A zero-strain coherent epitaxial p2-o3 dual-phase layered oxide positive electrode material and a preparation method thereof

Abstract

The application discloses a zero-strain coherent epitaxial P2-O3 dual-phase layered oxide positive electrode material and a preparation method thereof, and belongs to the technical field of sodium ion batteries. x Li y Ni z Fe m Mn p O2, trace lithium ions enter octahedral sites of transition metal layers to realize lattice modulation and eliminate P2-O3 two-phase a / b axis mismatch; Fe and Mn spontaneously distribute during high-temperature cooling to form atom-level coherent epitaxial P2-O3 dual-phase along the c axis in single particles, and the interface is zero-strain and has no lattice mismatch. The material has advantages of barrier-free ion transmission, ultra-small volume deformation, ultra-long cycle, wide temperature range stability and the like, the capacity retention rate is 88% after 2000 cycles at 3000 mA / g, and the retention rate is 86% after 1000 cycles at -20 DEG C. The preparation process adopts conventional precursors and air sintering, is low in cost, easy to scale, and suitable for large-scale energy storage and power battery sodium ion batteries.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery materials and electrochemical technology, specifically relating to a zero-strain coherent epitaxial P2-O3 biphase layered oxide cathode material and its preparation method. Background Technology

[0002] Sodium-ion batteries are considered a strong alternative to lithium-ion batteries in large-scale energy storage due to the abundance and low cost of sodium resources. Layered transition metal oxides (NaxTMO2, where TM represents a transition metal element) are among the most promising cathode materials for sodium-ion batteries due to their high theoretical capacity, tunable composition, and mature synthesis process. Based on the coordination environment of sodium ions and the oxygen stacking sequence, layered sodium-based oxides are mainly classified into two structures: P2 type and O3 type. In the P2 type structure, sodium ions occupy prism sites, the oxygen stacking sequence is ABBA, and it has open Na+ ions. + The P2-O3 phase exhibits good diffusion channels, rate performance, and structural stability, but its low sodium content leads to insufficient initial coulombic efficiency. In the O3-type structure, sodium ions occupy octahedral sites, and the oxygen stacking sequence is ABCABC, resulting in high sodium content and theoretical capacity. However, it undergoes complex multi-stage phase transitions (O3→P3→O3', etc.) during charge and discharge, leading to structural degradation and capacity decay. To comprehensively utilize the complementary advantages of the P2 and O3 phases, researchers have proposed a P2-O3 biphase mixing strategy. Existing technologies for achieving P2-O3 biphase mixing mainly include two configurations: core-shell and random blending. However, these schemes all face the following common technical problems: First, lattice stress mismatch exists at the interface between the two phases. The P2 and O3 phases have different oxygen packing sequences and interlayer spacings, resulting in lattice mismatch strain at the interface. In core-shell structures, the incoherent interface between the P2 shell and the O3 core leads to high interfacial resistance; in random blend structures, the random distribution of the two phases forms a large number of semi-coherent or incoherent interfaces, inducing interfacial Na+ stress. + Scattering reduces ion transport efficiency.

[0003] Second, insufficient phase transition synergy. During charge and discharge, the P2 and O3 phases undergo different phase transition paths, and their transition directions and amplitudes are not synchronized, leading to strain concentration at the interface, which triggers the initiation and propagation of microcracks, ultimately causing particle pulverization and rapid degradation of electrochemical performance.

[0004] Third, the formation mechanism of the two phases in existing two-phase mixing schemes remains unclear. Most studies only control the P2-O3 ratio by adjusting the sodium content or synthesis temperature, lacking a deep understanding of the thermodynamic driving force of two-phase formation, and thus failing to achieve atomic-level precise control of the interface structure.

[0005] The core of the above problem is that existing technologies cannot completely eliminate the lattice mismatch between the P2-O3 phases, and cannot construct a barrier-free phase boundary ion transport channel. Summary of the Invention

[0006] This invention addresses the shortcomings of existing P2-O3 biphase cathode materials, such as interfacial lattice mismatch, hindered ion transport, poor phase transition synergy, and insufficient wide-temperature adaptability. It provides a thermodynamically driven zero-strain coherent epitaxial P2-O3 biphase layered oxide cathode material and its preparation method. By modulating the lithium-ion lattice and distributing the Fe / Mn thermodynamic composition, the two-phase atomic-level coherent epitaxial coexistence is achieved, thus solving the aforementioned technical problems.

[0007] To address the aforementioned problems, this invention provides a zero-strain coherent epitaxial P2-O3 two-phase layered oxide cathode material with the general chemical formula Na. x Li y Ni z Fe m Mn p O2, where 0.7≤x≤0.9, 0.05≤y≤0.15, 0.1≤z≤0.3, 0.05≤m≤0.15, 0.5≤p≤0.7, and y+z+m+p=1; the cathode material has an atomically coherent epitaxial structure of P2 and O3 phases along the c-axis within a single primary particle; a zero-strain coherent interface is formed between the P2 and O3 phases, with the lattice strain at the interface approaching zero; the P2 phase is a manganese-rich region, and the O3 phase is an iron-rich region, formed by the thermodynamic spontaneous compositional distribution of iron and manganese elements during the cooling process after high-temperature sintering; lithium ions occupy octahedral sites in the transition metal layer, achieving lattice modulation of the transition metal oxide framework layer through Li-O-TM interaction, eliminating the lattice parameter differences between the P2 and O3 phases in the a / b axis direction.

[0008] Furthermore, in the general chemical formula, x=0.8, y=0.1, z=0.2, m=0.1, p=0.6, specifically Na. 0.8 Li 0.1 Ni 0.2 Fe 0.1 Mn 0.6 O2.

[0009] Furthermore, the lithium ions do not occupy the prism sites of the sodium layer, but only the octahedral sites of the transition metal layer.

[0010] Furthermore, the O3 phase in the iron-rich region and the P2 phase in the manganese-rich region coexist in the c-axis direction, and the transition metal oxide framework layer extends continuously without interruption when crossing the phase interface.

[0011] Furthermore, the volume change of the cathode material during the charging and discharging process is ≤0.87%, and the irreversible phase transition from P2 to O2 is suppressed.

[0012] Furthermore, the Na of the positive electrode material + The diffusion activation energy Ea = 0.522 eV, which is lower than that of pure P2 phase or pure O3 phase cathode materials, and Na + The P2 and O3 phases are inserted and extracted in parallel through their respective ab planes.

[0013] The key innovation of this invention lies in the synergistic design of three technical elements: (i) The precise occupancy of lithium ions at octahedral sites in the transition metal layer (occupancy rate 5-15%) modulates the lattice parameters of the transition metal oxide framework layer through Li-O-TM covalent interactions. (ii) Fe 3+ and Mn 4+ Thermodynamic spontaneous compositional distribution within the transition metal layer leads to enrichment and formation of O3 and P2 phases, respectively. (iii) The slow cooling process with controlled temperature control after sintering (rate 1-5℃ / min, preferably 2℃ / min) provides a sufficient diffusion kinetic window for the above component distribution, so that the P2-O3 dual phases coexist along the c-axis in an atomically coherent epitaxial manner.

[0014] All three are indispensable: if (i) is missing, the a / b axes of the two phases will be mismatched and a coherent interface cannot be formed (see Comparative Example 4 for details); if (iii) is missing, Fe / Mn cannot be fully distributed and only a random blended two-phase structure will be formed.

[0015] This invention also provides a method for preparing the above-mentioned zero-strain coherent epitaxial P2-O3 two-phase layered oxide cathode material, comprising the following steps: (1) Dissolve sodium source, lithium source, nickel source, iron source, manganese source and complexing agent in solvent according to stoichiometric ratio to obtain precursor mixture; (2) The precursor mixture was heated and evaporated to dryness to obtain a precursor dry gel; (3) The precursor dry gel is ground and pre-calcined in air atmosphere. After cooling, it is ground uniformly and then sintered in air atmosphere at 800℃~900℃. After sintering, the temperature is slowly reduced to below 100℃ at a programmed temperature control rate of 1-5℃ / min and then naturally cooled to room temperature to obtain the cathode material. The lattice modulation is achieved by the occupancy of the transition metal layer by lithium ions, and the thermodynamic spontaneous composition distribution of iron and manganese is used during the cooling process after sintering to drive the co-epitaxial growth of P2-O3 dual phase along the c-axis.

[0016] Further, the sodium source is a soluble sodium salt, preferably one or more of sodium acetate, sodium carbonate, and sodium nitrate, and more preferably anhydrous sodium acetate; the lithium source is a soluble lithium salt, preferably one or more of lithium acetate, lithium carbonate, and lithium nitrate, and more preferably anhydrous lithium acetate; the nickel source is a soluble nickel salt, preferably one or more of nickel acetate, nickel nitrate, and nickel chloride, and more preferably nickel acetate tetrahydrate; the iron source is a soluble iron salt, preferably one or more of ferric nitrate, ferric acetate, and ferric chloride, and more preferably ferric nitrate nonahydrate; the manganese source is a soluble manganese salt, preferably one or more of manganese acetate, manganese nitrate, and manganese carbonate, and more preferably manganese acetate tetrahydrate; the complexing agent is citric acid; the pre-calcination temperature is 400-500℃, and the pre-calcination time is 3-5 hours; the sintering temperature is 800-900℃, and the sintering time is 12-18 hours.

[0017] Furthermore, by mass, the sodium source is in excess by 2-5% to compensate for high-temperature volatilization.

[0018] The present invention also provides a sodium-ion battery, wherein the positive electrode active material of the sodium-ion battery comprises the zero-strain coherent epitaxial P2-O3 biphase layered oxide positive electrode material as described in any one of claims 1 to 6.

[0019] The beneficial effects of this invention are: (1) The zero-strain coherent interface eliminates interphase Na + Scattering. The ab surfaces of both the P2 and O3 phases are simultaneously exposed to the electrolyte, Na... + Ions intercalate and deintercalate in parallel through the respective ab planes of the two phases, without needing to cross the high-impedance phase interface along the c-axis. Variable-temperature electrochemical impedance spectroscopy (VIS) analysis indicates that the material exhibits ultra-low Na+ content. + The diffusion activation energy Ea = 0.522 eV, which is lower than that of pure P2 phase and pure O3 phase.

[0020] (2) The coherent interface acts as an atomic-level rivet, effectively absorbing and dissipating the anisotropic volumetric strain of the two phases during charge and discharge. In-situ X-ray diffraction tests show that the volume change of the material during a complete charge and discharge cycle is only 0.87%, and the irreversible phase transition from P2 to O2 is effectively suppressed.

[0021] (3) Electrochemical tests show that the positive electrode material retains 88% of its capacity after 2000 cycles at a current density of 3000 mA / g; it works normally at a current density of 1500 mA / g under a high temperature of 40℃; and it retains 86% of its capacity after 1000 cycles at a current density of 150 mA / g under a low temperature of -20℃, achieving a balance between wide temperature range, ultra-fast charging and long cycle stability. Attached Figure Description

[0022] Figure 1 The P2-O3 biphase layered oxide cathode material Na prepared in Example 1 0.8 Li 0.1 Ni 0.2 Fe 0.1 Mn 0.6 XRD pattern of O2 and Rietveld refinement results of P2-O3 two-phase.

[0023] Figure 2 The P2-O3 biphase layered oxide cathode material Na prepared in Examples 1-4 0.8 Li 0.1 Ni 0.2 Fe 0.1 Mn 0.6 XRD patterns of O2 at different sintering temperatures (750℃ and 950℃), and comparison of XRD patterns of samples with different compositions. Figure 3 The P2-O3 biphase layered oxide cathode material Na prepared in Example 1 0.8 Li 0.1 Ni 0.2 Fe 0.1 Mn 0.6 SEM image of O2.

[0024] Figure 4 The P2-O3 biphase layered oxide cathode material Na prepared in Example 1 0.8 Li 0.1 Ni 0.2 Fe 0.1 Mn 0.6 EDS elemental distribution diagram of O2.

[0025] Figure 5 The P2-O3 biphase layered oxide cathode material Na prepared in Example 1 0.8 Li 0.1 Ni 0.2 Fe 0.1 Mn 0.6 Aberration-corrected HAADF-STEM images and GPA strain analysis plots of O2.

[0026] Figure 6 The P2-O3 biphase layered oxide cathode material Na prepared in Example 1 0.8 Li 0.1 Ni 0.2 Fe 0.1 Mn 0.6 O2 rate performance diagram at different current densities.

[0027] Figure 7 The P2-O3 biphase layered oxide cathode material Na prepared in Example 10.8 Li 0.1 Ni 0.2 Fe 0.1 Mn 0.6 Cycling performance of O2 at a current density of 3000 mA / g for 2000 cycles.

[0028] Figure 8 The P2-O3 biphase layered oxide cathode material Na prepared in Example 1 0.8 Li 0.1 Ni 0.2 Fe 0.1 Mn 0.6 Cycling performance of O2 at 40°C and a current density of 1500 mA / g for 1000 cycles.

[0029] Figure 9 The P2-O3 biphase layered oxide cathode material Na prepared in Example 1 0.8 Li 0.1 Ni 0.2 Fe 0.1 Mn 0.6 Cycling performance of O2 at -20℃ with a current density of 150 mA / g for 1000 cycles.

[0030] Figure 10 The P2-O3 biphase layered oxide cathode material Na prepared in Example 1 0.8 Li 0.1 Ni 0.2 Fe 0.1 Mn 0.6 In-situ XRD pattern of O2. Detailed Implementation

[0031] The present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0032] Example 1: Na 0.8 Li 0.1 Ni 0.2 Fe 0.1 Mn 0.6 Preparation of O2 cathode material A thermodynamically driven zero-strain coherent epitaxial P2-O3 two-phase layered oxide cathode material is prepared through the following steps: 1) According to Na 0.8 Li 0.1 Ni 0.2 Fe 0.1 Mn 0.6The stoichiometric ratio of O2 is determined by dissolving anhydrous sodium acetate (with an excess of 2-5% sodium source to compensate for high-temperature volatilization loss), anhydrous lithium acetate, nickel acetate tetrahydrate, ferric nitrate nonahydrate, manganese acetate tetrahydrate, and citric acid in 0.3 L of deionized water and stirring at room temperature until completely dissolved to obtain a precursor mixture. 2) The precursor mixture was heated to dryness in an 80°C water bath to obtain a precursor dry gel; 3) After grinding the precursor dry gel into powder, it is first pre-calcined at 450℃ for 4 h in air atmosphere, then taken out and ground evenly, and then sintered at 850℃ for 15 h in air atmosphere. After sintering, the temperature is slowly reduced to below 100℃ at a programmed temperature control rate of 2℃ / min, and then naturally cooled to room temperature to obtain P2-O3 biphase layered oxide cathode material.

[0033] The product Na prepared in this example 0.8 Li 0.1 Ni 0.2 Fe 0.1 Mn 0.6 Taking O2 as an example, X-ray diffraction tests confirmed that... Figure 1 As shown, Rietveld refinement was performed using a two-phase model of P2 (P63 / mmc space group) and O3 (R-3m space group), with a refinement factor Rwp = 6.4%, lattice parameters a = 2.8903 Å, c = 11.079 Å, and V = 80.15 Å3, confirming the formation of the P2-O3 dual-phase structure. The XRD pattern showed a characteristic diffraction peak of the O3 phase at approximately 41.9°, clearly distinguishable from the pure P2 phase sample.

[0034] like Figure 5 As shown, aberration-corrected scanning transmission electron microscopy (HAADF-STEM) atomic-level imaging of a single primary particle cross-section reveals a coexisting structure of the P2 and O3 phases along the c-axis. Geometric phase analysis (GPA) shows that the εxx and εyy strains at the interface are approximately 0.5%, confirming the formation of a zero-strain coherent interface. Atomic-level EDS analysis indicates that the O3 phase is a Fe-rich region and the P2 phase is a Mn-rich region, confirming the two-phase formation mechanism driven by Fe / Mn thermodynamic composition partitioning.

[0035] The Na prepared in this embodiment 0.8 Li 0.1 Ni 0.2 Fe 0.1 Mn 0.6 Electrochemical tests were conducted on O2 cathode material as the positive electrode active material for sodium-ion batteries. The preparation method is as follows: Using Na... 0.8 Li 0.1 Ni 0.2 Fe 0.1 Mn 0.6O2 is used as the active material, acetylene black as the conductive agent, and polyvinylidene fluoride (PVDF) as the binder. The mass ratio of active material, acetylene black, and PVDF is 8:1:1. After thoroughly mixing them in this ratio, an appropriate amount of N-methyl-2-pyrrolidone (NMP) is added, and the mixture is stirred evenly and coated onto an aluminum foil current collector. The coated positive electrode is then dried in a vacuum oven at 120℃ for 12 hours before use. Using a 1 mol / L NaClO4 propylene carbonate solution (containing 5% fluoroethylene carbonate) as the electrolyte, a sodium sheet as the negative electrode, and GF / A glass fiber as the separator, a CR2032 sodium-ion button battery is assembled in an argon-filled glove box.

[0036] After electrochemical performance testing, such as Figure 6 As shown, within a voltage range of 2.0–4.3 V, the initial discharge specific capacity at a current density of 30 mA / g is approximately 120 mAh / g; after 2000 cycles at a current density of 3000 mA / g, the capacity retention is 88%. In-situ XRD testing indicates that the volume change during charge and discharge is only 0.87%. After 1000 cycles at a current density of 150 mA / g at a low temperature of -20°C, the capacity retention is 86%.

[0037] Tests on the cyclic samples revealed that the coherent phase boundaries remained stable during long cycles, with the interface still exhibiting zero strain, confirming that the coherent interface acts as an atomic-level rivet.

[0038] Example 2: Na 0.9 Li 0.1 Ni 0.21 Fe 0.07 Mn 0.62 Preparation of O2 cathode material A P2-O3 two-phase layered oxide cathode material is prepared by the following steps: 1) According to Na 0.9 Li 0.1 Ni 0.21 Fe 0.07 Mn 0.62 To obtain the precursor mixture, the following stoichiometric ratios were determined: anhydrous sodium acetate (sodium source in excess 2-5%), anhydrous lithium acetate, nickel acetate tetrahydrate, ferric nitrate nonahydrate, manganese acetate tetrahydrate, and citric acid were dissolved in 0.3 L of deionized water and stirred at room temperature until completely dissolved. 2) The precursor mixture was heated to dryness in an 80°C water bath to obtain a precursor dry gel; 3) After grinding the precursor dry gel into powder, it is first pre-calcined at 450℃ for 4 h in air atmosphere, then taken out and ground evenly, and then sintered at 850℃ for 15 h in air atmosphere. After sintering, the temperature is slowly reduced to below 100℃ at a programmed temperature control rate of 2℃ / min, and then naturally cooled to room temperature to obtain the cathode material.

[0039] Compared to Example 1, this embodiment increased the sodium content (from 0.8 to 0.9) and decreased the iron content (from 0.1 to 0.07). To maintain the normalization of the transition metal layer, the Ni content was adjusted accordingly (from 0.20 to 0.21) and Mn content (from 0.60 to 0.62) to investigate the regulatory effects of sodium and iron content on the P2-O3 biphase ratio. XRD results show that a higher sodium content is beneficial to increasing the O3 phase ratio. At the same time, due to the decrease in iron content, the driving force of Fe / Mn composition distribution is weakened, which affects the formation of the P2-O3 biphase coherent interface.

[0040] The electrochemical testing method was the same as in Example 1. The test results showed that a higher sodium content improved the initial coulombic efficiency, but the decrease in iron content had a certain impact on the integrity of the biphase coherent interface and long-term cycling stability.

[0041] Example 3: Na 0.7 Li 0.1 Ni 0.2 Fe 0.13 Mn 0.57 Preparation of O2 cathode material A P2-O3 two-phase layered oxide cathode material is prepared by the following steps: 1) According to Na 0.7 Li 0.1 Ni 0.2 Fe 0.13 Mn 0.57 To obtain the precursor mixture, the following stoichiometric ratios were determined: anhydrous sodium acetate (sodium source in excess 2-5%), anhydrous lithium acetate, nickel acetate tetrahydrate, ferric nitrate nonahydrate, manganese acetate tetrahydrate, and citric acid were dissolved in 0.3 L of deionized water and stirred at room temperature until completely dissolved. 2) The precursor mixture was heated to dryness in an 80°C water bath to obtain a precursor dry gel; 3) After grinding the precursor dry gel into powder, it is first pre-calcined at 450℃ for 4 h in air atmosphere, then taken out and ground evenly, and then sintered at 850℃ for 15 h in air atmosphere. After sintering, the temperature is slowly reduced to below 100℃ at a programmed temperature control rate of 2℃ / min, and then naturally cooled to room temperature to obtain the cathode material.

[0042] Compared to Example 1, this embodiment reduced the sodium content (from 0.8 to 0.7) and slightly increased the iron content (from 0.1 to 0.13). To maintain the normalization of the transition metal layer, the Mn content was correspondingly slightly reduced (from 0.60 to 0.57) to investigate the change in the P2-O3 biphase ratio under low sodium content conditions. XRD results showed that the lower sodium content increased the proportion of the P2 phase and decreased the proportion of the O3 phase, with the P2 phase becoming the dominant phase. The slight increase in iron content had little effect on the Fe / Mn composition distribution.

[0043] The electrochemical testing method was the same as in Example 1. Test results showed that a lower sodium content led to a decrease in the initial coulombic efficiency, but an increased proportion of the P2 phase was beneficial for improving rate performance.

[0044] Example 4 (Comparative Example): Na 0.8 Ni 0.22 Fe 0.11 Mn 0.67 Preparation of O2 cathode material A lithium-free layered oxide cathode material, as a comparative example, was prepared through the following steps: 1) According to Na 0.8 Ni 0.22 Fe 0.11 Mn 0.67 To obtain the precursor mixture, anhydrous sodium acetate (2-5% excess sodium source), nickel acetate tetrahydrate, ferric nitrate nonahydrate, manganese acetate tetrahydrate, and citric acid were dissolved in 0.3 L of deionized water and stirred at room temperature until completely dissolved. No lithium salt was added in this embodiment. 2) The precursor mixture was heated to dryness in an 80°C water bath to obtain a precursor dry gel; 3) After grinding the precursor dry gel into powder, it is first pre-calcined at 450℃ for 4 h in air atmosphere, then taken out and ground evenly, and then sintered at 850℃ for 15 h in air atmosphere. After sintering, the temperature is slowly reduced to below 100℃ at a programmed temperature control rate of 2℃ / min, and then naturally cooled to room temperature to obtain the cathode material.

[0045] The purpose of this comparative example is to verify the key driving role of lithium ions in the formation of the P2-O3 dual phase. XRD results show that lithium-free Na... 0.8 Ni 0.22 Fe 0.11 Mn 0.67 The O2 sample exhibits a pure P2 phase structure, with no characteristic diffraction peak of the O3 phase at approximately 41.9°. The (002) peak is located at approximately 15.82° (corresponding to d002=5.599 Å), which is lower than the 16.22° (d002=5.459 Å) of Example 1, indicating that the c-axis interlayer spacing of the lithium-free sample is larger.

[0046] The above results directly confirm that, under the same sodium content and transition metal composition, the lack of lithium ion occupancy and lattice modulation effect on the transition metal layer means that the difference in Fe / Mn composition alone is insufficient to drive the formation of the O3 phase; lithium ion occupancy of octahedral sites in the transition metal layer is a necessary condition for the coherent epitaxial coexistence of P2-O3 dual phases.

[0047] The electrochemical testing method was the same as in Example 1. The test results showed that pure P2 phase Na... 0.8 Ni 0.22 Fe 0.11 Mn 0.67 The rate performance and cycle stability of O2 were lower than those of the P2-O3 coherent material in Example 1, further verifying the effect of the coherent dual-phase structure on improving electrochemical performance.

[0048] Summary and comparison of results from various embodiments

[0049] The comparison of the above four sets of examples shows that the introduction of lithium ions is a necessary condition for realizing the P2-O3 biphase coherent epitaxial co-existing structure; in the lithium-containing system, the P2-O3 biphase ratio can be further controlled by adjusting the sodium and iron content; the lithium-free comparative sample only forms a pure P2 phase and does not have the zero-strain coherent interface and barrier-free parallel ion transport characteristics.

[0050] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A zero-strain coherent epitaxial P2-O3 two-phase layered oxide cathode material, characterized in that, The general chemical formula is Na x Li y Ni z Fe m Mn p O2, where 0.7≤x≤0.9, 0.05≤y≤0.15, 0.1≤z≤0.3, 0.05≤m≤0.15, 0.5≤p≤0.7, and y+z+m+p=1; the cathode material has an atomically coherent epitaxial structure of P2 and O3 phases along the c-axis within a single primary particle; a zero-strain coherent interface is formed between the P2 and O3 phases, and the absolute values ​​of the strain components εxx and εyy, as measured by GPA, are both ≤1%; the P2 phase is a manganese-rich region, and the O3 phase is an iron-rich region, which is driven by the thermodynamic spontaneous compositional distribution of iron and manganese elements during the cooling process after high-temperature sintering; lithium ions occupy octahedral sites in the transition metal layer, and achieve lattice modulation of the transition metal oxide framework layer through Li-O-TM interaction, eliminating the lattice parameter differences between the P2 and O3 phases in the a / b axis direction.

2. The zero-strain coherent epitaxial P2-O3 two-phase layered oxide cathode material according to claim 1, characterized in that, In the general chemical formula, x=0.8, y=0.1, z=0.2, m=0.1, p=0.6, specifically Na. 0.8 Li 0.1 Ni 0.2 Fe 0.1 Mn 0.6 O2.

3. The zero-strain coherent epitaxial P2-O3 two-phase layered oxide cathode material according to claim 1, characterized in that, The lithium ions do not occupy the prism sites of the sodium layer, but only the octahedral sites of the transition metal layer.

4. The zero-strain coherent epitaxial P2-O3 two-phase layered oxide cathode material according to claim 1, characterized in that, The O3 phase in the iron-rich region and the P2 phase in the manganese-rich region coexist in the c-axis direction, and the transition metal oxide framework layer extends continuously without interruption when crossing the phase interface.

5. The zero-strain coherent epitaxial P2-O3 dual-phase layered oxide cathode material according to claim 1, characterized in that, The volume change of the cathode material during the charging and discharging process is ≤0.87%, and the irreversible phase transition from P2 to O2 is suppressed.

6. The zero-strain coherent epitaxial P2-O3 two-phase layered oxide cathode material according to claim 1, characterized in that, The positive electrode material Na + The diffusion activation energy Ea = 0.522 eV, which is lower than that of pure P2 phase or pure O3 phase cathode materials, and Na + The P2 and O3 phases are inserted and extracted in parallel through their respective ab planes.

7. A method for preparing the zero-strain coherent epitaxial P2-O3 two-phase layered oxide cathode material according to any one of claims 1 to 6, characterized in that, Includes the following steps: (1) Dissolve sodium source, lithium source, nickel source, iron source, manganese source and complexing agent in solvent according to stoichiometric ratio to obtain precursor mixture; (2) The precursor mixture was heated and evaporated to dryness to obtain a precursor dry gel; (3) The precursor dry gel is ground and pre-calcined in air atmosphere. After cooling, it is ground uniformly and then sintered in air atmosphere at 800℃~900℃. After sintering, the temperature is slowly reduced to below 100℃ at a programmed temperature control rate of 1-5℃ / min and then naturally cooled to room temperature to obtain the cathode material. The lattice modulation is achieved by the occupancy of the transition metal layer by lithium ions, and the thermodynamic spontaneous composition distribution of iron and manganese is used during the cooling process after sintering to drive the co-epitaxial growth of P2-O3 dual phase along the c-axis.

8. The preparation method according to claim 7, characterized in that: The sodium source is a soluble sodium salt, selected from one or more of sodium acetate, sodium carbonate, and sodium nitrate; the lithium source is a soluble lithium salt, preferably selected from one or more of lithium acetate, lithium carbonate, and lithium nitrate; the nickel source is a soluble nickel salt, selected from one or more of nickel acetate, nickel nitrate, and nickel chloride; the iron source is a soluble iron salt, selected from one or more of ferric nitrate, ferric acetate, and ferric chloride; the manganese source is a soluble manganese salt, selected from one or more of manganese acetate, manganese nitrate, and manganese carbonate; the complexing agent is citric acid; the pre-calcination temperature is 400-500℃, and the pre-calcination time is 3-5 hours; the sintering temperature is 800-900℃, and the sintering time is 12-18 hours.

9. The preparation method according to claim 7, characterized in that, The sodium source is in excess by 2-5% by mass to compensate for high-temperature volatilization.

10. A sodium-ion battery, characterized in that, The positive electrode active material of the sodium-ion battery comprises the zero-strain coherent epitaxial P2-O3 biphase layered oxide positive electrode material as described in any one of claims 1 to 6.

Images