A surface-doped layered battery cathode material, and a preparation method and application thereof

Abstract

The application relates to a surface-doped layered battery positive electrode material and a preparation method and application thereof, and belongs to the technical field of sodium ion battery materials. The method realizes surface modification of the positive electrode material by constructing a lattice-matched surface layered in-situ reconstruction layer in a non-proton solvent with the aid of a complexing agent. The specific steps comprise the following: firstly, a layered sodium ion oxide Na 1‑x TMO2 is prepared through solid-phase sintering, a sol-gel method or a coprecipitation method; subsequently, the positive electrode material is placed in a non-proton solvent solution containing a metal salt, surface ion exchange and reconstruction are carried out through microwave, heating, stirring or ultrasonic methods; finally, the mixture after reaction is filtered, washed and annealed. The application significantly improves the cycle stability, ion migration rate and air stability of the material through surface doping, effectively inhibits the irreversible reaction of lattice oxygen and the interface side reaction, and meanwhile, the high energy density of the material is maintained. The method has the advantages of simple process, low cost and easy large-scale production, and is suitable for surface modification of sodium ion battery positive electrode materials.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion and lithium-ion batteries, specifically to a surface-doped layered battery cathode material, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries have become a research hotspot in the energy storage field in recent years due to their abundant resources, low cost, and high safety. With the global energy structure transitioning towards sustainable development, the application potential of sodium-ion batteries in large-scale energy storage, low-speed electric vehicles, and communication base stations is increasingly prominent. However, the cathode materials for sodium-ion batteries, especially layered transition metal oxides (Na₂O₃), remain a challenge. x TMO2 still faces many challenges in practical applications, including complex phase transition behavior, high sensitivity to water and oxygen, and interfacial side reactions. These problems directly lead to short battery cycle life and rapid capacity decay, which seriously restricts its commercialization process.

[0003] Layered oxide cathode material (Na) x TMO2 has attracted much attention due to its high specific capacity and energy density. Based on the oxygen coordination environment of sodium ions, Na... x TMO2 mainly consists of two phases: O3 (octahedral coordination) and P2 (triangular prism coordination). O3 phase materials have a higher initial sodium content, enabling the release of more sodium ions, but their compact structure results in a lower sodium ion migration rate. P2 phase materials, on the other hand, have a larger interlayer spacing, facilitating rapid sodium ion transport, but their lower initial sodium content limits their capacity. Although these materials theoretically exhibit excellent performance, in practical applications, irreversible lattice oxygen evolution under high voltage, phase transitions, and interfacial side reactions severely impact their electrochemical performance. Furthermore, the high sensitivity of layered oxide cathode materials to water and oxygen increases the difficulty of their preparation and storage. When exposed to air, the material surface readily reacts with water and carbon dioxide, generating byproducts such as sodium carbonate, further deteriorating their electrochemical performance.

[0004] To improve the performance of layered oxide cathode materials, existing technologies mainly employ modification methods such as doping or coating. For example, doping with different metal ions (such as alkali metals, alkaline earth metals, or rare earth elements) stabilizes the crystal structure, or coating with a protective layer reduces interfacial side reactions. However, these methods still have limitations: doping modification may lead to the substitution of some electrochemically active ions, thereby reducing the overall capacity of the material; coating modification is difficult to control the uniformity of the coating layer and is prone to structural damage and reduced cycle performance in water-sensitive materials. Therefore, there is an urgent need for a modification method that can effectively suppress lattice oxygen evolution and improve interfacial stability while maintaining high material capacity, in order to promote the widespread application of sodium-ion batteries.

[0005] The present invention is proposed based on the above background, aiming to construct a lattice-matched surface layered in-situ reconstruction layer through a surface doping modification method, thereby protecting the bulk material during charge and discharge, suppressing lattice oxygen oxidation-reduction and interfacial side reactions, and improving cycle stability and air stability. Summary of the Invention

[0006] The purpose of this invention is to provide a surface-doped layered battery cathode material, its preparation method, and its application. By using a complexing agent in an aprotic solvent to assist in the construction of a lattice-matched surface-doped in-situ reconstructed layer, a protective layer is formed on the bulk material during charge and discharge, suppressing lattice oxygen oxidation-reduction and interfacial side reactions, and improving cycle stability and air stability.

[0007] The technical solution of the present invention is as follows:

[0008] A surface-doped layered battery cathode material with the general structural formula: Na 1-x I y TMO2, where 0≤x<1, 0≤y<1. I is one or more of alkali metals, alkaline earth metals, transition metals, and rare earth metals such as Li, Ca, Sr, Ba, Zn, K, La to Lu; TM is one or more of metals such as Ni, Co, Mn, Fe, Cu, Al, Mg, and Cr.

[0009] The method for preparing the surface-doped layered battery cathode material includes the following steps:

[0010] S01: Well-crystallized layered sodium oxide (Na) produced by solid-state sintering, sol-gel, co-precipitation, or commercially available methods. 1-x TMO2 (x≥0), where TM is one or more of Ni, Fe, Mn, Co, Cu, Ti, Li, Al, Mg, and Cr, and its configuration is O3 phase or P2 phase.

[0011] S02: The positive electrode material Na 1-x TMO2 is placed in a metal salt (I n+ In the aprotic solvent solution containing acetone, dimethylformamide, N-methylpyrrolidone, or dimethyl sulfoxide, and the auxiliary complexing agent, the metal salts are sulfates, halides, nitrates, phosphates, or complex salts soluble in organic solvents; the dopant ions are alkali metals, alkaline earth metals, or rare earth elements; and the auxiliary complexing agents are small molecules or polymers such as phytic acid, dopamine, tannic acid, and polyvinylpyrrolidone. Target ions are introduced onto the surface of the layered material through microwave, heating, stirring, and ultrasound methods to achieve ion exchange and surface reconstruction. The reaction process is controlled by adjusting the concentration, time, temperature, and mechanical strength to achieve a uniform distribution of target ions on the surface of the cathode particles.

[0012] S03: The mixture after the above reaction is filtered, washed, and annealed. The solvent used for washing can be the same solvent used to dissolve the metal salt during ion exchange, the purpose of which is to remove unreacted metal ions and anions adsorbed on the surface. Whether an additional sodium source is needed depends on the selected cathode material. The annealing temperature range is 400–1000℃, the annealing time is 0.5–10 hours, and the annealing atmosphere can be air, argon, or oxygen.

[0013] Based on the common characteristics of layered cathode materials in sodium-ion batteries, this invention proposes a simple and efficient method for modifying layered cathode materials through surface ion reactions that maintain capacity, based on the high water and oxygen sensitivity of the materials. This method can directly modify the finished cathode materials, alleviate surface side reactions between the materials and the electrolyte, inhibit lattice oxygen evolution, improve water and oxygen stability, extend cycle life, and improve the electrochemical performance of the materials.

[0014] Another object of the present invention is to provide an application of surface-doped layered battery cathode material in sodium-ion batteries.

[0015] The beneficial effects of this invention are as follows:

[0016] 1. Significantly improves cycle stability: through solvent-assisted I n+ Surface reconstruction, partial I n+ It penetrates the surface of the positive electrode particles, forming a uniform layer of Na. 1-x I y The TMO2 gradient symbiotic layer significantly improves structural stability, suppresses irreversible reactions of lattice oxygen, and enhances cycle stability.

[0017] 2. Improved ion mobility and rate performance: The addition of I enhances the interlayer forces of layered oxides, and I with a larger radius is selected. n+ This causes local lattice changes, thereby improving ion mobility and rate performance.

[0018] 3. Improved air stability: The enrichment of I-surface reduces the erosion of O3-type sodium cathode by air, thereby further improving air stability. Attached Figure Description

[0019] Figure 1 Scanning electron microscope (SEM) image of the surface-modified material, scale bar 1 μm. The image shows that the surface of the modified material is smooth and dense, with no obvious impurities or defects, indicating that the surface modification process did not damage the morphology of the material, and the modified layer is uniformly covered on the particle surface.

[0020] Figure 2X-ray diffraction (XRD) pattern of the surface-modified material. The diffraction pattern shows that the modified material has good crystallinity, and the diffraction peaks are completely matched with the standard spectrum of the O3 phase layered structure. No other impurity phases were detected, proving that the modification process did not introduce additional impurities or change the crystal structure of the material.

[0021] Figure 3 Scanning electron microscope (STEM) images and energy-dispersive X-ray spectroscopy (EDS) elemental distribution maps of the modified cathode particle surface, scale bar 100 nm. The STEM images clearly show the microstructure of the particle surface, and the EDS spectra show that transition metal elements (such as Ni, Fe, Mn) are uniformly distributed inside the particles, while target ions (such as rare earth elements) show a significant enrichment effect at the particle edges, indicating that the surface reconstruction layer was successfully constructed.

[0022] Figure 4 Comparison of half-cell cycle performance of O3 cathode materials before and after rare earth metal surface modification at 1C rate. The results show that the cycle stability of the modified battery is significantly improved, and the capacity retention is significantly higher than that of the unmodified sample, proving that surface modification effectively suppresses lattice oxygen evolution and interfacial side reactions, thereby improving the electrochemical stability of the material.

[0023] Figure 5 Comparison of half-cell cycle performance of O3 cathode materials before and after rare earth metal surface modification at different rates. The results show that the discharge capacity of the modified battery is better than that of the original sample at all rates, indicating that surface modification not only improves cycle stability but also significantly improves the rate performance of the material, making high-power applications possible. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0025] Example 1:

[0026] Layered transition metals NaNi with O3 phase 0.33 Fe 0.33 Mn 0.33 Taking O2 as an example, the raw material is prepared by solid-state sintering, and the target ion for doping is one of alkali metals, alkaline earth metals, or rare earth elements. If a commercially available product is purchased, step 2 can be performed directly. The preparation includes the following steps:

[0027] S1: Preparation of NaNi by high-temperature solid-state sintering method 0.33 Fe 0.33 Mn 0.33O2 (NFM) bulk material: NiO, Fe2O3, MnO2, and Na2CO3 are added to a high-energy ball mill in stoichiometric ratio, with Na2CO3 in 5 mol% excess. The ball mill speed is 1000–1800 r / min, and the milling time is 30–180 min. The mixed powder is pressed into tablets and calcined in a muffle furnace at 800–1000℃ for 10–24 hours in air atmosphere to obtain the NFM cathode material, which is immediately stored in an argon-filled glove box.

[0028] S2: An ion-exchange liquid phase environment is prepared using an aprotic solvent (N-methylpyrrolidone solution). Based on the mass of the added cathode material, the complexing agent polyvinylpyrrolidone and the target ion (Lu) are formulated. 3+ ) nitrate solution.

[0029] S3: Grind and sieve the sintered NFM particles and add them to the mixed solution. Heat to 80°C and stir vigorously for 4 hours. Wash repeatedly with pure aprotic solvent to remove residual ions, filter and vacuum dry.

[0030] S4: Calcine at 400-1000℃ for 0.5-10h to obtain the final material, and store it in an argon glove box.

[0031] Figure 1 and Figure 2 The scanning electron microscope (SEM) and X-ray diffraction (XRD) images of the surface-modified material with metal ions show that the modified material has a smooth surface free of impurities, exhibits good crystallinity according to XRD, conforms to the layered structure of the O3 phase, and has no additional impurities. Table 1 shows that Example 1 has the highest coulombic efficiency and 100-cycle retention, demonstrating good reversibility of the electrochemical process. Furthermore, the capacity of Example 1 is comparable to that of the original material, proving that surface ion modification does not affect the effective capacity, and improves cycle stability while maintaining the energy density advantage of the O3-type cathode.

[0032] Example 2:

[0033] To demonstrate that different types of ions (alkali metals, alkaline earth metals, and rare earth elements) can achieve the present invention, this embodiment differs from Example 1 in that alkaline earth element ions are used for modification, and in step 2, calcium salts of equimolar concentration are used instead of rare earth salts as the target ion. Pure aprotic solvents are used, while the remaining steps are the same as in Example 1. As can be seen from the performance in Table 1, compared with Example 1, the 0.1C capacity is slightly improved, and the capacity retention rate after 100 cycles is slightly reduced, but it is still significantly higher than that of Comparative Example 1, indicating that the present invention has universal applicability in the selection of metal ions.

[0034] Example 3:

[0035] To demonstrate that this invention can be achieved with various types of aprotic solvents, this embodiment differs from Example 1 in that acetone is used instead of N-methylpyrrolidone as the aprotic solvent in step 2. All other steps are the same as in Example 1. As can be seen from the performance data in Table 1, compared to the examples, the 0.1C capacity and 100-cycle capacity retention are slightly lower, but the cycling stability is still significantly higher than that of Comparative Example 1, demonstrating the universality of this invention in selecting aprotic solvents.

[0036] Comparative Example 1:

[0037] To demonstrate the importance of ion exchange in modifying O3 phase cathode materials, this comparative example did not involve ion exchange. Instead, transition metal oxides and sodium carbonate were added to a high-energy ball mill in stoichiometric ratio, with sodium carbonate in excess at 3 mol%. The ball mill was operated at 1700 r / min for 45 min. The mixed powder was pressed into tablets and calcined in a muffle furnace at 900°C for 15 hours in air to obtain NFM cathode material, which was immediately stored in an argon-filled glove box. As shown in Table 1, Comparative Example 1 and Example 1 have similar 0.1C capacities, but the retention rates after 100 cycles differ significantly, at 69.1% and 90.4% respectively, and the coulombic efficiency also decreased by approximately 4%. This indicates that the present invention achieves a significant improvement in overall performance while maintaining the capacity advantage of the raw materials.

[0038] Comparative Example 2:

[0039] To demonstrate the importance of ion exchange in constructing a surface gradient symbiotic layered structure for modifying O3-phase cathode materials, this comparative example employs uniform doping of the target ion bulk phase, adding the target ion during the ball milling precursor process. Specifically, 2% of the target ion oxide was added during the ball milling step 1 of Example 1, mixed with transition metal oxide and sodium carbonate, and then ball milled. The mixed powder was pressed into tablets and calcined at 900°C for 15 hours in air. As can be seen from the performance in Table 1, compared with Example 1, the coulombic efficiency was slightly reduced, the 100-cycle retention rate was significantly reduced, and the 0.1C capacity decreased significantly by 14%, with the initial capacity decreasing to 140.18 mAh g–1. Although this modification method achieved a higher cycle retention rate than Comparative Example 1, it sacrificed capacity and reduced energy density.

[0040] Performance testing

[0041] The modified O3 cathode material obtained in the above examples and comparative examples was mixed with conductive agent acetylene black and binder (PVDF) at a mass ratio of 8:1:1, and an appropriate amount of N-methylpyrrolidone was added and stirred evenly to form a slurry. This slurry was then coated onto aluminum foil, vacuum dried at 120°C for 12 hours, and cut to form the cathode sheet for coin cell half-cells. Coin cells were assembled in a glove box. The counter electrode of the half-cell was sodium metal, the separator was a Whatman glass fiber membrane, and the electrolyte concentration was 1 mol / L. -1The solution used was NaClO4, with propylene carbonate (PC) as the solvent and an additional 5% fluoroethylene carbonate (FEC). After assembling coin cells and allowing them to stand at room temperature for 12 hours, the cells were activated at a low current rate within the voltage range of 2–4.2 V, using 1C (1C = 130 mAh g). -1 The performance test results of the cathode material after charge-discharge cycles at various rates are shown in Table 1 below:

[0042]

[0043] As shown in Table 1, the method for surface-doped layered battery cathode material provided by the present invention and the cathode material prepared therefrom significantly improve cycle stability, ion mobility and rate performance, high current charge-discharge cycle capability and air stability while maintaining the high specific capacity and high coulombic efficiency of O3 material.

[0044] Advantages and positive effects of the present invention

[0045] 1. Significantly improves cycle stability. For example... Figure 4 The cycling performance of the half-cell at 1C was demonstrated. The cycling stability of the half-cells of the ion-doped samples was significantly improved in the low current and voltage range of 2–4.2V. The retention rate of the half-cells after 200 cycles at 1C was improved by about 37.2% (from 45.0% to 82.2%), and the retention rate after 400 cycles at 2C was improved by about 40% (from 33.6% to 73.4%).

[0046] 2. Improve ion mobility and rate performance. For example... Figure 5 The results demonstrate that the ion-doped sample exhibits superior capacity compared to the original sample at different discharge rates. The discharge capacities at 0.1, 0.2, 0.5, 1, 2, 5, and 10C rates are 165.5, 158.6, 150.9, 143.6, 134.1, 121.2, and 110.6 mAh g⁻¹, respectively. –1 Especially noteworthy is its ability to maintain 110mAh / g even at 10C rate. –1 The above capacity, while the 10C capacity of the original material is less than 80 mAh g. –1 .

[0047] 3. Improve high-current charge-discharge cycle capability. At 10C = 1300mAh / g –1 Under current, the initial capacity of the ion-modified surface sample reached 120 mAh g. –1 After 500 cycles, the capacity retention rate was 70.8%, while the capacity retention rate of the unmodified sample was 18.4%, representing an improvement of 51%.

[0048] 4. Improved coulombic efficiency. As shown in Table 1, the O3-type bulk material provides excellent specific capacity. The ion-modified sample provides a higher first-cycle coulombic efficiency (coulombic efficiency = discharge capacity / charge capacity) than the original material under the same capacity, thus improving cycle reversibility.

[0049] 5. Improved air stability. O3-type sodium electrodes are highly sensitive to air, but the electrochemical performance of the ion-modified sample remained stable after 24 hours of exposure to air, with a capacity loss of 7.5%; under the same conditions, the original material experienced a capacity loss of 21.4%.

[0050] From a chemical principle perspective, solvent-assisted I n+ Surface reconstruction, partial I n+ It penetrates the surface of the positive electrode particles, forming a uniform layer of Na. 1-x I y Ni 0.33 Fe 0.33 Mn 0.33 O2 gradient co-occurrence layer. Argon-ion beam-assisted X-ray photoelectron spectroscopy at different depths revealed a decreasing concentration of I element from the surface to the bulk phase, confirming the formation of a gradient lattice co-doped structure. The addition of I enhanced the interlayer forces of the layered oxide, significantly improving structural stability and suppressing irreversible reactions of lattice oxygen. Simultaneously, during electrochemical charge-discharge, Na… + During the embedding and extraction process, I n+ It provides interlayer support, preventing structural collapse under conditions of high sodium desodiumation and improving cycle stability. When a larger radius I is selected... n+ This causes localized lattice changes, improving ion mobility and rate performance. Furthermore, due to the enrichment of I on the surface, the concentration of I in areas easily exposed to air reduces air erosion of the O3-type sodium cathode, thus further enhancing air stability.

[0051] In summary, surface ion doping is a universal method that improves the reversibility and overall electrochemical performance of the cathode structure.

[0052] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions, and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention, and within the spirit and principles of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A surface-doped layered battery cathode material, characterized in that, The general structural formula of the material is Na 1-x I y yTM1-xO2, wherein 0≤x<1, 0<y<1, I is Lu, and TM is Ni, Fe, Mn; The cathode material is an O3-phase layered sodium-ion battery cathode material. The I element is enriched and distributed on the surface of the cathode material particles, and its distribution decreases in a gradient from the particle surface to the particle interior, thereby forming Na on the particle surface that matches the internal layered main structure lattice. 1-x I y Ni 0.33 Fe 0.33 Mn 0.33 O2 gradient co-occurrence layered reconstruction structure.

2. The method for preparing a surface-doped layered battery cathode material according to claim 1, characterized in that, Includes the following steps: a) Obtain layered sodium ion oxide Na using solid-state sintering, sol-gel, or co-precipitation methods. 1-x TMO2, where 0 ≤ x < 1; b) The positive electrode material Na 1-x TMO2 is placed in an aprotic solvent solution containing metal salts and auxiliary complexing agents, and the target ions are involved in the construction of the reconstructed layer on the surface of the layered material by heating and stirring. c) The mixture after the above reaction is filtered, washed, and annealed.

3. The preparation method according to claim 2, characterized in that, The aprotic solvent mentioned in step b) is acetone, dimethylformamide, N-methylpyrrolidone, or dimethyl sulfoxide.

4. The preparation method according to claim 2, characterized in that, The metal salt mentioned in step b) is a sulfate, halide, nitrate, phosphate, or double salt that can be dissolved in an organic solvent.

5. The preparation method according to claim 2, characterized in that, The target ion mentioned in step b) is Lu 3+ .

6. The preparation method according to claim 2, characterized in that, The auxiliary complexing agent mentioned in step b) is phytic acid, tannic acid, dopamine, or polyvinylpyrrolidone (PVP) in small or high molecular weight form.

7. The preparation method according to claim 2, characterized in that, The annealing temperature range in step c) is 400–1000°C, the annealing time is 0.5–10 hours, and the annealing atmosphere is selected from air, argon, or oxygen atmosphere.

8. The application of the surface-doped layered battery cathode material according to claim 1 in sodium-ion batteries.

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