A surface-coated and bulk-gradient co-doped sodium-ion battery cathode material, a preparation method thereof and a sodium-ion battery

Abstract

The application discloses a surface-coated and bulk-gradient co-doped sodium ion battery positive electrode material and a preparation method and a sodium ion battery thereof, and belongs to the technical field of sodium ion batteries. The sodium ion battery positive electrode material comprises a layered transition metal oxide and a coating layer coated on the surface of the layered transition metal oxide; the layered transition metal oxide is a nickel-iron-manganese-based layered transition metal oxide, and the coating layer is an amorphous boron oxide coating layer; the chemical formula of the surface-coated and bulk-gradient co-doped sodium ion battery positive electrode material is Na(Ni 1 / 3 Fe 1 / 3 Mn 1 / 3 ) x B y M z O2, 0.9≤x≤0.98, 0.01≤y<0.2, 0.01≤z<0.1, and x+y+z=1, and M is selected from W or Nb; wherein the doping concentration of boron elements and metal M elements presents a gradient decreasing trend from the surface layer to the inside along the radial direction; the surface boron oxide coating and the bulk-gradient doping of boron elements and metal M elements are simultaneously realized; the surface coating layer effectively reduces the interface side reaction, and the bulk-gradient doping stabilizes the crystal structure from the inside; and the two synergistically act to jointly inhibit the structural degradation in the cycle process.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, and specifically relates to a surface-coated and bulk-phase gradient co-doped sodium-ion battery cathode material, its preparation method, and a sodium-ion battery. Background Technology

[0002] Among numerous energy storage technologies, lithium-ion batteries (LIBs) have been widely used in 4C electronic products, new energy vehicles, smart grids, and military special equipment due to their outstanding advantages such as lightweight design, high energy density, and long cycle life. However, the raw material costs for lithium-ion batteries continue to rise. In contrast, sodium-ion batteries (SIBs), with abundant resources and low cost, have become one of the most promising alternative technologies.

[0003] In the cathode material system of sodium-ion batteries, layered transition metal oxides (with the general chemical formula Na) x MO2 (where M represents transition metal elements such as Mn, Ni, Fe, Ti, Cu, and V) has become a current research focus due to its core advantages such as high specific capacity, excellent rate performance, and high energy density. The crystal structure of this type of material exhibits a characteristic layered stacking configuration, with transition metal layers (MO2) and sodium ion layers alternating along specific lattice directions to form a periodic arrangement. It is mainly divided into two major systems: O3 type and P2 type. Among them, the O3 type layered oxide NaNi... 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 has attracted much attention due to its high specific capacity and cost advantages. However, this material suffers from interfacial side reactions and bulk structure degradation during cycling, leading to rapid capacity decay and limiting its practical application. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a surface-coated and bulk-phase gradient co-doped sodium-ion battery cathode material, its preparation method, and a sodium-ion battery. This invention utilizes a surface passivation layer to reduce interfacial side reactions by coating the surface of a layered transition metal oxide with bulk gradient co-doping; and employs bulk doping to suppress transition metal migration and phase transitions, and enhance lattice stability. The synergistic effect of surface coating and bulk doping significantly improves the material's structural stability and electrochemical performance. This invention provides a coated sodium-ion battery cathode material that reduces irreversible phase transitions during cycling, suppresses interfacial side reactions, and improves cycle stability.

[0005] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a surface-coated and bulk-phase gradient co-doped sodium-ion battery cathode material, comprising a layered transition metal oxide and a coating layer coated on the surface of the layered transition metal oxide; wherein the layered transition metal oxide is a nickel-iron-manganese layered transition metal oxide, and the coating layer is an amorphous boron oxide coating layer; The chemical formula of the surface-coated and bulk-gradient co-doped sodium-ion battery cathode material is Na(Ni) 1 / 3 Fe 1 / 3Mn 1 / 3 ) x B y M z O2, where 0.9≤x≤0.98, 0.01≤y<0.2, 0.01≤z<0.1, and x+y+z=1, M is selected from W, Nb, Ti or Zr; where the doping concentration of boron and metal M decreases radially from the surface to the interior.

[0006] The layered transition metal oxide cathode material provided by this invention has a spatial distribution feature of "high surface doping-low bulk doping". Through the synergistic effect of boron oxide surface coating and bulk gradient doping, the layered transition metal oxide cathode material provided by this invention can simultaneously improve the interfacial stability and bulk structure integrity of the material, thereby significantly improving the long-cycle stability and rate performance of sodium-ion batteries.

[0007] Furthermore, the surface-coated and bulk-coated sodium-ion battery cathode material has an O3 structure, belongs to the hexagonal crystal system, and has a space group of R-3m. The surface-coated and bulk-coated sodium-ion battery cathode material also has a layered crystal structure.

[0008] Furthermore, the mass of the coating layer is 1.0% of the mass of the layered transition metal oxide.

[0009] Secondly, the present invention provides a method for preparing the above-mentioned surface-coated and bulk-phase gradient co-doped sodium-ion battery cathode material, comprising the following steps: S1. A nickel-iron-manganese spherical precursor was prepared by a co-precipitation method; S2. The nickel-iron-manganese spherical precursor is uniformly mixed with a sodium source, ground, and calcined to obtain a layered transition metal oxide matrix material; S3. The layered transition metal oxide matrix material is uniformly mixed with boride, ground and calcined to obtain the surface-coated and bulk-gradient co-doped sodium-ion battery cathode material.

[0010] Further, in step S1, the preparation method of the nickel-iron-manganese spherical precursor includes the following steps: mixing a nickel-iron-manganese mixed salt solution and a complexing agent solution, adjusting the pH of the mixed solution to 10.0-10.5, adding a precipitant solution to the mixed solution under stirring conditions, continuing the stirring reaction, and then aging, centrifuging, and drying to obtain the nickel-iron-manganese spherical precursor; in the nickel-iron-manganese mixed salt solution, Ni∶Mn∶Fe=1∶1∶1 in molar amounts; the complexing agent solution is ammonia water; the precipitant solution is sodium hydroxide solution; the total molar amount of nickel, manganese, and iron in the nickel-manganese-iron mixed salt solution and the molar ratio of sodium hydroxide is 1∶2.5.

[0011] Further, in step S2, the sodium source includes at least one of sodium carbonate, sodium acetate, sodium fluoride, sodium nitrate, and sodium hydroxide; the molar ratio of the total molar amount of nickel, manganese, and iron in the nickel-iron-manganese spherical precursor to the molar amount of sodium atoms in the sodium source is 1.05:1; the calcination includes the following steps: under an oxygen or air atmosphere, first heating to 500 ℃ at a heating rate of 2 ℃ / min and maintaining the temperature for 6 h, then heating to 900 ℃ at a heating rate of 5 ℃ / min and maintaining the temperature for 15 h.

[0012] During the calcination process, the pre-calcination stage (500 ℃) is beneficial to promote the pre-reaction of raw materials, eliminate impurities (such as water of crystallization or organic matter), promote the decomposition of precursors and the initial insertion of sodium, and initially form the lattice structure of transition metal oxides, providing a structural basis for subsequent high-temperature crystallization.

[0013] Further, in step S2, the grinding method is ball milling, and the ball milling conditions include: a rotation speed of 150-500 r / min and a time of 2-6 h.

[0014] The layered transition metal oxide matrix material provided by this invention is mainly prepared through two steps: a precursor preparation method via co-precipitation and sodium calcination. The co-precipitation method ensures uniform precursor composition by achieving atomic-level homogeneous mixing of metal elements, laying the foundation for synthesizing highly crystalline, single-phase O3-type layered structures. The preparation method provided by this invention is simple, uses inexpensive raw materials, and is suitable for large-scale industrial production.

[0015] Further, in step S3, the mass of the boride is 1.0% of the mass of the layered transition metal oxide matrix material.

[0016] Further, in step S3, the boride includes at least one of tungsten boride, niobium boride, titanium boride, and zirconium boride; the calcination includes the following steps: under an oxygen atmosphere, the temperature is first raised to 650 ℃ at a heating rate of 5 ℃ / min and held at a constant temperature for 9 h.

[0017] In step S3, high-temperature treatment (650 °C) is crucial for achieving surface coating and bulk gradient doping. During this process, borides (such as niobium boride, tungsten boride, etc.) react with or decompose and oxidize on the substrate surface in an oxygen atmosphere, forming a surface coating layer dominated by boron oxide. Simultaneously, boron and metallic M diffuse from the material surface into the bulk phase under thermal drive. By precisely controlling the calcination temperature and time, a gradient distribution of these elements from high concentration on the surface to low concentration in the bulk phase is achieved, thereby suppressing interfacial side reactions and enhancing the stability of the bulk structure. When the calcination temperature is too high, the surface coating layer may over-crystallize or even react adversely with the substrate, destroying the integrity of the coating layer, and the distribution of bulk doped elements is difficult to control; when the calcination temperature is too low, the reaction kinetics are insufficient, the coating layer is incomplete, element diffusion is restricted, and it is difficult to form an effective concentration gradient. When the calcination time is too short, the element diffusion is insufficient, the gradient distribution is not obvious, and the synergistic effect is weak. When the calcination time is too long, the elements may diffuse excessively into the bulk phase homogenization, lose the gradient characteristics, and may cause excessive grain growth, affecting the electrochemical activity of the material.

[0018] The sodium-ion battery cathode material provided by this invention, which combines surface coating and bulk gradient co-doping, achieves gradient synergistic doping of boron molar concentration and metallic M molar concentration from the surface to the bulk phase using an in-situ sintering process. Through precise occupancy of lattice sites and electronic structure regulation, it suppresses irreversible phase transitions and structural collapse during charging and discharging.

[0019] Further, in step S3, the grinding method is ball milling, and the ball milling conditions include: a rotation speed of 150-500 r / min and a time of 2-6 h.

[0020] This invention improves upon existing technologies by providing a coated sodium-ion battery cathode material. This material improves the cycle performance of sodium-ion batteries by coating the surface of layered transition metal oxides with borides, gradient doping of boron and metal ions in the bulk phase, and reducing phase transitions. Specifically, this invention primarily forms a surface passivation layer by coating the surface of layered transition metal oxides with borides, reducing interfacial side reactions during cycling and improving cycle stability. During in-situ sintering, boron ions and metal M ions penetrate from the surface into the bulk phase. Boron occupies tetrahedral sites in the sodium layer, effectively suppressing the irreversible migration of transition metals to the sodium layer during charging and discharging, blocking irreversible phase transition paths, and effectively inhibiting Na+ phase transitions. +Phase transition during intercalation / delamination: Metal M ions occupy transition metal layer sites, enhancing the bonding strength and lattice stability of the transition metal layer. This prevents layer slippage and structural collapse during charge / discharge, thus maintaining the stability of the transition metal layer. In the engineering application of layered transition metal oxide cathode materials for sodium-ion batteries, addressing key bottlenecks such as irreversible phase transitions under high voltage, intensified interfacial side reactions, and insufficient structural stability, this invention proposes a surface coating-bulk gradient doping synergistic control strategy to significantly improve the material's structural stability and electrochemical performance.

[0021] Thirdly, the present invention provides a sodium-ion battery, comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode comprises a positive electrode material, and the positive electrode material comprises the sodium-ion battery positive electrode material with surface coating and bulk gradient co-doping.

[0022] Compared with the prior art, the present invention has the following advantages and technical effects: (1) This invention achieves simultaneous surface boron oxide coating and bulk boron and metal M element gradient doping through one-step boride calcination treatment. The surface coating layer effectively reduces interfacial side reactions, while the bulk gradient doping stabilizes the crystal structure from the inside. The two work synergistically to suppress structural degradation during cycling. Experimental results show that the capacity retention of this surface-coated and bulk gradient co-doped sodium-ion battery cathode material can reach 90.1% after 100 cycles at 1C.

[0023] (2) The surface-coated and bulk gradient co-doped sodium-ion battery cathode material prepared by the present invention can alleviate the local stress during the sodium ion insertion and extraction process and suppress the phase transition, thereby obtaining a smooth charge-discharge curve and high reversible capacity. Experimental results show that the discharge capacity of this layered transition metal oxide cathode material can reach 143.9-146.2 mAh / g at 1C.

[0024] (3) The preparation method of the surface coating and bulk gradient co-doped sodium-ion battery cathode material provided by the present invention has a simple process, controllable raw material cost, and is suitable for large-scale industrial production. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments 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.

[0026] Figure 1 XRD patterns of sodium-ion battery cathode materials prepared in Examples 1-2 and Comparative Examples 4-5; Figure 2 SEM image of the surface-coated and bulk gradient co-doped sodium-ion battery cathode material prepared in Example 1; Figure 3 STEM image of the surface-coated and bulk gradient co-doped sodium-ion battery cathode material prepared in Example 1; Figure 4 The electron energy loss spectra (EELS) of the sodium-ion battery cathode materials prepared in Example 1 and Comparative Example 2 are shown, where a is the electron energy loss spectrum of Nb element in Example 1, b is the electron energy loss spectrum of B element in Example 1, c is the electron energy loss spectrum of Nb element in Comparative Example 2, and d is the electron energy loss spectrum of B element in Comparative Example 2. Figure 5 The first-week charge-discharge curves of the button batteries prepared in Application Examples 1-2 and Comparative Application Examples 4-5 are shown. Figure 6 To compare the first-week charge-discharge curves of the button batteries prepared in Examples 1-3; Figure 7 The graph shows the long-cycle performance of the button cells prepared in Application Example 1, Application Example 2 and Comparative Application Example 1 at 2.0-4.0 V and 1 C. Figure 8 The graph shows the long-cycle performance of the button cells prepared in Application Example 1 and Comparative Application Examples 6-7 at 2.0-4.0 V and 1 C. Detailed Implementation

[0027] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0028] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0029] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0030] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be obvious to those skilled in the art. This specification and embodiments are merely exemplary.

[0031] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0032] This invention provides a surface-coated and bulk-phase gradient co-doped sodium-ion battery cathode material, comprising a layered transition metal oxide and a coating layer covering the surface of the layered transition metal oxide; the layered transition metal oxide is a nickel-iron-manganese layered transition metal oxide, and the coating layer is an amorphous boron oxide coating layer; wherein, boron and metallic M form a continuous concentration gradient distribution from the surface to the bulk phase in the layered transition metal oxide cathode material, and the doping concentrations of boron and metallic M decrease radially from the surface to the interior; the chemical formula of the surface-coated and bulk-phase gradient co-doped sodium-ion battery cathode material is Na(Ni) 1 / 3 Fe 1 / 3 Mn 1 / 3 ) x B y M z O2, where 0.9≤x≤0.98, 0.01≤y<0.2, 0.01≤z<0.1, and x+y+z=1, and M is selected from W or Nb.

[0033] In some preferred embodiments, the surface-coated and bulk-gradient co-doped sodium-ion battery cathode material has an O3 structure, belongs to the hexagonal crystal system, and has a space group of R-3m.

[0034] In some preferred embodiments, the mass of the coating layer is 1.0% of the mass of the layered transition metal oxide.

[0035] This invention also provides a method for preparing the above-mentioned surface-coated and bulk gradient co-doped sodium-ion battery cathode material, comprising the following steps: S1. A nickel-iron-manganese spherical precursor was prepared by a co-precipitation method; S2. The nickel-iron-manganese spherical precursor is uniformly mixed with a sodium source, ground, and calcined to obtain a layered transition metal oxide matrix material; S3. The layered transition metal oxide matrix material is uniformly mixed with boride, ground and calcined to obtain the surface-coated and bulk-gradient co-doped sodium-ion battery cathode material.

[0036] In some preferred embodiments, step S1, the method for preparing the nickel-iron-manganese spherical precursor includes the following steps: (1) Prepare a mixed salt solution of nickel, iron and manganese, a complexing agent solution and a precipitant solution respectively.

[0037] (2) The reaction vessel is pretreated. After pretreatment, a nickel-iron-manganese mixed salt solution and a complexing agent solution are added to the reaction vessel, and the pH value of the mixed solution is adjusted to 10-10.5. Under stirring conditions, a precipitant solution is added to the reaction vessel by titration. Stirring is continued to allow the reaction to proceed. Then, after aging, centrifugation and drying, spherical nickel-iron-manganese hydroxide precursors are obtained.

[0038] The specific parameters for the nickel-iron-manganese spherical precursor prepared by the co-precipitation method include: the total concentration of metal ions in the nickel-iron-manganese mixed salt solution is 1.0-1.5 mol·L⁻¹. -1 (Example: 1 mol·L) -1 ); using 1.5 mol·L -1 Ammonia was used as a complexing agent; a concentration of 4.0-5.0 mol·L⁻¹ was employed. -1 Sodium hydroxide solution was used as a precipitant (exemplary: 4 mol·L⁻¹). -1 During the reaction, the pH value of the reaction system was precisely controlled within the range of 10.0-10.5 by real-time monitoring and coordinated regulation of the dropping rate of NaOH solution and ammonia water.

[0039] In this embodiment of the invention, the feeding of materials into the reactor is achieved by controlling the feed flow rates of the nickel-iron-manganese mixed salt solution, the complexing agent solution, and the precipitant solution, wherein the feed flow rate of the nickel-iron-manganese mixed salt solution is 0.6-1.8 L·h. -1 The feed flow rate of the complexing agent solution is 0.2-0.6 L·h. -1 The feed flow rate of the precipitant solution is 0.3-1.2 L·h. -1 When adding materials to the reactor, the molar ratio of the metal salt in the nickel-iron-manganese mixed salt solution to the precipitant in the precipitant solution is controlled to be 1:2.5. The reaction conditions include: reacting for 6 h at a stirring speed of 700 r / min and a temperature of 55 ℃.

[0040] In some preferred embodiments, in step S2, the sodium source includes at least one of sodium carbonate, sodium acetate, sodium fluoride, sodium nitrate, and sodium hydroxide.

[0041] In some preferred embodiments, in step S2, the grinding method is ball milling, which is performed using equipment such as a planetary ball mill. The ball milling conditions include: a rotation speed of 150-500 r / min and a time of 2-6 h.

[0042] In some preferred embodiments, step S2, the calcination includes the following steps: placing the mixture in a tube furnace, heating it to 500 °C at a heating rate of 2 °C / min and holding it at a constant temperature for 6 h under an oxygen / air atmosphere, then heating it to 900 °C at a heating rate of 5 °C / min and holding it at a constant temperature for 15 h, then naturally cooling it to 150 °C, removing it and quickly transferring it to a glove box filled with argon gas for storage, to obtain the layered transition metal oxide matrix.

[0043] In some preferred embodiments, in step S3, the boride includes at least one of tungsten boride, niobium boride, titanium boride, and zirconium boride; In some preferred embodiments, in step S3, the grinding method is ball milling, which is performed using equipment such as a planetary ball mill. The ball milling conditions include: a rotation speed of 150-500 r / min and a time of 2-6 h.

[0044] In some preferred embodiments, step S3, the calcination includes the following steps: placing the mixture in a tube furnace, heating it to 650 °C at a heating rate of 5 °C / min under an oxygen atmosphere and maintaining the temperature constant for 9 h, then naturally cooling it to 150 °C, removing it and quickly transferring it to a glove box filled with argon for storage, thus obtaining a sodium-ion battery cathode material with surface coating and bulk gradient co-doping. Choosing an oxygen atmosphere helps ensure that the transition metal maintains a high valence state, stabilizes the O3 layered structure, and promotes the effective conversion of the boride precursor and the formation of the coating layer.

[0045] The present invention achieves the following through co-precipitation and calcination process: (1) an amorphous boron oxide coating layer is formed in situ on the material surface, which serves as a physical barrier to effectively reduce the interfacial side reactions between the electrode material and the electrolyte; (2) boron and metal M elements (such as W and Nb) form a gradient distribution in the bulk phase. B elements suppress the migration of transition metal ions by occupying specific lattice sites, while the gradient-doped metal elements enhance the bonding strength of the transition metal layer. The two work together to stabilize the crystal structure and effectively suppress irreversible phase transitions during charging and discharging.

[0046] This invention also provides a sodium-ion battery, including a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive electrode material, which includes the surface-coated and bulk-gradient co-doped sodium-ion battery positive electrode material.

[0047] Among them, positive electrode material and negative electrode material refer to active materials, namely positive electrode active material and negative electrode active material, which are used to store and release charge during charging and discharging, thereby realizing the electrochemical energy storage and conversion function of the battery.

[0048] In some preferred embodiments, the negative electrode sheet includes a negative electrode material, which includes carbon material or metallic sodium; the electrolyte includes a sodium salt and a solvent, wherein the sodium salt includes at least one of sodium hexafluorophosphate, sodium perchlorate and sodium trifluoromethanesulfonate, and the solvent includes at least one of propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate and fluoroethylene carbonate.

[0049] The room temperature in this invention refers to 25±2 ℃.

[0050] Example 1: A method for preparing a surface-coated and bulk-gradient co-doped sodium-ion battery cathode material. S1. Weigh out nickel sulfate, manganese sulfate, and ferric sulfate in a molar ratio of Ni∶Mn∶Fe = 1∶1∶1. Dissolve the above raw materials in deionized water to prepare a solution with a total molar concentration of nickel, manganese, and ferric sulfate of 1 mol·L⁻¹. -1 A mixed nickel-manganese-iron salt solution A; additionally, deionized water was used to prepare an ammonia molar concentration of 1.5 mol·L⁻¹. -1 The ammonia solution B and sodium hydroxide solution have a molar concentration of 4 mol·L⁻¹. -1 Sodium hydroxide solution C was prepared. A mixed nickel-manganese-iron salt solution A was transferred to a reaction vessel. Under a nitrogen atmosphere, ammonia solution B was added to the reaction vessel to adjust the pH of the mixed solution to 10-10.5. Sodium hydroxide solution C was slowly added to the reaction vessel by titration at a stirring speed of 700 r / min, controlling the molar ratio of total nickel-manganese-iron to sodium hydroxide in the mixed nickel-manganese-iron salt solution to be 1:2.5. The reaction was carried out for 6 h at a stirring speed of 700 r / min and a temperature of 55 ℃. After the reaction, the sample was aged (720 min), filtered, and vacuum dried (80 ℃, 720 min) to obtain Ni. 1 / 3 Fe 1 / 3Mn 1 / 3 (OH)2 is the spherical precursor of nickel-iron-manganese hydroxide; S2. The nickel-iron-manganese hydroxide spherical precursor prepared in step S1 is uniformly mixed with a sodium source at a molar ratio of 1.05:1 (total molar amount of nickel, manganese, and iron to sodium atoms). The mixture is ball-milled for 4 h at 400 r / min to obtain a mixture. The mixture is placed in a tube furnace and heated to 500 °C at a heating rate of 2 °C / min under an oxygen atmosphere and held at that temperature for 6 h. Then, the temperature is increased to 900 °C at a heating rate of 5 °C / min and held at that temperature for 15 h. The mixture is then allowed to cool naturally to 150 °C, removed, and quickly transferred to an argon-filled glove box for storage to obtain the layered transition metal oxide matrix material NaNi. 1 / 3 Fe 1 / 3 Mn 1 / 3 O2.

[0051] S3. 5 g of the layered transition metal oxide matrix material NaNi prepared in S2 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 and 0.05 g NbB2 (niobium boride) were mixed uniformly; the mixture was ball-milled for 4 h at 400 r / min to obtain a final mixture. This mixture was then placed in a tube furnace and heated to 650 °C at a rate of 5 °C / min under an oxygen atmosphere, and sintered at 650 °C for 9 h. After natural cooling to 150 °C, the mixture was removed and quickly transferred to an argon-filled glove box for storage. This yielded a surface-coated and bulk-gradient co-doped sodium-ion battery cathode material, Na(Ni) 1 / 3 Fe 1 / 3 Mn 1 / 3 ) x B y Nb z O2, where x=0.97, y=0.02, z=0.01.

[0052] Example 2: A method for preparing a sodium-ion battery cathode material with surface coating and bulk gradient co-doping, comprising the following steps: S1. Same as Example 1; S2. Same as Example 1; S3. 5 g of the layered transition metal oxide matrix material NaNi prepared in S2 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 and 0.05 g BW (tungsten boride) were mixed uniformly; the mixture was ball-milled for 4 h at 400 r / min to obtain a final mixture. The mixture was then heated to 650 °C at a rate of 5 °C / min under an oxygen atmosphere and sintered at 650 °C for 9 h. After natural cooling to 150 °C, the mixture was removed and quickly transferred to an argon-filled glove box for storage. This yielded a surface-coated and bulk-gradient co-doped sodium-ion battery cathode material, Na(Ni)1 / 3 Fe 1 / 3 Mn 1 / 3 ) x B y W z O2, where x=0.98, y=0.01, z=0.01.

[0053] Comparative Example 1 S1. Weigh out nickel sulfate, manganese sulfate, and ferric sulfate in a molar ratio of Ni∶Mn∶Fe = 1∶1∶1. Dissolve the above raw materials in deionized water to prepare a solution with a total molar concentration of nickel, manganese, and ferric sulfate of 1 mol·L⁻¹. -1 A mixed nickel-manganese-iron salt solution A; additionally, deionized water was used to prepare an ammonia molar concentration of 1.5 mol·L⁻¹. -1 Ammonia solution B is used to prepare a sodium hydroxide solution with a molar concentration of 4 mol·L⁻¹. -1 Sodium hydroxide solution C was prepared; nickel-manganese-iron mixed salt solution A was transferred to a reaction vessel, and ammonia solution B was added to the reaction vessel under a nitrogen atmosphere. The pH value of the mixed solution was adjusted to 10-10.5 using ammonia solution B. Sodium hydroxide solution C was slowly added to the reaction vessel by titration at a stirring speed of 700 r / min. The molar ratio of total nickel-manganese-iron to sodium hydroxide in the nickel-manganese-iron mixed salt solution was controlled to be 1:2.5. The reaction was carried out at a stirring speed of 700 r / min and a temperature of 55 ℃ for 6 h. After the reaction was completed, the sample was aged (720 min), filtered, and vacuum dried (80 ℃, 720 min) to obtain spherical nickel-iron-manganese hydroxide precursor. S2. The nickel-iron-manganese hydroxide spherical precursor prepared in step S1 was uniformly mixed with a sodium source at a molar ratio of 1.05:1 and ball-milled for 4 h at a speed of 400 r / min to obtain a mixture. The mixture was placed in a tube furnace and heated to 500 ℃ at a heating rate of 2 ℃ / min under an oxygen atmosphere and held at that temperature for 6 h. Then, the temperature was increased to 900 ℃ at a heating rate of 5 ℃ / min and held at that temperature for 15 h. The mixture was then allowed to cool naturally to 150 ℃, removed, and quickly transferred to an argon-filled glove box for storage to obtain the layered transition metal oxide matrix material NaNi. 1 / 3 Fe 1 / 3 Mn 1 / 3 O2.

[0054] Comparative Example 2 This comparative example provides a cathode material prepared by a solid-state method in which boron and tungsten elements are uniformly distributed in the bulk phase. The preparation method is as follows: According to the chemical formula Na(Ni) 1 / 3 Fe 1 / 3 Mn 1 / 3 ) 0.98 B 0.01 W0.01 The stoichiometric ratio of O2 is determined by weighing Na2CO3 (sodium carbonate) and Ni. 1 / 3 Fe 1 / 3 Mn 1 / 3 Using (OH)₂ (nickel-iron-manganese hydroxide) and BW (tungsten boride) as raw materials, all raw materials were thoroughly mixed in one go and ball-milled for 4 h at a speed of 400 r / min to obtain a mixture. Subsequently, the uniformly mixed material was calcined in an oxygen atmosphere: the temperature was increased to 500 ℃ at a heating rate of 2 ℃ / min and held at that temperature for 6 h, and then increased to 900 ℃ at a heating rate of 5 ℃ / min and held at that temperature for 15 h. The prepared sample showed that B and W elements were uniformly distributed in the particulate bulk phase, without a concentration gradient from the surface to the interior.

[0055] Comparative Example 3 This comparative example provides a cathode material with only a surface boron oxide coating layer and no significant bulk boron gradient doping.

[0056] S1. Same as Example 1; S2. Same as Example 1; S3. 5 g of the layered transition metal oxide matrix material NaNi prepared in S2 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 and 0.05 g H3BO3 (boric acid) were mixed evenly; the mixture was ball-milled for 4 h at a speed of 400 r / min to obtain a mixture. The mixture was heated to 650 ℃ at a heating rate of 5 ℃ / min under an oxygen atmosphere and sintered at 650 ℃ for 9 h. The mixture was then cooled naturally to 150 ℃, removed and quickly transferred to a glove box filled with argon for storage, resulting in a cathode material with only surface boron oxide coating and no significant bulk boron gradient doping.

[0057] This comparative example is identical to Example 1 in steps S1 and S2, but in step S3, boric acid was used as the boron source instead of the reducing boron source (such as oxygen-deficient boron oxide or metal borides) used in Example 1. Boric acid decomposes at high temperatures to generate B2O3, which is chemically inert and non-reducing, and cannot undergo a significant redox reaction with the matrix material. Therefore, it cannot create effective vacancy defects (such as oxygen vacancies or transition metal vacancies) on the material surface. Lacking these defects as diffusion channels, boron atoms can only remain on the material surface to form a coating layer and cannot diffuse into the bulk phase, thus obtaining a sample that "only has a surface coating without significant bulk boron gradient doping".

[0058] Comparative Example 4 S1. Same as Example 1; S2. Same as Example 1; S3. 5 g of the layered transition metal oxide matrix material NaNi prepared in S2 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 and 0.05 g NbB2 (niobium boride) were mixed uniformly; the mixture was heated to 650 °C at a rate of 5 °C / min in air, and sintered at 650 °C for 9 h. After natural cooling to 150 °C, the mixture was removed and quickly transferred to an argon-filled glove box for storage, thus preparing the surface-coated and bulk-gradient co-doped sodium-ion battery cathode material Na(Ni) 1 / 3 Fe 1 / 3 Mn 1 / 3 ) x B y Nb z O2, where x=0.97, y=0.02, z=0.01.

[0059] Comparative Example 5 S1. Same as Example 2; S2. Same as Example 2; S3. 5 g of the layered transition metal oxide matrix material NaNi prepared in S2 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 and 0.05 g BW (tungsten boride) were mixed uniformly; the mixture was heated to 650 °C at a rate of 5 °C / min in air, and sintered at 650 °C for 9 h. The mixture was then allowed to cool naturally to 150 °C, removed, and quickly transferred to an argon-filled glove box for storage. This yielded a surface-coated and bulk-gradient co-doped sodium-ion battery cathode material, Na(Ni) 1 / 3 Fe 1 / 3 Mn 1 / 3 ) x B y W z O2, where x=0.98, y=0.01, z=0.01.

[0060] Comparative Example 6 Same as Example 1, except that the calcination temperature in S3 is 450°C. Comparative Example 7 Same as Example 1, except that the calcination temperature in S3 is 850°C. Material characterization test The samples obtained in Examples 1-2 and Comparative Examples 1-5 were characterized.

[0061] (1) X-ray diffraction (XRD) test Figure 1 The images show the XRD patterns of the sodium-ion battery cathode materials prepared in Examples 1-2 and Comparative Examples 4-5. Figure 1As can be seen from the data, the sodium-ion battery cathode materials prepared in Examples 1-2 and Comparative Examples 4-5 exhibit the R-3m space group of the O3 phase structure, which belongs to the hexagonal crystal system.

[0062] (2) Scanning electron microscopy (SEM) test Figure 2 This is a SEM image of the surface-coated and bulk gradient co-doped sodium-ion battery cathode material prepared in Example 1. Figure 2 It can be seen that the surface-coated and bulk-gradient co-doped sodium-ion battery cathode material prepared in Example 1 has a spherical morphology.

[0063] (3) Scanning transmission electron microscopy (STEM) test Figure 3 This is a STEM image of the surface-coated and bulk gradient co-doped sodium-ion battery cathode material prepared in Example 1. Figure 3 As can be seen from the data, the bulk phase of this sodium-ion battery cathode material, which is co-doped with the surface coating and the bulk phase gradient, is an O3 phase structure, and the surface is a coating layer.

[0064] (4) Electron Energy Loss Spectroscopy (EELS) test Figure 4 The electron energy loss spectra (EELS) of the sodium-ion battery cathode materials prepared in Example 1 and Comparative Example 2 are shown below. In Example 1, a is the electron energy loss spectrum of Nb, b is the electron energy loss spectrum of B, c is the electron energy loss spectrum of Nb, and d is the electron energy loss spectrum of B in Comparative Example 2. Figure 4 As can be seen from a and b, the distribution of B and Nb elements in the layered transition metal oxide cathode material prepared in Example 1 gradually weakens from the surface to the bulk phase, exhibiting a gradient distribution. This gradient distribution can alleviate local stress and suppress layer slip and structural collapse during charging and discharging. The cathode material of Comparative Example 2, after being characterized in the same way, proves that the doped elements are uniformly distributed in the bulk phase in the material prepared by the solid-state method.

[0065] Application examples and performance characterization tests Application Example 1 Preparation of the positive electrode: The sodium-ion battery positive electrode material prepared in Example 1, conductive carbon black (SuperP), and binder polyvinylidene fluoride (PVDF) were weighed at a mass ratio of 8:1:1 and uniformly dispersed in N-methylpyrrolidone (NMP) solvent to obtain a mixed slurry. The obtained mixed slurry was uniformly coated on the positive electrode current collector aluminum foil, vacuum dried (720 min), and then cut into circular positive electrode sheets with a diameter of 12 mm. Assembly of the sodium-ion battery: The CR2025 button cell was assembled, using the above-mentioned circular positive electrode sheet as the positive electrode, a metallic sodium sheet as the negative electrode, and 1 mol / L NaClO4 as the electrolyte (FEC EC:PC = 1:1:1 Vol%) in the mixed solvent. Together with other necessary battery components (separator and casing, etc.), the button cell was assembled in a glove box filled with high-purity argon gas.

[0066] Application Example 2 Similar to Application Example 1, except that the sodium-ion battery cathode material prepared in Example 1 is replaced with the sodium-ion battery cathode materials prepared in Examples 2-4.

[0067] Compare and contrast examples 1-7 Similar to Application Example 1, except that the sodium-ion battery cathode material prepared in Example 1 was replaced with the sodium-ion battery cathode materials prepared in Comparative Examples 1-7.

[0068] The batteries assembled from Application Examples 1-2 and Comparative Application Examples 1-7 were subjected to performance testing within a voltage window of 2.0-4.0 V. Specifically, charge-discharge performance tests were performed in a battery testing system, including: the first charge-discharge test and a 100-cycle test. The test temperature was 25 °C, and the rate was 1 C, where 1 C = 150 mA·g. -1 The voltage window is 2.0~4.0 V. The performance test results of the button batteries prepared in Application Example 1-2 and Comparative Application Example 1-7 are shown in Table 1 below. The first-week charge-discharge curves of the button batteries prepared in Application Example 1-2 and Comparative Example 4-5 are shown below. Figure 5 As shown, the first-week charge-discharge curves of the button batteries prepared in Examples 1-3 are compared. Figure 6 As shown.

[0069] Table 1 As shown in Table 1, the first charge / discharge capacity data of Comparative Application Example 1 (using the layered transition metal oxide matrix material in Comparative Example 1) exhibits the highest first charge / discharge capacity (160 mAh·g). -1 Discharge 147 mAh·g -1This aligns with fundamental principles of materials science: unmodified materials possess more complete electrochemical active sites and lower ion diffusion barriers, thus exhibiting optimal initial electrochemical activity. However, according to the consensus on layered oxide materials for sodium-ion batteries, this unmodified NaNi... 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 materials face serious interfacial side reactions and bulk structure degradation during cycling, resulting in poor long-term cycling stability.

[0070] Of all the modified materials, Example 1 (NbB2, oxygen atmosphere) and Example 2 (WB, oxygen atmosphere) achieved 146.2 mAh·g, respectively. -1 and 143.9 mAh·g -1 The initial discharge capacity data is of significant scientific importance. Notably, the two samples prepared in an oxygen atmosphere exhibited significantly better capacity performance than their corresponding samples prepared in an air atmosphere (Comparative Example 4 and Comparative Example 5). This phenomenon can be explained by the principles of materials thermodynamics: the high oxygen partial pressure environment provided by the oxygen atmosphere not only ensured the full conversion of the boride precursor into an amorphous B₂O₃ coating layer, but more importantly, it provided the necessary thermodynamic driving force for the directional diffusion of B and Nb or W elements into the bulk phase of the material, thus forming a more ideal functionally graded structure.

[0071] The experimental data from Comparative Application Example 2 (a cathode material with uniformly distributed boron and tungsten elements in the bulk phase prepared using Comparative Example 2) and Comparative Application Example 3 (a cathode material with only a surface boron oxide coating layer and no significant bulk boron gradient doping prepared using Comparative Example 3) provide important comparative references for this invention. The initial discharge capacities of these two comparative application examples (141.1 and 143.7 mAh·g, respectively) -1 Although the capacity was lower than that of Comparative Application Example 1, it remained at a high level. This phenomenon reveals a typical trade-off in material modification: surface coating and bulk doping introduce certain kinetic barriers, leading to a slight decrease in initial capacity, but this is a necessary price to pay for long-term cycling stability. Of particular note is that the capacity performance of Comparative Application Example 3 was slightly better than that of Comparative Application Example 2, suggesting that in short-term electrochemical performance, the contribution of interfacial stability may be more significant than that of uniform bulk doping.

[0072] Figure 7 The graph shows the long-cycle performance of the button cells prepared in Application Example 1, Application Example 2, and Comparative Application Example 1 at 2.0-4.0 V and 1 C. (By...) Figure 7As can be seen, Application Example 1 achieved a capacity retention of 90.1% after 100 cycles, leading to a more profound conclusion: simply pursuing high initial capacity is not the ultimate goal of material design; the key is to achieve excellent structural stability while maintaining appropriate capacity. Although Application Examples 1 and 2 had slightly lower initial capacities than Comparative Application Example 1, they achieved a qualitative leap in capacity retention and cycle life through the carefully designed "surface coating-bulk gradient doping" synergistic structure of this invention.

[0073] Figure 8 The graph shows the long-cycle performance of the button cells prepared in Application Example 1 and Comparative Application Examples 6-7 at 2.0-4.0 V and 1 C. Figure 8 It can be seen that the material prepared using the preferred process of this invention (Application Example 1) exhibits the highest reversible capacity, optimal coulombic efficiency, and best long-term cycling stability. In contrast, Application Example 6 (calcination temperature too low), although having a slightly higher initial charge capacity than Application Example 7 (calcination temperature too high), shows significantly lower coulombic efficiency, faster capacity decay, and the worst long-term cycling stability. This indicates that while excessively low temperatures may retain more active capacity, incomplete surface coating and the lack of a bulk gradient lead to intensified interfacial side reactions and structural instability; while excessively high temperatures result in capacity reduction and cycling degradation due to coating layer damage and excessive bulk diffusion. Neither can achieve the stable electrochemical performance achieved through surface-bulk phase synergistic optimization as in Application Example 1.

[0074] From a materials engineering perspective, these electrochemical data strongly validate the core innovation of this invention: by selecting specific borides (NbB2 or WB) and using a precisely controlled oxygen atmosphere process, a synergistic enhancement structure of surface coating and bulk gradient doping was successfully constructed. This structural design not only solves the interfacial instability problem, but more importantly, it suppresses phase transitions and structural degradation during cycling at the material bulk level, achieving optimized improvement in overall electrochemical performance.

[0075] It is worth noting that the experimental results of Comparative Examples 2 and 3 demonstrate the unique value of the technical solution of the present invention from the opposite perspective. That is, neither uniform doping nor simple surface coating can achieve the synergistic enhancement effect described in the present invention. This provides important theoretical guidance and technical direction for the design of future sodium-ion battery cathode materials.

[0076] In summary, the high-entropy cathode material and its preparation method provided in this application can maintain a high capacity while suppressing phase transitions during cycling and improving cycle stability.

[0077] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A surface-coated and bulk-gradient co-doped sodium-ion battery cathode material, characterized in that, It includes a layered transition metal oxide and a coating layer covering the surface of the layered transition metal oxide; the layered transition metal oxide is a nickel-iron-manganese layered transition metal oxide, and the coating layer is an amorphous boron oxide coating layer; The chemical formula of the surface-coated and bulk-gradient co-doped sodium-ion battery cathode material is Na(Ni) 1 / 3 Fe 1 / 3 Mn 1 / 3 ) x B y M z O2, where 0.9≤x≤0.98, 0.01≤y<0.2, 0.01≤z<0.1, and x+y+z=1, M is selected from W, Nb, Ti or Zr; wherein the doping concentration of boron and metal M decreases radially from the surface to the interior.

2. The surface-coated and bulk gradient co-doped sodium-ion battery cathode material according to claim 1, characterized in that, The surface-coated and bulk-gradient co-doped sodium-ion battery cathode material has an O3 structure, belongs to the hexagonal crystal system, and has a space group of R-3m.

3. The surface-coated and bulk gradient co-doped sodium-ion battery cathode material according to claim 1, characterized in that, The mass of the coating layer is 1.0% of the mass of the layered transition metal oxide.

4. A method for preparing a sodium-ion battery cathode material with surface coating and bulk gradient co-doping as described in any one of claims 1-3, characterized in that, Includes the following steps: S1. A nickel-iron-manganese spherical precursor was prepared by a co-precipitation method; S2. The nickel-iron-manganese spherical precursor is uniformly mixed with a sodium source, ground, and calcined to obtain a layered transition metal oxide matrix material; S3. The layered transition metal oxide matrix material is uniformly mixed with boride, ground and calcined to obtain the surface-coated and bulk-gradient co-doped sodium-ion battery cathode material.

5. The preparation method according to claim 4, characterized in that, In step S1, the preparation method of the nickel-iron-manganese spherical precursor includes the following steps: mixing a nickel-iron-manganese mixed salt solution and a complexing agent solution, adjusting the pH of the mixed solution to 10.0-10.5, adding a precipitant solution to the mixed solution under stirring conditions, continuing the stirring reaction, and then aging, centrifuging, and drying to obtain the nickel-iron-manganese spherical precursor; in the nickel-iron-manganese mixed salt solution, Ni∶Mn∶Fe=1∶1∶1 in molar amounts; the complexing agent solution is ammonia water; the precipitant solution is sodium hydroxide solution; the total molar amount of nickel, manganese, and iron in the nickel-manganese-iron mixed salt solution and the molar ratio of sodium hydroxide is 1∶2.

5.

6. The preparation method according to claim 4, characterized in that, In step S2, the sodium source includes at least one of sodium carbonate, sodium acetate, sodium fluoride, sodium nitrate, and sodium hydroxide.

7. The preparation method according to claim 4, characterized in that, In step S2, the calcination includes the following steps: under an oxygen or air atmosphere, first heat the temperature to 500 ℃ at a heating rate of 2 ℃ / min and maintain the temperature for 6 h, then heat the temperature to 900 ℃ at a heating rate of 5 ℃ / min and maintain the temperature for 15 h.

8. The preparation method according to claim 4, characterized in that, In step S3, the mass of the boride is 1.0% of the mass of the layered transition metal oxide matrix material.

9. The preparation method according to claim 4, characterized in that, In step S3, the boride includes at least one of tungsten boride, niobium boride, titanium boride, and zirconium boride; the calcination includes the following steps: Under an oxygen atmosphere, the temperature was first increased to 650 °C at a heating rate of 5 °C / min and then held at a constant temperature for 9 h.

10. A sodium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, and an electrolyte. The positive electrode includes a positive electrode material, which includes the surface-coated and bulk-gradient co-doped sodium-ion battery positive electrode material according to any one of claims 1-3.

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