Sodium-ion battery positive electrode material, preparation method thereof and sodium-ion battery

Abstract

The invention provides a sodium ion battery positive electrode material. The positive electrode material comprises an inner core and a coating layer arranged on the surface of the inner core, the chemical general formula of the inner core is Na2Fe < 1-x > M < x > P2O7; wherein M is a doped metal element, and X is greater than or equal to 0.001 and less than or equal to 0.1; the coating layer comprises a carbon material, or the surface of the coating layer is coated with a carbon layer. Metal elements are doped in Na2FeP2O7, and a coating layer of an inner core contains a carbon material or the surface of the coating layer is coated with a carbon layer; the introduction of metal elements can effectively reduce the band gap of Na2FeP2O7, reduce the diffusion energy barrier of sodium ions, and improve the electronic conductivity and ion diffusion rate of the material. Besides, the doped metal elements can also inhibit the positive electrode material from generating unfavorable phase change in the circulation process, so that the circulation stability and the overall electrochemical performance of the material are remarkably improved, the overall conductivity of the material can be further improved by the coated carbon layer, and the effective coating thickness can prevent electrolyte in the battery from corroding the positive electrode material.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, specifically to a sodium-ion battery cathode material, its preparation method, and a sodium-ion battery containing the cathode material. Background Technology

[0002] With the growing prominence of the global energy crisis and environmental issues, new energy storage technologies have received widespread attention. Sodium-ion batteries, due to the abundance and wide distribution of sodium resources, their low cost, and their similar working principle to lithium-ion batteries, have shown great application potential in large-scale energy storage, low-speed electric vehicles, and other fields, making them one of the current research hotspots in the new energy field. As a core component of sodium-ion batteries, the cathode material's performance directly determines key indicators such as energy density, cycle life, and rate performance. Currently, researched cathode materials for sodium-ion batteries mainly include layered oxides, polyanionic compounds, and Prussian blue analogs. Among these, polyanionic iron-based phosphate materials are favored by researchers due to their advantages such as structural stability, environmental friendliness, and low cost. However, traditional iron-based phosphate cathode materials suffer from poor electronic conductivity and low sodium-ion migration rates. Therefore, there is significant room for improvement in the battery's rate performance and cycle stability at high rates to suit ultra-high rate applications. To address these shortcomings, existing technologies often employ doping or coating methods to modify the materials. For example, metal doping can adjust the material's crystal structure to improve ion diffusion efficiency; carbon coating can construct electron transport channels to enhance conductivity. However, in existing technologies, the selection of doping elements and the control of doping amounts are not precise enough, and the thickness and structural design of the carbon coating layer are unreasonable. This often makes it difficult to simultaneously achieve synergistic optimization of electronic conductivity, ion migration rate, and structural stability, failing to meet the application requirements of sodium-ion batteries for high conductivity and high-rate cycle stability. Therefore, developing a sodium-ion battery cathode material with high conductivity and high-rate cycle stability is of significant practical importance. Summary of the Invention

[0003] Therefore, it is necessary to provide a sodium-ion battery cathode material with high conductivity and high cycle stability at high rates.

[0004] This invention provides a sodium-ion battery cathode material, the cathode material comprising a core and a coating layer disposed on the surface of the core; the core has the general chemical formula Na₂Fe. 1-x M xP2O7; where M is the doped metal element, 0.001≤X≤0.1; the coating layer contains carbon material, or the surface of the coating layer is coated with a carbon layer. When X=0, the cathode material is the undoped bulk, i.e., Na2FeP2O7. The cathode material of the present invention is equivalent to doping Na2FeP2O7 with metal elements, and the coating layer of the core contains carbon material or the surface of the coating layer is coated with a carbon layer. The introduction of metal elements can effectively reduce the band gap of Na2FeP2O7, reduce the diffusion barrier of sodium ions, and improve the electronic conductivity and ion diffusion rate of the material. In addition, the doped metal elements can also suppress the adverse phase transition of the cathode material during cycling, thereby significantly improving the cycle stability and overall electrochemical performance of the material. Moreover, the carbon coating layer can further improve the overall electronic conductivity of the material, and the effective coating thickness can prevent the electrolyte in the battery from corroding the cathode material. Furthermore, the range of doping metal elements was chosen to be 0.001 ≤ X ≤ 0.1, primarily to balance effectively improving material performance while avoiding the negative effects of excessive doping. Within this range, the crystal structure is effectively stabilized, harmful phase transitions during charging and discharging are suppressed, and the electronic structure and cell parameters are significantly optimized to promote bulk sodium ion diffusion. Secondly, appropriate doping can improve the intrinsic electronic conductivity of the material and reduce polarization during charging and discharging. Then, inefficient or harmful doping should be avoided. If the doping amount is too low, the electronic effect is too weak, resulting in negligible modification; if it is too high, excessive doping may lead to lattice distortion or the formation of impurity phases, causing a decrease in capacity and structural instability.

[0005] Preferably, the doped metal element M is selected from Cu, Ni, Co, Ag, Zn, Ti, V, and Mn. Cu is preferred for doping because Cu... 2+ The ionic radius is smaller than that of Fe. 2+ Because of their similar chemical properties, Cu 2+ It can replace Fe 2+ Cu enters the crystal lattice and forms a relatively stable crystal structure without changing the valence states of other elements. Appropriate concentrations of Cu doping can induce optimization of local cell parameters in Na₂FeP₂O₇ without affecting the overall crystal structure, thereby improving the overall electrochemical performance of sodium batteries.

[0006] Preferably, the thickness of the coating layer is 3nm-10nm. If the coating layer is too thin, its conductivity is limited; if the coating layer is too thick, it hinders the diffusion of sodium ions into the core. A coating layer thickness of 3nm-10nm ensures good conductivity without significantly hindering sodium ion diffusion.

[0007] Preferably, the carbon material or carbon layer is selected from one or a mixture of two of amorphous carbon, graphite, carbon nanotubes, graphene, and few-layer graphene.

[0008] Preferably, the sodium-ion battery cathode material satisfies at least one of the following conditions: (1) The particle size of the positive electrode material is 20 nm ~ 200 nm; (2) The specific surface area of ​​the cathode material is 29 m². 2 / g -31.5m 2 / g; (3) The ID / IG ratio of the cathode material is 2.36-2.40.

[0009] Reducing the particle size of sodium-ion battery cathode materials to 20nm~200nm can greatly shorten the migration distance of sodium ions from the inside of the particles to the surface, thereby accelerating the charging and discharging speed and improving the rate performance.

[0010] The specific surface area of ​​the sodium-ion battery cathode material was designed to be 29 m². 2 / g -31.5m 2 / g, the specific surface area is directly related to the contact area between the electrode and the electrolyte. Ensuring a sufficient electrode / electrolyte contact interface allows for full wetting of the electrode material, providing a broad "channel" for the rapid migration of sodium ions, and offering more reaction interfaces for the insertion and extraction of sodium ions, further supporting the battery's high power output and fast charging capability.

[0011] The ID / IG ratio of the sodium-ion battery cathode material was designed to be 2.36-2.40. ID / IG is an important parameter in Raman spectroscopy, derived from the intensity ratio of the D and G peaks in carbon materials. The ID / IG ratio reflects the degree of disorder in the material; a higher ratio indicates more structural defects and a higher degree of disorder. Disordered carbon structures generally possess better mechanical toughness, better adapting to the volume expansion and contraction caused by sodium ion insertion / extraction, thereby improving the material's structural stability and cycle life. However, an excessively high ratio can weaken the electronic conductivity of carbon materials.

[0012] Preferably, the lattice parameter a of the positive electrode material is 6.438 Å to 6.448 Å; and / or, the lattice parameter b of the positive electrode material is 9.435 Å to 9.455 Å.

[0013] Na₂FeP₂O₇ is a triclinic crystal system. The lattice parameters a and b describe the unit length of this three-dimensional crystal structure in two of the three directions. a, b, and c together define the size of an open three-dimensional sodium ion migration channel. Defining the lattice parameters within this range offers the core advantage of providing optimized sodium ion diffusion channels, ensuring the best sodium ion migration rate, and thus supporting excellent rate performance.

[0014] This invention also provides a method for preparing any of the above-mentioned sodium-ion battery cathode materials, comprising the following steps: Step 1: Add anhydrous sodium dihydrogen phosphate (NaH2PO4), ferrous oxalate dihydrate (FeC2O4·2H2O), metal salt, and glucose (C6H2PO4). 12 Mix O6·H2O and add solvent. Step 2: The above mixture is ball-milled to obtain a primary precursor material; Step 3: After the above-mentioned precursor material is dried, it is calcined twice to obtain the cathode material with a core and a surface coating layer.

[0015] The products prepared by this method have good uniformity and high purity; their structure and morphology are controllable; the process is relatively mature; and the raw material cost is low, making it very suitable for large-scale commercial production.

[0016] Preferably, in step one, the metal salt is one or more of oxalate, sulfate, acetate, and nitrate.

[0017] Preferably, in step three, the temperatures of the two calcinations are 300-400 ℃ and 600-700 ℃, respectively.

[0018] The present invention also provides a sodium-ion battery comprising any of the above-mentioned cathode materials. Detailed Implementation

[0019] To facilitate understanding of the present invention, a more comprehensive description of the present invention will be provided below.

[0020] As a core component of sodium-ion batteries, the performance of cathode materials directly determines key indicators such as energy density, cycle life, and rate performance of the battery.

[0021] Some embodiments of the present invention provide a sodium-ion battery cathode material, the cathode material comprising a core and a coating layer disposed on the surface of the core; the core has the general chemical formula Na₂Fe. 1-x M xP2O7; where M is the doped metal element, and 0.001≤X≤0.1; the coating layer contains carbon material, or the surface of the coating layer is coated with a carbon layer. In this embodiment, by doping Na2FeP2O7 with a metal element, the introduction of the metal element can effectively reduce the band gap of Na2FeP2O7, reduce the diffusion barrier of sodium ions, and improve the electronic conductivity and ion diffusion rate of the material. The range of the doped metal element is selected as 0.001≤X≤0.1, which mainly balances the relationship between effectively improving the material performance and avoiding the negative effects of excessive doping. Within this range, the crystal structure can be effectively stabilized, harmful phase transitions during charging and discharging can be suppressed, and the electronic structure and cell parameters can be significantly optimized to promote the diffusion of sodium ions. Secondly, an appropriate amount of doping can improve stability and conductivity without significantly sacrificing the reversible capacity of the material. Then, inefficient or harmful doping should be avoided. If the doping amount is too low, the modification effect is negligible; if it is too high, excessive doping may lead to a reduction in active sites, lattice distortion or the generation of impurity phases, which may cause a decrease in capacity and structural instability.

[0022] The specific value of X can be 0.001, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09 or 0.1.

[0023] In addition, the coating layer containing carbon materials or the coating layer surface being coated with a carbon layer can further improve the overall conductivity of the material, and the effective coating thickness can prevent the electrolyte in the battery from corroding the positive electrode material.

[0024] The doped metal element M can be selected from one of Cu, Ni, Co, Ag, Zn, Ti, V, and Mn. Cu is preferred because Cu... 2+ The ionic radius is slightly smaller than that of Fe. 2+ Furthermore, both have similar chemical properties, Cu 2+ It can replace Fe 2+ Cu enters the crystal lattice and forms a relatively stable crystal structure without changing the valence states of other elements. Appropriate concentrations of Cu doping can induce optimization of local cell parameters in Na₂FeP₂O₇ without affecting the overall crystal structure, thereby improving the overall electrochemical performance of sodium batteries.

[0025] In some embodiments, the thickness of the coating layer is controlled between 3 nm and 10 nm (inclusive). If the coating layer is too thin, for example, less than 3 nm, the conductivity is limited; if the coating layer is too thick, for example, greater than 10 nm, it affects the diffusion of sodium ions into the core. Therefore, controlling the thickness of the coating layer between 3 nm and 10 nm ensures good conductivity without significantly hindering the diffusion of sodium ions.

[0026] The thickness of the coating layer can be 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm or 10nm.

[0027] In some embodiments, the carbon material or carbon layer may be selected from one or a mixture of two of amorphous carbon, graphite, carbon nanotubes, graphene, and few-layer graphene.

[0028] In some embodiments, the particle size of the sodium-ion battery cathode material is controlled to be 20 nm to 200 nm. This significantly shortens the migration distance of sodium ions from the interior of the particles to the surface, thereby accelerating the charge and discharge speed. It also improves rate performance and provides more reactive sites, offering more reaction interfaces for the insertion and extraction of sodium ions.

[0029] The specific particle size can be 20nm, 30nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 110nm, 120nm, 130nm, 140nm, 150nm, 160nm, 170nm, 180nm, 190nm, or 200nm.

[0030] The specific surface area of ​​the sodium-ion battery cathode material was designed to be 29 m². 2 / g -31.5m 2 / g, specific surface area directly relates to the contact area between the electrode and the electrolyte. Ensuring a sufficient electrode / electrolyte contact interface allows for thorough wetting of the electrode material, providing a broad "channel" for the rapid migration of sodium ions. The large contact area enables sodium ions to react rapidly at numerous sites on the material surface simultaneously, further supporting the battery's high power output and fast charging capabilities.

[0031] The specific surface area value mentioned above can be taken as 29m². 2 / g, 29.5m 2 / g、30m 2 / g, 30.5m 2 / g、31m 2 / g or 31.5m 2 / g.

[0032] The ID / IG ratio of the sodium-ion battery cathode material was designed to be 2.36-2.40. ID / IG is an important parameter in Raman spectroscopy, derived from the intensity ratio of the D and G peaks in carbon materials. The ID / IG ratio reflects the degree of disorder in the material; a higher ratio indicates more structural defects and a higher degree of disorder. Disordered carbon structures typically possess abundant sodium-ion channels, facilitating rapid insertion and extraction of sodium ions. However, structural defects also lead to electron scattering; therefore, an excessively high ratio can reduce electronic conductivity.

[0033] The specific values ​​for ID / IG can be 2.36, 2.37, 2.38, 2.39, or 2.4.

[0034] In some embodiments, the lattice parameter a of the sodium-ion battery cathode material is designed to be 6.438 Å ~ 6.448 Å, and the lattice parameter b is designed to be 9.435 Å ~ 9.455 Å. Na₂FeP₂O₇ is a triclinic crystal system. The lattice parameters a and b describe the unit length in two of the three directions of this three-dimensional crystal structure. a, b, and c together define the size of an open three-dimensional sodium-ion migration channel. Defining the lattice parameters within this range has the core advantage of providing optimized sodium-ion diffusion channels, ensuring the best sodium-ion migration rate, and thus supporting excellent rate performance.

[0035] In some embodiments, the present invention also provides a method for preparing any of the above-mentioned sodium-ion battery cathode materials, comprising the following steps: Step 1: Add anhydrous sodium dihydrogen phosphate (NaH2PO4), ferrous oxalate dihydrate (FeC2O4·2H2O), metal salt, and glucose (C6H2PO4). 12 Mix O6·H2O and add solvent. Step 2: The above mixture is ball-milled to obtain a primary precursor material; Step 3: After the above-mentioned precursor material is dried, it is calcined twice to obtain a sodium-ion battery cathode material with a core and a surface coating layer.

[0036] The products prepared by this method have good uniformity and high purity; their structure and morphology are controllable; the process is relatively mature; and the raw material cost is low, making it very suitable for large-scale commercial production.

[0037] The solvents mentioned above are one or a mixture of several of anhydrous ethanol, isopropanol, and water, and the metal salts are one or more of oxalate, sulfate, acetate, and nitrate.

[0038] In step three, the calcination is carried out in two stages. The first calcination is carried out at a temperature of 300℃-400℃ for 3-4 hours, and the second calcination is carried out at a temperature of 600-700℃ for 6-12 hours.

[0039] In addition, in some embodiments, the present invention also provides a sodium-ion battery comprising any of the above-described positive electrode materials, and a sodium-ion battery comprising a positive electrode material prepared by any of the above-described methods.

[0040] In summary, the core uses Na2Fe 1-x M xP2O7 solid solution, through trace doping with specific metal elements, precisely controls the crystal cell parameters, reduces the energy barrier during sodium ion migration, and significantly improves ion diffusion efficiency. The carbon coating layer on the surface constructs a continuous electron transport channel, effectively solving the problem of poor intrinsic conductivity of sodium iron pyrophosphate material, taking into account both sodium ion insertion and extraction channels, while also suppressing particle agglomeration and side reactions with the electrolyte, thus extending the battery's cycle life. The material's particle size, specific surface area, carbon layer ID / IG value, and other physicochemical parameters are precisely controlled within the optimal range, achieving rapid sodium ion diffusion and equilibrium at the reaction interface. In addition, the preparation method adopts a ball milling combined with two calcinations process, the raw materials are easy to obtain, the operation is simple and controllable, the production cost is low, and it is suitable for large-scale industrial production. Sodium-ion batteries containing this cathode material have excellent comprehensive performance and obvious cost advantages, providing key material support for the commercialization of sodium-ion batteries and having broad application prospects.

[0041] The following is a comparison of the three sets of embodiments (see the table below):

[0042] The unmodified reference material (Example 1) exhibits poor rate capacity and poor cycle life due to its poor intrinsic conductivity and unstable structure, resulting in low practical value.

[0043] Single carbon coating modification (Example 2) significantly improved the rate performance of the material by constructing a conductive network, but its improvement on the stability of the material's bulk structure was limited, resulting in poor long-cycle capacity retention (30%).

[0044] Single copper doping modification (Example 3) starts from the material's crystal structure itself, which not only greatly improves the intrinsic conductivity (making the rate performance even better than the carbon-coated sample), but more importantly, greatly stabilizes the three-dimensional framework, thus achieving an amazing long lifespan that can still maintain 86% of the capacity after 12,000 ultrafast charge-discharge cycles.

[0045] In the description of this specification, references to terms such as "some embodiments," "preferred," "other embodiments," or "other embodiments," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in a suitable manner in any one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0046] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A sodium-ion battery cathode material, characterized in that, The positive electrode material includes a core and a coating layer disposed on the surface of the core; The general chemical formula of the core is Na₂Fe. 1-x M x P2O7, where M is the doped metal element, 0.001≤X≤0.1; The coating layer contains carbon material, or the surface of the coating layer is coated with a carbon layer.

2. The sodium-ion battery cathode material as described in claim 1, characterized in that, The metallic element M is selected from one of Cu, Ni, Co, Ag, Zn, Ti, V, and Mn.

3. The sodium-ion battery cathode material as described in claim 1, characterized in that, The thickness of the coating layer is 3nm-10nm.

4. The sodium-ion battery cathode material as described in claim 1, characterized in that, The carbon material or carbon layer is selected from one or a mixture of two of amorphous carbon, graphite, carbon nanotubes, graphene, and few-layer graphene.

5. The sodium-ion battery cathode material as described in claim 1, characterized in that, The cathode material satisfies at least one of the following conditions: (1) The particle size of the positive electrode material is 20 nm ~ 200 nm; (2) The specific surface area of ​​the cathode material is 29 m². 2 / g -31.5m 2 / g; (3) The ID / IG ratio of the cathode material is 2.36-2.

4.

6. The sodium-ion battery cathode material as described in claim 1, characterized in that, The lattice parameter a of the cathode material is 6.438 Å to 6.448 Å; and / or, the lattice parameter b of the cathode material is 9.435 Å to 9.455 Å.

7. The method for preparing the sodium-ion battery cathode material according to any one of claims 1 to 6, characterized in that, Includes the following steps: Step 1: Add anhydrous sodium dihydrogen phosphate (NaH2PO4), ferrous oxalate dihydrate (FeC2O4·2H2O), metal salt, and glucose (C6H2PO4). 12 Mix O6·H2O and add solvent. Step 2: The above mixture is ball-milled to obtain a primary precursor material. Step 3: After the above-mentioned precursor material is dried, it is calcined twice to obtain the cathode material with a core and a surface coating layer.

8. The method for preparing the sodium-ion battery cathode material as described in claim 7, characterized in that, In step one, the metal salt is one or more of oxalate, sulfate, acetate, and nitrate.

9. The method for preparing the sodium-ion battery cathode material as described in claim 7, characterized in that, In step three, the temperatures of the two calcinations are 300-400 ℃ and 600-700 ℃, respectively.

10. A sodium-ion battery, characterized in that, The sodium-ion battery includes the positive electrode material according to any one of claims 1-6, or the sodium-ion battery includes the positive electrode material prepared by any one of claims 7-9.

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