Sodium secondary battery and electrical device
Patent Information
- Application Number
- AU2024433669
- Authority / Receiving Office
- AU · AU
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2024-03-14
- Filing Date
- 2024-09-11
- Publication Date
- 2026-10-29
Smart Images
Figure 00000000_0000_ABST
Abstract
Description
TECHNICAL FIELD The present application belongs to the field of batteries, and specifically relates to a sodium secondary battery and an electrical apparatus. BACKGROUND Secondary batteries are widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as in many fields such as electric tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, and aerospace. Sodium-ion batteries are a type of secondary battery, and layered transition metal oxides are positive electrode active materials commonly used for sodium-ion batteries. Under a high voltage, the surface of layered transition metal oxides for sodium-ion batteries has high activity and is prone to side reactions with the electrolyte, thereby reducing the cycle performance of the batteries. SUMMARY In view of the technical problems existing in the background, the present application provides a sodium secondary battery, aiming to improve the cycle performance of the battery at a high voltage. To achieve the above object, in a first aspect, the present application provides a sodium secondary battery. The sodium secondary battery comprises a positive electrode active material; the positive electrode active material comprises a sodium-ion layered transition metal oxide; the sodium-ion layered transition metal oxide is doped with an R element; and the R element comprises at least one of Ti, Al, Mg, Zr or Zn, wherein the difference between the molar percentage r of the R element in metal elements other than Na in a surface selected area and the molar percentage s of the R element in metal elements other than Na in a bulk selected area is 5%-30%. The present application at least includes the following beneficial effects: the sodium secondary battery of the present application has high cycle performance at a high voltage. In some embodiments, the difference is 10%-25%. Thus, the specific capacity and cycle performance of the battery at a high voltage can be improved. In some embodiments, the value of r is 10%-30%. Thus, the cycle performance of the battery at a high voltage can be improved. In some embodiments, the value of s is 0-10%. Thus, the cycle performance of the battery at a high voltage can be improved. In some embodiments, the positive electrode active material comprises NaxMnaQbO2-c+8Fc, wherein a+b=l, 0.5<x<l.l, 0<a, 0<b, 0<c<0.2, -0.1<5<l, and Q comprises the R element. Thus, the specific capacity and cycle performance of the battery at a high voltage can be improved. In some embodiments, the Q further comprises at least one of Li, B, Si, K, Ca, P, Cr, V, Fe, Co, Ni, Cu, Ga, Y, Nb, Mo, In, Sn, Sb, La, Ce, Ta, W or Bi. In some embodiments, the positive electrode active material comprises an 03 phase, the space group comprises , ancj t^e interlayer spacing is 0.53nm-0.55nm. Thus, the initial charge capacity of the battery can be improved. In some embodiments, the positive electrode active material comprises a P2 phase, the space group comprises P63 / mmc, and the interlayer spacing is 0.54nm-0.57nm. Thus, the rate performance of the battery can be improved. In some embodiments, the pH value of a soaking liquid of the positive electrode active material is less than or equal to 13. In some embodiments, the positive electrode active material satisfies at least one of the following conditions: the Dv50 of the positive electrode active material is 3 pm-30 pm; the BET specific surface area of the positive electrode active material is 0.1 m2 / g-5 m2 / g; the tap density of the positive electrode active material is 1 g / cm3-3 g / cm3; and the compaction density of the positive electrode active material at a pressure of 300 MPa is 3.0 g / cm3-4.0 g / cm3. In a second aspect, the present application provides an electrical apparatus comprising the sodium secondary battery according to the first aspect of the present application. Additional aspects and advantages of the present application will be partly given in the following description, and partly become apparent from the following description, or be learned through practice of the present application. DESCRIPTION OF DRAWINGS By reading the detailed description of the preferred embodiments below, various other advantages and benefits will become apparent to those of ordinary skill in the art. The drawings are used for the purpose of illustrating the preferred embodiments only and are not to be considered a limitation to the present application. Moreover, in all of the drawings, the same components are indicated by the same reference numerals. In the drawings: FIG. 1 is a schematic structural view of a bulk selected area and a surface selected area of a positive electrode active material particle according to an embodiment of the present application; FIG. 2 is a schematic view of a battery cell according to an embodiment of the present application; FIG. 3 is an exploded view of the battery cell shown in FIG. 2 according to an embodiment of the present application; FIG. 4 is a schematic view of a battery module according to an embodiment of the present application; FIG. 5 is a schematic view of a battery pack according to an embodiment of the present application; FIG. 6 is an exploded view of the battery pack shown in FIG. 5 according to an embodiment of the present application; FIG. 7 is a schematic view of an electrical apparatus using the battery as a power source according to an embodiment of the present application; FIG. 8 is a schematic structural view of a bulk selected area and a surface selected area of the positive electrode active material obtained in Example 1; FIG. 9 is a schematic structural view of a bulk selected area and a surface selected area of the positive electrode active material obtained in Comparative Example 2; FIG. 10 is an SEM image of the positive electrode active material disassembled from the coin cell of Example 1 after 50 cycles. FIG. 11 is an SEM image of the positive electrode active material disassembled from the coin cell of Comparative Example 2 after 50 cycles. FIG. 12 is a spectrum of the sample of the positive electrode active material obtained in Example 2 after being thinned by focused ion beam (FIB) and then vacuum-transferred to a scanning transmission electron microscope (Talos F200S STEM). FIG. 13 is an EDS peak diagram of Ti element at various positions in FIG. 11. Description of reference numerals: 11 battery cell; 11 case; 13 cover plate; 2 battery module; 3 battery pack; 31 upper box; 32 lower box. DETAILED DESCRIPTION The embodiments of the technical solution of the present application will be described in detail below. The following embodiments are only used to more clearly illustrate the technical solutions of the present application, therefore only as examples, and cannot be used to limit the scope of protection of the present application. Reference to “an embodiment” herein means that a particular feature, structure, or characteristic described with reference to an embodiment can be included in at least one embodiment of the present application. The appearance of this phrase in various positions in the description does not necessarily mean that they all refer to the same embodiment, nor are they independent or alternative embodiments mutually exclusive with other embodiments. It is understood explicitly and implicitly by those skilled in the art that the embodiments described herein can be combined with other embodiments. The "range" disclosed in the present application is defined in a form of a lower limit and an upper limit, and a given range is limited by a selected lower limit and a selected upper limit, which define the boundaries of a particular range. The range defined in this way may include end values, and may be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that ranges of 60-110 and 80-120 are also contemplated. Furthermore, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4 and 5 are listed, the following ranges are all contemplated: 1-3, 1-4, 1-5,2-3, 2-4 and 2-5. In the present application, unless otherwise stated, the numerical range "a-b" means an abbreviated representation of any real number combination between a and b, where a and b are real numbers, and the range defined in this manner may include the endpoint values a and b. For example, the numerical range "0-5" means that all real numbers between "0-5" have been listed in the text, and "0-5" is only an abbreviated representation of these numerical combinations. In addition, when a parameter is expressed as an integer greater than or equal to 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and the like. Unless otherwise specified, all embodiments and optional embodiments of the present application may be combined with each other to form new technical solutions. Unless otherwise specified, all technical features and optional technical features of the present application may be combined with each other to form new technical solutions. Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), meaning that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, a method may further include step (c), which means that step (c) may be added to the method in any order. For example, the method may include steps (a), (b) and (c), or may further include steps (a), (c) and (b), or may further include steps (c), (a) and (b), and the like. Secondary batteries have not only been applied to energy storage power source systems such as hydraulic, firepower, wind and solar power stations but have also widely applied to many fields, e.g., electric transportation vehicles such as electric bicycles, electric motorcycles, and electric automobiles, military equipment and aerospace. Compared with traditional lithium-ion batteries, sodium-ion batteries have a strong price advantage and have broad application prospects in large-scale power storage systems. Layered transition metal oxides have become one of the popular positive electrode active materials for sodium-ion batteries due to a high electrical conductivity, a high energy density, a large capacity and a long cycle life. Under high-voltage charging conditions, more sodium ions are extracted from the layered transition metal oxide, giving it a higher specific capacity, but this causes the surface of the layered transition metal oxide to have a higher activity, and this highly active surface is prone to side reactions with the electrolyte, resulting in reduced specific capacity and cycle performance of the battery. The positive electrode active material in the sodium secondary battery of the present application is doped with an R element (the R element includes at least one of Ti, Al, Mg, Zr or Zn), and the difference between the molar percentage r of the R element in metal elements other than Na in the surface selected area and the molar percentage s of the R element in metal elements other than Na in the bulk selected area is controlled to be 5%-30%. On the one hand, this ensures that the surface selected area has sufficient R element to reduce the surface activity thereof while reducing the content of the R element in the bulk selected area, suppressing side reactions between the positive electrode active material and the electrolyte, thereby improving the cycle performance of the battery at a high voltage; on the other hand, this avoids an excessively high content of the R element in the surface selected area, which would affect the transport of Na ions and reduce the specific capacity of the battery. The sodium secondary battery disclosed in the embodiments of the present application can be used in electrical apparatuses that use batteries as power sources or in various energy storage systems that use batteries as energy storage elements. The electrical apparatus can include, but is not limited to, a mobile phone, a tablet, a laptop, an electric toy, an electric tool, a storage battery car, an electric vehicle, a ship, a spacecraft, etc. The electric toys can include a fixed or mobile electric toy, such as a game machine, an electric automobile toy, an electric ship toy and an electric aircraft toy, and the spacecraft can include an aircraft, a rocket, a space shuttle, a spacecraft and the like. A first aspect of the present application provides a sodium secondary battery, the sodium secondary battery comprising a positive electrode active material, the positive electrode active material comprising a sodium-ion layered transition metal oxide, the sodium-ion layered transition metal oxide being doped with an R element, the R element comprising at least one of Ti, Al, Mg, Zr or Zn, wherein the difference between the molar percentage r of the R element in metal elements other than Na in a surface selected area and the molar percentage s of the R element in metal elements other than Na in a bulk selected area is 5%-30%. Herein, the term "surface selected area" may mean a region from the outermost surface of the positive electrode active material particle to a length of about 0.001 %-l % of the total distance from the particle center to the surface, for example, a region of about 0.5% of the total distance from the outermost surface to the particle center, or a region at a depth of about 1 nm-100 nm from the outermost surface toward the particle center, for example, a region at a depth of about 50 nm. Additionally, the term "bulk selected area" may mean a region extending from the outermost surface of the surface selected area toward the particle center for800nm-2000nm and further extending for 1 OOnm in depth, for example, a region extending 1 OOOnm from the outermost surface toward the particle center and further extending for lOOnm in depth. In the present application, for example, referring to FIG. 1, the "surface selected area" of the positive electrode active material can be defined as a region within 100 nm extending radially from the surface of the positive electrode active material; the "bulk selected area" of the positive electrode active material is defined as a region from a point 1000 nm extending radially from the surface of the positive electrode active material to a further region of 100 nm in diameter; the molar percentage r of the R element in metal elements other than Na in the surface selected area and the molar percentage s of the R element in metal elements other than Na in the bulk selected area can be measured by the following method: with transmission electron microscopy (TEM) combined with X-ray energy dispersive spectroscopy (EDS) for selected area analysis: a sample thinned by focused ion beam (FIB) is vacuum-transferred to a scanning transmission electron microscope (Talos F200S STEM), the microscope is adjusted to an STEM mode, and simultaneous high-angle annular dark-field (HAADF) and EDS imaging are performed on the surface selected area and the bulk selected area of the sample; after background subtraction of the collected EDS spectral image data, the contents of all metal elements are obtained from the EDS spectra, and the molar percentages of the R element in metal elements other than Na in the surface selected area and the bulk selected area are calculated according to nR / llother metal elements other than Na* The positive electrode active material in the sodium secondary battery of the present application is doped with the above R element, and the molar percentage r of the R element in the surface selected area is higher than the molar percentage s of the R element in the bulk selected area, and the difference satisfies an appropriate range, which not only greatly reduces the surface activity of the positive electrode active material and suppresses side reactions with the electrolyte, thereby improving the cycle performance of the battery at a high voltage, but also improves the specific capacity of the battery. The process of improving the specific capacity and cycle performance of the battery at a high voltage is speculated as follows: the positive electrode active material in the sodium secondary battery of the present application is doped with element R, which includes at least one of Ti, Al, Mg, Zr or Zn, and such elements are not prone to a valence change, and the difference (i.e., r-s) between the molar percentage r of the R element in the surface selected area and the molar percentage s of the R element in the bulk selected area satisfies an appropriate range. On the one hand, this ensures that the surface selected area has sufficient R element to reduce its surface activity, while reducing the content of the R element in the bulk selected area, suppressing side reactions between the positive electrode active material and the electrolyte, reducing metal dissolution and oxygen loss in the positive electrode active material, improving the structural stability of the positive electrode active material at a high voltage, thereby improving the cycle performance of the battery at a high voltage. On the other hand, this avoids an excessively high content of the R element in the surface selected area, which would affect the transport of Na ions and reduce the specific capacity of the battery. In some embodiments of the present application, the value of the molar percentage r of the R element in metal elements other than Na in the surface selected area of the positive electrode active material is 10%-30%. Thus, since the R element used in the present application is not prone to a valence change, by enriching the above content of the R element on the surface of the positive electrode active material, the surface activity of the positive electrode active material can be further reduced, the side reactions between the positive electrode active material and the electrolyte can be reduced, the cycle performance at a high voltage can be improved, while ensuring the transport of Na ions and improving the specific capacity of the battery. In other embodiments of the present application, the value of the molar percentage r of the R element in metal elements other than Na in the surface selected area of the positive electrode active material is 10%-25%. In some embodiments of the present application, the value of the molar percentage s of the R element in metal elements other than Na in the bulk selected area of the positive electrode active material is 0-10%. Thus, by controlling the content of the R element in the bulk selected area of the positive electrode active material within the above range, the bulk defects and impurity phases can be reduced, and the structural stability can be improved. In other embodiments of the present application, the value of the molar percentage s of the R element in metal elements other than Na in the bulk selected area of the positive electrode active material is 0%-5%. In some embodiments of the present application, the positive electrode active material comprises NaxMnaQbO2-c+6Fc, wherein a+b=l, 0.5<x<l.l, 0<a, 0<b, 0<c<0.2, -0.1<5<l, and Q comprises the R element. Thus, the specific capacity and cycle performance of the battery at a high voltage can be improved. In some embodiments of the present application, in the above positive electrode active material NaxMnaQbO2-c+8Fc, x may be 0.5<x<l.l, for example, 0.6<x<l, 0.7<x<0.9, 0.7<x<0.8, etc. In the present application, the molar percentage r of the R element in metal elements other than Na in the surface selected area of the positive electrode active material NaxMnaQbO2-c+8Fc is equal to the molar content of the R element in the surface selected area / (molar content of Q element in the surface selected area + molar content of Mn element in the surface selected area); the molar percentage s of the R element in metal elements other than Na in the bulk selected area is equal to the molar content of the R element in the bulk selected area / (molar content of Q element in the bulk selected area + molar content of Mn element in the bulk selected area). It should be noted that in the positive electrode plates, batteries or electrical apparatuses, since sodium ions are consumed during the processes, such as formation and cycling of the battery, it may occur that the measured sodium element content x in the positive electrode active material is less than 1. At the same time, if a sodium supplement agent is used in the positive electrode plate and the negative electrode plate, after the battery undergoes formation and cycling, it may occur that the measured sodium element content x in the positive electrode active material is greater than 1. In some embodiments of the present application, in the above positive electrode active material NaxMnaQbO2-c+8Fc, a may be 0.8>a>0, for example, 0.001<a<0.8, 0.005<a<0.8, 0.1<a<0.8, 0.2<a<0.7, 0.3<a<0.6, 0.4<a<0.5, etc. Thus, the positive electrode active material contains manganese in this content, which can effectively improve the structural stability of the positive electrode active material and improve the cycle stability of the battery containing the same. In other embodiments of the present application, in the above positive electrode active material NaxMnaQbO2-c।sFc, a may be 0.3<a<0.5. In some embodiments of the present application, in the above positive electrode active material NaxMnaQbO2-c+8Fc, Q may include the R element, for example, at least one of Ti, Al, Mg, Zr or Zn. Thus, these elements are not prone to a valence change, and doping them into the positive electrode active material can reduce side reactions between the positive electrode active material and the electrolyte and improve the structural stability of the positive electrode active material. Further, in the above positive electrode active material NaxMnaQbO2-c+3Fc, b may be 0.8>b>0, for example, 0.001<b<0.8, 0.005<b<0.8, 0.1<b<0.8, 0.2<b<0.7, 0.3<b<0.6, 0.4<b<0.5, etc. Thus, the positive electrode active material contains the Q element in this content, which can effectively improve the structural stability of the positive electrode active material and improve the cycle stability of the battery containing the same. In other embodiments of the present application, in the above positive electrode active material NaxMnaQbO2-c+8Fo, b may be 0.4<b<0.7. In some embodiments of the present application, in the above positive electrode active material NaxMnaQbO2-c+sFc, Q may fiirther include at least one of Li, B, Si, K, Ca, P, Cr, V, Fe, Co, Ni, Cu, Ga, Y, Nb, Mo, In, Sn, Sb, La, Ce, Ta, W or Bi. In other embodiments of the present application, in the above positive electrode active material NaxMnaQbO2-o+sFc, Q may further include at least one of Fe, Ni, Cu, Co or Li In some embodiments of the present application, in the above positive electrode active material NaxMnaQbO2-c+sFc, 5 is -0.1<5<l, and c is 0<c<0.2,for example-0.09<5<0.99, -0.08<8<0.95, -0.07<5<0.92, -0.06<5<0.9, -0.05<5<0.85, -0.04<5<0.8, -0.03<5<0.75, -0.02<5<0.7, -0.01<5<0.65, -0.01<5<0.6, 0<5<0.55, 0.05<5<0.5, 0.1<5<0.45, 0.15<5<0.4, 0.2<5<0.35, 0.25<5<0.3, etc.; and 0.01<c<0.2, 0.02<c<0.18, 0.05<c<0.15, 0.08<c<0.12, 0.1<c<0.12, etc.. Specifically, in the positive electrode active material NaxMnaQbO2-c+sFc, doping the oxygen sites with F in this content can effectively stabilize the oxygen in the positive electrode active material, thereby preventing the structural damage caused by lattice oxygen release, improving the stability of the positive electrode active material, and thus improving the cycle performance of the battery. It should be noted that in the positive electrode plates, batteries or electrical apparatuses, since oxygen elements in the positive electrode active material are lost during cycling of the batteries, it may occur that the measured oxygen element content 2+5-e in the positive electrode active material is less than 2. In some embodiments of the present application, the positive electrode active material comprises an 03 phase, the space group comprisesand the interlayer spacing is 0.53nm-0.55nm,for example the interlayer spacing may be 0.532nm-0.55nm, 0.535nm-0.548nm, 0.537nm-0.545nm, 0.540nm-0.542nm, etc. Specifically, the formed 03-phase positive electrode active material has a high Na content, resulting in a high battery capacity. In some embodiments of the present application, the positive electrode active material comprises a P2 phase, the space group comprises P63 / mmc, and the interlayer spacing is 0.54nm-0.57nm,for example the interlayer spacing may be 0.542nm-0.57nm, 0.545nm-0.568nm, 0.547nm-0.565nm, 0.55nm-0.562nm, 0.552nm-0.56nm, 0.555nm-0.558nm, etc. Specifically, the formed P2-phase positive electrode active material has a large interlayer spacing, which can increase the transport rate of Na ions, maintain the integrity of the layered structure, and provide the battery with excellent rate performance and cycle performance. It should be noted that the phase, space group and interlayer spacing of the positive electrode active material in the present application can be characterized by X-ray diffraction. Specifically, the method for measuring the interlayer spacing doos of the (003) plane and the space group of the positive electrode active material includes: grinding the sample to be tested in an agate mortar in a dry room or glove box and sieving through a 350-mesh sieve; taking an appropriate amount of the sieved sample and placing it in the middle of the groove of the sample holder, so that the loose sample powder is slightly higher than the plane of the sample holder; pressing the sample surface lightly with a glass slide to level the sample surface flush with the frame plane and scraping off excess powder. After sample preparation, a Brucker D8A_A25 X-ray powder diffractometer from Brucker AxS, Germany, is used with CuKa radiation as the radiation source, wavelength X=1.5406 A, scanning 20 range of 5°-60°, scanning speed of 4° / min. After the test, the angle corresponding to the (003) plane is obtained, and according to Bragg's equation 2d • sinO = X, and since each unit cell of the (003) plane contains three transition metal layers, the interlayer spacing doo3 of the (003) plane can be obtained. The space group and phase of the sample can be confirmed by comparing the XRD diffraction peaks of the sample with the standard cards in XRD analysis software. In some embodiments of the present application, the pH value of the soaking liquid of the positive electrode active material is less than or equal to 13, for example, the pH value of the soaking liquid of the positive electrode active material may be 7-13, 7.2-13, 7.5-13, 8-13, 8.5-13, 9-13, 10-12.9, 10.5-12.7, 11.0-12.7, 11.2-12.5, 11.5-12.3, 11.7-12.0, etc. Controlling the pH value of the soaking liquid of the positive electrode active material within the above range can reduce gel formation during the slurry mixing process, facilitating subsequent slurry coating of the positive electrode active material. Specifically, the pH value of the soaking liquid of the positive electrode active material can be measured by the following method: at a temperature of 25°C, the positive electrode active material is dispersed in pure water for soaking, filtered to obtain a soaking liquid, and the soaking liquid is titrated with a standard hydrochloric acid solution. In some embodiments of the present application, the Dv50 of the positive electrode active material is 3 pm-30 pm, for example, the Dv50 of the positive electrode active material may be 3 pm-29 pm, 4 pm-28 pm, 5 pm-25 pm, 6 pm-22 pm, 8 pm-18 pm, 9 pm-16 pm, 10 pm-15 pm, 11 pm-14 pm, 12 pm-13 pm, etc. In other embodiments of the present application, the Dv50 of the positive electrode active material is 5 pm-15 pm. In the present application, Dv50 refers to the particle size corresponding to a cumulative volume distribution percentage of 50%, for example, measured according to standard GB / T 190772016 using a laser particle size analyzer (e.g., Malvern Master Size 3000). In some embodiments of the present application, the BET specific surface area of the positive electrode active material is 0.1 m2 / g-5 m2 / g, for example, the specific surface area may be 0.2 m2 / g-4.5 m2 / g, 0.5 m2 / g-4 m2 / g, 0.8 m2 / g-3.5 m2 / g, 1 nr / g-3 m2 / g, 1.5 m2 / g-2.5 m2 / g, 2 m2 / g-2.5 m2 / g, etc. In other embodiments of the present application, the BET specific surface area of the positive electrode active material is 0.3 nr / g-3 nr / g. In the present application, the BET specific surface area of the positive electrode active material has its conventional meaning in the art and can be measured using conventional instruments and methods in the art For example, it can be measured by the following method: using a Micromeritics Gemini VII 2390 multi-station fully automatic specific surface area and porosity analyzer, taking about 7 g of a sample into a 9 cc long tube with a bulb, degassing at 200°C for 2 h, and then placing it into the main unit for testing to obtain the BET specific surface area data of the positive electrode active material. In some embodiments of the present application, the tap density of the positive electrode active material is 1 g / cm3-3 g / cm\ for example, the tap density may be 1 g / cm’-2.8 g / cm3, 1.5 g / cm<2.5 g / cm3, 1.7 g / cm3-2.4 g / cm3, 1.8 g / cm3-2. g / cnr’, etc. In other embodiments of the present application, the tap density of the positive electrode active material is 1.5 g / cm3-2.5 g / cm3. In the present application, tap density refers to the mass per unit volume of a powder in a container after tapping under specified conditions. The method for measuring the tap density of the positive electrode active material is as follows: putting the weighed positive electrode active material into a tapping apparatus measuring cylinder, and fixing the cylinder on a support; rotating the cam, and the guide rod driving the support to slide up and down and impact on the anvil; vibrating at 250±15 times per minute for 12 minutes; measuring the volume of the positive electrode active material in the cylinder, and the ratio of the mass of the positive electrode active material to the volume is the tap density of the positive electrode active material. The formula for tap density is: pbt = mo / V, wherein pbt—the tap density, g / cm3; mo—the mass of the positive electrode active material, g; V—the volume of the positive electrode active material after tapping (volume of the measuring cup), cm3. In some embodiments of the present application, the positive electrode active material has a compaction density at a pressure of 300 MPa of 3 g / cm3-4 g / cm3, for example, the compaction density of the positive electrode active material at a pressure of 300 MPa may be 3.0 g / cm3-3.8 g / cm3, 3.2 g / cm3-3.5 g / cm3, 3.2 g / cm3-3.4 g / cm3, etc. In the present application, "compaction density" has its conventional meaning in the art, and can be measured using conventional instruments and methods in the art. For example, the following test method can be used: a quantitative amount m of powder is placed in a special compaction die, the die is placed on a compaction density tester, a pressure of 300 MPa is applied, and the thickness volume v of the powder under a pressure of 300 MPa is measured on the equipment; then the compaction density is calculated by density = mass m / volume v (refer to GB / T24533-2009 for details). Specifically, when at least one of the Dv50, specific surface area and compaction density at a pressure of 300 MPa of the positive electrode active material of the present application is within the above ranges, the conduction distance in the positive electrode active material is short and the surface side reactions are reduced, which promotes the positive electrode active material to release its gram capacity and improves the capacity retention rate of the battery containing the same. Further, the preparation method for the above positive electrode active material may include: mixing a sodium-ion layered transition metal oxide with a raw material containing the R element, and sintering to obtain the positive electrode active material, wherein the R element includes at least one of Ti, Al, Mg, Zr or Zn, and the difference between the molar percentage r of the R element in metal elements other than Na in the surface selected area and the molar percentage s of the R element in metal elements other than Na in the bulk selected area is 5%-30%. As an example, the raw material containing the R element may include at least one of oxides, carbonates or hydroxides of the R element. In some embodiments of the present application, the sintering is carried out in an air atmosphere, the sintering temperature is 600°C-800°C, and the temperature-maintaining time is 5 h-25 h, for example, the sintering temperature may be 620°C-780°C, 650°C-750°C, 680°C-720°C, 700°C-720°C, etc.; the temperature-maintaining time may be 7 h-22 h, 9 h-20 h, 10 h-18 h, 12 h-15 h, etc. In some embodiments of the present application, the sodium-ion layered transition metal oxide can be obtained by the following method: mixing the corresponding metal sources (metal sources other than the R element and sodium element) and a sodium source according to the content in the chemical formula of the sodium-ion layered transition metal oxide, for example, the metal sources may include at least one of oxides, hydroxides or carbonates of the corresponding metals, and the sodium source may include at least one of sodium carbonate, sodium bicarbonate or sodium hydroxide; then placing the mixture thus mixed in a sintering furnace, calcining in an air atmosphere at a temperature of 800°C-1000°C for 10 h-30 h, cooling to room temperature and pulverizing to obtain the sodium-ion layered transition metal oxide, for example, the sintering temperature may be 800°C-980°C, 850°C-950°C, 880°C-920°C, 900°C-920°C, etc.; the temperature-maintaining time may be 10 h-28 h, 12 h-25 h, 15 h-22 h, 17 h-20 h, etc. A battery refers to a battery that can continue to be used by activating the active material through charging after discharge. It can be understood that the sodium secondary battery proposed in the present application is a sodium-ion battery. Typically, a battery includes a positive electrode plate, a negative electrode plate, a separator, and an electrolyte. During charging and discharging, active ions are intercalated and deintercalated back and forth between the positive electrode plate and the negative electrode plate. The separator is arranged between the positive electrode plate and the negative electrode plate and plays a role in isolation. The electrolyte plays the role of conducting ions between the positive electrode plate and the negative electrode plate. In some embodiments of the present application, the positive electrode plate includes a positive electrode current collector and a positive electrode active material layer, the positive electrode active material layer is provided on at least one side of the positive electrode current collector, and the positive electrode active material layer includes the above positive electrode active material. As an example, the positive electrode current collector has two opposite surfaces in its own thickness direction, and the positive electrode active material layer is provided on one or both of the opposite surfaces of the positive electrode current collector. In some embodiments of the present application, the positive electrode current collector can be a metal foil or a composite current collector. For example, an aluminum foil may be adopted as the metal foil. The composite current collector may include a polymer base layer and a metal layer formed on at least one surface of the polymer base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on the polymer base layer (e.g., polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.). In some embodiments of the present application, the positive electrode active material layer may optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer or a fluorinated acrylate resin. In some embodiments of the present application, based on the total mass of the positive electrode active material layer, the mass percentage of the binder is 0.5%-3%, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, etc. In some embodiments of the present application, the positive electrode active material layer may optionally include a conductive agent. As an example, the conductive agent may include at least one of super conductive carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene or carbon nanofibers. In some embodiments of the present application, based on the total mass of the positive electrode active material layer, the mass percentage of the conductive agent is 0.8%-4%, for example, 0.8%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, etc. In some embodiments of the present application, the positive electrode plate can be prepared by the following method: dispersing the above components for preparing the positive electrode plate, such as the positive electrode active material, the conductive agent, the binder and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry on the positive electrode current collector, and after drying, cold pressing, etc., to obtain the positive electrode plate. In the battery, the negative electrode plate generally includes a negative electrode current collector and a negative electrode active material layer provided on the negative electrode current collector, the negative electrode active material layer including a negative electrode active material. As an example, the negative electrode current collector has two opposite surfaces in its own thickness direction, and the negative electrode active material layer is provided on one or both of the opposite surfaces of the negative electrode current collector. In some embodiments of the present application, the negative-electrode current collector can be a metal foil or a composite current collector. For example, as the metal foil, a copper foil can be used. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be formed by forming a metal material (copper, a copper alloy, nickel, a nickel alloy, titanium, a titanium alloy, silver, a silver alloy, etc.) on a polymer material substrate (such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.). In some embodiments of the present application, the negative electrode active material may be a negative electrode active material for batteries known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may include at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites or silicon alloys. The tinbased material may include at least one of elemental tin, a tin oxide compound, or a tin alloy. However, the present application is not limited to these materials, and other conventional materials useful as negative-electrode active materials for batteries can also be used. These negative electrode active materials may be used alone or in combination of two or more. In some embodiments of the present application, the negative electrode active material layer may optionally include a conductive agent. The conductive agent may include at least one of super conductive carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene or carbon nanofibers. In some embodiments of the present application, the negative electrode active material layer may optionally include other auxiliary agents, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)), etc. In some embodiments of the present application, the negative electrode plate can be prepared by the following method: dispersing the above components for preparing the negative electrode plate, such as the negative electrode active material, the conductive agent, the binder, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry on the negative electrode current collector, and after drying, cold pressing, etc., to obtain the negative electrode plate. In some embodiments of the present application, based on the total mass of the negative electrode active material layer, the mass percentage of the binder in the negative electrode plate is l%-3%, for example, 1.2%-2.8%, 1.5%-2.5%, 1.8%-2.2%, 2%-2.2%, etc. Thus, the peeling of the negative electrode plate can be reduced, thereby improving the cycle performance of the battery containing the same. The present application has no particular limitation on the type of electrolyte, which can be selected according to requirements. For example, the electrolyte may be in a liquid state, a gel state or an all solid state. In some embodiments of the present application, an electrolyte solution is adopted as the electrolyte. The electrolytic solution includes an electrolyte salt and a solvent. In some embodiments of the present application, the electrolyte sodium salt may include at least one of sodium hexafluorophosphate, sodium difluorooxalatoborate, sodium tetrafluoroborate, sodium bis(oxalato)borate, sodium perchlorate, sodium hexafluoroarsenate, sodium bis(fluorosulfonyl)imide, sodium trifluoromethanesulfonate, or sodium bis(trifluoromethanesulfonyl)imide. In some embodiments of the present application, the solvent may include at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, or diethyl sulfone. In some embodiments of the present application, the electrolytic solution may optionally include additives. For example, the additive may include a negative-electrode film-forming additive, or a positive-electrode film-forming additive, or may include an additive that capable of improving some performances of the battery, e.g., an additive for improving the overcharge performance of the battery, an additive for improving the high-temperature performance or low-temperature performance of the battery, and the like. The battery of the present application may be in the form of a battery cell, a battery module, or a battery pack. In some embodiments of the present application, the positive electrode plate, the negative electrode plate and the separator can be formed into a wound cell by a winding process. In some embodiments, the battery cell may include an outer package. The outer package may be used to encapsulate the electrode assembly and the electrolyte mentioned above. In some embodiments, the outer package of the battery cell may be a hard shell, such as a hard plastic shell, an aluminum shell, and a steel shell. The outer package of the battery cell may also be a soft bag, such as a pouch-type soft bag. The material of the soft bag may be a plastic, and examples of the plastic may include polypropylene, polybutylene terephthalate, polybutylene succinate, and the like. The present application has no particular limitation on the shape of the battery cell, which may be cylindrical, prismatic, or any other shape. For example, FIG. 2 shows a prismatic battery cell 1 as an example. In some embodiments, referring to FIG. 3, the outer package may include a case 11 and a cover plate 13. The case 11 may include a bottom plate and side plates connected to the bottom plate, and the bottom plate and side plates enclose to form an accommodating cavity. The case 11 has an opening communicating with the accommodating cavity, and the cover plate 13 can cover the opening to seal the accommodating cavity. The positive electrode plate, separator and negative electrode plate can be formed into an electrode assembly 10 by stacking and winding processes. The electrode assembly 10 is encapsulated in the accommodating cavity. The electrolyte infiltrates into the electrode assembly 10. The number of electrode assemblies 10 contained in the battery cell 1 may be one or more, which can be selected by those skilled in the art according to actual needs. In some embodiments, the battery cells can be assembled into a battery module, and the number of battery cells in the battery module may be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module. FIG. 4 shows a battery module 2 as an example. Referring to FIG. 4, in the battery module 2, multiple battery cells 1 may be arranged in sequence along the length direction of the battery module 2. Of course, any other arrangements are also possible. The multiple battery cells 1 can be further fixed by fasteners. Optionally, the battery module 2 may further include a shell having an accommodating space, and the plurality of battery cells 1 are accommodated in the accommodating space. In some embodiments, the above battery module can be assembled into a battery pack, and the number of battery modules in the battery pack may be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery pack. FIG. 5 and FIG. 6 show a battery pack 3 as an example. Referring to FIG. 5 and FIG. 6, the battery pack 3 may include a battery box and a plurality of battery modules 2 disposed in the battery box. The battery box includes an upper box 31 and a lower box 32, and the upper box 31 can cover the lower box 32 and form a closed space for accommodating the battery modules 2. The plurality of battery modules 2 can be arranged in the battery box in any manner. In addition, the present application also provides an electrical apparatus, which includes the battery provided in the present application. The battery cell, battery module or battery pack can be used as a power source for the electrical apparatus, or as an energy storage unit of the electrical apparatus. The electrical apparatus may include mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships, satellites, energy storage systems, etc., but is not limited thereto. As the electrical apparatus, a battery cell, battery module or battery pack can be selected according to the use requirements thereof. FIG. 7 is an example of an electrical apparatus. The electrical apparatus is an all-electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, or the like. To satisfy the need of the electrical apparatus for a high power and high energy density of secondary batteries, a battery pack or a battery module can be used. As another example, the electrical device may be a mobile phone, a tablet personal computer, a laptop, etc. The apparatus is generally required to be thin and light, and may use a battery cell as a power source. Examples of the present application will be described below. The embodiments described below are exemplary and are provided only to explain the present application, and should not be construed as limiting the present application. If the specific technology or conditions are not indicated in the embodiments, the technology or conditions described in the literature in the art or in accordance with the product specification shall be followed. The reagents or instruments used without manufacturer indicated are all commercially available conventional products. Example 1 Preparation of positive electrode active material Na^COs, Mn2O3, Fe2Os, NiO, and AI2O3 were weighed in a molar ratio of Na:Mn:Fe:Ni:Al of 1:0.45:0.3:0.2:0.05 to a total of 30 g of samples. The other samples except AI2O3 were pre-ground in an agate mortar and then added to a planetary ball mill and ball-milled for 1 h to obtain a precursor mixture. The obtained precursor mixture was uniformly placed in an open crucible, and then heated in a muffle furnace at a heating rate of 5°C / min from room temperature to 950°C, and held at 950°C for 15 h (first sintering). After natural cooling, the obtained layered transition metal oxide was mixed with the remaining weighed AbOjby grinding in an agate mortar and then added to a planetary ball mill and ball-milled for 1 h. The obtained mixture was uniformly placed in an open crucible, and then heated in a muffle furnace at a heating rate of 5°C / min from room temperature to 700°C, and held at 700°C for 6 h (second sintering). After natural cooling, an Aldoped positive electrode active material Na0.9Mn0.45Fe0.3Ni0.2Al0.05O2 was obtained, wherein the metal composition of the positive electrode active material was tested by inductively coupled plasma optical emission spectrometry. Example 2 Preparation of positive electrode active material Na2COa, Mn2O3, Fe2O3, NiO, and TiO2were weighed in a molar ratio of Na:Mn:Fe:Ni:Ti of 1:0.45:0.3:0.2:0.05 to a total of 30 g of samples. The other samples except TiO2were pre-ground in an agate mortar and then added to a planetary ball mill and ball-milled for 1 h to obtain a precursor mixture. The remaining steps were the same as in Example 1. Example 3 Preparation of positive electrode active material Na2COs, Mn2O3, Fe2O3, NiO, and ZnO were weighed in a molar ratio of Na:Mn:Fe:Ni:Zn of 1:0.45:0.3:0.2:0.05 to a total of 30 g of samples. The other samples except TiO2were pre-ground in an agate mortar and then added to a planetary ball mill and ball-milled for 1 h to obtain a precursor mixture. The remaining steps were the same as in Example 1. Example 4 Preparation of positive electrode active material Na2CO3, Mn2O3, Fe2O3, NiO, ZnO, and MgO were weighed in a molar ratio of Na:Mn:Fe:Ni:Zn:Mg of 1:0.45:0.3:0.2:0.04:0.01 to a total of 30 g of samples. The other samples except ZnO and MgO were pre-ground in an agate mortar and then added to a planetary ball mill and ball-milled for 1 h to obtain a precursor mixture. The remaining steps were the same as in Example 1. Example 5 Preparation of positive electrode active material Na2CO3, MmOs, Fe2O3, NiO, TiO2, and ZrO2 were weighed in a molar ratio of Na:Mn:Fe:Ni:Ti:Zr of 1:0.45:0.3:0.2:0.04:0.01 to a total of 30 g of samples. The other samples except TiO2 and ZrO2 were pre-ground in an agate mortar and then added to a planetary ball mill and ball-milled for 1 h to obtain a precursor mixture. The remaining steps were the same as in Example 1. Comparative Example 1 Na2CO3, M112O3, Fe2O3, and NiO were weighed in a molar ratio of Na:Mn:Fe:Ni of 1:0.45:0.3:0.25 to a total of 30 g of samples. The samples were pre-ground in an agate mortar and then added to a planetary ball mill and ball-milled for 1 h to obtain a precursor mixture. The obtained precursor mixture was uniformly placed in an open crucible, and then heated in a muffle furnace at a heating rate of 5°C / min from room temperature to 950°C, and held at 950°C for 15 h (first sintering). After natural cooling, a positive electrode active material Nao.9Mno.45Feo.3Nio.25O2 was obtained, wherein the metal composition was tested by inductively coupled plasma optical emission spectrometry. Comparative Example 2 Na2CO3, Mn2O3, Fe2O3, NiO, and AI2O3 were weighed in a molar ratio of Na:Mn:Fe:Ni:Al of 1:0.45:0.3:0.2:0.05 to a total of 30 g of samples. The samples including AI2O3 were pre-ground in an agate mortar and then added to a planetary ball mill and ball-milled for 1 h to obtain a precursor mixture. The obtained precursor mixture was uniformly placed in an open crucible, and then heated in a muffle furnace at a heating rate of 5°C / min from room temperature to 950°C, and held at 950°C for 15 h (first sintering). After natural cooling, an Al-uniformly distributed layered oxide positive electrode active material Na0.9Mn0.45Fe0.3Ni0.2Al0.05O2 was obtained, wherein the metal composition was tested by inductively coupled plasma optical emission spectrometry. Comparative Example 3 Na2CO3, Mn20.3, Fe2O3, NiO, and ZnO were weighed in a molar ratio of Na:Mn:Fe:Ni:Zn of 1:0.45:0.3:0.2:0.05 to a total of 30 g of samples. The samples including ZnO were pre-ground in an agate mortar and then added to a planetary ball mill and ball-milled for 1 h to obtain a precursor mixture. The obtained precursor mixture was uniformly placed in an open crucible, and then heated in a muffle furnace at a heating rate of 5°C / min from room temperature to 950°C, and held at 950°C for 15 h (first sintering). After natural cooling, a Zn-uniformly distributed layered oxide positive electrode active material Na0.87Mn0.45Fe0.3Ni0.2Zn0.05O2 was obtained, wherein the metal composition was tested by inductively coupled plasma optical emission spectrometry. Comparative Example 4 Na2CO3, Mn2O3, Fe2O3, NiO, and AI2O3 were weighed in a molar ratio of Na:Mn:Fe:Ni:Al of 1:0.45:0.3:0.2:0.05 to a total of 30 g of samples. The other samples except AI2O3 were pre-ground in an agate mortar and then added to a planetary ball mill and ball-milled for 1 h to obtain a precursor mixture. The obtained precursor mixture was uniformly placed in an open crucible, and then heated in a muffle furnace at a heating rate of 5°C / min from room temperature to 950°C, and held at 950°C for 15 h (first sintering). After natural cooling, the obtained layered transition metal oxide was mixed with the remaining weighed AI2O3 by grinding in an agate mortar and then added to a planetary ball mill and ball-milled for 1 h. The obtained mixture was uniformly placed in an open crucible, and then heated in a muffle furnace at a heating rate of 5°C / min from room temperature to 900°C, and held at 900°C for 6 h (second sintering). After natural cooling, an Al-slightly surface enriched layered oxide positive electrode active material Na0.9Mn0.45Fe0.3Ni0.2Al0.05O2 was obtained, wherein the metal composition was tested by 5 inductively coupled plasma optical emission spectrometry. Comparative Examples 5-6 The preparation method for the positive electrode active materials was similar to that of Example 2, except that the first and second sintering conditions were different, as shown in Table 1. 10 Table 1 R element composition First sintering Second sintering Example 1 Al 950°C maintained for 15 h 700°C maintained for 6 h Example 2 Ti 950°C maintained for 15 h 700°C maintained for 6 h Example 3 Zn 950°C maintained for 15 h 700°C maintained for 6 h Example 4 Zn and Mg 950°C maintained for 15 h 700°C maintained for 6 h Example 5 Ti and Zr 950°C maintained for 15 h 700°C maintained for 6 h Comparative Example 1 / 950°C maintained for 15 h / Comparative Example 2 Al 950°C maintained for 15 h / Comparative Example 3 Zn 950°C maintained for 15 h / Comparative Example 4 Al 950°C maintained for 15 h 900°C maintained for 6 h Comparative Example 5 Ti 950°C maintained for 15 h 900°C maintained for 6 h Comparative Example 6 Ti 950°C maintained for 15 h 500°C maintained for 6 h The molar percentage r of the R element in metal elements other than Na in the surface selected area and the molar percentage s of the R element in metal elements other than Na in the bulk selected area, composition, Dv50, compaction density, pH, phase and interlayer spacing of the positive electrode active materials of Examples 1-5 and Comparative Examples 1-6 were 15 characterized, and the first-cycle discharge specific capacity and cycle performance of coin cells using the obtained positive electrode active materials of Examples 1-5 and Comparative Examples 1-6 were also characterized. The characterization results are shown in Tables 2 and 3. Characterization methods: (1) Test for molar percentage r of R element in metal elements other than Na in surface selected area and molar percentage s of R element in metal elements other than Na in bulk selected area Transmission electron microscopy (TEM) combined with x-ray energy dispersive spectroscopy (EDS) for selected area analysis of the positive electrode active material: the sample thinned by focused ion beam (FIB) was vacuum-transferred to a scanning transmission electron microscope (Talos F200S STEM), the microscope was adjusted to an STEM mode, and simultaneous high-angle annular dark-field (HAADF) and EDS imaging were performed on the surface and bulk of the sample. After background subtraction of the collected EDS spectral image data, the contents of all metal elements were obtained from the EDS spectra, and the molar percentages of the R element in metal elements other than Na in the surface selected area and the bulk selected area were Calculated according to nR / Hotlier metal elements other than Na. (2) Composition test of positive electrode active material Inductively coupled plasma optical emission spectrometry was used: instrument standard reference EPA6010D-2014 "Inductively Coupled Plasma-Atomic Emission Spectrometry". The sample was digested into a solution by chemical treatment, then atomized into plasma to excite characteristic spectral lines of elements, and the elements were qualitatively and quantitatively analyzed according to the wavelength and intensity (proportional to concentration) of the spectral lines. (3) Dv50 test of positive electrode active material Instrument model: Malvern 2000 (MasterSizer 2000) laser particle size analyzer, reference standard procedure: GB / T19077-2016 / ISO 13320:2009. Specific test procedure: taking an appropriate amount of the sample (sample concentration to ensure 8-12% obscuration), adding 20 ml of deionized water, and ultrasonicating same for 5 min (53 KHz / 120 W) to ensure complete dispersion of the sample, then testing the sample according to GB / T19077-2016 / ISO 13320:2009. (4) Compaction density test of positive electrode active material Measured according to GB / T5162-2006. (5) pH test of positive electrode active material At 25°C, 2 g of the positive electrode active material was placed in a beaker, 18 mL of deionized water was added and stirred vigorously for 1 min, allowed to stand for 30 min, and the pH of the supernatant was measured with a pH meter (6) Phase and interlayer spacing test of positive electrode active material In a dry room or glove box, the sample to be tested was ground in an agate mortar and sieved through a 350-mesh sieve; an appropriate amount of the sieved sample was taken and placed in the middle of the groove of the sample holder, so that the loose sample powder was slightly higher than the plane of the sample holder; a glass slide was lightly pressed on the sample surface to level the sample surface flush with the frame plane, and the excess powder was scraped off. After sample preparation, a Brucker D8A_A25 X-ray powder diffractometer from Brucker AxS, Germany, was used with CuKa radiation as the radiation source, wavelength X=1.5406 A, scanning 20 range of 5°-60°, scanning speed of 4° / min. After the test, the angle corresponding to the (003) plane was obtained, and according to Bragg's equation 2d • sinO = X, and since each unit cell of the (003) plane contains three transition metal layers, the interlayer spacing d003 of the (003) plane was obtained. The space group and phase of the sample were confirmed by comparing the XRD diffraction peaks of the sample with the standard cards in XRD analysis software. (7) Test of first-cycle discharge specific capacity and cycle performance of coin cell At 25°C, the obtained positive electrode active material was prepared as a coin cell, then charged to 4.3 V at a constant current density of 10 mA / g, and then discharged to 2.0 V at a constant current density of 10 mA / g to obtain the discharge specific capacity Co of the coin cell. Then, the cell was cycled for 50 cycles at a constant current density of 10 mA / g, and the discharge specific capacity Ci at the 50th cycle was taken. The capacity retention rate after 50 cycles of the positive electrode active material = Ci / Co. In such a case, the coin cell was prepared according to the following steps: Preparation of positive electrode plate: the positive electrode active material, a conductive agent of carbon black (Super P), and a binder of polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 80:15:5 in an appropriate amount of a solvent of NMP with sufficient stirring to form a uniform positive electrode slurry; the positive electrode slurry was uniformly coated on the surface of an aluminum foil as the positive electrode current collector, dried, cold-pressed, and punched into a circular disk with a diameter of 14 mm to obtain the positive electrode plate. Negative electrode plate: a metallic sodium sheet was used. Preparation of electrolyte: ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in an equal volume to obtain an organic solvent, and then NaC104 was dissolved in the above organic solvent to prepare an electrolyte with a concentration of 1 mol / L. Separator: a porous polyethylene membrane was used as the separator. Preparation of coin cell: the above positive electrode plate, separator and negative electrode plate were stacked in order, with the separator between the positive and negative electrode plates to provide isolation, and the above prepared electrolyte was added to complete the coin cell The structural diagrams of the bulk selected area and surface selected area of the positive electrode active material obtained in Example 1 are shown in FIG. 8, and those of Comparative Example 2 are shown in FIG. 9. The SEM image of the positive electrode active material disassembled from the coin cell of Example 1 after 50 cycles is shown in FIG. 10. The particle surface showed no obvious cracks, and the contact with the electrolyte did not easily cause severe side reactions, indicating that the difference between the molar percentage r of Al element relative to other metals other than Na in the surface selected area and the molar percentage r of Al element relative to other metals other than Na in the bulk selected area of the positive electrode active material of Example 1 was 15%, which could reduce the surface activity of the positive electrode 5 active material, thereby reducing side reactions of the positive electrode active material with the electrolyte. The SEM image of the positive electrode active material disassembled from the coin cell of Comparative Example 2 after 50 cycles is shown in FIG. 11. The particle surface showed obvious cracks, and side reactions with the electrolyte were severe, indicating that the difference between the molar percentage r of Al element in other metals other than Na in the surface selected 10 area and the molar percentage r of Al element in other metals other than Na in the bulk selected area of the positive electrode active material of Comparative Example 2 was 1%, and the surface of the positive active material still had high activity, resulting in severe side reactions with the electrolyte. The spectrum of the sample of the positive electrode active material obtained in Example 2 after being thinned by focused ion beam (FIB) and then vacuum-transferred to a 15 scanning transmission electron microscope (Talos F200S STEM) is shown in FIG. 11, and the corresponding EDS peak diagrams of Ti element at various positions in FIG. 11 are shown in FIG. 12. It can be seen that the Ti element content gradually decreased from the surface selected area to the bulk selected area of the positive electrode active material. Table 2 Composition Phase Interlayer spacing (nm) Dv50 (pm) Compaction density at 300 MPa ( g / cm3) PH Molar percentage r of clement R in the selected surface region Molar ratio of element R in the selected bulk region r-s Example 1 Nao.9Mno.45F eo.3Nio.2Alo. 05O2 03 0.539 8.8 3.5 12.2 16.00% 1.00% 15.00% Example 2 Nao.89Mno.45Feo.3Nio.2Ti 0.05O2 03 0.536 7.9 3.4 12.6 22.00% 1.00% 21.00% Example 3 Nao.87Mno.45Feo.3Nio.2Zn 0.05O2 03 0.538 9.8 3.5 12.2 15.00% 2.00% 13.00% Example 4 Nao.9Mno.45F eo.3Nio.2Zno 04Mg0.01O2 03 0.537 8.5 3.6 12.5 12.00% 1.00% 11.00% Example 5 Nao.84Mno.45Feo.3Nio.2Ti o.o4Zro.oi02 03 0.537 8.4 3.5 12.4 18.00% 1.00% 17.00% Comparative Example 1 Nao.9Mno.45F eo.3Nio.2502 03 0.534 9.3 3.5 12.8 / / / Comparative Example 2 Nao.9Mno.45F eo.3Nio.2Alo. 05O2 03 0.537 8.9 3.5 12.9 6.00% 5.00% 1.00% Comparative Example 3 Nao.87 Mn0.45Fe0.3Ni0.2Zn0.05O2 03 0.538 8.6 3.7 13 6.00% 4.00% 2.00% Comparative Example 4 Nao.9M1io.45F eo.3Nio.2Alo. 05O2 03 0.536 8.7 3.4 12.7 8.00% 4.00% 4.00% Comparative Example 5 Nao.89 Mn0.45Fe0.3Ni0.2Ti0.05O2 03 0.537 7.9 3.4 12.6 8.00% 4.00% 4.00% Comparative Example 6 Nao.gs Mn0.45Fe0.3Ni0.2Ti0.05O2 03 0.539 7.9 3.4 12.6 33.00% 0.00% 33.00% Table 3 Discharge specific capacity (mAh / g) Capacity retention rate after 50 cycles (%) Example 1 152 87 Example 2 153 86 Example 3 156 88 Example 4 154 86 Example 5 156 87 Comparative Example 1 153 65 Comparative Example 2 149 72 Comparative Example 3 146 65 Comparative Example 4 147 75 Comparative Example 5 148 71 Comparative Example 6 142 85 Conclusion: From Table 2, it can be seen that the difference between the molar percentage r of the R element in metal elements other than Na in the surface selected area and the molar percentage s of the R element in metal elements other than Na in the bulk selected area of the positive electrode active materials of Examples 1-5 is 5%-30%; the positive electrode active material of Comparative Example 1 was not doped with an R element; the difference between the molar percentage r of the R element in metal elements other than Na in the surface selected area and the molar percentage s of the R element in metal elements other than Na in the bulk selected area of the positive electrode active material of Comparative Example 2 is 1%; the difference between the molar percentage r of the R element in metal elements other than Na in the surface selected area and the molar percentage s of the R element in metal elements other than Na in the bulk selected area of the positive electrode active material of Comparative Example 3 is 2%; the difference between the molar percentage r of the R element in metal elements other than Na in the surface selected area and the molar percentage s of the R element in metal elements other than Na in the bulk selected area of the positive electrode active material of Comparative Example 4 is 4%; the difference between the molar percentage r of the R element in metal elements other than Na in the surface selected area and the molar percentage s of the R element in metal elements other than Na in the bulk selected area of the positive electrode active material of Comparative Example 5 is 5%; the difference between the molar percentage r of the R element in metal elements other than Na in the surface selected area and the molar percentage s of the R element in metal elements other than Na in the bulk selected area of the positive electrode active material of Comparative Example 6 is 33%. From the data in Table 3, it can be seen that the cycle performance of the batteries of Examples 1-5 is superior to that of Comparative Examples 16, and the specific capacities of the batteries of Examples 1 -5 are higher than those of Comparative Examples 2-6, indicating that the positive electrode active material doped with the R element and 5 having the molar percentage r of the R element in metal elements other than Na in the surface selected area higher than the molar percentage s of the R element in metal elements other than Na in the bulk selected area of the positive electrode active material with a difference satisfying 5%-30% can improve the specific capacity and cycle performance of the battery at high voltage. The specific capacity of the battery of Comparative Example 1 was relatively high because there was 10 no enrichment of the R element on the surface selected area, which would not reduce Na ion transport, thus providing a higher specific capacity. It should be noted that the present application is not limited to the above embodiments. The above embodiments are merely examples, and any embodiments within the scope of technical solutions of the present application that have substantially the same configuration to the technical 15 concept and achieve the same functional effects are included within the technical scope of the present application. In addition, without departing from the scope of the main intention of the present application, various modifications that can be conceived by those skilled in the art are applied to the embodiments, and other modes constructed by combining some of the constituent elements of the embodiments are also included within the scope of the present application.
Claims
1. A sodium secondary battery, comprising a positive electrode active material, wherein the positive electrode active material comprises a sodium-ion layered transition metal oxide, the sodium-ion layered transition metal oxide is doped with an R element, and the R element comprises at least one of Ti, Al, Mg, Zr or Zn, wherein the difference between the molar percentage r of the R element in metal elements other than Na in a surface selected area and the molar percentage s of the R element in metal elements other than Na in a bulk selected area is 5%-30%.
2. The sodium secondary battery according to claim 1, wherein the difference is 10%-25%.
3. The sodium secondary battery according to claim 1 or 2, wherein a value of r is 10%-30%.
4. The sodium secondary battery according to any one of claims 1-3, wherein a value of s is 010%.
5. The sodium secondary battery according to any one of claims 1-4, wherein the positive electrode active material comprises NaxMnaQbO2-c+§Fc, wherein a+b=l, 0.5<x<l.l, 0<a<0.8, 0<b<0.8, 0<c<0.2, -0.1<5<l, and Q comprises the R element.
6. The sodium secondary battery according to claim 5, wherein the Q further comprises at least one of Li, B, Si, K, Ca, P, Cr, V, Fe, Co, Ni, Cu, Ga, Y, Nb, Mo, In, Sn, Sb, La, Ce, Ta, W or Bi.
7. The sodium secondary battery according to any one of claims 1-6, wherein the positivep 3 melectrode active material comprises an 03 phase, the space group comprises ,and the interlayer spacing is 0.53nm-0.55nm.
8. The sodium secondary battery according to any one of claims 1-6, wherein the positive electrode active material comprises a P2 phase, the space group comprises P63 / mmc, and the interlayer spacing is 0.54nm-0.57nm.
9. The sodium secondary battery according to any one of claims 1-8, wherein the pH value of a soaking liquid of the positive electrode active material is less than or equal to 13.
10. The sodium secondary battery according to any one of claims 1-9, wherein the positive electrode active material has a Dv50 of 3 pm-30 pm.
11. The sodium secondary battery according to any one of claims 1-10, wherein the positive electrode active material has a BET specific surface area of 0.1 nr / g-5 m2 / g.
12. The sodium secondary battery according to any one of claims 1-11, wherein the positive electrode active material has atap density of 1 g / cm3-3 g / cm3.
13. The sodium secondary battery according to any one of claims 1-12, wherein the positive electrode active material has a compaction density of 3.0 g / cm3-4.0 g / cm3at a pressure of 300 MPa.
14. An electrical apparatus, comprising the sodium secondary battery according to any one of claims 1-13.