Positive electrode active materials, their preparation methods and applications

Abstract

This application provides a positive electrode active material, its preparation method, and its application. The general formula of the positive electrode active material includes A3V. 2‑ x M x (P 1‑y E y O4)3, where A represents an alkali metal element, M represents a dopant element replacing V, M includes one or more transition metal elements and rare earth elements, E represents a dopant element replacing P, E includes one or more of As, Sb, and Bi, 0≤x≤1, 0<y≤1 / 3. By appropriately doping the P site with element E, the positive electrode active material can achieve a high charge / discharge voltage platform as well as good ionic conductivity and electronic conductivity, thereby improving the battery rate performance and energy density.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a positive electrode active material, its preparation method, and its application. Background Technology

[0002] As a key component of batteries, the cathode material has a significant impact on battery performance. Taking sodium-ion batteries as an example, among various cathode materials, sodium superionic conductor cathode materials are polyanionic materials. Due to their stable crystal structure, adjustable operating voltage, and high theoretical specific capacity, they have become a research focus in recent years. However, these materials suffer from drawbacks such as poor electronic conductivity, difficulty in active ion diffusion, and a low charge / discharge voltage plateau, which severely restrict their large-scale application in sodium batteries.

[0003] Industry experts believe that doping modification of the aforementioned materials could potentially solve their problems. However, existing doping methods have not shown significant effects on improving the charge / discharge voltage and the diffusion of active ions. Summary of the Invention

[0004] In view of this, this application provides a positive electrode active material, which can effectively improve the charge and discharge voltage and ion conductivity of the material by doping the phosphorus sites of alkali metal vanadium phosphate with at least one of As, Sb, Bi, etc.

[0005] Specifically, the first aspect of this application provides a positive electrode active material, the general molecular formula of which includes A3V. 2-x M x (P 1-y E y O4)3, where A represents an alkali metal element, M represents a dopant element that substitutes for V, wherein M includes one or more of transition metal elements and rare earth elements, E represents a dopant element that substitutes for P, wherein E includes one or more of As, Sb, and Bi, 0≤x≤1, 0<y≤1 / 3.

[0006] Doping the P element in the A3V2(PO4)3 lattice with an appropriate amount of E, an element of the same group with a larger ionic radius than P, can cause cell expansion and increase the size of the A3V2(PO4)3 lattice. + Ion shuttle channels, reducing A + The ion migration barrier can also improve the electronic conductivity of the material, which is conducive to improving the rate performance of the material. Furthermore, phosphorus doping of element E can also increase the operating voltage of doped A3V2(PO4)3 material, which is conducive to improving energy density.

[0007] A second aspect of this application provides a method for preparing a positive electrode active material, comprising the following steps:

[0008] (1) The positive electrode active material A3V to be prepared2-x M x (P 1-y E y The element sources of each element in O4)3 are mixed to obtain the precursor material; wherein, A represents an alkali metal element, M represents a doping element that substitutes for V, wherein M includes one or more of transition metal elements and rare earth elements, E represents a doping element that substitutes for P, wherein E includes one or more of As, Sb, and Bi, 0≤x≤1, 0<y≤1 / 3.

[0009] (2) The precursor material is sintered to obtain the positive electrode active material.

[0010] The above-mentioned method for preparing positive electrode active materials is simple, easy to operate, and suitable for large-scale production.

[0011] A third aspect of this application provides a positive electrode sheet comprising the positive electrode active material provided in the first aspect of this application. This positive electrode sheet can be used to provide a battery with good rate performance and high energy density.

[0012] The fourth aspect of this application provides a secondary battery that includes the positive electrode provided in the third aspect of this application. Attached Figure Description

[0013] Figure 1A The molecular configurations of Na3V2(PO4)3 (abbreviated as NVP) and its fully charged compound NaV2(PO4)3 are shown.

[0014] Figure 1B Na3V 1.5 Fe 0.5 (PO4)3 (abbreviated as Fe-NVP) and its fully charged compound NaV 1.5 Fe 0.5 The molecular configuration of (PO4)3.

[0015] Figure 1C Na3V in Example 1 1.5 Fe 0.5 (P 17 / 18 As 1 / 18 Molecular configurations of O4)3 (abbreviated as As1-Fe-NVP) and its fully charged compounds.

[0016] Figure 1D Na3V in Example 2 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 Molecular configurations of O4)3 (abbreviated as As3-Fe-NVP) and its fully charged compounds.

[0017] Figure 1ENa3V in Example 3 1.5 Fe 0.5 (P 2 / 3 As 1 / 3 Molecular configurations of O4)3 (abbreviated as As6-Fe-NVP) and its fully charged compounds.

[0018] Figure 2 The X-ray diffraction (XRD) spectra of the positive electrode active materials provided in Examples 1-3 and Comparative Examples 1-2 of this application are summarized.

[0019] Figure 3 The cycle performance curves at 0.5C of batteries made with the positive electrode active materials provided in Examples 1-2, 4-5, and 10 of this application and batteries made with the material of Comparative Example 1 are summarized.

[0020] Figure 4 The discharge specific capacity of batteries made using the positive electrode active materials provided in Examples 1-2, 4-5, and 10 of this application and batteries made using the material of Comparative Example 1 are summarized as the curves of the change in discharge specific capacity with the number of cycles at different rates. Detailed Implementation

[0021] Sodium vanadium phosphate (Na3V2(PO4)3, abbreviated as NVP) is a common sodium fast ion conductor (NASICON) type cathode material. It has a relatively stable crystal structure, good safety performance, and high theoretical specific capacity. However, it has poor ion conduction and electronic conduction, resulting in poor rate performance and a low charge and discharge voltage platform, which is not conducive to improving the energy density of sodium batteries.

[0022] To address the aforementioned problems of NVP, this application provides a positive electrode active material, the general formula of which includes A3V. 2-x M x (P 1-y E y O4)3, where A represents an alkali metal element, M represents a dopant element that replaces V, wherein M includes one or more transition metal elements and rare earth elements, E represents a dopant element that replaces P, wherein E includes one or more of As, Sb, and Bi; x represents the molar ratio of V element replaced by the M element, y represents the molar ratio of P element replaced by the E element, 0≤x≤1, 0<y≤1 / 3.

[0023] The element E mentioned above is a member of the same group with an ionic radius larger than that of P. E is more easily incorporated into the lattice of A3V2(PO4)3, replacing some of the P positions and causing cell expansion. This can expand the A... + Ion shuttle channels lower their migration energy barriers and reduce the repulsive effect of surrounding atoms, thereby improving A +The increased ion migration rate is beneficial for improving rate performance. Furthermore, the crystal structure of A3V2(PO4)3 contains PO4 tetrahedra and VO6 octahedra, connected by a common oxygen (O) atom at the vertex. Phosphorus doping with electrons alters the charge of the surrounding O atoms, forcing a change in the charge environment around the V atoms. This induces redox reactions in V at high potentials, increasing the operating voltage of the doped A3V2(PO4)3 material and thus improving its energy density. Simultaneously, phosphorus doping with electrons also reduces the band gap of the doped A3V2(PO4)3 material, allowing for greater electron accumulation near the Fermi level and improving electronic conductivity. Moreover, controlling the amount of electron doping to avoid excessive levels ensures good structural stability while achieving a balance between good electronic conductivity, good ion conductivity, and a high charge / discharge voltage platform, ultimately contributing to a battery with good safety, good rate performance, and high energy density.

[0024] Furthermore, when A3V2(PO4)3 is doped at the P-site using an appropriate amount of element E and at the V (vanadium)-site using an appropriate amount of metal element M, the voltage plateau of the resulting dual-doped material can be significantly higher than that of the single-doped A3V2(PO4)3 by utilizing the synergistic effect of the V-site doping of element M and the P-site doping of element E. 1-y E y O4)3 is further improved, and compared with A3V doped with a single M element, it is also improved. 2-x M x (PO4)3 materials, the charge-discharge voltage plateau of the double-doped materials is significantly improved, A + The ion migration barrier was significantly reduced.

[0025] In the embodiments of this application, M may include one or more of Cr (chromium), Mn (manganese), Fe (iron), Co (cobalt), Ni (nickel), Cu (copper), Zn (zinc), Ti (titanium), Mo (molybdenum), Nb (niobium), Zr (zirconium), La (lanthanum), Ce (cerium), etc., but is not limited thereto.

[0026] In some embodiments of this application, M includes one or more of Cr, Mn, Fe, Co, Ni, Cu, Zn, Ti, Mo, Nb, and Zr. These elements are less expensive than the lanthanide elements La and Ce. Among them, Ti, Mo, Nb, and Zr have larger ionic radii than V ions, and the introduction of these doping elements also causes lattice expansion in A3V2(PO4)3, affecting the A... +Ion migration is beneficial. The ionic radii of Cr, Mn, Fe, Co, Ni, Cu, and Zn are slightly smaller than those of V ions. While doping A3V2(PO4)3 with these elements alone would reduce the cell volume, introducing the aforementioned element E can increase the cell volume, thus improving the material's diffusion performance. In other embodiments of this application, to better balance the cost of element M and its impact on the material's ionic conductivity and operating voltage, element M can be selected from one or more of Ti, Cr, Mn, and Fe. These elements also contribute to increasing the operating voltage of doped materials and are easier to substitute at vanadium sites; among them, Fe has the lowest cost.

[0027] In this embodiment of the application, the element E includes at least one of As, Sb, and Bi. The ionic radii of As, Sb, and Bi are all larger than those of P. Using these elements for P-site doping (i.e., replacing P atom positions in the A3V2(PO4)3 crystal structure with E atoms) can appropriately expand the A3V... 2-x M x (P 1-y E y The smaller unit cell volume of O4)3 reduces the migration barrier of As ions, while the ionic radii of As, Sb, and Bi are not excessively larger than those of P. This approach ensures higher crystal structure stability while maintaining high ionic conductivity. In some embodiments, element E is As and / or Bi. As and P are both nonmetallic elements with similar properties, and As is more likely to substitute for P.

[0028] In some embodiments, the M element is Fe and the E element is As. In this case, A3V 2-x M x (P 1-y E y O4)3 has a low preparation cost, and the substitution of P element by As can increase the migration rate of A ions to improve the rate performance of the material, as well as increase the working voltage to improve the energy density of the material.

[0029] In this embodiment, element A may specifically include one or more of Li, Na, K, etc. Element A can be selected according to the specific type of secondary battery. For example, when the above-mentioned battery active material A3V... 2-x M x (P 1-y E y If O4)3 is used in a sodium secondary battery, then element A is Na.

[0030] In some embodiments of this application, the value range of x can be 0.001 ≤ x ≤ 1, or more specifically, 0.01 ≤ x ≤ 1. Controlling the doping amount of element M within the above range can prevent excessively high doping levels of element M from causing the aforementioned A3V... 2-x Mx (P 1-y E y O4)3 loses its electrochemical activity, which also avoids the situation where the doping amount of element M is too low and cannot play a role in improving electronic conductivity. In some embodiments, x is in the range of 0.001-0.5, preferably in the range of 0.01-0.5, more preferably in the range of 0.1-0.5, and more preferably in the range of 0.3-0.5.

[0031] In some embodiments of this application, the value range of y is: 1 / 18 ≤ y ≤ 1 / 3. Controlling the doping amount of element E within this range allows for the production of A3V... 2-x M x (P 1-y E y The cell volume of O4)3 is significantly increased, which improves ionic conductivity, without causing excessive changes in its crystal structure and increasing the risk of structural collapse. This ensures good cycle performance of the material, significantly increases the working voltage, and also makes its band gap extremely narrow, which is conducive to improving conductivity.

[0032] In some embodiments of this application, y is 1 / 6. In this case, A3V 2-x M x (P 1-y E y O4)3 materials have a very low A-ion migration barrier, resulting in excellent rate performance and cycling performance. In other embodiments, y is 1 / 3. In this case, A3V 2-x M x (P 1-y E y O4)3 material has the highest operating voltage and the best electronic conductivity. In some other embodiments, y is 1 / 18. In this case, A3V 2- x M x (P 1-y E y O4)3 material has a high degree of crystallinity and excellent structural stability. In some embodiments, the value of y is in the range of 1 / 18 ≤ y ≤ 1 / 6; in other embodiments, the value of y is in the range of 1 / 6 ≤ y ≤ 1 / 3.

[0033] Accordingly, this application provides a method for preparing the above-mentioned positive electrode active material. Specifically, the method for preparing the positive electrode active material may include the following steps:

[0034] (1) The positive electrode active material A3V to be prepared 2-x M x (P 1-y E yThe element sources of each element in O4)3 are mixed to obtain the precursor material; wherein, A represents an alkali metal element, M represents a doping element that substitutes for V, wherein M includes one or more of transition metal elements and rare earth elements, E represents a doping element that substitutes for P, wherein E includes one or more of As, Sb, and Bi, 0≤x≤1, 0<y≤1 / 3.

[0035] (2) The precursor material is sintered to obtain the positive electrode active material.

[0036] The above-mentioned method for preparing positive electrode active materials is simple, easy to operate, and suitable for large-scale production.

[0037] In step (1), the element sources used to prepare the positive electrode active material include an alkali metal source, a vanadium source, a phosphorus source, an M source containing doped metal element M, and an E source containing doped element E. The alkali metal source, vanadium source, M source, phosphorus source, and E source can be weighed according to a molar ratio of A, V, M, P, and E of 3:(2-x):x:(1-y):y. Furthermore, considering that alkali metal elements are prone to loss during subsequent sintering, the alkali metal source can be in excess by up to 10%.

[0038] The alkali metal source can include one or more of sodium, lithium, and potassium sources, specifically one or more of alkali metal nitrates, oxalates, acetates, acetylacetonates, carbonates, sulfates, phosphates, and hydroxides. The vanadium source can be one or more of trivalent, tetravalent, and pentavalent vanadium sources, specifically one or more of oxides of vanadium (such as vanadium pentoxide, vanadium tetroxide, vanadium trioxide, etc.), hydroxides, metavanadates (such as ammonium metavanadate, sodium metavanadate, etc.), nitrates, sulfates, phosphates, oxalates, acetates, acetylacetonate vanadium, and acetylacetonate vanadium oxide. The M source containing the doped metal element M can be similar to the vanadium source described above, and can be selected from one or more of the oxides, hydroxides, nitrates, sulfates, phosphates, oxalates, acetates, and acetylacetonate salts of element M. The phosphorus source can be a polyanionic group source of P, such as an oxyacid source of P; in some embodiments, the phosphorus source can include H3PO4, Na3PO4, and Li. + Na + K + NH4 +The source can be one or more of the following: dihydrogen phosphate, dihydrogen phosphate, or orthophosphate. The E source can be one or more oxides, acids, or salts corresponding to As, Sb, or Bi. In some embodiments, the E source can be an E-doped anion source, such as one or more of the following: oxyacids or oxysalts corresponding to As, Sb, or Bi. Among the above-mentioned element sources, the phosphorus source can be the same substance as the alkali metal source, vanadium source, or M source; for example, sodium phosphate can simultaneously serve as both a phosphorus and sodium source.

[0039] In this application, the mixing method can be a liquid-phase method or a solid-phase method. The solid-phase method can be one or more of mechanical stirring, ball milling, mechanical fusion, etc. The liquid-phase method can be a sol-gel method, a hydrothermal / solvothermal method, a high-energy liquid-phase ball milling method, etc. The solvent used in the liquid-phase method can be one or more of water, ethanol, acetone, etc.

[0040] In some embodiments, the precursor material is prepared by solid-state ball milling, which may specifically include: ball milling an alkali metal source, a vanadium source, a phosphorus source, an M source, and an E-doped anion source under solvent-free conditions to obtain the precursor material.

[0041] In other embodiments, the precursor material is prepared using a sol-gel method, which may specifically include: mixing an alkali metal source, a vanadium source, a phosphorus source, an M source, and an E-doped anion source in a solvent; heating and stirring the resulting mixed solution until the solvent evaporates to dryness to obtain the precursor material. During the heating and stirring process, each element source reacts to obtain the precursor material, which can be sintered to transform into the desired positive electrode active material. In this embodiment, the heating and stirring temperature can be 30-200℃, for example, 40-100℃, specifically determined according to the boiling point of the solvent used; the stirring speed can be 300-900 rpm, and the stirring time can be 1-6 hours.

[0042] In this embodiment, the sintering is carried out under an inert gas atmosphere, the sintering temperature is 400-900℃, and the sintering time is 10-30 hours. The inert gas can be one or more of nitrogen, argon, helium, etc., with argon and helium being preferred.

[0043] This application also provides a positive electrode sheet comprising the aforementioned positive electrode active material. This positive electrode sheet can be used to prepare secondary batteries with excellent performance, such as good rate capability and high energy density.

[0044] In this embodiment, the positive electrode generally includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, wherein the positive active material layer contains the aforementioned positive active material. Furthermore, the positive active material layer may also contain a binder and, optionally, a conductive agent. The binder and conductive agent are conventional choices in the battery industry.

[0045] The positive current collector can be any material suitable for use as a current collector in a positive electrode sheet, including but not limited to elemental metal foil, alloy foil, metal-plated polymer film, or the aforementioned materials coated with carbon. Specifically, elemental metal foil can be aluminum foil, alloy foil can be aluminum alloy foil, and the metal plated on the surface of the polymer film can be an elemental aluminum layer or an aluminum alloy layer.

[0046] This application also provides a secondary battery comprising the aforementioned positive electrode. Specifically, the secondary battery may be a lithium secondary battery, a sodium secondary battery, or a potassium secondary battery.

[0047] This secondary battery, using the aforementioned doped A3V2(PO4)3 as the positive electrode active material, exhibits high energy density, excellent rate performance, and good cycle performance. It can be used in 3C electronic products (such as mobile phones and tablets), transportation vehicles (such as cars and ships), or other electrical equipment to improve their performance and market competitiveness.

[0048] The secondary battery can be a liquid battery using a liquid electrolyte, or a semi-solid or solid battery using a semi-solid or solid electrolyte. In some embodiments, the secondary battery may include the aforementioned positive electrode, negative electrode, and a separator and electrolyte disposed between the positive and negative electrodes. In other embodiments, the secondary battery may include a positive electrode, a negative electrode, and a semi-solid or solid electrolyte disposed between the positive and negative electrodes. Furthermore, when using a semi-solid or solid electrolyte, the positive and negative electrodes may also contain semi-solid or solid electrolyte materials.

[0049] The technical solution of this application will be further described in detail below through several embodiments.

[0050] Example 1

[0051] A positive electrode active material with the general formula Na3V 1.5 Fe 0.5 (P 17 / 18 As 1 / 18 O4)3 (abbreviated as As1-Fe-NVP).

[0052] The preparation process of the above As1-Fe-NVP is as follows:

[0053] (1) Weigh out sodium source (specifically sodium nitrate), vanadium source (specifically vanadium pentoxide), Fe source (specifically ferrous oxide), phosphorus source (specifically phosphoric acid), and As source (specifically diammonium arsenate) according to the molar ratio of Na:V:Fe:P:As = 3:1.5:0.5:17 / 18:1 / 18, mix them in solvent ethanol, heat and stir at 80°C until the solvent evaporates to dryness, and obtain the precursor material;

[0054] (2) The above precursor material was sintered at 800°C for 12 hours under a nitrogen atmosphere to obtain Na3V. 1.5 Fe 0.5 (P 17 / 18 As 1 / 18 O4)3.

[0055] Example 2

[0056] A positive electrode active material with the general formula Na3V 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 O4)3 (abbreviated as As3-Fe-NVP).

[0057] The preparation method of As3-Fe-NVP differs from that in Example 1 in that the amounts of P source and As source are changed so that the molar ratio of Fe element:P element:As element is 0.5:5 / 6:1 / 6.

[0058] Example 3

[0059] A positive electrode active material with the general formula Na3V 1.5 Fe 0.5 (P 2 / 3 As 1 / 3 O4)3 (abbreviated as As6-Fe-NVP).

[0060] The preparation method of As6-Fe-NVP differs from that in Example 1 in that the amount of P source and As source is changed so that the molar ratio of Fe element:P element:As element is 0.5:2 / 3:1 / 3.

[0061] Comparative Example 1

[0062] A positive electrode active material with the general formula Na3V2(PO4)3(NVP).

[0063] The preparation method of NVP differs from that in Example 1 in that As source and Fe source are not introduced.

[0064] Comparative Example 2

[0065] A positive electrode active material with the general formula Na3V 1.5 Fe 0.5 (PO4)3 (abbreviated as Fe-NVP).

[0066] The preparation method of Fe-NVP differs from that in Example 1 in that no As source is introduced.

[0067] Figure 1A The molecular configurations of Na3V2(PO4)3 (abbreviated as NVP) and its fully charged compound NaV2(PO4)3 are shown. Figure 1B Na3V 1.5 Fe 0.5 (PO4)3 (abbreviated as Fe-NVP) and its fully charged compound NaV 1.5 Fe 0.5 The molecular configuration of (PO4)3. Figure 1C As1-Fe-NVP and its fully charged compound NaV from Example 1 1.5 Fe 0.5 (P 17 / 18 As 1 / 18 The molecular configuration of O4)3.

[0068] Figure 1A In the Na3V2(PO4)3 unit cell, 12 V atoms are located at the center of each VO6 octahedron, 18 P atoms are located at the center of each PO4 tetrahedron, and 72 O atoms are used to connect the octahedrons and tetrahedrons, forming the framework structure of the Na3V2(PO4)3 compound. The remaining 18 Na atoms are uniformly occupied in the vacancies of the framework structure. The unit cell contains two types of Na sites in different oxygen environments: the 6b site at the center of the octahedron and the 18e site at the center of the tetrahedron, labeled Na1 and Na2, respectively. The Na3V2(PO4)3 unit cell has 6 Na atoms at the Na1 sites (see the dashed arrow) and 12 Na atoms at the Na2 sites. Sodium ions at the Na1 sites are difficult to extract; typically, sodium ions at the Na2 sites are extracted / intercalated during charging and discharging, thus completing the crystal structure transformation between the empty state and the fully charged state of Na3V2(PO4)3. In the fully charged NaV2(PO4)3 unit cell, all six Na ions occupy the Na1 site.

[0069] Figure 1B In Fe-NVP, it can be seen as... Figure 1A Based on the molecular configuration of Na3V2(PO4)3, by uniformly replacing 3 of the 12 P atoms with 3 Fe atoms, the fully charged compound of Fe-NVP is NaV. 1.5 Fe 0.5The difference between (PO4)3 and Fe-NVP configurations lies in the removal of the Na atom at the Na2 position.

[0070] Figure 1C As1-Fe-NVP can be seen as in Figure 1B Based on the Fe-NVP molecular configuration, one P atom in the third layer is replaced by an As atom (indicated by the solid arrow). The fully charged compound NaV of As1-Fe-NVP is then formed. 1.5 Fe 0.5 (P 17 / 18 As 1 / 18 The difference between O4)3 and the As1-Fe-NVP configuration is that the Na atom at the Na2 position is removed (the dashed arrow points to the Na atom at the Na1 position, and the remaining spherical atom is the Na2 position atom).

[0071] Figure 1D As3-Fe-NVP and its fully charged compound NaV in Example 2 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The molecular configuration of O4)3. In the molecular configuration of As3-Fe-NVP, it is equivalent to replacing 3 out of 12 P atoms with 3 Fe atoms on the basis of NVP. In the 6 layers of P atoms, one P atom is selected every other layer and replaced with an As atom, for a total of 3 As atoms (indicated by the solid arrow).

[0072] Figure 1E As6-Fe-NVP and its fully charged compound NaV in Example 3 1.5 Fe 0.5 (P 2 / 3 As 1 / 3 The molecular configuration of O4)3. In the molecular configuration of As6-Fe-NVP, it is equivalent to replacing 3 out of 12 P atoms with 3 Fe atoms on the basis of NVP, and selecting one P atom in each of the 6 layers of P atoms to replace it with As, for a total of 6 As atoms (indicated by the solid arrow).

[0073] according to Figures 1A to 1E The molecular configurations of the compounds shown can be used to predict the cell volume, empty / fully charged cell volume change rate (%), open-circuit voltage, band gap, and sodium ion migration barrier of NVP, Fe-NVP, As1-Fe-NVP, As3-Fe-NVP, and As6-Fe-NVP using first-principles prediction methods. The relevant results are summarized in Table 1 below.

[0074] The change rate of unit cell volume between empty and fully charged states is illustrated using As1-Fe-NVP as an example, with fully charged NaV1 as the reference. 1.5 Fe 0.5 (P 17 / 18 As 1 / 18 The cell volume of O4)3 and the empty state of Na3V 1.5 Fe 0.5 (P 17 / 18 As 1 / 18 The absolute value of the difference in unit cell volume between O4)3(As1-Fe-NVP) and its empty state unit cell volume gives the rate of change of its empty / fully charged state unit cell volume. A certain empty state compound A3V... 2-x M x (P 1-y E y The open-circuit voltage of O4)3 can be calculated by determining the lattice constants and total energy of the cell structure of the empty-state compound and its fully charged-state compound, based on their molecular configurations. 2-x M x (P 1-y E y The open-circuit voltage of the O4)3 compound. The band gap can be calculated from the density of states distribution curve of the material; the Na ion migration barrier can be calculated by the NEB method (nuEgeE elastic banE) or a modified NEB method.

[0075] Table 1

[0076]

[0077] As can be seen from Table 1, for the cell volume of each compound, when NVP is doped with Fe, the cell volume is reduced because Fe has a smaller atomic radius than V; when As, which has a larger atomic radius than P, is added to Fe-doped NVP, the cell volume gradually increases with the increase of As doping concentration, which indicates that the introduction of As is beneficial to cell expansion.

[0078] The changes in the empty / fully charged state volume of materials resulting from single Fe doping and co-doping with Fe and As are not the same. As shown in Table 1, As3-Fe-NVP has the smallest change in the empty / fully charged state volume, indicating the highest stability of its crystal structure. Furthermore, although the rate of change in the empty / fully charged state volume of Fe-NVP, As1-Fe-NVP, and As3-Fe-NVP is greater than that of NVP, their crystal structures are still considered stable, ensuring that the structure will not collapse due to excessive deformation during charging and discharging.

[0079] Regarding the open-circuit voltage of each compound, Table 1 shows that compared to undoped NVP and Fe-NVP (Fe-NVP with single iron doping), the open-circuit voltage of the Fe and As co-doped materials (Examples 1-3) is significantly improved. When this material is used in sodium batteries, it helps to improve the battery energy density. Furthermore, as the As doping concentration increases, the open-circuit voltage of the Fe and As co-doped NVP continuously increases, further contributing to the improvement of battery energy density. Since the open-circuit voltage of a material is generally higher than its operating voltage, when the open-circuit voltage of material A is higher than that of material B, the operating voltage of material A is also generally higher than that of material B. Therefore, by comparing the open-circuit voltages of different materials, the energy density of the manufactured battery can be predicted.

[0080] Regarding the band gaps of each compound, Table 1 shows that the undoped NVP has a wider band gap, indicating a larger resistance; while the band gap of Fe-NVP doped with iron can be reduced to 0.82 eV; co-doping with Fe and As can further reduce the band gap, among which As3-Fe-NVP and As6-Fe-NVP have almost no band gap, indicating that they have high conductivity.

[0081] Regarding the band gaps of each compound, Table 1 shows that Na has a relatively high migration barrier in NVP, approximately 0.612 eV. However, after introducing Fe as a single dopant, the migration barrier of Na ions actually increases. This is mainly because Fe has a small ionic radius, and its single doping causes the cell volume of NVP to shrink. In contrast, the migration barrier of Na ions in Fe and As co-doped As1-Fe-NVP, As3-Fe-NVP, and As6-Fe-NVP is significantly reduced, and the ionic conductivity is improved, which in turn facilitates the improvement of the charging and discharging speed of the material.

[0082] also, Figure 2 The measured XRE spectra of NVP, Fe-NVP, As1-Fe-NVP, As3-Fe-NVP, and As6-Fe-NVP were also compiled. Figure 2 It can be seen that compared with the main XRE diffraction peak of undoped NVP, the main peak position shifts slightly to the right after Fe doping, and the lattice parameters maintain a high degree of consistency with the overall NVP. Based on Fe doping, with the addition of As, at low concentrations (e.g., As1-Fe-NVP), the main peak shifts to the right overall. As the doping concentration increases (e.g., As1-Fe-NVP), the full width at half maximum (FWHM) of some high-intensity peaks slightly broadens, while some lower-intensity peaks also show varying degrees of enhancement. This indicates that co-doping with Fe and As can change the crystal plane growth direction of NVP, making previously difficult-to-grow planes possible to grow. This demonstrates that co-doping with Fe and As can control the morphology and size of NVP.

[0083] Example 4

[0084] A positive electrode active material with the general formula Na3V 1.5 Fe 0.5 (P 5 / 6 Sb 1 / 6 O4)3.

[0085] The Na3V 1.5 Fe 0.5 (P 5 / 6 Sb 1 / 6 The preparation method of O4)3 is the same as that of Na3V in Example 2. 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The difference in the preparation method of O4)3 is that the As source is replaced with the Sb source (specifically sodium antimonate).

[0086] Example 5

[0087] A positive electrode active material with the general formula Na3V 1.5 Fe 0.5 (P 5 / 6 Bi 1 / 6 O4)3.

[0088] The Na3V 1.5 Fe 0.5 (P 5 / 6 Bi 1 / 6 The preparation method of O4)3 is the same as that of Na3V in Example 2. 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The difference in the preparation method of O4)3 is that the As source is replaced with a Bi source (specifically, bismuth ammonium citrate).

[0089] Example 6

[0090] A positive electrode active material with the general formula Na3V 1.8 Fe 0.2 (P 5 / 6 As 1 / 6 O4)3.

[0091] The Na3V 1.8 Fe 0.2 (P 5 / 6 As 1 / 6 The preparation method of O4)3 is the same as that of Na3V in Example 2. 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The difference in the preparation method of O4)3 is that the amount of Fe source and V source is changed so that the molar ratio of V:Fe element is 1.8:0.2.

[0092] Example 7

[0093] A positive electrode active material with the general formula Na3V 1.99 Fe 0.01 (P 5 / 6 As 1 / 6 O4)3.

[0094] The Na3V 1.99 Fe 0.01 (P 5 / 6 As 1 / 6 The preparation method of O4)3 is the same as that of Na3V in Example 2. 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The difference in the preparation method of O4)3 is that the amount of Fe source and V source is changed so that the molar ratio of V:Fe element is 1.99:0.01.

[0095] Example 8

[0096] A positive electrode active material with the general formula Na3V 1.5 Ti 0.5 (P 5 / 6 As 1 / 6 O4)3.

[0097] The Na3V 1.5 Ti 0.5 (P 5 / 6 As 1 / 6 The preparation method of O4)3 is the same as that of Na3V in Example 2. 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The difference in the preparation method of O4)3 is that the Fe source is replaced with a Ti source (specifically titanium oxide).

[0098] Example 9

[0099] A positive electrode active material with the general formula Na3V 1.5 Cr 0.5 (P 5 / 6 As 1 / 6 O4)3.

[0100] The Na3V 1.5 Cr 0.5 (P 5 / 6 As 1 / 6 The preparation method of O4)3 is the same as that of Na3V in Example 2. 1.5 Fe 0.5 (P 5 / 6 As 1 / 6The difference in the preparation method of O4)3 is that the Fe source is replaced with the Cr source (specifically chromium oxide).

[0101] Example 10

[0102] A positive electrode active material with the general formula Na3V 1.5 Nb 0.5 (P 5 / 6 As 1 / 6 O4)3.

[0103] The Na3V 1.5 Nb 0.5 (P 5 / 6 As 1 / 6 The preparation method of O4)3 is the same as that of Na3V in Example 2. 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The difference in the preparation method of O4)3 is that the Fe source is replaced with the Nb source (specifically sodium niobate).

[0104] Example 11

[0105] A positive electrode active material with the general formula Na3V2(P 17 / 18 As 1 / 18 O4)3.

[0106] The Na3V2(P 17 / 18 As 1 / 18 The preparation method of O4)3 differs from that of Example 1 in that the doping source - Fe source - was not used.

[0107] Example 12

[0108] A positive electrode active material with the general formula Na3V2(P 5 / 6 As 1 / 6 O4)3.

[0109] The Na3V2(P 5 / 6 As 1 / 6 The preparation method of O4)3 differs from that of Example 2 in that the doping source - Fe source - was not used.

[0110] Example 13

[0111] A positive electrode active material with the general formula Na3V2(P 2 / 3 As 1 / 3 O4)3.

[0112] The Na3V2(P 2 / 3 As 1 / 3 The preparation method of O4)3 differs from that of Example 3 in that the doping source - Fe source - was not used.

[0113] Example 14

[0114] A positive electrode active material with the general formula Na3VFe(P) 5 / 6 As 1 / 6 O4)3.

[0115] The Na3VFe(P 5 / 6 As 1 / 6 The preparation method of O4)3 is the same as that of Na3V in Example 2. 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The difference in the preparation methods of O4)3 lies in changing the amount of Fe source and V source so that the molar ratio of V to Fe is 1:1.

[0116] Example 15

[0117] A positive electrode active material with the general formula Na3V 1.5 Fe 0.5 (P 35 / 36 As 1 / 36 O4)3, its preparation method is the same as Na3V in Example 2. 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The difference in the preparation method of O4)3 lies in changing the amount of P source and As source so that the molar ratio of Fe element:P element:As element is 0.5:35 / 36:1 / 36.

[0118] Example 16

[0119] A positive electrode active material with the general formula Na3V 1.5 Mn 0.5 (P 5 / 6 As 1 / 6 O4)3.

[0120] Example 17

[0121] A positive electrode active material with the general formula Na3V 1.5 Mn 0.5 (P 5 / 6 Bi 1 / 6 O4)3.

[0122] Example 18

[0123] A positive electrode active material with the general formula Na3V1Mn1(P 5 / 6 Bi 1 / 6 O4)3.

[0124] Example 19

[0125] A positive electrode active material with the general formula Na3V 1.5 Mn 0.5 (P 5 / 6 Sb 1 / 6 O4)3.

[0126] Example 20

[0127] A positive electrode active material with the general formula Na3V 1.5 Ti 0.5 (P 5 / 6 Bi 1 / 6 O4)3.

[0128] Table 2 below also summarizes the relevant electrochemical performance of the positive electrode active materials of Examples 4-20.

[0129] Table 2

[0130]

[0131] As shown in Table 2, compared to the undoped Na3V2(PO4)3 in Comparative Example 1, the dual-doped sodium vanadium phosphate materials provided in Examples 4-10 and 14-20 of this application can maintain good structural stability while also achieving higher open-circuit voltage, narrower band gap, and lower Na ion migration barrier, thus achieving higher energy density and better rate performance. Furthermore, a comparison of Examples 11-13 with Examples 1-3 in Table 1 shows that when Na3V2(PO4)3 is doped with the same amount of As without iron, the resulting material has a larger cell volume, smaller volume changes between the fully charged and empty states, and lower sodium ion migration energy, which is beneficial for improving battery rate performance. However, the open-circuit voltage and conductivity of the material are somewhat reduced. Moreover, when Na3V2(PO4)3 is doped with the same amount of As, Sb, or Bi, further doping with Mn at the V-site is more beneficial for improving the open-circuit voltage of the material.

[0132] To further support the beneficial effects of the embodiments of this application, the materials from the above embodiments and comparative examples were fabricated into batteries, and their electrochemical performance was tested. The relevant results are shown in Table 3.

[0133] The battery manufacturing process is as follows: ① Preparation of positive electrode sheet: The positive electrode active material of each embodiment or comparative example, the conductive agent - acetylene black, and the binder - polyvinylidene fluoride (PVEF) are added to the solvent - NMP (N-methylpyrrolidone) in a mass ratio of 88:6:6, and stirred evenly to obtain a positive electrode slurry; the positive electrode slurry is coated on the positive electrode current collector - aluminum foil, and after drying, rolling, and slitting, a positive electrode sheet is obtained. ② Preparation of negative electrode sheet: The negative electrode active material (specifically hard carbon) and the binder (specifically styrene-butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC-Na) in a mass ratio of 2:3) are mixed in deionized water in a mass ratio of 95:5, and stirred evenly to obtain a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector - copper foil, and after drying, rolling, and slitting, a negative electrode sheet is obtained. ③ Battery assembly: The above-mentioned positive electrode sheet, separator and negative electrode sheet are alternately stacked to obtain dry cell. The dry cell is then wound and placed in an aluminum-plastic film outer packaging foil. Electrolyte is injected, and after vacuum sealing, standing, formation and shaping, the sodium full cell is completed.

[0134] The following electrochemical performance tests were performed on the batteries of each embodiment and comparative example:

[0135] 1) Cycling Performance: Each battery was subjected to charge-discharge cycle tests at 25℃ with a current rate of 0.5C, and a voltage range of 2.5-4.3V. During charging, the batteries were first charged at a constant current of 0.5C until the cutoff voltage reached 4.3V, and then charged at a constant voltage until the cutoff current reached 0.05C. During discharging, the batteries were discharged at a constant current of 0.5C until the voltage reached 2.5V. The specific capacity of the first discharge cycle and the capacity retention rate after 50 cycles were recorded for each battery. The specific capacity of the first discharge cycle is equal to the ratio of the first discharge capacity of each sodium full cell to the mass of the positive electrode active material in the battery; the capacity retention rate after 50 cycles is equal to the ratio of the discharge capacity after 50 cycles to the discharge capacity of the first cycle.

[0136] Furthermore, the discharge curves obtained from the above-mentioned 0.5C constant current charge-discharge curves of each battery were integrated and divided by the first-cycle discharge capacity to obtain the average voltage of each battery, i.e., the voltage plateau. The relevant results are summarized in Table 3, and the cycling curves of some embodiments and comparative examples are summarized in... Figure 3 .

[0137] 2) Rate Performance: The discharge capacity of each battery was tested at 25°C at different rates (0.5C, 1C, 5C, 10C) as a function of cycle number, with a voltage range of 2.5-4.3V. Rate performance curves for some examples and comparative examples are shown below. Figure 4As shown in the figure. When calculating the discharge specific capacity, the ratio of the discharge capacity at a certain current density to the mass of the positive electrode active material is used as the discharge specific capacity at that current density. Table 3 summarizes the first-cycle discharge specific capacity of each battery at 10C rate, and the ratio of the first-cycle discharge capacity at 10C rate to the first-cycle discharge capacity at 0.5C rate.

[0138] Table 3

[0139]

[0140] As shown in Table 3, compared to the sodium battery made using the materials of Comparative Example 1, the sodium battery made using the materials provided in Examples 1-20 of this application has a higher voltage plateau and a higher first-cycle discharge capacity at 10C / 0.5C, indicating better rate performance. At the same time, the battery's cycle performance is not significantly reduced. Furthermore, the batteries in Examples 1-7 and 14-15 also have a higher voltage plateau and better rate performance compared to the battery in Comparative Example 2.

[0141] The above description is an exemplary embodiment of this application. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principles of this application, and these improvements and modifications are also considered to be within the scope of protection of this application.

Claims

1. A positive electrode active material, characterized in that, The general formula of the positive electrode active material includes A3V. 2-x M x (P 1-y E y O4)3, where A represents an alkali metal element, M represents a dopant element that substitutes for V, wherein M includes one or more of transition metal elements and rare earth elements, E represents a dopant element that substitutes for P, wherein E includes one or more of As, Sb, and Bi, 0 < x ≤ 1, 0 < y ≤ 1 / 3.

2. The positive electrode active material according to claim 1, characterized in that, The M includes one or more of Cr, Mn, Fe, Co, Ni, Cu, Zn, Ti, Mo, Nb, Zr, La, and Ce.

3. The positive electrode active material according to claim 2, characterized in that, The M includes one or more of Cr, Mn, Fe, and Ti.

4. The positive electrode active material according to claim 1, characterized in that, E is As and / or Bi.

5. The positive electrode active material according to claim 1, characterized in that, The value of x is in the range of 0.01≤x≤1.

6. The positive electrode active material according to claim 5, characterized in that, The value of x is in the range of 0.01 ≤ x ≤ 0.

5.

7. The positive electrode active material according to any one of claims 1-6, characterized in that, The value range of y is 1 / 18 ≤ y ≤ 1 / 3.

8. A method for preparing a positive electrode active material, characterized in that, Includes the following steps: (1) The positive electrode active material A3V to be prepared 2-x M x (P 1-y E y The element sources of each element in O4)3 are mixed to obtain the precursor material; wherein, A represents an alkali metal element, M represents a doping element that substitutes for V, and M includes one or more of transition metal elements and rare earth elements, E represents a doping element that substitutes for P, and E includes one or more of As, Sb, and Bi, 0 < x ≤ 1, 0 < y ≤ 1 / 3. (2) The precursor material is sintered to obtain the positive electrode active material.

9. The preparation method according to claim 8, characterized in that, The precursor material is prepared by the sol-gel method, specifically including: mixing an alkali metal source, a vanadium source, a phosphorus source, an M source, and an E source in a solvent, heating and stirring the resulting mixed solution until the solvent evaporates to dryness, and obtaining the precursor material.

10. The preparation method according to claim 8 or 9, characterized in that, The sintering is carried out under an inert gas atmosphere, the holding temperature of the sintering is 400-900℃, and the sintering time is 10-30h.

11. A positive electrode plate, characterized in that, The positive electrode sheet comprises the positive electrode active material according to any one of claims 1-7, or comprises the positive electrode active material prepared by the preparation method according to any one of claims 8-10.

12. A secondary battery, characterized in that, The secondary battery includes the positive electrode sheet according to claim 11.

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