Positive electrode active material, method for preparing the same, positive electrode sheet, battery, and electrical device

Abstract

The present invention discloses a positive electrode active material, a preparation method thereof, a positive electrode sheet, a battery, and an electrical device. The positive electrode active material includes: a core and a coating layer. The core includes Na<subgt;a< / subgt;Fe<subgt;b< / subgt>M<subgt;c< / subgt;(PO<subgt;4< / subgt;)<subgt;2< / subgt>P<subgt;2< / subgt>O<subgt;7< / subgt>, where 3 < a ≤ 4, 2 < b < 3, 0 ≤ c ≤ 0.1, and M is selected from one or more of Mo, W, V, Zr, Cr, Mg, Ni, Co, Mn, B, and Li; the coating layer is located on at least a part of the surface of the core and includes Na<subgt;1+x< / subgt>Al<subgt;x< / subgt>Ti<subgt;2‑x< / subgt>(PO<subgt;4< / subgt;)<subgt;3< / subgt>, where 0 < x ≤ 0.05. The positive electrode active material has good air stability and ionic conductivity and can be used in sodium batteries to improve electrochemical properties such as the rate performance and cycle performance of the battery.

Description

Technical Field

[0001] The present invention belongs to the field of batteries, and specifically relates to a positive electrode active material, a preparation method thereof, a positive electrode sheet, a battery, and an electrical device. Background Art

[0002] Secondary battery systems represented by lithium / sodium ion batteries are increasingly widely used in electrical devices and energy storage fields, and their market demand has also increased year by year. Different usage scenarios of secondary batteries also put forward targeted requirements for their energy density, cycle life, safety, and usage cost. Taking the application in electric vehicles as an example, the high energy density of lithium ion batteries can meet the long-range driving requirements of automobiles, but lithium ion batteries are costly and the crust abundance of lithium is low. Sodium ion batteries are considered as secondary battery systems that can replace lithium ion batteries in electrical devices and energy storage fields due to their significant advantages such as low cost, high cycle life, and excellent safety. However, in the sodium ion battery system, different positive electrode materials face different technical problems, and the performance of the positive electrode active material still needs to be further improved. Summary of the Invention

[0003] The present invention aims to solve at least one of the technical problems in the related art to some extent. For this reason, an object of the present invention is to provide a positive electrode active material, a preparation method thereof, a positive electrode sheet, a battery, and an electrical device. The air stability and ionic conductivity of the positive electrode active material are good, and it can be used in sodium batteries to improve the electrochemical performance such as the rate performance and cycle performance of the battery.

[0004] In a first aspect of the present invention, a positive electrode active material is provided, comprising:

[0005] a core, the core comprising Na a Fe b M c (PO4)2P2O7, 3 < a ≤ 4, 2 < b < 3, 0 ≤ c ≤ 0.1, M is selected from one or more of Mo, W, V, Zr, Cr, Mg, Ni, Co, Mn, B, Li;

[0006] a coating layer, the coating layer being located on at least a part of the surface of the core and comprising Na 1+x Al x Ti 2-x (PO4)3, 0 < x ≤ 0.5.

[0007] The positive electrode active material of the first aspect of the present invention has at least the following beneficial effects: The sodium iron pyrophosphate phosphate has good structural stability, and by doping the M element in the sodium iron pyrophosphate phosphate, the ionic conductivity of the sodium iron pyrophosphate phosphate itself can be improved, which is beneficial to improving the charge-discharge performance and rate performance of the positive electrode active material; further, setting the coating layer can not only improve the air stability of the positive electrode active material, but also reduce the side reaction between the core material and the electrolyte, effectively alleviating the dissolution of transition metals in Na a Fe b M c (PO4)2P2O7, which is beneficial to improving the cycle and rate performance of the positive electrode active material. Moreover, the coating layer material Na 1+x Al x Ti 2-x (PO4)3 is a fast ion conductor, which can further improve the ionic conductivity of the positive electrode active material; in addition, when forming the coating layer, it is also beneficial to convert part of the residual alkali (such as sodium salts of sodium pyrophosphate, sodium phosphate, etc.) on the surface of the core into Na 1+x Al x Ti 2-x (PO4)3 through chemical reaction, thereby being beneficial to reducing the amount of surface residual alkali of the positive electrode active material. In summary, the air stability and ionic conductivity of this positive electrode active material are both good, and using it in a sodium battery can improve the electrochemical performance such as the rate performance and cycle performance of the battery.

[0008] In addition, the positive electrode active material according to the above embodiments of the present invention may further have the following additional technical features:

[0009] In some embodiments of the present invention, a / b≥1.33.

[0010] In some embodiments of the present invention, 2 < b + c ≤ 3.

[0011] In some embodiments of the present invention, the Na a Fe b M c (PO4)2P2O7 includes a first phase, and in the X-ray diffraction pattern of the positive electrode active material, the strongest peak corresponds to the first phase.

[0012] In some embodiments of the present invention, the 2θ1 corresponding to the peak height of the strongest peak is 33.6° ± 0.2°.

[0013] In some embodiments of the present invention, the full width at half maximum of the strongest peak is 0.16° to 0.24°.

[0014] In some embodiments of the present invention, the unit cell volume corresponding to the strongest peak is

[0015] In some embodiments of the present invention, in the X-ray diffraction pattern of the positive electrode active material, diffraction characteristic peaks exist at 2θ2 = 34.8° ± 0.2° and / or 2θ3 = 10.6° ± 0.2°.

[0016] In some embodiments of the present invention, in the X-ray diffraction pattern of the positive electrode active material, the peak height of the strongest peak is I1, the peak height of the diffraction characteristic peak at 2θ2 is I2, and the peak height of the diffraction characteristic peak at 2θ3 is I3. I1, I2, and I3 satisfy: I1 / (I1 + I2 + I3) ≥ 85%.

[0017] In some embodiments of the present invention, the core further includes: a conductive carbon material, and the conductive carbon material is dispersed in the core and / or distributed on the surface of the core.

[0018] In some embodiments of the present invention, based on the total mass of the positive electrode active material, the mass ratio of the conductive carbon material is 1 wt% to 4 wt%.

[0019] In some embodiments of the present invention, based on the mass of Na a Fe b M c (PO4)2P2O7, the mass ratio of the conductive carbon material is 1 wt% to 4 wt%.

[0020] In some embodiments of the present invention, based on the total mass of the positive electrode active material, the mass ratio of Na a Fe b M c (PO4)2P2O7 is 94.0 wt% - 98.8 wt%.

[0021] In some embodiments of the present invention, the pH value of the positive electrode active material is ≤ 10.5, optionally ≤ 10.

[0022] In the second aspect of the present invention, a method for preparing the above positive electrode active material is provided, including:

[0023] Mixing a sodium source, an iron source, an M source, a phosphate source, and a pyrophosphate source and performing a first calcination treatment to obtain a core material;

[0024] Mixing the core material with a titanium source and an aluminum source and performing a second calcination treatment to obtain a positive electrode active material with a coating layer formed on the surface of the core,

[0025] wherein the core includes Na a Fe b M c(PO4)2P2O7, 3 < a ≤ 4, 2 < b < 3, 0 ≤ c ≤ 0.1, M is selected from one or more of Mo, W, V, Zr, Cr, Mg, Ni, Co, Mn, B, Li; the coating layer includes Na 1+x Al x Ti 2-x (PO4)3, 0 < x ≤ 0.5.

[0026] The method for preparing the positive electrode active material according to the second aspect of the present invention has at least the following beneficial effects: By incorporating the M source material, it is beneficial to improve the ionic conductivity of the core matrix; by forming the coating layer, not only can the air stability of the prepared positive electrode active material be improved, but also the side reaction between the core material and the electrolyte can be reduced, effectively alleviating the dissolution of transition metals in Na a Fe b M c (PO4)2P2O7. At the same time, the prepared coating layer material Na 1+x Al x Ti 2-x (PO4)3, as a fast ion conductor, can further improve the ionic conductivity of the positive electrode active material; furthermore, when forming the coating layer, part of the residual alkali on the surface of the core (such as sodium salts of pyrophosphate, phosphate, etc.) can also be converted into Na 1+x Al x Ti 2-x (PO4)3, thereby reducing the amount of surface residual alkali of the positive electrode active material; in addition, during the preparation process, qualitative and quantitative analysis of the composition and content of the residual alkali on the surface of the core material can also be realized, which is conducive to obtaining a positive electrode active material with a lower amount of surface residual alkali by adjusting the amounts of titanium source and aluminum source. In summary, this method not only has a simple process and easy control of the preparation process, but also has low cost and is suitable for large-scale production, and the prepared positive electrode active material has a low amount of surface residual alkali, good air stability and ionic conductivity, and using it in sodium batteries can improve the electrochemical performance such as the rate performance and cycle performance of the battery.

[0027] In some embodiments of the present invention, a sodium source, an iron source, an M source, a phosphate source, and a pyrophosphate source are mixed with water to obtain a slurry, the slurry is dried, and the dried product is subjected to the first calcination treatment.

[0028] In some embodiments of the present invention, in the sodium source and the iron source, the molar ratio of sodium to iron ≥ 1.33.

[0029] In some embodiments of the present invention, the first calcination treatment is carried out in an inert gas and / or nitrogen atmosphere, the calcination temperature is 400°C to 500°C, and the time is 16h to 24h.

[0030] In some embodiments of the present invention, the second calcination treatment is carried out in an inert gas and / or nitrogen atmosphere, the calcination temperature is 400°C to 500°C, and the time is 4h to 8h.

[0031] Based on the molecular weight of Na a Fe b M c (PO4)2P2O7, the total addition amount of the M source is 500 ppm to 5000 ppm;

[0032] Based on the mass of Na a Fe b M c (PO4)2P2O7, the total addition amount of the titanium source and the aluminum source is 1000 ppm to 4000 ppm.

[0033] In some embodiments of the present invention, the D 50 particle size of the slurry is 0.2 μm to 0.5 μm.

[0034] In some embodiments of the present invention, the drying is spray drying, the inlet air temperature range of the spray drying is 160°C to 240°C, and the outlet air temperature range is 80°C to 120°C.

[0035] In some embodiments of the present invention, the slurry further includes:

[0036] In some embodiments of the present invention, the sodium source includes one or more of sodium hydroxide, sodium carbonate, sodium acetate, sodium phosphate, sodium dihydrogen phosphate, and sodium pyrophosphate.

[0037] In some embodiments of the present invention, the iron source includes one or more of iron oxalate, iron acetate, iron nitrate, iron phosphate, and iron pyrophosphate.

[0038] In some embodiments of the present invention, the M source includes one or more of a tungsten source, a molybdenum source, a vanadium source, a zirconium source, a chromium source, a nickel source, a cobalt source, a magnesium source, a manganese source, a lithium source, and a boron source.

[0039] In some embodiments of the present invention, the phosphoric acid source includes one or more of phosphorus pentoxide, phosphoric acid, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, iron phosphate, and sodium phosphate.

[0040] In some embodiments of the present invention, the pyrophosphoric acid source includes sodium pyrophosphate and / or iron pyrophosphate.

[0041] In some embodiments of the present invention, the titanium source includes one or more of titanium dioxide, titanium oxysulfate, tetrabutyl titanate, and aluminum titanate.

[0042] In some embodiments of the present invention, the aluminum source includes one or more of aluminum oxide, aluminum nitrate, aluminum phosphate, and aluminum titanate.

[0043] In the third aspect of the present invention, a positive electrode sheet is provided, including: the above-mentioned positive electrode active material, or the positive electrode active material prepared by the above-mentioned preparation method. Using the positive electrode sheet in a sodium battery can improve the electrochemical performance such as the rate performance and cycling performance of the battery.

[0044] In the fourth aspect of the present invention, a battery is provided, including: the above-mentioned positive electrode sheet. The battery has both good rate performance and cycling performance.

[0045] In the fifth aspect of the present invention, an electrical device is provided, including: the above-mentioned battery. Description of the Drawings

[0046] Figure 1 It is a schematic structural diagram of positive electrode active material particles according to an embodiment of the present invention.

[0047] Figure 2 It is a scanning electron microscope image of the positive electrode active material prepared according to Example 1 of the present invention.

[0048] Figure 3 It is a transmission electron microscope image of the positive electrode active material prepared according to Example 1 of the present invention.

[0049] Figure 4 It is an XRD spectrum of the positive electrode active material prepared according to Example 1 of the present invention. Detailed Description of the Embodiments

[0050] Embodiments of the present invention will be described in detail below. The embodiments described below are exemplary and are intended to explain the present invention and should not be construed as limiting the present invention.

[0051] In the present invention, the terms "first" and "second" are used only for descriptive purposes and should not be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include at least one of such features.

[0052] At present, the cathode active materials of sodium-ion batteries with commercialization prospects mainly include transition metal oxides, Prussian blue compounds, and polyanionic cathodes. However, although transition metal oxides have relatively high theoretical capacity, they have poor cycling performance, complex volume phase changes, and irreversible oxygen release at high potentials. Prussian blue compounds are easy to synthesize and low in cost, but the lattice water that cannot be removed in their structure will occupy the insertion sites of sodium ions, resulting in a decrease in the actual specific capacity of the material. In addition, the lattice water will escape into the electrolyte, leading to a decrease in the first-cycle and cycling efficiency of the cathode active material. The polyanionic cathode active material has excellent cycling stability and high safety. By improving the polyanionic cathode active material, it is beneficial to further improve the electrochemical performance of sodium batteries.

[0053] In view of this, in the first aspect of the present invention, a cathode active material is proposed, including: a core and a coating layer, wherein the core includes Na a Fe b M c (PO4)2P2O7, where 3 < a ≤ 4, 2 < b < 3, 0 ≤ c ≤ 0.1, and M is selected from one or more of Mo, W, V, Zr, Cr, Mg, Ni, Co, Mn, B, and Li; the coating layer is located on at least a part of the surface of the core and includes Na 1+x Al x Ti 2-x (PO4)3, where 0 < x ≤ 0.5.

[0054] For example, with reference to Figure 1 it can be understood that the cathode active material includes a core 10 and a coating layer 20. The core 10 includes Na a Fe b M c (PO4)2P2O7, and the coating layer 20 includes Na 1+x Al x Ti 2-x (PO4)3. Exemplarily, the value of a can be 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, or 4, etc.; the value of b can be 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, or 2.95, etc.; the value of c can be 0, 0.05, 0.08, 0.1, 0.2, 0.3, 0.35, 0.4, or 0.5, etc.

[0055] Among them, in the polyanionic cathode material, sodium iron pyrophosphate phosphate has the characteristics of high rate and high power density. Its stable charge-discharge platform is above 3.0V. The covalent bond structure of P-O endows this type of material with a stable crystal framework, which can avoid the release of oxygen, and thus support the multiple cyclic deintercalation and intercalation of sodium ions in the bulk phase of the material, and the cycle stability is good. That is, the structural stability and cycle stability of sodium iron pyrophosphate phosphate are good. On this basis, by doping element M into sodium iron pyrophosphate phosphate, the ionic conductivity of sodium iron pyrophosphate phosphate itself can be improved, which is beneficial to improving the charge-discharge performance and rate performance of the cathode active material. Further, setting the coating layer can not only improve the air stability of the cathode active material, but also reduce the side reaction between the core material and the electrolyte, and effectively alleviate the residual sodium in the core material a Fe b M c (PO4)2P2O7 dissolution of transition metals, which is beneficial to improving the cycle and rate performance of the cathode active material. And the coating material Na 1+x Al x Ti 2-x (PO4)3 is a fast ion conductor, which can further improve the ionic conductivity of the cathode active material. In addition, when forming the coating layer, it is also beneficial to convert part of the residual alkali on the surface of the core (such as sodium salts of sodium pyrophosphate, sodium phosphate, etc.) into Na 1+x Al x Ti 2-x (PO4)3 through chemical reactions, which is beneficial to reducing the amount of surface residual alkali of the cathode active material.

[0056] Thus, the cathode active material of the first aspect of the present invention has at least the following beneficial effects: both the air stability and the ionic conductivity are good, and when it is used in a sodium battery, it can improve the electrochemical performance such as the rate performance and cycle performance of the battery.

[0057] According to the embodiments of the present invention, conventional instruments such as an X-ray diffractometer, an ion polisher, a scanning electron microscope (SEM), and a transmission electron microscope (TEM) can be combined to judge whether the cathode active material particles have the structure of a core and a coating layer, and whether Na a Fe b M c (PO4)2P2O7 material exists in the core, and whether Na 1+x Al x Ti 2-x (PO4)3 material exists in the coating layer. For example, the phase composition of the cathode active material can be characterized by an X-ray diffractometer.

[0058] In some embodiments of the present invention, a / b can be ≥ 1.33. For example, a / b can be 1.33, 1.35, 1.38, 1.40, 1.42, 1.45, 1.5, 1.55, 1.60, 1.65 or 1.70, etc. In the core material Na a Fe b M c (PO4)2P2O7 preparation process, the final product obtained is usually mainly Na a Fe b M c (PO4)2P2O7, and includes a mixed product of sodium iron phosphate and sodium pyrophosphate iron. Sodium iron phosphate is a high-temperature stable phase. If the iron content is too high, it is easy to make Na a Fe b M c (PO4)2P2O7 material contains more sodium iron phosphate-based impurity phases. In the present invention, by controlling a / b to meet the given range, that is, the molar ratio of sodium element to iron element meets the given range, it is beneficial to form a crystal structure of iron-deficient phase and reduce the content of sodium iron phosphate-based impurity phases in the finally prepared Na a Fe b M c (PO4)2P2O7 material, so as to obtain a Na a Fe b M c (PO4)2P2O7 material with higher phase purity. Optionally, the molar ratio of sodium to iron can be 1.33 to 1.80, which is beneficial to simultaneously consider the yield, purity and specific capacity of the core material Na a Fe b M c (PO4)2P2O7.

[0059] In some embodiments of the present invention, the value of b + c can further satisfy: 2 < b + c ≤ 3. For example, the value of b + c can be 2.1, 2.2, 2.4, 2.5, 2.7, 2.9, or 3.0, etc., so that the value range of b + c further meets the given range, which is beneficial to further improve the ionic conductivity of sodium pyrophosphate iron itself on the basis of considering the original performance of sodium pyrophosphate iron, and reduce the sodium iron phosphate-based impurity phases, and further beneficial to further improve the electrochemical performance such as the rate performance and cycle performance of the positive electrode active material.

[0060] In some embodiments of the present invention, Na a Fe b M c (PO4)2P2O7 can include a first phase. In the X-ray diffraction pattern of the positive electrode active material, the strongest peak can correspond to the first phase, and the phase is Na4Fe3(PO4)2P2O7. Thus, it can further make Naa Fe b M c (PO4)2P2O7 has a high phase purity, which is beneficial to improving the electrochemical properties such as the rate performance and cycle performance of the cathode active material while taking into account the original properties of sodium iron pyrophosphate phosphate.

[0061] In some embodiments of the present invention, the 2θ angle corresponding to the peak height of the strongest peak can be 33.6° ± 0.2°, and the strongest peak can be the diffraction peak of the (222) crystal plane of the first phase. The first phase with the peak position meeting the given conditions has good rate performance, high power density, and good structural stability and cycle stability, which can make the finally prepared cathode active material have better air stability, rate performance, and cycle performance, etc.

[0062] In some embodiments of the present invention, the full width at half maximum of the strongest peak can be 0.16° to 0.24°, for example, it can be 0.16°, 0.18°, 0.20°, 0.22°, 0.24°, etc. Among the cathode active materials meeting the given conditions, Na a Fe b M c (PO4)2P2O7 has a high phase purity and a low impurity phase content, which is beneficial to improving the electrochemical properties such as the rate performance and cycle performance of the cathode active material while taking into account the original properties of sodium iron pyrophosphate phosphate.

[0063] In some embodiments of the present invention, the unit cell volume corresponding to the strongest peak can be For example, it can be etc. The unit cell volume corresponding to the strongest peak can be obtained by performing material structure refinement on an X-ray diffractometer in combination with the built-in SmartLab Studio II software and analyzing it using the Rietveld technique. The cathode active material with the unit cell volume corresponding to the strongest peak meeting the given range has relatively good structural stability, and Na a Fe b M c (PO4)2P2O7 has a high phase purity and a low impurity phase content, which is beneficial to improving the electrochemical properties such as the rate performance and cycle performance of the cathode active material while taking into account the original properties of sodium iron pyrophosphate phosphate.

[0064] In some embodiments of the present invention, the 2θ angle corresponding to the peak height of the strongest peak can be 33.6° ± 0.2°, the strongest peak can be the diffraction peak of the (222) crystal plane of the first phase, the full width at half maximum of the strongest peak can be 0.16° to 0.24°, and the unit cell volume corresponding to the strongest peak can be Meeting the given conditions can further make Na a Feb M c (PO4)2P2O7 has a high phase purity and a low content of impurity phases.

[0065] In some embodiments of the present invention, in the X-ray diffraction pattern of the positive electrode active material, diffraction characteristic peaks may exist at 2θ2 = 34.8° ± 0.2° and / or 2θ3 = 10.6° ± 0.2°. Among them, the positive electrode active material may include a sodium iron phosphate impurity phase and / or a sodium pyrophosphate iron impurity phase, and corresponding diffraction characteristic peaks will appear in the X-ray diffraction pattern of the positive electrode active material. The 2θ angle corresponding to the peak height of the diffraction characteristic peak of the sodium iron phosphate impurity phase is 34.8° ± 0.2°, and the 2θ angle corresponding to the peak height of the diffraction characteristic peak of the sodium pyrophosphate iron impurity phase is 10.6° ± 0.2°.

[0066] In some embodiments of the present invention, in the X-ray diffraction pattern of the positive electrode active material, the peak height of the strongest peak is I1, the peak height of the diffraction characteristic peak at 2θ2 is I2, and the peak height of the diffraction characteristic peak at 2θ3 is I3. I1, I2, and I3 satisfy: I1 / (I1 + I2 + I3) ≥ 85%. For example, the value of I1 / (I1 + I2 + I3) can be 85.0%, 85.6%, 86.0%, 86.5%, 86.8%, 87.0%, 87.5%, 88.0%, 89.0%, 90.0%, 91.0%, 92.0%, 93.0%, 94.0%, or 95.0%, etc. The sodium iron phosphate type impurity phase is a non-electrochemically active phase, and this substance will destroy the electrochemical interface formed between the current collector and the active material during charge and discharge, reducing the battery cycle performance; while the sodium pyrophosphate iron type impurity phase has electrochemical activity, but its charge and discharge efficiency exceeds 100%, which may lead to the decomposition of the electrolyte and the occurrence of other side reactions, and is also not conducive to obtaining a positive electrode active material with good cycle performance. By controlling Na a Fe b M c (PO4)2P2O7, the relative peak height of the first phase satisfies the given range, which is beneficial to further making Na a Fe b M c (PO4)2P2O7 have a high phase purity and a low content of impurity phases, thereby being beneficial to improving the electrochemical properties such as the rate performance and cycle performance of the positive electrode active material while taking into account the original performance of sodium pyrophosphate iron phosphate. Further, I1 / (I1 + I2 + I3) can be ≥ 90.0%.

[0067] In some embodiments of the present invention, the core of the positive electrode active material may further include: a conductive carbon material, which may be dispersed in the core and / or distributed on the surface of the core. Further incorporating the conductive carbon material into the core can further improve the conductivity of the positive electrode active material. The conductive carbon material cooperates with the M-doped element, which is beneficial to further improving the charge-discharge capacity and rate performance of the positive electrode active material.

[0068] In some embodiments of the present invention, based on the total mass of the positive electrode active material, the mass ratio of the conductive carbon material may be 1.0 wt% to 4.0 wt%, for example, it may be 1.0 wt%, 1.5 wt%, 2.0 wt%, 2.4 wt%, 2.8 wt%, 3 wt%, 3.5 wt% or 4.0 wt%, etc. The mass ratio of the conductive carbon material in the positive electrode active material can be detected by a carbon-sulfur analyzer. The addition of an appropriate amount of conductive carbon is beneficial to improving the conductivity of the positive electrode material, thereby improving the charge-discharge capacity and cycle stability of the material; the introduction of too much conductive carbon will cause the specific surface area of the positive electrode material to be too large, and it is easy to form a jelly during the material pulping process. Therefore, controlling the amount of the conductive carbon material in the positive electrode active material to meet the above range can, on the basis of maintaining the excellent electrochemical performance of sodium iron M (pyro)phosphate, make the designed positive electrode material have excellent slurry processing performance.

[0069] In some embodiments of the present invention, based on the total mass of the positive electrode active material, the mass ratio of Na a Fe b M c (PO4)2P2O7 may be 94.0 wt% - 98.8 wt%, for example, it may be 94.0 wt%, 94.5 wt%, 95.5 wt%, 97.0 wt%, 97.8 wt%, 98.0 wt%, 98.3 wt% or 98.8 wt%, etc.; the mass ratio of Na 1+x Al x Ti 2-x (PO4)3 may be 0.2 wt% - 2 wt%, for example, it may be 0.2 wt%, 0.5 wt%, 0.8 wt%, 1 wt%, 1.2 wt%, 1.5 wt%, 1.8 wt% or 2 wt%, etc. Controlling the relative amounts of the two to meet the given range can not only effectively improve the stability and ionic conductivity of the positive electrode active material, as well as electrochemical properties such as rate performance and cycle performance, but also reduce the risk of affecting the specific capacity of the positive electrode active material when the content of Na 1+x Al x Ti 2-x (PO4)3 is relatively high. In addition, in Na a Fe b M cIn the preparation process of (PO4)2P2O7, the existing forms of residual alkali on its surface mainly include sodium carbonate and various sodium phosphate salts containing phosphate groups, specifically including sodium phosphate, sodium pyrophosphate, sodium hydrogen phosphate, and sodium dihydrogen phosphate. When using Na a Fe b M c to prepare Na 1+ x Al x Ti 2-x (PO4)3, controlling the relative amounts of the two to meet the given range can also significantly reduce the amount of residual alkali on the surface of the positive electrode active material. Thus, it is beneficial to further improve the air stability, ionic conductivity, rate performance, and cycle performance of the positive electrode active material on the basis of taking into account the original performance of sodium iron pyrophosphate phosphate.

[0070] In some embodiments of the present invention, the pH value of the positive electrode active material can be ≤ 10.5, and further can be ≤ 10. The pH of the positive electrode active material meeting the given range is beneficial to further improve the rate performance, cycle stability, and other electrochemical performances of the battery.

[0071] In the second aspect of the present invention, a method for preparing the above-mentioned positive electrode active material is proposed, including:

[0072] Mixing a sodium source, an iron source, an M source, a phosphate source, and a pyrophosphate source and performing a first calcination treatment to obtain a core material;

[0073] Mixing the core material with a titanium source and an aluminum source and performing a second calcination treatment to obtain a positive electrode active material with a coating layer formed on the surface of the core,

[0074] wherein, the core includes Na a Fe b M c (PO4)2P2O7, 3 < a ≤ 4, 2 < b < 3, 0 ≤ c ≤ 0.1, M is selected from one or more of Mo, W, V, Zr, Cr, Mg, Ni, Co, Mn, B, and Li; the coating layer includes Na 1+x Al x Ti 2-x (PO4)3, 0 < x ≤ 0.5.

[0075] The method for preparing the positive electrode active material in the second aspect of the present invention has at least the following beneficial effects: by incorporating the M source material, it is beneficial to improve the ionic conductivity of the core matrix; by forming the coating layer, not only can the air stability of the prepared positive electrode active material be improved, but also the side reaction between the core material and the electrolyte can be reduced, effectively alleviating the Na a Fe b Mc (Dissolution of transition metals in (PO4)2P2O7. Meanwhile, the obtained coating material Na 1+x Al x Ti 2-x (PO4)3, as a fast ion conductor, can further improve the ionic conductivity of the cathode active material. Moreover, when forming the coating layer, it can also convert some residual alkalis (such as sodium salts of pyrophosphate, sodium phosphate, etc.) on the surface of the core into Na 1+x Al x Ti 2-x (PO4)3 coating material, thereby not only reducing the amount of residual alkalis on the surface of the cathode active material; in addition, during the preparation process, qualitative and quantitative analysis of the composition and content of the residual alkalis on the surface of the core material can also be realized, which is conducive to obtaining a cathode active material with a lower amount of residual alkalis on the surface by adjusting the amounts of titanium source and aluminum source. In summary, this method is not only simple in process, easy to control in the preparation process, but also low in cost and suitable for large-scale production, and the obtained cathode active material has a low amount of residual alkalis on the surface, good air stability and ionic conductivity, and using it in sodium batteries can improve the electrochemical properties such as the rate performance and cycle performance of the battery. It should be noted that the characteristics and effects described for the above-mentioned cathode active material of the present invention also apply to the method for preparing the cathode active material, and will not be elaborated here one by one.

[0076] In some embodiments of the present invention, a sodium source, an iron source, an M source, a phosphate source, a pyrophosphate source and water can be mixed to obtain a slurry, the slurry is dried, and the dried product is subjected to a first calcination treatment. Using this method is conducive to achieving sufficient and uniform mixing of each raw material component.

[0077] In some embodiments, the slurry can also be subjected to treatments such as stirring, grinding and / or sanding before drying, which is conducive to further obtaining a dried product with a small particle size and a narrow particle size distribution range. Among them, when spray drying is selected, grinding or sanding the slurry is also conducive to the smooth progress of spray drying.

[0078] In some embodiments, the drying method can include but is not limited to spray drying. Spray drying not only has a high drying efficiency, but also can flexibly control the particle size and particle size distribution range of the dried product by adjusting process parameters such as the temperature and pressure of spray drying. It should be noted that the specific conditions of the spray drying are not particularly limited, and those skilled in the art can flexibly select according to actual needs. For example, the inlet air temperature range of spray drying can be 160°C to 240°C, and the outlet air temperature range can be 80°C to 120°C.

[0079] In some embodiments, the D of the slurry 50The particle size can be 0.2 μm to 0.5 μm, for example, it can be 0.2 μm, 0.3 μm, 0.4 μm or 0.5 μm. This not only facilitates the smooth progress of spray drying, but also helps to obtain a dried product with a relatively narrow particle size distribution range. Among them, D 50 The particle size can be measured by a laser particle size analyzer.

[0080] In some embodiments of the present invention, after the first calcination treatment, it can further include: cooling and pulverizing the calcined product, thereby further regulating the particle size and particle size distribution of the finally prepared cathode active material.

[0081] In some embodiments of the present invention, the core material can further include a carbon source. Among them, the carbon source can be uniformly dispersed in the core material, distributed on the surface of the core material, or can be distributed in the core material and on the surface of the core material at the same time. In addition, the introduction method of the carbon source is not particularly limited, and those skilled in the art can flexibly select according to actual needs. For example, the carbon source can be mixed and introduced together with the sodium source, iron source, M source, phosphate source, pyrophosphate source before the first calcination treatment, or the carbon source can be introduced into the core material by physical coating or chemical vapor deposition after the core material is prepared. The carbon source is suitable as a conductive agent or can be converted into a conductive agent to improve the conductivity of the cathode active material. Correspondingly, the carbon source can be either a conductive agent or a carbon source that can form a conductive carbon material through calcination treatment and other methods.

[0082] In some embodiments, the sodium source, iron source, M source, phosphate source, pyrophosphate source, and carbon source can be mixed with water to obtain a slurry, the slurry is dried, and the dried product is subjected to the first calcination treatment. In this embodiment, the carbon source can include, but is not limited to, one or more of glucose, sucrose, starch, graphene, and carbon nanotubes, and a carbon source that can dissolve in water can be optionally selected, thereby further improving the distribution uniformity of the carbon source and other components.

[0083] In some embodiments of the present invention, in the sodium source and iron source, the molar ratio of sodium to iron can be ≥1.33. During the calcination treatment, sodium loss may occur, and if the iron content is too high, it is easy to cause more sodium iron phosphate-based heterophases in the calcined Na a Fe b M c (PO4)2P2O7 material. By controlling the molar ratio of sodium to iron to meet the given range, it is beneficial to form a crystal structure with an iron-deficient phase and reduce the Na of the prepared core material a Fe b M cThe content of sodium iron phosphate-based heterophase in (PO4)2P2O7, thereby obtaining a core material with higher phase purity. Optionally, the molar ratio of sodium to iron can be 1.33 to 1.80, which is beneficial for taking into account the core material Na a Fe b M c The yield, purity and specific capacity of (PO4)2P2O7.

[0084] In some embodiments of the present invention, the first calcination treatment can be carried out in an inert gas and / or nitrogen atmosphere. For example, it can be carried out in an argon atmosphere, a nitrogen atmosphere, or a mixed atmosphere of argon and nitrogen in any ratio. The calcination temperature can be 400°C to 500°C, such as 400°C, 420°C, 450°C, 480°C or 500°C, etc., and the time can be 16h to 24h, such as 16h, 18h, 20h, 22h or 24h, etc. Controlling the first calcination treatment to meet the given conditions can improve the purity of the core material, reduce the risk of the decrease in the main phase content and the increase in the heterophase content caused by the relatively high sintering temperature. Furthermore, using the prepared cathode active material in a battery can obtain better electrochemical performance.

[0085] In some embodiments of the present invention, based on the molecular weight of Na a Fe b M c (PO4)2P2O7, the total addition amount of the M source can be 500 ppm to 5000 ppm, such as 500 ppm, 800 ppm, 1500 ppm, 2000 ppm, 3000 ppm, 3500 ppm, 4000 ppm or 5000 ppm, etc. This is beneficial for improving the air stability, ionic conductivity, and electrochemical properties such as rate performance and cycling performance of the cathode active material while taking into account the original performance of sodium iron pyrophosphate, and is also beneficial for reducing the heterophase content in the cathode active material, such as sodium iron phosphate-based heterophases, etc.

[0086] In some embodiments of the present invention, there are no particular limitations on the specific types of sodium source, iron source, M source, phosphate source, and pyrophosphate source. Those skilled in the art can flexibly select according to actual situations. For example, the sodium source can include, but is not limited to, one or more of sodium hydroxide, sodium carbonate, sodium acetate, sodium phosphate, sodium dihydrogen phosphate, and sodium pyrophosphate; the iron source can include, but is not limited to, one or more of iron oxalate, iron acetate, iron nitrate, iron phosphate, and iron pyrophosphate; the M source can include, but is not limited to, one or more of tungsten source, molybdenum source, vanadium source, zirconium source, chromium source, nickel source, cobalt source, magnesium source, manganese source, lithium source, and boron source. Additionally, the form of providing the M source can include, but is not limited to, providing it in the form of salts, oxides, metals, etc.; the phosphate source can include, but is not limited to, one or more of phosphorus pentoxide, phosphoric acid, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, iron phosphate, and sodium phosphate; the pyrophosphate source can include, but is not limited to, sodium pyrophosphate and / or iron pyrophosphate.

[0087] In some embodiments of the present invention, the second calcination treatment can be carried out in an inert gas and / or nitrogen atmosphere. For example, it can be carried out in an argon atmosphere, a nitrogen atmosphere, or a mixed atmosphere of argon and nitrogen in any proportion. The calcination temperature can be 400°C to 500°C, such as 400°C, 420°C, 450°C, 480°C, or 500°C, etc., and the time can be 4h to 8h, such as 4h, 5h, 6h, 7h, or 8h, etc. Controlling the second calcination treatment to meet the given conditions can improve the purity of the core material, reduce the risk of the main phase content decreasing and the impurity phase content increasing easily caused by a relatively high sintering temperature. Furthermore, using the prepared cathode active material in a battery can obtain better electrochemical performance.

[0088] In some embodiments of the present invention, based on the mass of Na a Fe b M c (PO4)2P2O7, the total addition amount of the titanium source and the aluminum source is 1000 ppm to 4000 ppm. For example, it can be 1000 ppm, 1600 ppm, 2000 ppm, 2400 ppm, 2600 ppm, 2800 ppm, 3000 ppm, 3400 ppm, 3600 ppm, 3800 ppm, or 4000 ppm, etc. This can effectively reduce the surface residual alkali amount of the core material and convert the surface residual alkali into the fast ion conductor Na 1+x Al x Ti 2-x (PO4)3 coated on the surface of the core material. Furthermore, it can not only improve the ionic conductivity of the cathode active material, but also be beneficial to suppressing the side reaction between the cathode active material and the electrolyte, improving the air stability and chemical stability of the cathode active material, and obtaining a cathode active material with improved rate performance and cycle performance.

[0089] In some embodiments of the present invention, before the second calcination treatment, the composition and content of the residual alkali on the surface of the core material can be qualitatively and quantitatively analyzed, and then the relative amounts of the core material, titanium source, and aluminum source can be determined. This is conducive to converting as much of the residual alkali on the surface of the core material as possible into the fast ion conductor Na 1+x Al x Ti 2-x (PO4)3 coated on the surface of the core material. In the present invention, the main composition of the core material is Na a Fe b M c (PO4)2P2O7, which is rich in phosphate and pyrophosphate groups. The main forms of the residual alkali on the core surface are sodium carbonate and various sodium salts containing phosphate groups, specifically including sodium phosphate, sodium pyrophosphate, sodium hydrogen phosphate, and sodium dihydrogen phosphate.

[0090] In some embodiments of the present invention, there are no particular restrictions on the specific types of the titanium source and the aluminum source, and those skilled in the art can flexibly select according to the actual situation. For example, the titanium source can include, but is not limited to, one or more of titanium dioxide, titanium oxysulfate, tetrabutyl titanate, and aluminum titanate; the aluminum source can include, but is not limited to, one or more of aluminum oxide, aluminum nitrate, aluminum phosphate, and aluminum titanate.

[0091] In the third aspect of the present invention, a positive electrode sheet is proposed, which includes: the above positive electrode active material, or the positive electrode active material prepared by the above preparation method. The features and effects described for the above positive electrode active material and the preparation method of the positive electrode active material also apply to this positive electrode sheet, and will not be elaborated here. Generally speaking, using this positive electrode sheet in a sodium battery can improve the electrochemical performance such as the rate performance and cycle performance of the battery.

[0092] Under normal circumstances, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one side of the positive electrode current collector. Among them, the positive electrode active material layer may include a positive electrode active material, a conductive agent, and a binder. When preparing the positive electrode active material layer, the positive electrode active material, the conductive agent, and the binder dispersion can be formed into a positive electrode active paste in an organic solvent (such as N-methylpyrrolidone, etc.). Then, the positive electrode active paste is coated on at least one side of the positive electrode current collector, and through processes such as drying and cutting, the positive electrode active material layer is obtained. Among them, the positive electrode active material is the positive electrode active material described in the foregoing part or the positive electrode active material prepared by the method for preparing the positive electrode active material described in the foregoing part. In addition, the relative amounts of the positive electrode active material, the conductive agent, and the binder, as well as the specific types of the conductive agent and the binder are not particularly limited, and those skilled in the art can flexibly select according to actual needs. For example, it can be a conventional selection in the art. In some examples, the conductive agent may include, but is not limited to, one or more of conductive carbon black, graphene, carbon nanotubes, acetylene black, etc., and the binder may include, but is not limited to, polyvinylidene fluoride (PVDF), etc. The specific material and structure of the positive electrode current collector are also not particularly limited, and those skilled in the art can flexibly select according to actual needs. For example, it can be a conventional selection in the art. In some examples, the positive electrode current collector may include, but is not limited to, metal foil, such as aluminum foil, etc. In addition, it should be noted that the sources of the positive electrode active material, the binder, and the conductive agent are not particularly limited, and they can be obtained either by preparation or by purchase.

[0093] In the fourth aspect of the present invention, a battery is proposed, including: the above-mentioned positive electrode sheet. The features and effects described for the above-mentioned positive electrode sheet are equally applicable to this battery, and will not be elaborated here one by one. Generally speaking, this battery has both good rate performance and cycling performance.

[0094] It can be understood that the battery also includes a negative electrode plate, an electrolyte, a separator, etc. The specific structures or compositions of the negative electrode plate, the electrolyte, and the separator are not particularly limited either. Those skilled in the art can flexibly select according to actual needs. For example, the separator can include, but is not limited to, a polyethylene (PE) film, a polypropylene (PP) film, a PP / PE / PP composite film, a composite ceramic separator, a coated separator, etc.; for another example, the electrolyte can include an organic solvent and an electrolyte salt. Taking sodium ions as an example, the electrolyte salt can be a sodium salt, and both the organic solvent and the sodium salt can be conventional selections in the art; optionally, additives can also be added to the electrolyte, such as conventional additives including, but not limited to, film-forming additives, stabilizers, etc. The negative electrode plate can be either a metal foil or include a negative electrode current collector and a negative electrode active material layer provided on at least one side of the negative electrode current collector. Among them, the specific material and structure of the negative electrode current collector are not particularly limited either. Those skilled in the art can flexibly select according to actual needs, such as conventional selections in the art. In some examples, the positive electrode current collector can include, but is not limited to, a metal foil, such as a copper foil, etc. In addition, the composition and material selection of the negative electrode active material layer are not particularly limited either. Those skilled in the art can flexibly select according to actual needs, such as conventional selections in the art.

[0095] In the fifth aspect of the present invention, an electrical device is proposed, including: the above-mentioned battery. It should be noted that the features and effects described for the above-mentioned battery also apply to this electrical device, and will not be elaborated here one by one. In addition, the specific type of the electrical device is not particularly limited either. Those skilled in the art can flexibly select according to actual needs. For example, it can include, but is not limited to, vehicles, electronic devices, household appliances, etc.

[0096] The present invention will be described below with reference to specific embodiments. It should be noted that these embodiments are merely descriptive and do not limit the present invention in any way. For those not specifying specific techniques or conditions in the embodiments, the techniques or conditions described in the literature in the art or according to the product specifications are followed. For reagents or instruments not indicating the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0097] Example 1

[0098] (1) Prepare the positive electrode active material:

[0099] Dissolve sodium dihydrogen phosphate, iron phosphate, iron oxalate, ammonium dihydrogen phosphate, glucose, manganese acetate, and cobalt acetate with a batch amount of 450 g in pure water. Based on the total batch amount, the mass ratio of the added carbon source is 4 wt%; among them, taking Na a Fe b M cBased on the molecular weight of (PO4)2P2O7, the doping amounts of Co and Mn elements are both 2000 ppm; the molar ratio of sodium to iron is 3.8:2.8, and the designed molecular formula of the material is Na 3.8 Fe 2.8 Mn 0.023 Co 0.021 (PO4)2P2O7. After stirring the slurry evenly, it is poured into a stirring mill and stirred at a frequency of 22 Hz for 1 h. After the stirring mill discharges the material, the slurry is poured into a sand mill and ground at a rotation speed of 2000 rpm for 3 h. When the D 50 <0.3 μm of the obtained slurry, the slurry is spray-dried, and the outlet air temperature of the spray drying is 110 °C.

[0100] The obtained spray material is subjected to a first calcination treatment at a reaction temperature of 480 °C under nitrogen protection, and the calcination time is 20 h, and a first-fired material (i.e., the sodium iron pyrophosphate core material) can be obtained. After the first-fired material is cooled, it is uniformly mixed with aluminum nitrate and titanium dioxide in a high-speed mixer to prepare Na a Fe b M c (PO4)2P2O7 as the benchmark, and the mass doping amounts of aluminum nitrate and titanium dioxide are both 800 ppm. The mixture is subjected to a second calcination in a nitrogen atmosphere, the calcination temperature is 500 °C, and the calcination time is 6 h. After the second calcination is completed, a core-shell structured cathode active material with a sodium aluminum titanate phosphate coating on the surface of the core can be obtained. Figure 2 and Figure 3 The morphological characterization can clearly show that there is a nano-coating layer structure on the surface of the cathode matrix, and the thickness of the coating layer is about 2-5 nm.

[0101] (2) Preparation of the battery:

[0102] The prepared cathode material, acetylene black and polyvinylidene fluoride (PVDF) are mixed according to a mass ratio of 90:5:5, coated on an aluminum foil and dried, and stamped into a cathode pole piece with a diameter of 12 mm and a thickness of 120 μm under a pressure of 100 MPa. Then, the cathode pole piece is placed in a vacuum drying oven and dried at 120 °C for 12 h. The anode uses a Na metal sheet with a diameter of 17 mm and a thickness of 1 mm; the separator uses a Celgard 2400 porous membrane with a thickness of 25 μm; the electrolyte uses a mixed solvent of ethylene carbonate and propylene carbonate with a mass ratio of 1:1, and the electrolyte salt uses NaPF6, and the concentration of NaPF6 in the electrolyte is 1 mol / L, and an R2025 type button sodium ion half-cell is assembled.

[0103] Examples 2 to 12 and Comparative Examples 1 to 3

[0104] The differences between Examples 2 to 12 and Comparative Examples 1 to 3 and Example 1 are shown in Table 1 in detail. Among them, in Example 2, the positive electrode active material prepared in Example 1 was exposed to air for 5 days and then the battery was assembled.

[0105] Microscopic morphology characterization: The scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to observe the microscopic morphology of the prepared positive electrode active material.

[0106] Surface residual alkali characterization: The pH value of the prepared positive electrode active material was measured by potentiometric titration. By comparing the relative amounts of hydrochloric acid consumed in the forward titration and sodium hydroxide consumed in the reverse titration, the existence form of residual sodium in the positive electrode active material was judged. Through mechanism analysis, the components and contents of the surface residual alkali of the positive electrode active material were qualitatively and quantitatively analyzed. Taking the surface residual alkali characterization of the core material obtained by the first calcination as an example, when n(HCl) = n(NaOH), the existence form of residual lithium is sodium phosphate salt containing phosphate group (specifically including sodium phosphate, sodium pyrophosphate, sodium hydrogen phosphate and sodium dihydrogen phosphate); when n(HCl) > n(NaOH), the existence form of residual lithium is sodium carbonate and sodium phosphate salt containing phosphate group; when n(HCl) < n(NaOH), the existence form of residual lithium is sodium phosphate salt containing phosphate group and sodium pyrophosphate. Then, the types of sodium phosphate salt containing phosphate group were further classified in detail according to the pH of the composite material: for the composite material with pH greater than 10, the existence form of sodium phosphate salt containing phosphate group on the surface is Na3PO4 and Na2HPO4; for the composite material with pH less than 10, the existence form of sodium phosphate salt containing phosphate group on the surface is Na2HPO4 and NaH2PO4.

[0107] Elemental analysis: An inductively coupled plasma emission spectrometer was used to characterize the elemental composition of the first calcination product and the prepared positive electrode active material. The types of elements on the surface of the positive electrode active material were characterized by combining scanning electron microscopy - energy dispersive spectrometer (SEM-EDS).

[0108] XRD characterization: An X-ray diffractometer (XRD) was used to characterize the phase composition, crystal structure, etc. of the prepared positive electrode active material, and to determine the phase corresponding to the strongest peak, the peak height, and the peak heights of the diffraction characteristic peaks corresponding to 2θ angles of 34.8° ± 0.2° and 2θ angles of 10.6° ± 0.2°, as well as the full width at half maximum and the unit cell volume of the strongest peak. Taking Example 1 as an example, in the XRD pattern of the positive electrode active material, the phase corresponding to the strongest peak (i.e., the main phase) is Na4Fe3(PO4)2P2O7, and this diffraction peak is the diffraction peak corresponding to the (222) crystal plane, and its peak height is I1. The phase corresponding to the position of 2θ angle of 34.8° ± 0.2° is NaFePO4, and its peak height is I2. The phase corresponding to the position of 2θ angle of 10.6° ± 0.2° is Na 3.12 Fe 2.44P2O7, with its peak height being I3. Among them, the relative content PCT1 of the main phase = I1 / (I1 + I2 + I3)×100%, the relative content PCT2 of the impurity phase NaFePO4 = I2 / (I1 + I2 + I3)×100%, and the relative content PCT3 of the impurity phase Na 3.12 Fe 2.44 P2O7 = I3 / (I1 + I2 + I3)×100%.

[0109] Charge-discharge performance and cycle performance tests: Within the upper and lower cut-off voltage ranges of the battery, at 25°C, perform 4 charge-discharge cycles successively at charge-discharge rates of 0.1C, 0.2C, 0.5, and 1C, and record the charge-discharge specific capacities at different charge-discharge rates; then, perform 80 cycles at a charge-discharge rate of 0.1C, and calculate the cycle capacity retention rate after 80 cycles. Taking Example 1 as an example, the test voltage range is 2.0 - 4.0V. First, perform charge-discharge cycles at a charge-discharge rate of 0.1C, and record the charge specific capacity and discharge specific capacity at a rate of 0.1C respectively. Then, perform charge-discharge cycles at a charge-discharge rate of 0.2C, and record the charge specific capacity and discharge specific capacity at a rate of 0.2C, record the charge specific capacity and discharge specific capacity at a rate of 1C. Then, perform charge-discharge cycles again at a charge-discharge rate of 0.1C, and calculate the discharge capacity retention rate after 80 cycles.

[0110] Perform relevant tests on the core materials, cathode active materials, and batteries prepared in Examples 1 - 12 and Comparative Examples 1 - 3. The test results are shown in Tables 1 - 2 and Figures 2 to 4 .

[0111]

[0112]

[0113] Results and conclusions:

[0114] Combined with the microscopic morphology characterization, XRD characterization, and elemental analysis, it can be known that the cathode active material prepared by the above examples of the present invention has a core-shell structure, and the core materials all include Na a Fe b M c (PO4)2P2O7 (3 < a ≤ 4, 2 < b < 3, 0 ≤ c ≤ 0.1), and the coating layer material includes Na 1+x Al x Ti 2-x (PO4)3 (0 < x ≤ 0.5). Taking Example 1 as an example, Figure 2 shows the scanning electron microscope image of the cathode active material prepared in Example 1, Figure 3 shows the transmission electron microscope image of the cathode active material prepared in Example 1. From Figures 2 to 3It can be seen that a coating layer structure is formed on the surface of its core matrix; Figure 4 The XRD pattern of the positive electrode active material prepared in Example 1 is shown. From this XRD pattern, it is observed that the main phase of the positive electrode active material prepared in Example 1 is Na4Fe3(PO4)2P2O7, and at the same time, there are Na 3.12 Fe 2.44 (P2O7)2 impurity phase and NaFePO4 impurity phase.

[0115] Furthermore, by combining Examples 1 to 12, Comparative Examples 1 to 3 and Tables 1 to 2, it can be known that the positive electrode active material prepared in the above examples of the present invention has good air stability and low surface residual alkali content, and the charge and discharge performance and cycle stability after being used in sodium batteries are relatively good. Among them, by combining Example 1, 6, 10 and Comparative Examples 1 to 3, it can be known that under the same conditions, on the surface of the core material Na a Fe b M c (PO4)2P2O7, forming a Na 1+x Al x Ti 2-x (PO4)3 coating layer material is beneficial to reducing the surface residual alkali of the core material and improving the electrochemical performance of the battery; by combining Examples 1 to 2, after the positive electrode active material prepared in Example 1 is exposed to air for 5 days, its phase composition, surface residual alkali and electrochemical performance do not decrease significantly, indicating that the positive electrode active material prepared in the examples of this application has good air stability; in addition, by combining Examples 1 and 3, it can be known that doping conductive carbon materials in the core is beneficial to further improving the performance of the battery; by combining Examples 1 and 4 to 5, it can be known that under the same conditions, doping M element in the core material Na a Fe b M c (PO4)2P2O7 is beneficial to further improving the electrochemical performance of the battery; by combining Examples 1 and 8, it can be known that under the same conditions, appropriately increasing the coating layer material is beneficial to further improving the electrochemical performance of the battery; by combining Examples 1, 6, 7, 10, appropriately increasing the sodium-iron ratio in the core material is also beneficial to improving the electrochemical performance of the battery; furthermore, by combining Example 1 and Examples 11 to 12, controlling the appropriate calcination temperature is also beneficial to increasing the main phase ratio and purity of the core material, and thus improving the electrochemical performance of the battery.

[0116] In the description of this specification, the descriptions referring to terms such as "one embodiment", "some embodiments", "examples", "specific examples", or "some examples", etc., mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0117] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A positive electrode active material, characterized in that, Comprising: Core, the core includes Na a Fe b M c (PO4)2P2O7, 3 < a ≤ 4, 2 < b < 3, 0 < c ≤ 0.1, M is selected from one or more of Mo, W, V, Zr, Cr, Mg, Ni, Co, Mn, B, Li; A coating layer located on at least a portion of the surface of the core and comprising Na 1+x Al x Ti 2-x (PO4)3,0 <x≤0.5;在形成包覆层时,内核表面的部分残碱转换为Na 1+x Al x Ti 2-x (PO4)3; 2 < b + c ≤ 2.844; In the X-ray diffraction pattern of the positive electrode active material, the peak height of the strongest peak is I1, the peak height of the diffraction characteristic peak at 2θ2 is I2, and the peak height of the diffraction characteristic peak at 2θ3 is I3. I1, I2, and I3 satisfy: I1 / (I1 + I2 + I3) ≥ 85%; Diffraction characteristic peaks exist at 2θ2 = 34.8° ± 0.2° and 2θ3 = 10.6° ± 0.2°.

2. The positive electrode active material according to claim 1, characterized in that, a / b > 1.

33.

3. The positive electrode active material according to claim 1 or 2, characterized in that, The Na a Fe b M c (PO4)2P2O7 includes a first phase, and in the X-ray diffraction pattern of the positive electrode active material, the strongest peak corresponds to the first phase.

4. The positive electrode active material according to claim 3, wherein Satisfying at least one of the following conditions: 2θ1 corresponding to the peak height of the strongest peak is 33.6° ± 0.2°; The full width at half maximum of the strongest peak is 0.16° to 0.24°; The unit cell volume corresponding to the strongest peak is 1240 Å 3 ~1260 Å 3 .

5. The positive electrode active material according to claim 1 or 2, characterized in that The core further comprises: a conductive carbon material, and the conductive carbon material is dispersed in the core and / or distributed on the surface of the core.

6. The positive electrode active material according to claim 5, wherein Based on the total mass of the positive electrode active material, the mass ratio of the conductive carbon material is 1 wt% to 4 wt%.

7. The positive electrode active material according to any one of claims 1 or 2, characterized in that, Satisfying at least one of the following conditions: Based on the total mass of the positive electrode active material, the Na a Fe b M c The mass proportion of (PO4)2P2O7 is 94.0wt%-98.8wt%, Na 1+x Al x Ti 2-x The mass ratio of (PO4)3 is 0.2wt%-2wt%; The pH value of the positive electrode active material ≤ 10.

5.

8. The positive electrode active material according to claim 7, wherein The pH value of the positive electrode active material ≤ 10.

9. A method for preparing the cathode active material according to any one of claims 1 to 8, characterized in that, Comprising: Mixing a sodium source, an iron source, an M source, a phosphate source, and a pyrophosphate source and performing a first calcination treatment to obtain a core material; Mixing the core material with a titanium source and an aluminum source and performing a second calcination treatment to obtain a positive electrode active material with a coating layer formed on the surface of the core, Among them, the core includes Na a Fe b M c (PO4)2P2O7, 3 < a ≤ 4, 2 < b < 3, 0 < c ≤ 0.1, M is selected from one or more of Mo, W, V, Zr, Cr, Mg, Ni, Co, Mn, B, Li; the coating layer includes Na 1+x Al x Ti 2-x (PO4)3, 0 < x ≤ 0.

5.

10. The method according to claim 9, wherein Satisfying at least one of the following conditions: Mixing the sodium source, the iron source, the M source, the phosphate source, and the pyrophosphate source with water to obtain a slurry, drying the slurry, and performing the first calcination treatment on the dried product; In the sodium source and the iron source, the molar ratio of sodium to iron ≥ 1.33; The first calcination treatment is carried out in an inert gas and / or nitrogen atmosphere, the calcination temperature is 400°C to 500°C, and the time is 16 h to 24 h; The second calcination treatment is carried out in an inert gas and / or nitrogen atmosphere, the calcination temperature is 400°C to 500°C, and the time is 4 h to 8 h; Based on the molecular weight of Na a Fe b M c (PO4)2P2O7, the total addition amount of the M source is 500 ppm to 5000 ppm; Based on the mass of Na a Fe b M c (PO4)2P2O7, the total addition amount of the titanium source and the aluminum source is 1000 ppm to 4000 ppm.

11. The method according to claim 10, wherein Satisfying at least one of the following conditions: The D of the slurry 50 has a particle size of 0.2 µm to 0.5 µm; The drying is spray drying, and the inlet air temperature range of the spray drying is 160°C to 240°C, and the outlet air temperature range is 80°C to 120°C; The slurry further comprises: a carbon source.

12. The method according to any one of claims 9 to 11, characterized in that Satisfying at least one of the following conditions: The sodium source includes one or more of sodium hydroxide, sodium carbonate, sodium acetate, sodium phosphate, sodium dihydrogen phosphate, and sodium pyrophosphate; The iron source includes one or more of iron oxalate, iron acetate, iron nitrate, iron phosphate, and iron pyrophosphate; The M source includes one or more of a tungsten source, a molybdenum source, a vanadium source, a zirconium source, a chromium source, a nickel source, a cobalt source, a magnesium source, a manganese source, a lithium source, and a boron source; The phosphate source includes one or more of phosphorus pentoxide, phosphoric acid, ammonium dihydrogen phosphate, sodium dihydrogen phosphate, iron phosphate, and sodium phosphate; The pyrophosphate source includes sodium pyrophosphate and / or iron pyrophosphate; The titanium source includes one or more of titanium dioxide, titanium oxysulfate, tetrabutyl titanate, and aluminum titanate; The aluminum source includes one or more of aluminum oxide, aluminum nitrate, aluminum phosphate, and aluminum titanate.

13. A positive electrode plate, characterized in that, Comprising: The positive electrode active material according to any one of claims 1 to 8, or the positive electrode active material prepared by the method according to any one of claims 9 to 12.

14. A battery, characterized in that, Comprising: The positive electrode sheet according to claim 13.

15. An electrical device, characterized in that, Comprising: The battery according to claim 14.

Images