transfer while reducing the salt / material ratio in the system to an extremely low level. This allows the final product to maintain a pure bulk structure and interfacial state without requiring solvent washing for impurity removal. This wash-free dry synthesis strategy effectively avoids the high sensitivity of sodium-ion battery cathode materials to polar solvents, eliminating the risk of proton exchange reactions and surface lattice collapse that may be induced by the washing process at its physical source. It also fully preserves the initial morphology and intrinsic electrochemical activity established by the single crystal particles during the high-temperature crystallization stage. Furthermore, the highly simplified process is highly conducive to continuous industrial production, and the material's excellent air stability significantly improves its storage life in complex environments.

[0025] The "embedded nano-heterogeneous single crystal" structure obtained by the preparation method described in this invention has a significantly different physical mechanism compared with conventional modified materials:

[0026] (1) Stress Adaptation and “Functional Crack” Model: There is a significant mismatch in lattice parameters and elastic modulus between the internally embedded secondary phase and the layered main phase. During the severe volume shrinkage / expansion caused by charge-discharge cycles, a local stress gradient will be generated at the heterogeneous interface. This stress field will induce the main phase to generate controlled transgranular microcracks that do not lead to pulverization.

[0027] (2) Kinetic segmentation effect: The generated microcracks do not reduce electronic conductivity, but instead serve as efficient permeation channels for liquid electrolytes, effectively dividing the 10 μm-scale large single crystal into several submicron-scale kinetic active units, significantly shortening the solid-phase transport path of sodium ions, and realizing the high capacity of the large single crystal at high magnification.

[0028] (3) Interface passivation and shielding effect: The embedded phase replaces the highly active Ni at the cross-section. 3+ / Ni 4+ Plasma directly contacts the electrolyte, acting as a physical shielding layer, which significantly reduces side reaction gas production (CO2, O2) and transition metal dissolution under high voltage.

[0029] This invention provides the application of the aforementioned monocrystalline sodium-ion layered cathode material in sodium-ion batteries.

[0030] The present invention also provides a sodium-ion battery comprising a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode comprises the aforementioned monocrystalline sodium-ion layered positive electrode material.

[0031] The beneficial effects of this invention are:

[0032] 1. The quasi-monocrystalline sodium-ion layered cathode material prepared by this invention adopts a monocrystalline framework structure, which has extremely high compaction density and mechanical strength, effectively suppressing intergranular fragmentation during cycling. Simultaneously, by utilizing the lattice mismatch between the embedded secondary phase and the layered main phase, controlled transgranular microcracks are guided to form during charge and discharge. These microcracks serve as functional electrolyte permeation channels, significantly shortening the diffusion distance of sodium ions within the solid phase. Furthermore, the monocrystalline structure reduces the specific surface area of ​​the material, and the embedded phase generates an interface passivation shielding effect at the crystal cross-section, significantly reducing electrolyte side reactions and gas generation behavior under high voltage. This significantly improves the rate performance and cycle stability of sodium-ion batteries.

[0033] 2. This invention breaks through the limitations of traditional pursuit of "absolutely pure phase" single crystals, innovatively proposing the active introduction of a specific proportion of secondary phases within the single crystal. By utilizing heterogeneous interfaces to guide the generation of "adaptive" transgranular microcracks, the originally destructive volumetric strain is transformed into a beneficial factor for improving kinetics. It successfully solves the problem of limited rate performance in large-size single crystals, utilizing endogenous heterogeneous phases to guide the generation of "functional microcracks," significantly improving ion penetration while maintaining morphological integrity. It also enhances stability under high voltage and high temperature. The internally embedded structure avoids particle fragmentation through stress guidance, resulting in a 10%-25% improvement in capacity retention after 500 cycles compared to conventional single crystals. Attached Figure Description

[0034] 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 only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 The images show the X-ray diffraction (XRD) patterns of the sodium ion cathode materials prepared in Examples 1-4 and the comparative examples.

[0036] Figure 2 The NaNi444-type single-crystal sodium-ion layered cathode material containing a NiO secondary phase in bulk phase in Example 1 is described. 0.4 Fe 0.2 Mn 0.4 Cross-sectional scanning electron microscope (SEM) image of O2.

[0037] Figure 3 The NaNi quasi-single-crystal sodium-ion layered cathode material in Example 5, which contains a CuO secondary phase in the bulk phase. 0.35 Cu 0.05 Fe 0.2 Mn 0.3 Ti 0.1 Cross-sectional SEM image of O2.

[0038] Figure 4 The NaNi quasi-single-crystal layered cathode material containing a secondary MgO phase in the bulk phase in Example 6 is an example of this. 0.35 Mg 0.05 Fe 0.2 Mn 0.4 Cross-sectional SEM image of O2.

[0039] Figure 5 NaNi, a single-crystal layered cathode material with no secondary phase intercalation in the comparative example, is used. 0.4 Fe 0.2 Mn0.4 Cross-sectional SEM image of O2. Detailed Implementation

[0040] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] Example 1

[0042] A method for preparing a single-crystal sodium-ion layered cathode material, comprising a co-precipitation-high-temperature solid-state method to prepare a single-crystal sodium-ion layered cathode material NaNi containing a NiO secondary phase in the bulk phase. 0.4 Fe 0.2 Mn 0.4 O2, the specific steps are as follows:

[0043] (1) 4g of transition metal precursor Ni 0.4 Fe 0.2 Mn 0.4 (OH)2, 1.8754 g of NaNO3 (50% molar fraction) and 0.0588 g of B2O3 flux were placed in an agate mortar and wet-milled for 1 h with 40 mL of anhydrous ethanol to ensure uniform distribution of trace salts, thus obtaining the first precursor.

[0044] (2) The first precursor powder was placed in a muffle furnace and treated at 850°C for 15 hours. The resulting black powder was then ground to obtain a single crystal intermediate.

[0045] (3) Take 4g of the single-crystal intermediate obtained in step (2), add 0.8490g of NaNO3 (50% molar fraction) to it, grind and mix it evenly to obtain secondary precursor powder; place the secondary precursor powder in a muffle furnace and sinter it at 950℃ for 15h. After natural cooling, black powder is obtained, which is the single-crystal sodium ion layered cathode material NaNi containing NiO secondary phase in the bulk phase of this invention. 0.4 Fe 0.2 Mn 0.4 O2.

[0046] The NaNi-like single-crystal sodium ion layered cathode material prepared in this embodiment 0.4 Fe 0.2 Mn 0.4 The XRD pattern of O2 is as follows: Figure 1 As shown in the image, the main phase is O3, with a small amount of NiO secondary phase; cross-sectional SEM image ( Figure 2 The particle interior is shown to be a NiO secondary phase.

[0047] Example 2

[0048] A method for preparing a single-crystal sodium-ion layered cathode material, comprising a co-precipitation-high-temperature solid-state method to prepare a single-crystal sodium-ion layered cathode material NaNi containing a ZnO secondary phase in the bulk phase. 0.3 Zn 0.1 Fe 0.2 Mn 0.4 O2, the specific steps are as follows:

[0049] (1) 4g of transition metal precursor Ni 0.3 Zn 0.1 Fe 0.2 Mn 0.4 The first precursor was obtained by mixing (OH)2, 1.3931 g of NaNO3 (60% mole fraction) and 0.0539 g of B2O3 flux evenly.

[0050] (2) The first precursor powder was placed in a muffle furnace and treated at 800°C for 15 hours. The resulting black powder was ground to obtain a single crystal intermediate.

[0051] (3) Take 4g of the single-crystal intermediate obtained in step (2), add 0.8226g of NaNO3 (40% molar fraction) to it, grind and mix it evenly to obtain secondary precursor powder; place the secondary precursor powder in a muffle furnace and sinter at 800℃ for 10h. After cooling to room temperature, the bulk ZnO secondary phase-containing quasi-single-crystal sodium ion layered cathode material NaNi is obtained. 0.3 Zn 0.1 Fe 0.2 Mn 0.4 O2.

[0052] The NaNi-like single-crystal sodium ion layered cathode material prepared in this embodiment 0.3 Zn 0.1 Fe 0.2 Mn 0.4 The XRD pattern of O2 is as follows: Figure 1 As shown, the main phase is O3 phase, and it contains a small amount of ZnO secondary phase.

[0053] Example 3

[0054] A method for preparing a single-crystal sodium-ion layered cathode material involves using a co-precipitation-high-temperature solid-state method to prepare a single-crystal sodium-ion layered cathode material NaNi containing a CuO secondary phase in the bulk phase. 0.3 Cu 0.1 Fe 0.2 Mn 0.4 O2, the specific steps are as follows:

[0055] (1) 4g of transition metal precursor Ni 0.3 Cu 0.1 Fe 0.2 Mn 0.4 CO3, 0.9050g of NaNO3 (50% mole fraction) and 0.0491g of H3BO3 flux were mixed evenly to obtain the first precursor.

[0056] (2) The first precursor powder was placed in a muffle furnace and kept at 450°C for 5 hours, and then calcined at 900°C for 15 hours. The resulting black powder was ground to obtain a single crystal intermediate.

[0057] (3) Take 4g of the single-crystal intermediate obtained in step (2), add 1.0536g of NaNO3 (50% molar fraction) to it, and grind and mix it evenly in an agate mortar to obtain secondary precursor powder; place the secondary precursor powder in a muffle furnace and sinter it at 900℃ for 15h. After cooling to room temperature, the bulk single-crystal sodium ion layered cathode material NaNi containing CuO secondary phase is obtained. 0.3 Cu 0.1 Fe 0.2 Mn 0.4 O2.

[0058] The NaNi-like single-crystal sodium ion layered cathode material prepared in this embodiment 0.3 Cu 0.1 Fe 0.2 Mn 0.4 The XRD pattern of O2 is as follows: Figure 1 As shown, the main phase is O3 phase, and it contains a small amount of CuO secondary phase.

[0059] Example 4

[0060] A method for preparing a single-crystal sodium-ion layered cathode material, comprising a co-precipitation-high-temperature solid-state method to prepare a single-crystal sodium-ion layered cathode material NaNi containing a secondary MgO phase in the bulk phase. 0.3 Mg 0.1 Fe 0.2 Mn 0.4 O2, the specific steps are as follows:

[0061] (1) 4g of transition metal precursor Ni 0.3 Mg 0.1 Fe 0.2 Mn 0.4 CO3, 1.1237 g of NaNO3 (60% mole fraction) and 0.0512 g of NaBH4 flux were mixed evenly to obtain the first precursor.

[0062] (2) The first precursor powder was placed in a muffle furnace and kept at 450°C for 2 hours, and then calcined at 900°C for 18 hours. The resulting black powder was ground to obtain a single crystal intermediate.

[0063] (3) Take 4g of the single-crystal intermediate obtained in step (2), add 0.8567g of NaNO3 (40% molar fraction) to it, and grind and mix it evenly in an agate mortar to obtain secondary precursor powder; place the secondary precursor powder in a muffle furnace and sinter it at 800℃ for 18h. After cooling to room temperature, the bulk MgO secondary phase-containing quasi-single-crystal sodium ion layered cathode material NaNi is obtained. 0.3 Mg 0.1 Fe 0.2 Mn 0.4 O2.

[0064] The NaNi-like single-crystal sodium ion layered cathode material prepared in this embodiment 0.3 Mg 0.1 Fe 0.2 Mn 0.4 The XRD pattern of O2 is as follows: Figure 1 As shown, the main phase is O3 phase, and it contains a small amount of MgO secondary phase.

[0065] Example 5

[0066] A method for preparing a single-crystal sodium-ion layered cathode material involves using a co-precipitation-high-temperature solid-state method to prepare a single-crystal sodium-ion layered cathode material NaNi containing a CuO secondary phase in the bulk phase. 0.35 Cu 0.05 Fe 0.2 Mn 0.3 Ti 0.1 O2, the specific steps are as follows:

[0067] (1) 4g of transition metal precursor Ni 0.35 Cu 0.05 Fe 0.2 Mn 0.3 Ti 0.1 (OH)2, 2.0320 g of NaNO3 (70% mole fraction) and 0.0603 g of NaBH4 flux were mixed evenly to obtain the first precursor;

[0068] (2) The first precursor powder was placed in a muffle furnace and kept at 400°C for 6 hours, and then calcined at 850°C for 15 hours. The resulting black powder was ground to obtain a single crystal intermediate.

[0069] (3) Take 4g of the single-crystal intermediate obtained in step (2), add 0.9695g of NaNO3 (30% molar fraction) to it, and grind and mix it evenly in an agate mortar to obtain secondary precursor powder; place the secondary precursor powder in a muffle furnace and sinter it in air at 850℃ for 15h. After cooling to room temperature, the bulk CuO secondary phase-containing quasi-single-crystal sodium ion layered cathode material NaNi is obtained. 0.35 Cu 0.05 Fe 0.2 Mn 0.3 Ti 0.1 O2.

[0070] The NaNi-like single-crystal sodium ion layered cathode material prepared in this embodiment 0.35 Cu 0.05 Fe 0.2 Mn 0.3 Ti 0.1 SEM image of O2 cross section as follows Figure 3 As shown, the particle size is approximately 8 μm, and the particles exhibit a single-crystal-like morphology with distinct edges and corners. The interior of the particles consists of a CuO secondary phase.

[0071] Example 6

[0072] A method for preparing a single-crystal sodium-ion layered cathode material, comprising a co-precipitation-high-temperature solid-state method to prepare a single-crystal sodium-ion layered cathode material NaNi containing a secondary MgO phase in the bulk phase. 0.35 Mg 0.05 Fe 0.2 Mn 0.4 O2, the specific steps are as follows:

[0073] (1) 4g of transition metal precursor Ni 0.35 Mg 0.05 Fe 0.2 Mn 0.4 CO3, 0.5476 g of Na2O (50% mole fraction) and 0.0455 g of H3BO3 flux were mixed evenly to obtain the first precursor.

[0074] (2) The first precursor powder was placed in a muffle furnace and kept at 400°C for 6 hours, and then calcined at 850°C for 15 hours. The resulting black powder was ground to obtain a single crystal intermediate.

[0075] (3) Take 2g of the single crystal intermediate obtained in step (2), add 0.3205g of Na2O (50% molar fraction) to it, and grind it finely and mix it evenly in an agate mortar to obtain secondary precursor powder; place the secondary precursor powder in a muffle furnace and sinter it in air at 850℃ for 15h. After cooling to room temperature, the bulk sodium ion layered cathode material NaNi containing MgO secondary phase is obtained.0.35 Mg 0.05 Fe 0.2 Mn 0.4 O2.

[0076] The NaNi-like single-crystal sodium ion layered cathode material prepared in this embodiment 0.35 Mg 0.05 Fe 0.2 Mn 0.4 SEM image of O2 cross section as follows Figure 4 As shown, the particle size is approximately 6 μm, and the particles exhibit a single-crystal-like morphology with distinct edges and corners. The interior of the particles consists of the MgO secondary phase.

[0077] Example 7

[0078] A method for preparing a single-crystal sodium-ion layered cathode material, comprising a co-precipitation-high-temperature solid-state method to prepare a single-crystal sodium-ion layered cathode material NaNi containing a ZnO secondary phase in the bulk phase. 0.30 Zn 0.10 Fe 0.2 Mn 0.4 O2, the specific steps are as follows:

[0079] (1) 4g of transition metal precursor Ni 0.30 Zn 0.10 Fe 0.2 Mn 0.4 CO3, 0.9036 g of Na2CO3 (50% mole fraction) and 0.0490 g of H3BO3 flux were mixed evenly to obtain the first precursor;

[0080] (2) The first precursor powder was placed in a muffle furnace and kept at 450°C for 6 hours, and then calcined at 900°C for 15 hours. The resulting black powder was ground to obtain a single crystal intermediate.

[0081] (3) Take 2g of the single-crystal intermediate obtained in step (2), add 0.5258g of Na2CO3 (50% molar fraction) to it, and grind and mix it evenly in an agate mortar to obtain secondary precursor powder; place the secondary precursor powder in a muffle furnace and sinter it in air at 900℃ for 15h. After cooling to room temperature, the bulk ZnO secondary phase-containing quasi-single-crystal sodium ion layered cathode material NaNi is obtained. 0.30 Zn 0.10 Fe 0.2 Mn 0.4 O2.

[0082] Comparative Example 1

[0083] A method for preparing sodium-ion layered cathode material, comprising a co-precipitation-high-temperature solid-state method to prepare a single-crystal sodium-ion layered cathode material NaNi without secondary phase intercalation.0.4 Fe 0.2 Mn 0.4 O2, the specific steps are as follows:

[0084] (1) 4g of transition metal precursor Ni 0.4 Fe 0.2 Mn 0.4 CO3, 1.8180g of Na2CO3 and 0.0424g of H3BO3 flux were mixed evenly to obtain the first precursor.

[0085] (2) The precursor powder was placed in a muffle furnace and kept at 450°C for 5 hours, and then calcined at 850°C for 15 hours. After cooling to room temperature, the single-crystal sodium ion layered cathode material NaNi was obtained. 0.4 Fe 0.2 Mn 0.4 O2.

[0086] The single-crystal sodium-ion layered cathode material NaNi prepared in this embodiment 0.4 Fe 0.2 Mn 0.4 The XRD pattern of O2 is as follows: Figure 1 The image shown is of the pure O3 phase. Its cross-sectional SEM image (…) Figure 5 The results show that the particles have no secondary phases or obvious grain boundaries, indicating that they are single-crystal particles.

[0087] Example of implementation effect 1

[0088] Sodium-ion batteries were assembled using the monocrystalline sodium-ion layered cathode materials prepared in Examples 1-7 and the monocrystalline sodium-ion layered cathode material prepared in Comparative Example 1, and their performance was tested, as follows:

[0089] Half-cell assembly: 0.32g of the bulk-phase sodium-ion layered cathode material containing secondary phase embedding from Examples 1-7 and the material prepared in Comparative Example 1 were respectively mixed with 0.04g of conductive carbon black (Super P) and 0.04g of vinylidene fluoride (PVDF) at a mass ratio of 80:10:10 in an N-methylpyrrolidone (NMP) solution to form a slurry, which was then coated onto aluminum foil. After vacuum drying, the slurry was cut into electrode sheets with a diameter of 12 mm (loading of 5-10 mg / cm²). 2 Using a sodium metal sheet as the negative electrode, a 1 mol / L NaPF6 / polycarbonate (PC): ethylene carbonate (EC): dimethyl carbonate (DMC) solution (volume ratio 1:1:1) as the electrolyte, and a glass fiber diaphragm, a CR2032 coin cell half-cell was assembled in an argon glove box.

[0090] Charge and discharge test: The voltage range for charging and discharging the coin cell half-cell is 2.0-4.0 V. Before the cycle test, it was activated twice with a low current density of 15 mA / g (0.1C), and then cycled at 1C rate within the same voltage range. All electrochemical performance tests were performed at room temperature.

[0091] The electrochemical performance test results are shown in Table 1:

[0092] Table 1. Battery performance test results based on the materials in the examples and comparative examples.

[0093]

[0094] The comparison of test data in the table above shows that, in the battery specific capacity tests at 0.1C, 1C, and 5C full rate conditions, and the long-cycle capacity retention test results at 1C for 500 cycles, the sodium-ion monocrystalline nickel-iron-manganese base oxide cathode materials prepared in Examples 1-7 exhibit significantly improved specific capacity at low current, rate performance at high rate, and long-cycle stability. Compared to the sodium-ion monocrystalline nickel-iron-manganese base oxide cathode material in Comparative Example 1, the modified material prepared in this invention has significant advantages in electrochemical performance and can meet the application requirements of sodium-ion batteries for high capacity, high rate, and long lifespan.

[0095] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A type of single-crystal sodium-ion layered cathode material, characterized in that: The quasi-single-crystal sodium ion layered cathode material is a composite quasi-single-crystal particle with an embedded nano-heterogeneous structure and an average particle size D50 of 1-20 μm; the composite quasi-single-crystal particle is composed of a layered main phase and an oxide secondary phase embedded inside the main phase.

2. The quasi-single-crystal sodium-ion layered cathode material according to claim 1, characterized in that: The main phase is an O3-type layered transition metal oxide with the general chemical formula Na. x Ni a Fe b Mn c M d O2, where 0.66≤x≤1.0, 0.1≤a≤0.5, 0.1≤b≤0.5, 0.1≤c≤0.5, 0≤d≤0.2, a+b+c+d=1, and M is selected from at least one of Li, Mg, Ca, Zn, Cu, Al, Ti, Zr, V, Nb, Ta and Cr; The secondary phase is selected from at least one of nickel oxide, zinc oxide, copper oxide, and magnesium oxide.

3. The quasi-single-crystal sodium-ion layered cathode material according to claim 2, characterized in that: The secondary phase appears as short rectangular nanorods or near-spherical nanoparticles within the quasi-single crystal phase; a coherent or semi-coherent heterostructure interface is formed between the secondary phase and the layered main phase.

4. The method for preparing the quasi-single-crystal sodium-ion layered cathode material according to any one of claims 1-3, characterized in that, Includes the following steps: (1) The transition metal precursor, the first sodium source and the flux are mixed and ground evenly to obtain the first precursor; (2) The first precursor was sintered for the first time, cooled down and then ground to obtain a single crystal intermediate; (3) Add a second part of sodium source to the single crystal intermediate and mix it evenly. Then perform a second sintering. After cooling, crush and sieve to obtain a single crystal sodium ion layered cathode material.

5. The preparation method according to claim 4, characterized in that: In step (1), the transition metal precursor is the transition metal hydroxide precursor Ni. a Fe b Mn c M d (OH)2 or carbonate precursor Ni a Fe b Mn c M d CO3, wherein the first portion of sodium source is selected from at least one of sodium carbonate, sodium nitrate, sodium hydroxide and sodium oxalate; the amount of the first portion of sodium source and transition metal precursor added is 0.4-0.9 based on the molar ratio of sodium to total transition metal, expressed as Na / TM value.

6. The preparation method according to claim 5, characterized in that: In step (1), the flux is selected from at least one of sodium nitrate, sodium borohydride, sodium borate, boron oxide, borohydride, borane, boric acid and borax; the amount of flux added is 0.1%-3% of the total mass of the precursor.

7. The preparation method according to claim 6, characterized in that: The temperature of the first sintering in step (2) is 600℃-950℃ and the holding time is 5-20 h; the single crystal intermediate is a single crystal intermediate containing surface precipitated oxide particles and P2 phase or P2 and O3 mixed phase main phase, and the surface precipitated oxide particles are selected from at least one of nickel oxide, zinc oxide, copper oxide and magnesium oxide.

8. The preparation method according to claim 7, characterized in that: In step (3), the amount of sodium source added in the second part is to supplement the total sodium amount until the Na / TM molar ratio is 0.66-1.0; the temperature of the second sintering is 800-950℃ and the holding time is 10-20 h.

9. The application of the monocrystalline sodium-ion layered cathode material according to any one of claims 1-3 in sodium-ion batteries.

10. A sodium-ion battery, comprising a positive electrode, a negative electrode, and an electrolyte, characterized in that: The positive electrode comprises the monocrystalline sodium ion layered positive electrode material as described in any one of claims 1-3.

Images