Positive electrode active material, its manufacturing method and use

Abstract

A positive electrode active material and a method for producing the same, a positive electrode including the positive electrode active material, a secondary battery including the positive electrode, and an electrical device including the secondary battery. The positive electrode active material has the general formula A3V 2-x M x (P 1-y E y O4)3, wherein A represents an alkali metal element, M represents a doping element substituting for V, M comprises one or more of a transition metal element and a rare earth element, E represents a doping element substituting for P, E comprises one or more of As, Sb, and Bi, 0≦x≦1, 0

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This disclosure claims priority to Chinese Patent Application No. 202210957755.5, entitled "POSITIVE ELECTRODE ACTIVE MATERIAL, AND PREPARATION METHOD THEREFOR AND USE THEREOF," filed on August 10, 2022, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to the technical field of batteries, and in particular to cathode active materials, methods for making same, and methods for using same. [Background technology]

[0003] Battery cathode materials are key components in batteries and have a significant impact on battery performance. For example, among the numerous cathode materials for sodium-ion batteries, sodium superionic conductor cathode materials, which are polyanionic materials, have been the focus of research in recent years due to their potential advantages, such as stable crystal structure, tunable operating voltage, and high theoretical specific capacity. However, such materials have several drawbacks, including low electronic conductivity, difficult diffusion of active ions, and low charge / discharge voltage plateaus, which severely limit the large-scale application of such materials in sodium batteries. Summary of the Invention [Problem to be solved by the invention]

[0004] In the industry, it is believed that doping modification of the above materials can solve the existing problems, but the existing doping methods have no obvious effect on improving the charge / discharge voltage and diffusing active ions. [Means for solving the problem]

[0005] In view of this, the present disclosure provides a positive electrode active material in which the charge / discharge voltage and ionic conductivity of the material can be effectively improved by doping the phosphorus site of alkali metal vanadium phosphate with at least one of As, Sb, and Bi.

[0006] Specifically, in a first aspect of the present disclosure, a positive electrode active material is provided. The positive electrode active material has the general formula: A3V 2-x M x (P 1-y E y O4)3, wherein A represents an alkali metal element, M represents a doping element substituting for V, M comprises one or more of a transition metal element and a rare earth element, E represents a doping element substituting for P, E comprises one or more of As, Sb, and Bi, and 0≦x≦1, 0 <y≦1 / 3である。

[0007] The proper substitution of element P in the lattice of A3V2(PO4)3 by doping with an element of the same group whose ionic radius is larger than P causes the expansion of the unit cell, A + Expands ion transport channels and + The doping of element E into the phosphorus site can reduce the energy barrier for ion migration and improve the material's electronic conductivity, which in turn can promote the improvement of the material's rate performance. Furthermore, doping element E into the phosphorus site can also improve the operating voltage of the doped A3V2(PO4)3 material, which contributes to improving the energy density.

[0008] In one embodiment, M comprises one or more of Cr, Mn, Fe, Co, Ni, Cu, Zn, Ti, Mo, Nb, Zr, La, and Ce.

[0009] In one embodiment, M comprises one or more of Cr, Mn, Fe, and Ti.

[0010] In one embodiment, E is one or more of As and Bi.

[0011] In one embodiment, M is Fe and E is As.

[0012] In one embodiment, A comprises one or more of Li, Na, and K.

[0013] In one embodiment, x is in the range 0.001≦x≦1.

[0014] In one embodiment, x is in the range 0.01≦x≦1.

[0015] In one embodiment, x is in the range 0.3≦x≦0.5.

[0016] In one embodiment, y is in the range 1 / 18≦y≦1 / 3.

[0017] In one embodiment, y is 1 / 6, 1 / 3, or 1 / 18.

[0018] In one embodiment, y is in the range 1 / 18≦y≦1 / 6.

[0019] In one embodiment, y is in the range 1 / 6≦y≦1 / 3. In a second aspect of the present disclosure, there is provided a method for producing a positive electrode active material, comprising the steps of:

[0020] Manufactured cathode active material A3V 2-x M x (P 1-y E y Element sources of various elements of O4)3 are mixed to obtain a precursor material, wherein A represents an alkali metal element, M represents a doping element to replace V, M includes one or more of a transition metal element and a rare earth element, E represents a doping element to replace P, E includes one or more of As, Sb, and Bi, and 0≦x≦1, 0 <y≦1 / 3である。

[0021] The precursor material is sintered to obtain the positive electrode active material.

[0022] The method for producing the positive electrode active material has a simple manufacturing process and is easy to operate, and is therefore suitable for use in mass production.

[0023] In one embodiment, the precursor material is produced by a sol-gel method as follows: an A source, a vanadium source, a phosphorus source, an M source, and an E source are mixed in a solvent, and the resulting mixed solution is heated and stirred until the solvent is evaporated to dryness to obtain the precursor material.

[0024] In one embodiment, the precursor material is prepared by solid-state ball milling as follows: An A source, a vanadium source, a phosphorus source, an M source, and an E element-containing doping anion source are ball milled in the absence of a solvent to obtain the precursor material.

[0025] In one embodiment, the sintering is carried out in an inert gas atmosphere, the sintering temperature is 400 to 900° C., and the sintering time is 10 to 30 hours.

[0026] In one embodiment, the inert gas comprises one or more of nitrogen, argon, and helium.

[0027] In a third aspect of the present disclosure, a positive electrode is provided. The positive electrode includes the positive electrode active material provided in the first aspect of the present disclosure. The positive electrode may be used to provide a battery with good rate performance and high energy density.

[0028] In a fourth aspect of the present disclosure, a secondary battery is provided, the secondary battery including the positive electrode provided in the third aspect of the present disclosure.

[0029] In a fifth aspect of the present disclosure, there is provided an electric device, the electric device including the secondary battery provided in the fourth aspect of the present disclosure. [Brief explanation of the drawings]

[0030] [Figure 1A] FIG. 1 shows the molecular structures of NaV(PO)(NVP) and its fully charged compound NaV(PO). [Figure 1B] FIG. 1 shows the molecular structures of Na3V1.5Fe0.5(PO4)3 (Fe-NVP) and its fully charged compound NaV1.5Fe0.5(PO4)3. [Figure 1C] FIG. 1 shows the molecular structures of Na3V1.5Fe0.5(P17 / 18As1 / 18O4)3 (As1-Fe-NVP) and its fully charged compound in Example 1. [Figure 1D] FIG. 1 shows the molecular structures of Na3V1.5Fe0.5(P5 / 6As1 / 6O4)3 (As3-Fe-NVP) and its fully charged compound in Example 2. [Figure 1E] FIG. 1 shows the molecular structures of Na3V1.5Fe0.5(P2 / 3As1 / 3O4)3 (As6-Fe-NVP) and its fully charged compound in Example 3. [Figure 2] 1 is a schematic flow diagram of a method for producing a positive electrode active material according to an embodiment of the present disclosure. [Figure 3] 1 is a summary of X-ray diffraction (XRD) patterns of the positive electrode active materials provided in Examples 1 to 3 and Comparative Examples 1 and 2 of the present disclosure. [Figure 4] 1 summarizes the cycle performance curves at 0.5 C for batteries fabricated using the positive electrode active materials provided in Examples 2, 4-5, and 10 of the present disclosure, as well as for a battery fabricated using the material of Comparative Example 1. [Figure 5] 1 summarizes the curves of specific discharge capacity versus cycle number at different rates for batteries fabricated using the positive electrode active materials provided in Examples 2, 4-5, and 10 of the present disclosure, as well as a battery fabricated using the material of Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0031] Sodium vanadium phosphate (Na3V2(PO4)3, NVP) is a common sodium superionic conductor (NASICON) cathode material. It has a stable crystal structure, good safety performance, and a high theoretical specific capacity. However, it has low ionic and electronic conductivity, resulting in poor rate performance and a low charge / discharge voltage plateau, and does not contribute to improving the energy density of sodium batteries.

[0032] In view of the problems associated with NVP, embodiments of the present disclosure provide a cathode active material having the general formula: A3V 2-x M x (P 1-y E y O4)3, wherein A represents an alkali metal element, M represents a doping element substituting for V, M comprises one or more of a transition metal element and a rare earth element, E represents a doping element substituting for P, E comprises one or more of As, Sb, and Bi, x represents a molar ratio at which the element V is replaced by the element M, y represents a molar ratio at which the element P is replaced by the element E, and 0≦x≦1, 0 <y≦1 / 3である。

[0033] Element E is an element of the same group with a larger ionic radius than P. E can easily dope into the lattice of A3V2(PO4)3, replacing P at some positions and causing the expansion of the unit cell. + Enlarge the transport channel of ions, reduce the energy barrier for migration, and reduce the repulsive effect of surrounding atoms, thus increasing A +This improves the ion transfer rate and rate performance. Furthermore, the crystal structure of A3V2(PO4)3 materials contains PO4 tetrahedra and VO6 octahedra, which are connected at their vertices by shared oxygen (O) atoms. Doping of element E into the phosphorus site changes the charge value of the surrounding O atoms, which forces a change in the charge environment around the V atoms, inducing the V atoms to undergo redox reactions at higher potentials, thus improving the operating voltage and energy density of the doped A3V2(PO4)3 materials. Furthermore, doping of element E into the phosphorus site also reduces the band gap of the M-doped A3V2(PO4)3 material, concentrating more electrons near the Fermi surface and improving the material's electronic conductivity. Furthermore, the doping amount of element E is controlled to ensure that the material not only has good structural stability but also good electronic conductivity, good ionic conductivity, and a high charge / discharge voltage plateau. Therefore, batteries may be promoted to have good safety, good rate performance, and high energy density.

[0034] Furthermore, when an appropriate amount of element E is used to dope the P site and an appropriate amount of metal element M is used to dope the vanadium (V) site in A3V2(PO4)3, the voltage plateau of the double-doped material is higher than that of A3V2(P 1-y E y O4)3 materials and A3V doped only with element M 2-x M x Compared with the (PO4)3 material, further improvements are possible. The charge-discharge voltage plateau of the double-doped material is obviously improved, and A + The energy barrier for ion migration is clearly reduced.

[0035] In one embodiment of the present disclosure, M may include one or more of chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), titanium (Ti), molybdenum (Mo), niobium (Nb), zirconium (Zr), La (lanthanum), and cerium (Ce), although the present disclosure is not limited thereto.

[0036] In some embodiments of the present disclosure, M includes one or more of Cr, Mn, Fe, Co, Ni, Cu, Zn, Ti, Mo, Nb, and Zr. The cost of these elements is lower than that of the lanthanide elements La and Ce. The ionic radii of Ti, Mo, Nb, and Zr are larger than that of the V ion. The introduction of these doping elements also causes the lattice of A3V2(PO4)3 to expand, thus increasing the A + The element M can promote ion migration. However, the ionic radii of Cr, Mn, Fe, Co, Ni, Cu, and Zn are slightly smaller than that of the V ion. When these elements are simply doped into A3V2(PO4)3, the unit cell volume is reduced, but after the introduction of the above-mentioned element E, the unit cell volume is increased, promoting improved diffusion performance of the material. In some other embodiments of the present disclosure, to better balance the cost of element M and its effect on the ionic conductivity and operating voltage of the material, element M can be one or more selected from Ti, Cr, Mn, and Fe. These elements also help improve the operating voltage of the doped material and can easily replace vanadium. Of these, Fe has the lowest cost.

[0037] In one embodiment of the present disclosure, E includes at least one of As, Sb, and Bi. The ionic radii of As, Sb, and Bi are all larger than the ionic radius of P. By doping them into the P site (i.e., substituting E atoms for the positions of P atoms in the crystal structure of A3V2(PO4)3), A3V 2-x M x (P 1-y E yThe volume of the unit cell of O4)3 is moderately enlarged, which can reduce the energy barrier for A ion migration. The ionic radii of As, Sb, and Bi are not excessively larger than the ionic radius of P, which can ensure that the material has not only higher stability of the crystal structure but also higher ionic conductivity. In some embodiments, the element E is As and / or Bi. As and P are nonmetallic elements with similar properties, and As is more easily substituted for P.

[0038] In some embodiments, the element M is Fe and the element E is As. In this case, A3V 2-x M x (P 1-y E y The production cost of O4)3 is low, and P is replaced by As, which improves the migration rate of A ions, improving the rate capability of the material, and the operating voltage, improving the energy density of the material.

[0039] In one embodiment of the present disclosure, the element A can specifically include one or more of Li, Na, K, etc. The element A can be selected according to the specific type of secondary battery. For example, the battery active material A3V 2-x M x (P 1-y E y When O4)3 is used in a sodium secondary battery, element A is Na.

[0040] In some embodiments of the present disclosure, x is in the range of 0.001≦x≦1, and further in the range of 0.01≦x≦1. By controlling the doping amount of M within the above range, A3V caused by an excessively high doping amount of element M can be prevented. 2-x M x (P 1-y E yLoss of electrochemical activity of O4)3 may be avoided, and the inability to improve electronic conductivity due to too low a doping amount of element M is also avoided. In some embodiments, s is in the range of 0.001 to 0.5, preferably in the range of 0.01 to 0.5, more preferably in the range of 0.1 to 0.5, and even more preferably in the range of 0.3 to 0.5.

[0041] In some embodiments of the present disclosure, y is in the range of 1 / 18≦y≦1 / 3. By controlling the doping amount of element E within the above range, A3V 2-x M x (P 1-y E y The unit cell volume of O4)3 is significantly increased, improving ionic conductivity without increasing the risk of structural collapse due to excessive changes in its crystal structure. This ensures that the material has good cycle performance, significantly improves the operating voltage of the material, and narrows the band gap of the material, thereby improving conductivity.

[0042] In some embodiments of the present disclosure, y is 1 / 6. In this case, A3V 2-x M x (P 1-y E y The energy barrier for A ion migration in the O4)3 material is extremely low, and the material has excellent rate and cycle performance. In some other embodiments, y is 1 / 3. In this case, A3V 2-x M x (P 1-y E y O4)3 materials have 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 materials have very high crystallinity and very good structural stability. In some embodiments, y is in the range of 1 / 18≦y≦1 / 6. In some other embodiments, y is in the range of 1 / 6≦y≦1 / 3.

[0043] Accordingly, an embodiment of the present disclosure provides a method for producing a positive electrode active material, as shown in Figure 2. Specifically, the method for producing a positive electrode active material includes the following steps:

[0044] 10: Manufactured cathode active material A3V 2-x M x (P 1-y E y Element sources of various elements of O4)3 are mixed to obtain a precursor material, wherein A represents an alkali metal element, M represents a doping element to replace V, M includes one or more of a transition metal element and a rare earth element, E represents a doping element to replace P, E includes one or more of As, Sb, and Bi, and 0≦x≦1, 0 <y≦1 / 3である。

[0045] 20: The precursor material is sintered to obtain the positive electrode active material.

[0046] The method for producing the positive electrode active material is suitable for use in mass production because the production process is simple and easy to operate.

[0047] In step 10, various element sources used to produce the positive electrode active material include an A source, a vanadium source, a phosphorus source, an M source containing a doping metal element M, and an E source containing a doping element E. The A source, vanadium source, M source, phosphorus source, and E source may be weighed according to a weight ratio of the elements A, V, M, P, and E of 3:(2-x):x:(1-y):y. Furthermore, considering that alkali metal elements are prone to element loss in the subsequent sintering process, the A source may be 10% in excess or in a small amount.

[0048] The A source may include one or more of a sodium source, a lithium source, and a potassium source, specifically one or more of alkali metal nitrates, oxalates, acetates, acetylacetonates, carbonates, sulfates, phosphates, and hydroxides. The vanadium source may be one or more of a trivalent vanadium source, a tetravalent vanadium source, and a pentavalent vanadium source. Specific examples of the vanadium source include vanadium oxides (such as vanadium pentoxide, vanadium tetroxide, and vanadium trioxide), hydroxides, metavanadates (such as ammonium metavanadate and sodium metavanadate), nitrates, sulfates, phosphates, oxalates, acetates, vanadium acetylacetonates, and vanadyl acetylacetonates. The source of M containing the doping metal element M may be similar to the vanadium source described above, or may be one or more selected from oxides, hydroxides, nitrates, sulfates, phosphates, oxalates, acetates, and acetylacetonates of element M. The phosphorus source may be a source of polyanionic groups for P, such as a source of oxyacid groups for P. In some embodiments, the phosphorus source is H3PO4, Na3PO4, and Li + , Na + , K. + , NH4 + The E source may include one or more of dihydrogen phosphate, hydrogen phosphate, and orthophosphate of As, Sb, and Bi. The E source may be one or more of oxides, acids, salts, etc. corresponding to As, Sb, and Bi. In some embodiments, the E source may be a doping anion source containing element E, such as one or more of oxyacids and oxysalts corresponding to As, Sb, and Bi. Among the above element sources, the phosphorus source may be the same substance as the alkali metal source, vanadium source, or M source; for example, sodium phosphate can function as both the phosphorus source and the sodium source.

[0049] In the present disclosure, 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 liquid phase high-energy ball milling method, etc. The solvent used in the liquid phase method can be one or more of water, ethanol, acetone, etc.

[0050] In some embodiments, the precursor material is prepared by solid-state ball milling, specifically as follows: an alkali metal source, a vanadium source, a phosphorus source, an M source, and an E-containing doping anion source are ball milled in the absence of a solvent to obtain the precursor material.

[0051] In some other embodiments, the precursor material is produced by a sol-gel method. This process is specifically as follows: An alkali metal source, a vanadium source, and a phosphorus source are mixed with an M source and a doping anion source containing element E in a solvent. The resulting mixed solution is heated and stirred until the solvent evaporates to dryness to obtain the precursor material. During the heating and stirring process, the various element sources react to obtain the precursor material. The precursor material may be converted into the required cathode active material after sintering. In one embodiment of the present disclosure, the heating and stirring temperature can be 30 to 200°C, for example, 40 to 100°C, depending on the boiling point of the solvent used. The stirring speed during heating and stirring can be 300 to 900 rpm, and the heating and stirring time can be 1 to 6 hours.

[0052] In one embodiment of the present disclosure, sintering is carried out under an inert gas atmosphere, the sintering temperature is 400 to 900°C, and the sintering time is 10 to 30 hours. The inert gas may be one or more of nitrogen, argon, and helium, with argon and helium being preferred.

[0053] The present disclosure also provides a positive electrode, which includes a positive electrode active material. The positive electrode may be used to manufacture a secondary battery having excellent performance, such as good rate capability and high energy density.

[0054] In one embodiment of the present disclosure, the positive electrode generally includes a positive current collector and a positive active material layer disposed on at least one side of the positive current collector, the positive active material layer including a positive active material. The positive active material layer may further include a binder and, optionally, a conductive agent. The binder and conductive agent may be selected from conventional options in the field of batteries.

[0055] The positive electrode current collector may be made of a variety of materials suitable for use as a positive electrode current collector, including, but not limited to, metal foil, alloy foil, metallized polymer film, or the aforementioned materials coated with carbon. The metal foil may be aluminum foil, the alloy foil may be aluminum alloy foil, or the metal plated on the surface of the polymer film may be an aluminum or aluminum alloy layer.

[0056] The present disclosure also provides a secondary battery, which includes a positive electrode. The secondary battery may be a lithium secondary battery, a sodium secondary battery, or a potassium secondary battery.

[0057] Because doped A3V2(PO4)3 is used as the positive electrode active material, the secondary battery has high energy density, good rate performance, and good cycle performance.

[0058] An embodiment of the present disclosure further provides an electronic device, the electrical device including a secondary battery, the electrical device may be a vehicle, a ship, a 3C product (including computers, communications, home appliances, etc.), or an energy storage system.

[0059] The secondary battery, which uses doped A3V2(PO4)3 as the positive electrode active material, has high energy density, good rate capability, and good cycle performance. When used in electrical devices, the secondary battery can improve the performance and market competitiveness of the electrical devices.

[0060] The secondary battery may 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 a positive electrode sheet, a negative electrode sheet, and a separator and electrolyte disposed between the positive and negative electrode sheets. In some other embodiments, the secondary battery may include a positive electrode sheet, a negative electrode sheet, and a semi-solid or solid electrolyte disposed between the positive and negative electrode sheets. Furthermore, when a semi-solid or solid electrolyte is used, the positive and negative electrode sheets may also include a semi-solid or solid electrolyte material.

[0061] The technical solutions of the present disclosure are further illustrated in detail by examples. [Example]

[0062] The positive electrode active material is of the general formula Na3V 1.5 Fe 0.5 (P 17 / 18 As 1 / 18 O4)3(As1-Fe-NVP).

[0063] As1-Fe-NVP was prepared as follows.

[0064] (1) A sodium source (specifically, sodium nitrate), a vanadium source (specifically, vanadium pentoxide), an Fe source (specifically, ferrous oxide), a phosphorus source (specifically, phosphoric acid), and an As source (specifically, ammonium arsenate) were weighed out in a molar ratio of Na:V:Fe:P:As=3:1.5:0.5:17 / 18:1 / 18, mixed in an ethanol solvent, and heated at 80°C with stirring until the solvent evaporated to dryness, thereby obtaining a precursor material.

[0065] (2) The precursor material was sintered at a sintering temperature of 800 °C for 12 h under a nitrogen atmosphere to form Na3V 1.5 Fe 0.5 (P 17 / 18 As 1 / 18 O4)3 was obtained. [Example]

[0066] The positive electrode active material is of the general formula Na3V 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 O4)3(As3-Fe-NVP).

[0067] The method for producing As3-Fe-NVP was the same as in Example 1, except that the amounts of the P source and As source were changed so that the molar ratio of Fe element:P element:As element was 0.5:5 / 6:1 / 6. [Example]

[0068] The positive electrode active material is of the general formula Na3V 1.5 Fe 0.5 (P 2 / 3 As 1 / 3 O4)3(As6-Fe-NVP).

[0069] The method for producing As6-Fe-NVP was the same as in Example 1, except that the amounts of the P source and As source were changed so that the molar ratio of Fe element:P element:As element was 0.5:2 / 3:1 / 3.

[0070] Comparative Example 1 The positive electrode active material has the general formula Na3V2(PO4)3(NVP).

[0071] The preparation method of NVP was the same as in Example 1, except that the As source and Fe source were not introduced.

[0072] Comparative Example 2 The positive electrode active material is of the general formula Na3V 1.5 Fe 0.5 It has (PO4)3(Fe-NVP).

[0073] The preparation method of Fe-NVP was the same as in Example 1, except that no As source was introduced.

[0074] Figure 1A shows the molecular structure of Na3V2(PO4)3 (NVP) and its fully charged compound NaV2(PO4)3. Figure 1B shows the molecular structure of Na3V 1.5 Fe 0.5 (PO4)3(Fe-NVP) and its fully charged compound NaV 1.5 Fe 0.5 Figure 1C shows the molecular structure of As-Fe-NVP and its fully charged compound NaV in Example 1. 1.5 Fe 0.5 (P 17 / 18 As 1 / 18 The molecular structure of O4)3 is shown.

[0075] In Figure 1A, in the Na3V2(PO4)3 unit cell, all 12 V atoms are located at the centers of various VO6 octahedra, all 18 P atoms are located at the centers of various PO4 tetrahedra, and 72 O atoms are used to connect the octahedra and tetrahedra to form the framework structure of the A3V2(PO4)3 compound. The remaining 18 Na atoms uniformly occupy the vacancies in the framework structure. The unit cell shown above 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, marked as the Na1 site and the Na2 site, respectively. The Na3V2(PO4)3 unit cell contains six Na atoms at the Na1 site (indicated by the dashed arrows) and 12 Na atoms at the Na2 site. The sodium ions in the Na1 site are difficult to extract, and the sodium ions in the Na2 site are usually deintercalated / intercalated during the charge / discharge process, completing the transformation of the crystal structure between uncharged Na3V2(PO4)3 and fully charged NaV2(PO4)3. Six Na ions in the unit cell of fully charged NaV2(PO4)3 occupy the Na1 site.

[0076] In Figure 1B, Fe-NVP may be considered as the molecular structure of Na3V2(PO4)3 in Figure 1A, with three Fe atoms uniformly substituted for three of the twelve V atoms. The fully charged compound of Fe-NVP is NaV 1.5 Fe 0.5 (PO4)3, which has a different structure from Fe-NVP in that the Na atom at the Na2 site has been removed.

[0077] In Figure 1C, As1-Fe-NVP can be considered as the molecular structure of Fe-NVP in Figure 1B, where one P atom in the third layer is replaced by one As atom (shown by the solid arrow). The fully charged compound of As1-Fe-NVP is NaV. 1.5 Fe 0.5 (P 17 / 18 As 1 / 18 O4)3, which has a structure different from As1-Fe-NVP in that the Na atom at the Na2 position has been removed (the dotted arrow indicates the Na atom at the Na1 position, and the remaining spherical atoms are Na2 atoms).

[0078] FIG. 1D shows the 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 structure of As3-Fe-NVP is shown below. The molecular structure of As3-Fe-NVP is equivalent to the molecular structure of NVP in which three of the 12 V atoms are evenly replaced by three Fe atoms, and one P atom in every other layer of the six layers of P atoms is replaced by an As atom, for a total of three As atom replacements (indicated by solid arrows).

[0079] FIG. 1E shows the As6-Fe-NVP and its fully charged compound NaV in Example 3. 1.5 Fe 0.5 (P 2 / 3 As 1 / 3The molecular structure of As6-Fe-NVP is shown in Fig. 1. The molecular structure of As6-Fe-NVP is equivalent to the molecular structure of NVP in which three of the 12 V atoms are evenly replaced by three Fe atoms, and one P atom in each of the six P layers is replaced by an As atom, for a total of six As atom replacements (indicated by solid arrows).

[0080] According to the molecular structures of the compounds shown in Figures 1A-1E, the unit cell volumes of NVP, Fe-NVP, As1-Fe-NVP, As3-Fe-NVP, and As6-Fe-NVP, as well as the unit cell volume change (%) in the uncharged state relative to the fully charged state, open circuit voltages, band gaps, and energy barriers for sodium ion migration can be predicted using first-principles calculations. The relevant results are summarized in Table 1 below.

[0081] The volume change rate of the unit cell in the uncharged state relative to the fully charged state is explained using As1-Fe-NVP as an example. The volume change rate of the unit cell in the uncharged state relative to the fully charged state is explained using As1-Fe-NVP as an example. 1.5 Fe 0.5 (P 17 / 18 As 1 / 18 Unit cell volume of O4)3 and uncharged Na3V 1.5 Fe 0.5 (P 17 / 18 As 1 / 18 The absolute difference between the unit cell volume of the uncharged compound A3V and that of the uncharged compound A3V is calculated by dividing the absolute difference between the unit cell volume of the uncharged compound A3V and that of the uncharged compound A3V. 2-x M x (P 1-y E y For the open circuit voltage of O4), the lattice constants and total energies of the unit cell structures of the uncharged and fully charged compounds are obtained from their molecular structures, and these are used to calculate A3V. 2-x M x (P 1-y E yThe open circuit voltage of O4)3 may be calculated. The band gap may be calculated from the density of states distribution curve of the material. The energy barrier for Na ion migration may be calculated by the Nudged Elastic Band (NEB) method or the modified NEB method. [Table 1]

[0082] As can be seen from Table 1, when NVP is doped with only Fe, the unit cell volume is reduced because the atomic radius of Fe is smaller than that of V. When Fe-doped NVP is further doped with As, the unit cell volume increases with increasing As doping concentration, indicating that the introduction of As promotes the expansion of the unit cell.

[0083] Doping with only Fe and co-doping with Fe and As results in different unit cell volume changes in the uncharged state relative to the charged state of the material. As can be seen from Table 1, As3-Fe-NVP has the smallest unit cell volume change in the uncharged state relative to the fully charged state and the most stable crystal structure. Furthermore, although the unit cell volume changes in the uncharged state relative to the fully charged state of Fe-NVP, As1-Fe-NVP, and As3-Fe-NVP are larger than those of NVP, their crystal structures are stable, ensuring that the structure does not collapse due to excessive deformation during the charge-discharge process.

[0084] As can be seen from Table 1, the open circuit voltage of the Fe and As co-doped materials (Examples 1-3) is significantly improved compared to undoped NVP and Fe-NVP doped only with iron. When this material is used in sodium batteries, the battery's energy density is enhanced. Furthermore, with increasing As doping concentration, the open circuit voltage of Fe and As co-doped NVP increases, thus improving the battery's energy density. Since the open circuit voltage of a material is generally higher than its operating voltage, if the open circuit voltage of material A is higher than that of material B, the operating voltage of material A will generally be higher than that of material B. Therefore, the energy density of the fabricated battery can be estimated by comparing the open circuit voltages of different materials.

[0085] As can be seen from Table 1, the band gap of each compound is wider for undoped NVP, indicating greater resistance. The band gap of Fe-NVP, which is doped only with iron, can be reduced to 0.82 eV, and co-doping with Fe and As can further reduce the band gap. As3-Fe-NVP and As6-Fe-NVP show almost no band gap and high conductivity.

[0086] As can be seen from Table 1, the energy barrier for Na ion migration in NVP is relatively high, approximately 0.612 eV. The energy barrier for Na ion migration increases after the introduction of single-doped Fe. This is mainly because the ionic radius of Fe is small, and the doping of iron alone reduces the unit cell volume of NVP. In contrast, the energy barrier for Na ion migration in As1-Fe-NVP, As3-Fe-NVP, and As6-Fe-NVP, which are co-doped with Fe and As, is clearly reduced, improving the ionic conductivity and thus the charge / discharge rate of the material.

[0087] Additionally, Figure 3 summarizes the measured XRE patterns of NVP, Fe-NVP, As1-Fe-NVP, As3-Fe-NVP, and As6-Fe-NVP. As can be seen from Figure 3, compared to the main diffraction peak in the XRE pattern of undoped NVP, after Fe doping, the position of the main peak shifts slightly to the right, and the lattice parameters are generally highly consistent with NVP. With Fe doping, the main peak shifts to the right at low As concentrations (e.g., As1-Fe-NVP). With increasing As doping concentrations (e.g., As1-Fe-NVP), the half-widths of some high-intensity peaks slightly broaden, and the intensities of some low-intensity peaks improve to various degrees. This indicates that co-doping with Fe and As can change the crystal growth direction of NVP, enabling growth on previously difficult planes. This suggests that co-doping with Fe and As can control the morphology and size of NVP. [Example]

[0088] The positive electrode active material is of the general formula Na3V 1.5 Fe 0.5 (P 5 / 6 Sb 1 / 6 O4)3.

[0089] Na3V 1.5 Fe 0.5 (P 5 / 6 Sb 1 / 6 The method for preparing NaV (O4)3 was the same as in Example 2, except that the As source was replaced with an Sb source (specifically, sodium antimonate). 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The manufacturing method was the same as that of O4)3. [Example]

[0090] The positive electrode active material is of the general formula Na3V 1.5 Fe 0.5 (P 5 / 6 Bi 1 / 6 O4)3.

[0091] Na3V 1.5 Fe 0.5 (P 5 / 6 Bi 1 / 6 The method for preparing NaV (O4)3 was the same as in Example 2, except that the As source was replaced with a Bi source (specifically, bismuth ammonium citrate). 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The manufacturing method was the same as that of O4)3. [Example]

[0092] The positive electrode active material is of the general formula Na3V 1.8 Fe 0.2 (P 5 / 6 As 1 / 6 O4)3.

[0093] Na3V 1.8 Fe 0.2 (P 5 / 6 As 1 / 6 The method for producing Na3V (O4)3 was the same as in Example 2, except that the amounts of the Fe source and the V source were changed so that the molar ratio of V element:Fe element was 1.8:0.2. 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The manufacturing method was the same as that of O4)3. [Example]

[0094] The positive electrode active material is of the general formula Na3V 1.99 Fe 0.01 (P 5 / 6 As 1 / 6 O4)3.

[0095] Na3V 1.99 Fe 0.01 (P 5 / 6 As 1 / 6 The method for producing Na3V (O4)3 was the same as in Example 2, except that the amounts of the Fe source and the V source were changed so that the molar ratio of V element:Fe element was 1.99:0.01. 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The manufacturing method was the same as that of O4)3. [Example]

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

[0097] Na3V 1.5 Ti 0.5 (P 5 / 6 As 1 / 6 The method for preparing NaV (O4)3 was the same as in Example 2, except that the Fe source was replaced with a Ti source (specifically titania). 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The manufacturing method was the same as that of O4)3. [Example]

[0098] The positive electrode active material is of the general formula Na3V 1.5 Cr 0.5 (P 5 / 6 As 1 / 6 O4)3.

[0099] Na3V 1.5 Cr 0.5 (P 5 / 6 As 1 / 6 The method for preparing NaV (O4)3 was the same as in Example 2, except that the Fe source was replaced with a Cr source (specifically, chromium oxide). 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The manufacturing method was the same as that of O4)3. [Example]

[0100] The positive electrode active material is of the general formula Na3V 1.5 Nb 0.5 (P 5 / 6 As 1 / 6 O4)3.

[0101] Na3V 1.5 Nb 0.5 (P 5 / 6 As1 / 6 The method for preparing NaV (O4)3 was the same as in Example 2, except that the Fe source was replaced with a Nb source (specifically, sodium niobate). 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The manufacturing method was the same as that of O4)3. [Example]

[0102] The positive electrode active material is a compound represented by the general formula Na3V2(P 17 / 18 As 1 / 18 O4)3.

[0103] Na3V2(P 17 / 18 As 1 / 18 The preparation method of O4)3 was the same as that of Example 1, except that no doping source—Fe source—was used. [Example]

[0104] The positive electrode active material is a compound represented by the general formula Na3V2(P 5 / 6 As 1 / 6 O4)3.

[0105] Na3V2(P 5 / 6 As 1 / 6 The preparation method of O4)3 was the same as that of Example 2, except that no doping source—Fe source—was used. [Example]

[0106] The positive electrode active material is a compound represented by the general formula Na3V2(P 2 / 3 As 1 / 3 O4)3.

[0107] Na3V2(P 2 / 3 As 1 / 3 The preparation method of O4)3 was the same as that of Example 3, except that no doping source—Fe source—was used. [Example]

[0108] The positive electrode active material is a compound represented by the general formula Na3VFe(P 5 / 6 As1 / 6 O4)3.

[0109] Na3VFe(P 5 / 6 As 1 / 6 The method for producing Na3V (O4)3 was the same as in Example 2, except that the amounts of the Fe source and the V source were changed so that the molar ratio of V element:Fe element was 1:1. 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The manufacturing method was the same as that of O4)3. [Example]

[0110] The positive electrode active material is of the general formula Na3V 1.5 Fe 0.5 (P 35 / 36 As 1 / 36 The production method was the same as that of Example 2, except that the amounts of the P source and the As source were changed so that the molar ratio of Fe:P:As was 0.5:35 / 36:1 / 36. 1.5 Fe 0.5 (P 5 / 6 As 1 / 6 The manufacturing method was the same as that of O4)3. [Example]

[0111] The positive electrode active material is of the general formula Na3V 1.5 Mn 0.5 (P 5 / 6 As 1 / 6 O4)3. [Example]

[0112] The positive electrode active material is of the general formula Na3V 1.5 Mn 0.5 (P 5 / 6 Bi 1 / 6 O4)3. [Example]

[0113] The positive electrode active material is a compound represented by the general formula Na3V1Mn1(P 5 / 6 Bi 1 / 6 O4)3. [Example]

[0114] The positive electrode active material is of the general formula Na3V 1.5 Mn 0.5 (P 5 / 6 Sb 1 / 6 O4)3. [Example]

[0115] The positive electrode active material is of the general formula Na3V 1.5 Ti 0.5 (P 5 / 6 Bi 1 / 6 O4)3.

[0116] Table 2 below summarizes the relevant electrochemical performance of the positive electrode active materials in Examples 4-20. [Table 2]

[0117] As can be seen from Table 2, compared to the undoped Na3V2(PO4)3 in Example 1, the doubly doped sodium vanadium phosphate materials provided in Examples 4-10 and 14-20 of the present disclosure not only maintain good structural stability but also have high open-circuit voltages, narrow band gaps, and low energy barriers to sodium ion migration, thus resulting in high energy density and good rate performance. Furthermore, by comparing Examples 11-13 in Table 1 with Examples 1-3, it can be seen that when Na3V2(PO4)3 is doped with the same amount of As only, without iron, the resulting material has a larger unit cell volume, a smaller unit cell volume change rate from the fully charged state to the uncharged state, and a reduced energy barrier to sodium ion migration. As a result, the rate performance of the battery is improved, but the open-circuit voltage and conductivity of the material are slightly reduced. Furthermore, when Na3V2(PO4)3 is doped with the same amount of As, Sb, or Bi, the open-circuit voltage of the material can be further improved after Mn is doped into the V site.

[0118] To further demonstrate the beneficial effects of the presently disclosed embodiments, the materials of the above examples and comparative examples were fabricated into batteries and their electrochemical performances were tested. The relevant results are shown in Table 3.

[0119] The battery manufacturing process was as follows: (1) Preparation of positive electrode sheet: The positive electrode active material of each example or comparative example, the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVEF) were added to the solvent NMP (N-methylpyrrolidone) in a weight ratio of 88:6:6 and stirred uniformly to obtain a positive electrode slurry. The positive electrode slurry was coated onto an aluminum foil positive electrode current collector, dried, rolled, and cut to obtain a positive electrode sheet. (2) Preparation of negative electrode sheet: The negative electrode active material (specifically, hard carbon) and the binder (specifically, styrene butadiene rubber (SBR) and carboxymethyl cellulose sodium (CMC-Na) in a weight ratio of 2:3) were mixed with ionized water in a weight ratio of 95:5 and stirred until uniform to obtain a negative electrode slurry. The negative electrode slurry was coated onto a copper foil negative electrode current collector, dried, rolled, and cut to obtain a negative electrode sheet. (3) Battery assembly: A positive electrode sheet, a separator, and a negative electrode sheet are stacked in order to obtain a dry battery. This dry battery is then rolled up and placed in an aluminum-plastic film exterior foil, and the electrolyte is poured in. After vacuum packaging, leaving it to stand, chemical formation, molding, and other processes, the manufacture of a sodium battery is completed.

[0120] Each example and comparative cell was tested for the following electrochemical performance.

[0121] 1) Cycle Performance: Each battery was subjected to a charge-discharge cycle test at a rate of 0.5C at 25°C. The voltage range was 2.5 to 4.3V. During charging, the battery was 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 battery was discharged at a constant current of 0.5C until the cutoff voltage reached 2.5V. The first-cycle discharge capacity per gram and the capacity retention after 50 cycles were recorded for each battery. The first-cycle discharge capacity per gram is equal to the ratio of the first-cycle discharge capacity of each sodium whole battery to the weight of the positive electrode active material in the battery. The capacity retention after 50 cycles is equal to the ratio of the discharge capacity after 50 cycles to the discharge capacity of the first cycle.

[0122] Furthermore, the discharge curves of the charge / discharge curves obtained by constant current charging / discharging at 0.5 C for each battery were integrated and divided by the discharge capacity to obtain the average voltage of each battery. The relevant results are summarized in Table 3 below, and the cycle curves of some examples and comparative examples are summarized in Figure 4.

[0123] 2) Rate Performance: The change in discharge capacity per gram of each battery with respect to cycles was tested at 25°C at 0.5C, 1C, 5C, and 10C. The voltage range was 2.5 to 4.3V. Rate performance curves for some examples and comparative examples are shown in Figure 5. When calculating the discharge capacity per gram, the ratio of the discharge capacity at a certain current density to the weight of the positive electrode active material was considered to be the discharge capacity per gram at that current density. Table 3 summarizes the first-cycle discharge capacity per gram at a 10C rate for each battery, as well as the ratio of the first-cycle discharge capacity at a 10C rate to the first-cycle discharge capacity at a 0.5C rate. [Table 3]

[0124] As can be seen from Table 3, compared with the sodium battery fabricated with the material of Comparative Example 1, the sodium batteries fabricated with the materials provided in Examples 1-20 of the present disclosure have a higher voltage plateau and a higher first cycle discharge capacity ratio at 10C / 0.5C, indicating higher rate performance of the batteries. Furthermore, the cycle performance of the batteries is not significantly reduced. Furthermore, the batteries of Examples 1-7 and 14-15 have a higher voltage plateau and better rate performance than the battery of Comparative Example 2.

[0125] The exemplary embodiments of the present disclosure have been described above. It should be noted that, without departing from the principle of the present disclosure, some improvements and modifications may be made by those skilled in the art, which still fall within the protection scope of the present disclosure.

Claims

1. A positive electrode active material, the positive electrode active material being represented by general formula A ３ V ２－ｘ M ｘ (P １－ｙ E ｙ O ４ ) ３ wherein A represents an alkali metal element, M represents a doping element substituting for V, M comprises one or more of a transition metal element and a rare earth element, E represents a doping element substituting for P, and E comprises one or more of As, Sb, and Bi, and 0≦x≦1, and 0<y≦1 / 3.

2. 10. The cathode active material of claim 1, wherein M comprises one or more of Cr, Mn, Fe, Co, Ni, Cu, Zn, Ti, Mo, Nb, Zr, La, and Ce.

3. 10. The cathode active material of claim 1, wherein M comprises one or more of Cr, Mn, Fe, and Ti.

4. 4. The cathode active material of claim 1, wherein E is one or more of As and Bi.

5. 2. The positive electrode active material of claim 1, wherein M is Fe and E is As.

6. 6. The cathode active material of claim 1, wherein A comprises one or more of Li, Na, and K.

7. The positive electrode active material according to claim 1 , wherein x is in the range of 0.001≦x≦1.

8. The positive electrode active material according to claim 1 , wherein x is in the range of 0.01≦x≦1.

9. The positive electrode active material according to claim 1 , wherein x is in the range of 0.3≦x≦0.

5.

10. The positive electrode active material according to claim 1 , wherein y is in the range of 1 / 18≦y≦1 / 3.

11. 10. The positive electrode active material of claim 1, wherein y is 1 / 6, 1 / 3, or 1 / 18.

12. The positive electrode active material according to claim 1 , wherein y is in the range of 1 / 18≦y≦1 / 6.

13. The positive electrode active material according to claim 1 , wherein y is in the range of 1 / 6≦y≦1 / 3.

14. A method for producing a positive electrode active material, comprising: The positive electrode active material A to be produced ３ V ２－ｘ M ｘ (P １－ｙ E ｙ O ４ ) ３ to obtain a precursor material (10), wherein A represents an alkali metal element, M represents a doping element substituting for V, M comprises one or more of a transition metal element and a rare earth element, E represents a doping element substituting for P, and E comprises one or more of As, Sb, and Bi, and 0≦x≦1, 0<y≦1 / 3; sintering the precursor material to obtain the positive electrode active material (20); A method comprising:

15. 15. The method of claim 14, wherein the precursor material is produced by a sol-gel method comprising mixing an A source, a vanadium source, a phosphorus source, an M source, and an E source in a solvent, and heating and stirring the resulting mixed solution until the solvent is evaporated to dryness, to obtain the precursor material.

16. 15. The manufacturing method of claim 14, wherein the precursor material is produced by solid-phase ball milling, comprising ball milling an A source, a vanadium source, a phosphorus source, an M source, and an E element-containing doping anion source in the absence of a solvent to obtain the precursor material.

17. The method according to any one of claims 14 to 16, wherein the sintering is carried out in an inert gas atmosphere, the sintering temperature is 400 to 900°C, and the sintering time is 10 to 30 hours.

18. A positive electrode comprising the positive electrode active material according to any one of claims 1 to 13, or the positive electrode active material produced by the method according to any one of claims 14 to 17.

19. A secondary battery comprising the positive electrode according to claim 18.

20. An electrical device comprising the secondary battery according to claim 19.

Images