Sodium secondary battery and electric device

Abstract

The present application discloses a sodium secondary battery and an electric device, the sodium secondary battery comprising a positive electrode active material, the positive electrode active material comprising a sodium ion layered transition metal oxide doped with an element R, the element R comprising at least one of Ti, Al, Mg, Zr or Zn, the difference value between the molar ratio r of the R element in the surface selection area to the metal elements except Na and the molar ratio s of the R element in the bulk phase selection area to the metal elements except Na is 5%-30%. Therefore, the cycle performance of the battery under high voltage can be improved.

Description

Technical Field

[0001] The present application belongs to the field of batteries, and specifically relates to a sodium secondary battery and an electrical device. Background Art

[0002] Secondary batteries are widely used in energy storage systems such as hydropower, thermal, wind, and solar power plants, as well as in power 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 commonly used as positive electrode active materials in sodium-ion batteries. Under high voltage, the highly active surface of layered transition metal oxides in sodium-ion batteries can easily cause side reactions with the electrolyte, thereby reducing the battery's cycling performance. Summary of the Invention

[0003] In view of the technical problems existing in the background technology, the present application provides a sodium secondary battery, aiming to improve its cycle performance under high voltage.

[0004] To achieve the above-mentioned objectives, the first aspect of the present application provides a sodium secondary battery, comprising a positive electrode active material, wherein the positive electrode active material comprises a sodium ion layered transition metal oxide, wherein the sodium ion layered transition metal oxide is doped with an R element, wherein the R element comprises at least one of Ti, Al, Mg, Zr or Zn, and the difference between the molar proportion r of the R element in the surface selected area and the molar proportion s of the R element in the bulk selected area to the metal elements other than Na is 5%-30%.

[0005] The present application at least includes the following beneficial effects: the sodium secondary battery of the present application has higher cycle performance at high voltage.

[0006] In some embodiments, the difference is 10%-25%. Thus, the specific capacity and cycle performance of the battery at high voltage can be improved.

[0007] In some embodiments, r is 10%-30%, thereby improving the cycle performance of the battery under high voltage.

[0008] In some embodiments, the value of s is 0-10%, thereby improving the cycle performance of the battery under high voltage.

[0009] In some embodiments, the positive electrode active material includes Na x Mn a Q b O 2-c+δ F c, a + b = 1, 0.5 ≤ x ≤ 1.1, 0 < a, 0 < b, 0 ≤ c ≤ 0.2, -0.1 ≤ δ ≤ 1, and Q includes element R. Thus, the specific capacity and cycling performance of the battery under high voltage can be improved.

[0010] In some embodiments, Q further includes 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.

[0011] In some embodiments, the phase of the positive electrode active material includes the O3 phase, the space group includes R3m, and the layer spacing is 0.53 nm - 0.55 nm. Thus, the first charge capacity of the battery can be improved.

[0012] In some embodiments, the phase of the positive electrode active material includes the P2 phase, the space group includes P63 / mmc, and the layer spacing is 0.54 nm - 0.57 nm. Thus, the rate performance of the battery can be improved.

[0013] In some embodiments, the pH value of the immersion solution of the positive electrode active material is less than or equal to 13.

[0014] 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 μm - 30 μm; the BET specific surface area of the positive electrode active material is 0.1 m 2 / g - 5 m 2 / g; the tap density of the positive electrode active material is 1 g / cm)] 3 - 3 g / cm 3 ; the compaction density of the positive electrode active material under a pressure of 300 MPa is 3.0 g / cm 3 - 4.0 g / cm 3

[0015] In the second aspect of the present application, the present application provides an electrical device including the sodium secondary battery described in the first aspect of the present application.

[0016] The additional aspects and advantages of the present application will be partially given in the following description, partially become apparent from the following description, or be understood through the practice of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] By reading the detailed description of the preferred embodiments below, various other advantages and benefits will become clear to those of ordinary skill in the art. The drawings are only for the purpose of illustrating the preferred embodiments and are not considered to be a limitation of the present application. Moreover, in all the drawings, the same reference numerals are used to represent the same components. In the drawings: ​

[0018] Figure 1 It is a schematic structural diagram of the bulk phase selection area and surface selection area of ​​the positive electrode active material particles according to one embodiment of the present application.

[0019] Figure 2 Schematic diagram of a battery cell according to one embodiment of the present application.

[0020] Figure 3 yes Figure 2 FIG. 1 is an exploded view of a battery cell according to an embodiment of the present application.

[0021] Figure 4 Schematic diagram of a battery module according to one embodiment of the present application.

[0022] Figure 5 Schematic diagram of a battery pack according to one embodiment of the present application.

[0023] Figure 6 yes Figure 5 An exploded view of a battery pack according to an embodiment of the present application is shown.

[0024] Figure 7 Schematic diagram of an electrical device using a battery as a power source according to one embodiment of the present application.

[0025] Figure 8 It is a schematic diagram of the structure of the bulk phase selection area and surface selection area of ​​the positive electrode active material obtained in Example 1.

[0026] Figure 9 This is a schematic diagram of the structure of the bulk phase selection area and surface selection area of ​​the positive electrode active material obtained in Comparative Example 2.

[0027] Figure 10 This is a SEM image of the positive electrode active material of the button battery obtained in Example 1 after being disassembled after 50 cycles.

[0028] Figure 11 This is a SEM image of the positive electrode active material of the button battery obtained in Comparative Example 2 after being disassembled after 50 cycles.

[0029] Figure 12 This is a spectrum of the positive electrode active material obtained in Example 2 after the sample was thinned by focused ion beam (FIB) and vacuum transferred to a scanning transmission electron microscope (Talos F200S STEM).

[0030] Figure 13 yes Figure 11 EDS peak diagram of Ti element at each position above.

[0031] Description of reference numerals:

[0032] 11 battery cell; 11 shell; 13 cover plate; 2 battery module; 3 battery pack; 31 upper box; 32 lower box. DETAILED DESCRIPTION

[0033] The following embodiments of the technical solution of the present application are described in detail. The following embodiments are only used to more clearly illustrate the technical solution of the present application and are therefore only used as examples and are not intended to limit the scope of protection of the present application.

[0034] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0035] " scope " disclosed in the present application is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and selected lower limit and upper limit define the boundary of special range.The scope that this mode limits can be to include end value, and can be combined arbitrarily, and promptly any lower limit can form a scope with any upper limit combination.For example, if the scope of 60-120 and 80-110 is listed for specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected.In addition, if the minimum range value 1 and 2 listed, and if the maximum range value 3,4 and 5 are listed, then the following scope can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5.In the present application, unless otherwise specified, numerical range " ab " represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers, and the scope that this mode limits can be to include end value a and b. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0036] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0037] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0038] 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), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating 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 include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0039] Secondary batteries are used not only in energy storage systems such as hydropower, thermal, wind, and solar power plants, but also in electric vehicles like electric bicycles, electric motorcycles, and electric cars, as well as in military equipment and aerospace. Sodium-ion batteries offer significant price advantages over traditional lithium-ion batteries and hold broad application prospects in large-scale energy storage systems.

[0040] Layered transition metal oxides are a popular cathode active material for sodium-ion batteries due to their high electrical conductivity, high energy density, large capacity, and long cycle life. Under high-voltage charging conditions, a high amount of sodium ions are released from the layered transition metal oxides, resulting in a high specific capacity. However, this results in a high surface activity of the layered transition metal oxides, which is prone to side reactions with the electrolyte, resulting in reduced battery specific capacity and cycle performance.

[0041] 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 ratio r of the R element in the surface selected area to the metal elements other than Na and the molar ratio s of the R element in the bulk selected area to the metal elements other than Na is controlled to be 5%-30%. On the one hand, it can ensure that the surface selected area has sufficient R elements to reduce its surface activity while reducing the content of the R element in the bulk selected area, thereby inhibiting the side reaction between the positive electrode active material and the electrolyte, thereby improving the cycle performance of the battery under high voltage; on the other hand, it avoids the excessive content of the R element in the surface selected area affecting the transmission of Na ions and reducing the specific capacity of the battery.

[0042] The sodium secondary battery disclosed in the embodiments of the present application can be used in various energy storage systems that use batteries as power sources or use batteries as energy storage elements. Electrical devices may include, but are not limited to, mobile phones, tablets, laptops, electric toys, power tools, battery cars, electric cars, ships, spacecraft, and the like. Among them, electric toys may include fixed or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, and the like, and spacecraft may include airplanes, rockets, space shuttles, and spacecraft, and the like.

[0043] In a first aspect, the present application provides a 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, and the difference between the molar ratio r of the R element in the surface selected area to the molar ratio s of the R element in the bulk selected area to the metal elements other than Na is 5%-30%.

[0044] As used herein, the term "surface selected region" may refer to a region extending from the outermost surface of a positive electrode active material particle to approximately 0.001% to 1% of the total distance from the center of the particle to the surface, for example, a region extending from the outermost surface to approximately 0.5% of the total distance from the outermost surface to the center of the particle, or a region extending from the outermost surface of a positive electrode active material particle to a depth of approximately 1 nm to 100 nm in the direction of the particle center, for example, a region extending from the outermost surface of a positive electrode active material particle to a depth of approximately 50 nm in the direction of the particle center. Furthermore, the term "bulk phase selected region" may refer to a region extending from the outermost surface of a surface selected region of a positive electrode active material particle to a depth of 800 nm to 2000 nm in the direction of the particle center to a further depth of 100 nm, for example, a region extending from the outermost surface of a positive electrode active material particle to a depth of 100 nm in the direction of the particle center to a further depth of 100 nm.

[0045] In this application, for example, reference is made to Figure 1 The "surface selected area" of the positive electrode active material can be defined as the area extending from the surface of the positive electrode active material along the radial direction within 100 nm; the "bulk selected area" of the positive electrode active material is defined as the area extending from the surface of the positive electrode active material along the radial direction 1000 nm to the area further extending by 100 nm in diameter; the molar ratio of the R element in the surface selected area of ​​the positive electrode active material to the metal elements other than Na, r, and the molar ratio of the R element in the bulk selected area of ​​the positive electrode active material to the metal elements other than Na, s, can be tested by the following method:

[0046] Transmission electron microscopy (TEM) combined with energy dispersive X-ray spectroscopy (EDS) was used for selected area analysis of the cathode active material: the sample thinned by focused ion beam (FIB) was vacuum transferred to a scanning transmission electron microscope (Talos F200S STEM), and the electron microscope was adjusted to STEM mode. High-angle annular dark field (HAADF) and EDS imaging were performed on the surface and bulk selected areas of the sample respectively; after removing the background of the collected EDS spectrum image data, the content of all metal elements was obtained from the EDS spectrum. According to n R / n 除Na外其他金属元素 The molar ratio of R element in the surface selected area and the bulk selected area of ​​the sample to the metal elements except Na was calculated.

[0047] The positive electrode active material in the sodium secondary battery of the present application is doped with the above-mentioned R element, and the molar ratio r of the R element in the surface selected area of ​​the positive electrode active material to the metal elements other than Na is higher than the molar ratio s of the R element in the bulk selected area to the metal elements other than Na, and the difference satisfies an appropriate range. This can not only greatly reduce the surface activity of the positive electrode active material and inhibit its side reaction with the electrolyte, thereby improving the cycle performance of the battery under high voltage, but also improve the battery specific capacity.

[0048] The above-mentioned process of improving the specific capacity and cycle performance of the battery under high voltage is speculated as follows: the positive electrode active material in the sodium secondary battery of the present application is doped with an element R, which includes at least one of Ti, Al, Mg, Zr or Zn. This type of element is not easy to change valence, and the difference between the molar ratio r of the R element in the surface selection area of ​​the above-mentioned positive electrode active material to the metal elements other than Na and the molar ratio s of the R element in the bulk selection area to the metal elements other than Na (i.e., rs) meets the appropriate range. On the one hand, it can ensure that the surface selection area has enough R elements to reduce its surface activity, while reducing the content of R elements in the bulk selection area, inhibiting the side reaction between the positive electrode active material and the electrolyte, reducing metal dissolution and oxygen deficiency in the positive electrode active material, and improving the structural stability of the positive electrode active material under high pressure, thereby improving the cycle performance of the battery under high pressure. On the other hand, it avoids the excessive content of R elements in the surface selection area, which affects the transmission of Na ions and reduces the specific capacity of the battery.

[0049] In some embodiments of the present application, the molar ratio r of the R element in the surface selected area of the above positive electrode active material to the metal elements other than Na is 10%-30%. Thus, the R element used in the present application is not prone to valence change. By enriching the R element with the above content on the surface of the positive electrode active material, the surface activity of the positive electrode active material can be further reduced, the side reaction between the positive electrode active material and the electrolyte can be reduced, the cycling performance under high voltage of the battery can be improved, and at the same time, the transmission of Na ions can be ensured, and the specific capacity of the battery can be increased. In some other embodiments of the present application, the molar ratio r of the R element in the surface selected area of the above positive electrode active material to the metal elements other than Na is 10%-25%.

[0050] In some embodiments of the present application, the molar ratio s of the R element in the bulk selected area of the above positive electrode active material to the metal elements other than Na 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 some other embodiments of the present application, the molar ratio s of the R element in the bulk selected area of the above positive electrode active material to the metal elements other than Na is 0%-5%.

[0051] In some embodiments of the present application, the positive electrode active material includes Na x Mn a Q b O 2-c+δ F c , a + b = 1, 0.5 ≤ x ≤ 1.1, 0 < a, 0 < b, 0 ≤ c ≤ 0.2, -0.1 ≤ δ ≤ 1, and Q includes the R element. Thus, the specific capacity and cycling performance under high voltage of the battery can be improved.

[0052] In some embodiments of the present application, the above positive electrode active material Na x Mn a Q b O 2-c+δ F c In, x can take 0.5 ≤ x ≤ 1.1, for example, 0.6 ≤ x ≤ 1, 0.7 ≤ x ≤ 0.9, 0.7 ≤ x ≤ 0.8, etc.

[0053] In the present application, the positive electrode active material Na x Mn a Q b O 2-c+δ F cThe molar proportion r of the surface selected area R element in the metal elements other than Na is equal to the molar content of the surface selected area R element / (molar content of the surface selected area Q element + molar content of the surface selected area Mn element); the molar proportion s of the positive electrode active material bulk selected area R element in the metal elements other than Na is equal to the molar content of the bulk selected area R element / (molar content of the bulk selected area Q element + molar content of the bulk selected area Mn element).

[0054] It should be noted that in the positive electrode, battery, or electrical equipment, sodium ions are consumed during the battery formation and cycling processes, so the measured sodium content x in the positive electrode active material may be less than 1. At the same time, if a sodium supplement is used in the positive and negative electrode sheets, the measured sodium content x in the positive electrode active material may be greater than 1 after the battery formation and cycling processes.

[0055] In some embodiments of the present application, the above-mentioned positive electrode active material Na x Mn a Q b O 2-c+δ F c In the formula (a), a can 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 includes manganese at this content, which can effectively improve the structural stability of the positive electrode active material and improve the cycle stability of the battery containing it. In other embodiments of the present application, the above-mentioned positive electrode active material Na x Mn a Q b O 2-c+δ F c Here, a can be 0.3≤a≤0.5.

[0056] In some embodiments of the present application, the above-mentioned positive electrode active material Na x Mn a Q b O 2-c+δ F c In the formula ( ), Q may include an R element, such as at least one of Ti, Al, Mg, Zr, or Zn. As such, these elements are less susceptible to valence changes. Doping them in the positive electrode active material can reduce side reactions between the positive electrode active material and the electrolyte, thereby improving the structural stability of the positive electrode active material.

[0057] Furthermore, the positive electrode active material Na x Mn a Q b O 2-c+δ F cIn the formula (a), b can 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. Therefore, the inclusion of the Q element in the positive electrode active material can effectively improve the structural stability of the positive electrode active material and improve the cycle stability of the battery containing the Q element. In other embodiments of the present application, the positive electrode active material Na x Mn a Q b O 2-c+δ F c Here b can be 0.4≤b≤0.7.

[0058] In some embodiments of the present application, the above-mentioned positive electrode active material Na x Mn a Q b O 2-c+δ F c Q may also 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, the above-mentioned positive electrode active material Na x Mn a Q b O 2-c+δ F c The Q may further include at least one of Fe, Ni, Cu, Co or Li.

[0059] In some embodiments of the present application, the above-mentioned positive electrode active material Na x Mn a Q b O 2-c+δ F c Where δ is -0.1≤δ≤1, and c is 0≤c≤0.2, for example, -0.09≤δ≤0.99, -0.08≤δ≤0.95, -0.07≤δ≤0.92, -0.06≤δ≤0.9, -0.05≤δ≤0.85, -0.04≤δ≤0.8, -0.03≤δ≤0.75, -0.02≤δ≤0.7, -0.01≤δ≤0.6 5, -0.01≤δ≤0.6, 0≤δ≤0.55, 0.05≤δ≤0.5, 0.1≤δ≤0.45, 0.15≤δ≤0.4, 0.2≤δ≤0.35, 0.25≤δ≤0.3, etc.; etc., 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.

[0060] Specifically, the positive electrode active material Na x Mn a Q b O 2-c+δ F c Doping the middle oxygen site with this content of F can effectively stabilize the oxygen in the positive electrode active material, thereby reducing the structural damage caused by lattice oxygen release in the positive electrode active material, improving the stability of the positive electrode active material, and further improving the cycle performance of the battery.

[0061] It should be noted that in the positive electrode sheet, battery or electrical equipment, due to the battery cycling process, the oxygen element in the positive electrode active material is lost, so the measured oxygen element content 2+δ-e in the positive electrode active material may be less than 2.

[0062] In some embodiments of the present application, the phase of the positive electrode active material includes an O3 phase, and the space group includes The interlayer spacing is 0.53nm-0.55nm, for example, the interlayer spacing can be 0.532nm-0.55nm, 0.535nm-0.548nm, 0.537nm-0.545nm, 0.540nm-0.542nm, etc. Specifically, the formed O3 phase has a high Na content in the positive active material, which increases the capacity of the battery.

[0063] In some embodiments of the present application, the phase of the positive electrode active material includes a P2 phase, a space group includes P63 / mmc, and an interlayer spacing of 0.54nm-0.57nm, for example, the interlayer spacing can 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 interlayer spacing of the formed P2 phase positive electrode active material is large, which can improve the transmission rate of Na ions and maintain the integrity of the layered structure, so that the battery has excellent rate performance and cycle performance.

[0064] It should be noted that the characterization method of the phase, space group and interlayer distance of the positive electrode active material in the present application can be performed by X-ray diffraction.

[0065] Specifically, the interlayer spacing d of the 003 crystal plane of the positive electrode active material is 003The space group test method includes: grinding the sample to be tested in an agate mortar in a dry room or glove box and then passing it through a 350 mesh sieve. Taking an appropriate amount of the sieved sample, place 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; taking a glass slide and gently pressing the sample surface to make the sample surface flat and consistent with the frame plane, and scraping off the excess powder. After the sample preparation is completed, a BruckerD8A_A25 X-ray powder diffractometer from Brucker AxS, Germany, is used to measure the CuK α The ray is the radiation source, and the wavelength of the ray is The scanning 2θ angle range is 5°-60°, and the scanning rate is 4° / min. After the test is completed, the angle corresponding to the 003 crystal plane is obtained according to the Bragg equation 2d·sinθ=λ, and each unit cell of the 003 crystal plane contains three layers of transition metal layers, and the interlayer spacing d of the 003 crystal plane can be obtained. 003 By comparing the XRD diffraction peaks of the sample with the standard card of the XRD analysis software, the space group and phase of the sample can be confirmed.

[0066] In some embodiments of the present application, the pH value of the soaking solution of the positive electrode active material is less than or equal to 13. For example, the pH value of the soaking solution of the positive electrode active material can 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. The pH value of the soaking solution of the positive electrode active material is controlled within the above range, which can reduce the generation of gel during the slurry adjustment process and facilitate the subsequent slurry adjustment and coating of the positive electrode active material.

[0067] Specifically, the pH value of the immersion solution of the positive electrode active material can be measured by the following method:

[0068] At a temperature of 25° C., the positive electrode active material is dispersed in pure water and immersed, and the immersion liquid is obtained by filtration. The immersion liquid is measured and titrated with a standard hydrochloric acid solution.

[0069] In some embodiments of the present application, the Dv50 of the positive electrode active material is 3 μm-30 μm. For example, the Dv50 of the positive electrode active material can be 3 μm-29 μm, 4 μm-28 μm, 5 μm-25 μm, 6 μm-22 μm, 8 μm-18 μm, 9 μm-16 μm, 10 μm-15 μm, 11 μm-14 μm, 12 μm-13 μm, etc. In other embodiments of the present application, the Dv50 of the positive electrode active material is 5 μm-15 μm.

[0070] In the present application, Dv50 refers to the particle size corresponding to when the cumulative volume distribution percentage reaches 50%, for example, with reference to the standard GB / T 19077-2016, and is measured using a laser particle size analyzer (such as Malvern Master Size 3000).

[0071] In some embodiments of the present application, the BET specific surface area of ​​the positive electrode active material is 0.1 m 2 / g-5m 2 / g, for example, the specific surface area of ​​the positive electrode active material can be 0.2m 2 / g-4.5m 2 / g,0.5m 2 / g-4m 2 / g,0.8m 2 / g-3.5m 2 / g,1m 2 / g-3m 2 / g,1.5m 2 / g-2.5m 2 / g,2m 2 / g-2.5m 2 In some other embodiments of the present application, the BET specific surface area of ​​the positive electrode active material is 0.3 m 2 / g-3m 2 / g.

[0072] In this application, the BET specific surface area of ​​the positive electrode active material has a meaning well known in the art and can be measured using instruments and methods well known in the art. For example, it can be tested by referring to the following method: using the American Micromeritics multi-station fully automatic specific surface area and pore analyzer GeminiVII2390, take about 7g of sample and put it into a 9cc long tube with a bulb, degas at 200℃ for 2h, and then put it into the host to test to obtain the BET specific surface area data of the positive electrode active material.

[0073] In some embodiments of the present application, the tap density of the positive electrode active material is 1 g / cm 3 -3g / cm 3 For example, the tap density of the positive electrode active material can be 1 g / cm 3 -2.8g / cm 3 , 1.5g / cm 3 -2.5g / cm 3 , 1.7g / cm 3 -2.4g / cm 3 , 1.8g / cm 3 -2.g / cm 3 In some other embodiments of the present application, the tap density of the positive electrode active material is 1.5 g / cm3 -2.5g / cm 3 .

[0074] In this application, tap density refers to the mass per unit volume of the powder in the container after being tapped under specified conditions. The tap density of the positive electrode active material is determined as follows:

[0075] Place the weighed positive electrode active material into the graduated cylinder of the vibrating device and secure the cylinder to the support. Rotate the cam, and the directional rod drives the support up and down, striking the anvil. Vibrate at 250 ± 15 vibrations per minute for 12 minutes. Measure the volume of the positive electrode active material in the graduated cylinder. The ratio of the positive electrode active material mass to volume is the tap density of the positive electrode active material.

[0076] The calculation formula of tap density is: ρbt=m0 / V

[0077] Where, ρbt is the tap density, g / cm 3

[0078] m0——mass of positive electrode active material, g

[0079] V - the volume of the positive electrode active material after compaction (the volume of the measuring cup), cm 3 .

[0080] In some embodiments of the present application, the compaction density of the positive electrode active material at a pressure of 300 MPa is 3 g / cm 3 -4g / cm 3 For example, the compaction density of the positive electrode active material at a pressure of 300 MPa can be 3.0 g / cm 3 -3.8g / cm 3 , 3.2g / cm 3 -3.5g / cm 3 , 3.2g / cm 3 -3.4g / cm 3 wait.

[0081] In this application, "compacted density" has a meaning well known in the art and can be measured using instruments and methods well known in the art. For example, the following test method can be used: a fixed amount m of powder is placed in a special compaction mold, the mold is placed on a compaction density instrument, a pressure of 300 MPa is set, and the thickness and volume v of the powder under the pressure of 300 MPa is measured on the equipment. The compacted density is calculated by density = mass m / volume v; (for details, refer to GB / T24533-2009).

[0082] Specifically, when the D of the positive electrode active material of the present application is v50. When at least one of the specific surface area and the compaction density under a pressure of 300 MPa is within the above range, the conduction distance within the positive electrode active material is small and the surface side reactions are few, which promotes the positive electrode active material to exert its specific capacity and improves the capacity retention rate of the battery containing it.

[0083] Furthermore, the preparation method of the positive electrode active material may include:

[0084] A sodium ion layered transition metal oxide is mixed and sintered with a raw material containing an R element to obtain a 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 ratio r of the R element in the surface selected area to the metal elements other than Na and the molar ratio s of the R element in the bulk selected area to the metal elements other than Na is 5%-30%.

[0085] As an example, the raw material containing the R element may include at least one of an oxide, a carbonate, or a hydroxide of the R element.

[0086] 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 holding time is 5h-25h. For example, the sintering temperature can be 620°C-780°C, 650°C-750°C, 680°C-720°C, 700°C-720°C, etc.; the holding time can be 7h-22h, 9h-20h, 10h-18h, 12h-15h, etc.

[0087] In some embodiments of the present application, the sodium ion layered transition metal oxide can be obtained by the following method:

[0088] The corresponding metal source (other metal sources except those containing R elements and sodium elements) and the sodium source are mixed according to the content of the chemical formula of the sodium ion layered transition metal oxide. For example, the metal source can be at least one of the oxide, hydroxide or carbonate of the corresponding metal, and the sodium source can be at least one of the corresponding sodium carbonate, sodium bicarbonate or sodium hydroxide. The mixed mixture is then placed in a sintering furnace, calcined at a temperature of 800°C-1000°C in an air atmosphere for 10h-30h, cooled to room temperature and then pulverized to obtain the sodium ion layered transition metal oxide. For example, the sintering temperature can be 800°C-980°C, 850°C-950°C, 880°C-920°C, 900°C-920°C, etc.; the holding time can be 10h-28h, 12h-25h, 15h-22h, 17h-20h, etc.

[0089] A battery is a battery that can be recharged to activate the active materials after discharge and continue to be used.

[0090] It can be understood that the sodium secondary battery proposed in this application is a sodium ion battery.

[0091] Typically, a battery consists of a positive electrode, a negative electrode, a separator, and an electrolyte. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The separator is placed between the positive and negative electrodes to provide isolation. The electrolyte conducts ions between the positive and negative electrodes.

[0092] In some embodiments of the present application, the positive electrode sheet 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-mentioned positive electrode active material.

[0093] As an example, the positive electrode current collector has two surfaces facing each other in its thickness direction, and the positive electrode active material layer is provided on either or both of the two facing surfaces of the positive electrode current collector.

[0094] In some embodiments of the present application, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. 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 base layer. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0095] In some embodiments of the present application, the positive electrode active material layer may further 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 fluorine-containing acrylate resin.

[0096] In some embodiments of the present application, based on the total mass of the positive electrode active material layer, the mass proportion of the binder is 0.5%-3%, for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, etc.

[0097] In some embodiments of the present application, the positive electrode active material layer may further optionally include a conductive agent. As an example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers.

[0098] In some embodiments of the present application, based on the total mass of the positive electrode active material layer, the mass proportion of the conductive agent is 0.8%-4%, for example, 0.8%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, etc.

[0099] In some embodiments of the present application, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the positive electrode collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0100] In a battery, the negative electrode sheet generally includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material.

[0101] As an example, the negative electrode current collector has two surfaces facing each other in its thickness direction, and the negative electrode active material layer is provided on either or both of the two facing surfaces of the negative electrode current collector.

[0102] In some embodiments of the present application, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used 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 material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0103] In some embodiments of the present application, the negative electrode active material may be a negative electrode active material for a battery that is well 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. The silicon-based material may include at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, or silicon alloys. The tin-based material may include at least one of elemental tin, tin oxides, or tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0104] In some embodiments of the present application, the negative electrode active material layer may further include a conductive agent, which may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers.

[0105] In some embodiments of the present application, the negative electrode active material layer may optionally further include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0106] In some embodiments of the present application, the negative electrode sheet can be prepared by the following method: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, and the binder, are dispersed in a solvent (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0107] In some embodiments of the present application, based on the total mass of the negative electrode active material layer, the binder accounts for 1%-3% by mass in the negative electrode sheet, for example, 1.2%-2.8%, 1.5%-2.5%, 1.8%-2.2%, 2%-2.2%, etc. This can reduce the shedding of the negative electrode sheet, thereby improving the cycle performance of the battery containing the binder.

[0108] The present application has no specific restrictions on the type of electrolyte, which can be selected according to needs. For example, the electrolyte can be liquid, gel or solid.

[0109] In some embodiments of the present application, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0110] In some embodiments of the present application, the electrolyte sodium salt may include at least one of sodium hexafluorophosphate, sodium difluorooxalatoborate, sodium tetrafluoroborate, sodium bisoxalatoborate, sodium perchlorate, sodium hexafluoroarsenate, sodium bis(fluorosulfonyl)imide, sodium trifluoromethylsulfonate or sodium bis(trifluoromethylsulfonyl)imide.

[0111] 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, methylpropyl carbonate, ethylpropyl 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, cyclopentane, dimethyl sulfone, methyl ethyl sulfone or diethyl sulfone.

[0112] In some embodiments of the present application, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0113] The battery of the present application may include a battery cell form, a battery module form, and a battery pack form.

[0114] In some embodiments of the present application, the positive electrode sheet, the negative electrode sheet and the separator can be made into a wound battery cell through a winding process.

[0115] In some embodiments, the battery cell may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0116] In some embodiments, the outer packaging of the battery cell may be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the battery cell may be a soft shell, such as a pouch-type soft shell. The soft shell may be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0117] The present application has no particular restrictions on the shape of the battery cell, which can be cylindrical, square or any other shape. For example, Figure 2 The battery cell 1 is a square structure as an example.

[0118] In some embodiments, reference Figure 3 , the outer packaging may include a shell 11 and a cover plate 13. The shell 11 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 11 has an opening connected to the receiving cavity, and the cover plate 13 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the isolation membrane and the negative electrode sheet can be formed into an electrode assembly 10 through a stacking and winding process. The electrode assembly 10 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 10. The number of electrode assemblies 10 contained in the battery cell 1 can be one or more, and those skilled in the art can select according to specific actual needs.

[0119] In some embodiments, battery cells may be assembled into a battery module. The number of batteries contained in the battery module may be one or more. The specific number may be selected by those skilled in the art based on the application and capacity of the battery module.

[0120] Figure 4 2 is an example of a battery module. Figure 4 In the battery module 2, the plurality of battery cells 1 may be arranged in sequence along the length direction of the battery module 2. Of course, they may also be arranged in any other manner. The plurality of battery cells 1 may further be fixed by fasteners.

[0121] Optionally, the battery module 2 may further include a housing having an accommodation space, and the plurality of battery cells 1 are accommodated in the accommodation space.

[0122] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0123] Figure 5 and Figure 6 The battery pack 3 is used as an example. Figure 5 and Figure 6 The battery pack 3 may include a battery box and multiple battery modules 2 disposed in the battery box. The battery box includes an upper box body 31 and a lower box body 32. The upper box body 31 can cover the lower box body 32 and form an enclosed space for accommodating the battery modules 2. The multiple battery modules 2 can be arranged in the battery box in any manner.

[0124] In addition, the present application also provides an electrical device, 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 device, or as an energy storage unit for the electrical device. The electrical device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0125] As the electrical device, a battery cell, a battery module or a battery pack can be selected according to its usage requirements.

[0126] Figure 7 This is an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0127] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be lightweight and thin, and may use a battery cell as a power source.

[0128] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0129] Example 1

[0130] Preparation of positive electrode active materials

[0131] A total of 30 g of Na2CO3, Mn2O3, Fe2O3, NiO, and Al2O3 were weighed according to the molar ratio of Na:Mn:Fe:Ni:Al of 1:0.45:0.3:0.2:0.05. The other samples except Al2O3 were pre-ground in an agate mortar and then added to a planetary ball mill for ball milling for 1 hour to obtain a precursor mixture. The obtained precursor mixture was evenly placed in an open crucible, and then heated from room temperature to 950°C at a heating rate of 5°C / min in a muffle furnace, and kept at a constant temperature of 950°C for 15h (first sintering). After cooling naturally, the obtained layered transition metal oxide was ground with the remaining Al2O3 in an agate mortar and then added to a planetary ball mill for ball milling for 1h. The obtained mixture was evenly placed in an open crucible, and then heated from room temperature to 700°C at a heating rate of 5°C / min in a muffle furnace, and kept at a constant temperature of 700°C for 6h (second sintering). After cooling naturally, Al-doped positive electrode active material Na 0.9 Mn 0.45 Fe 0.3 Ni 0.2 Al 0.05 O2, where the metal composition of the positive electrode active material is tested by elemental composition inductively coupled plasma optical emission spectrometry;

[0132] Example 2

[0133] Preparation of positive electrode active materials

[0134] A total of 30 g of Na₂CO₃, Mn₂O₃, Fe₂O₃, NiO, and TiO₂ were weighed at a molar ratio of Na:Mn:Fe:Ni:Ti of 1:0.45:0.3:0.2:0.05. The remaining samples, except for TiO₂, were pre-ground in an agate mortar and then added to a planetary ball mill for 1 h to obtain a precursor mixture. The remaining steps were the same as in Example 1.

[0135] Example 3

[0136] Preparation of positive electrode active materials

[0137] A total of 30 g of Na2CO3, Mn2O3, Fe2O3, NiO, and ZnO were weighed at a molar ratio of Na:Mn:Fe:Ni:Zn of 1:0.45:0.3:0.2:0.05. The remaining samples, except for TiO2, were pre-ground in an agate mortar and then added to a planetary ball mill for 1 hour to obtain a precursor mixture. The remaining steps were the same as in Example 1.

[0138] Example 4

[0139] Preparation of positive electrode active materials

[0140] A total of 30 g of Na2CO3, Mn2O3, Fe2O3, NiO, ZnO, and MgO were weighed at a molar ratio of Na:Mn:Fe:Ni:Zn:Mg of 1:0.45:0.3:0.2:0.04:0.01. The remaining samples, except ZnO and MgO, were pre-ground in an agate mortar and then added to a planetary ball mill for 1 hour to obtain a precursor mixture. The remaining steps were the same as in Example 1.

[0141] Example 5

[0142] Preparation of positive electrode active materials

[0143] A total of 30 g of Na2CO3, Mn2O3, Fe2O3, NiO, TiO2, and ZrO2 were weighed at a molar ratio of Na:Mn:Fe:Ni:Ti:Zr of 1:0.45:0.3:0.2:0.04:0.01. The remaining samples, except TiO2 and ZrO2, were pre-ground in an agate mortar and then added to a planetary ball mill for 1 hour to obtain a precursor mixture. The remaining steps were the same as in Example 1.

[0144] Comparative Example 1

[0145] A total of 30 g of Na2CO3, Mn2O3, Fe2O3, and NiO were weighed according to a molar ratio of Na:Mn:Fe:Ni of 1:0.45:0.3:0.25. The sample was pre-ground in an agate mortar and added to a planetary ball mill for 1 h to obtain a precursor mixture. The obtained precursor mixture was evenly placed in an open crucible, and then heated from room temperature to 950 ° C at a heating rate of 5 ° C / min in a muffle furnace, and kept at a constant temperature of 950 ° C for 15 h (first sintering). After cooling naturally, the positive electrode active material Na 0.9 Mn 0.45 Fe 0.3 Ni 0.25 O2, where the metal composition was tested by elemental composition inductively coupled plasma optical emission spectrometry.

[0146] Comparative Example 2

[0147] A total of 30 g of Na2CO3, Mn2O3, Fe2O3, NiO, and Al2O3 were weighed according to a molar ratio of Na:Mn:Fe:Ni:Al of 1:0.45:0.3:0.2:0.05. The samples including Al2O3 were pre-ground in an agate mortar and added to a planetary ball mill for ball milling for 1 hour to obtain a precursor mixture. The obtained precursor mixture was evenly placed in an open crucible, and then heated from room temperature to 950°C in a muffle furnace at a heating rate of 5°C / min, and kept at a constant temperature of 950°C for 15 hours (first sintering). After natural cooling, a layered oxide positive electrode active material Na with uniform Al distribution was obtained. 0.9 Mn 0.45 Fe 0.3 Ni 0.2 Al 0.05 O2, where the metal composition was tested by elemental composition inductively coupled plasma optical emission spectrometry.

[0148] Comparative Example 3

[0149] A total of 30 g of Na2CO3, Mn2O3, Fe2O3, NiO, and ZnO were weighed according to a molar ratio of Na:Mn:Fe:Ni:Zn of 1:0.45:0.3:0.2:0.05. The samples including ZnO were pre-ground in an agate mortar and added to a planetary ball mill for 1 hour to obtain a precursor mixture. The obtained precursor mixture was evenly placed in an open crucible, and then heated from room temperature to 950°C in a muffle furnace at a heating rate of 5°C / min, and kept at a constant temperature of 950°C for 15 hours (first sintering). After natural cooling, a layered oxide positive electrode active material Na with uniform Zn distribution was obtained. 0.87 Mn 0.45 Fe 0.3 Ni 0.2 Zn 0.05 O2, where the metal composition was tested by elemental composition inductively coupled plasma optical emission spectrometry.

[0150] Comparative Example 4

[0151] A total of 30 g of Na2CO3, Mn2O3, Fe2O3, NiO, and Al2O3 were weighed according to the molar ratio of Na:Mn:Fe:Ni:Al of 1:0.45:0.3:0.2:0.05. The other samples except Al2O3 were pre-ground in an agate mortar and then added to a planetary ball mill for 1 h to obtain a precursor mixture. The obtained precursor mixture was evenly placed in an open crucible and then heated from room temperature to 9 ° C in a muffle furnace at a heating rate of 5 ° C / min. 50 ℃, and kept at a constant temperature of 950 ℃ for 15h (first sintering), after cooling naturally, the obtained layered transition metal oxide was ground with the remaining Al2O3 in an agate mortar and then added to a planetary ball mill for ball milling for 1h. The obtained mixture was evenly placed in an open crucible, and then heated from room temperature to 900 ℃ in a muffle furnace at a heating rate of 5 ℃ / min, and kept at a constant temperature of 900 ℃ for 6h (second sintering), and after cooling naturally, a layered oxide positive electrode active material Na with only slightly enriched Al on the surface was obtained. 0.9 Mn 0.45 Fe 0.3 Ni 0.2 Al 0.05 O2, where the metal composition was tested by elemental composition inductively coupled plasma optical emission spectrometry.

[0152] Comparative Examples 5-6

[0153] The preparation method of the positive electrode active material is similar to that of Example 2, except that the first sintering and the second sintering conditions are different, see Table 1 for details.

[0154] Table 1

[0155]

[0156]

[0157] The molar ratio r of the R element in the surface selected area of ​​the positive electrode active materials in Examples 1-5 and Comparative Examples 1-6 and the molar ratio s of the R element in the bulk selected area of ​​the positive electrode active materials in Examples 1-5 and Comparative Examples 1-6 were characterized, and the first-cycle discharge specific capacity and the cycle performance of the button batteries obtained in Examples 1-5 and Comparative Examples 1-6 were characterized. The characterization results are shown in Tables 2 and 3.

[0158] Characterization methods:

[0159] (1) Test of the molar ratio of R element to metal elements other than Na in the selected area of ​​the positive electrode active material surface and the molar ratio of R element to metal elements other than Na in the selected area of ​​the bulk phase

[0160] Transmission electron microscopy (TEM) combined with energy dispersive X-ray spectroscopy (EDS) for selected area analysis of the cathode active material: the sample thinned by focused ion beam (FIB) was vacuum transferred to a scanning transmission electron microscope (Talos F200S STEM), and the electron microscope was adjusted to STEM mode to perform simultaneous high-angle annular dark field (HAADF) and EDS imaging of the surface and bulk of the sample. After removing the background of the collected EDS spectrum image data, the content of all metal elements was obtained from the EDS spectrum. According to n R / n 除Na外其他金属元素 The molar ratio of R element in the surface selected area and the bulk selected area of ​​the sample to the metal elements except Na was calculated.

[0161] (2) Positive electrode active material composition test

[0162] Inductively coupled plasma atomic emission spectrometry (ICP-AES) is used for testing. The instrument standard refers to EPA 6010D-2014, "Inductively Coupled Plasma Atomic Emission Spectrometry." The sample is chemically treated and dissolved into a solution. The atomized sample is then atomized into a plasma, where it is excited to produce characteristic spectral lines. The wavelength and intensity of these lines (proportional to concentration) are used to qualitatively and quantitatively analyze the elemental content.

[0163] (3) Dv50 test of positive electrode active material

[0164] Equipment model: Malvern 2000 (MasterSizer 2000) laser particle size analyzer, reference standard process: GB / T19077-2016 / ISO 13320:2009, specific test process: Take an appropriate amount of the sample to be tested (the sample concentration is sufficient to ensure 8-12% obscuration), add 20ml of deionized water, and ultrasonicate for 5min (53KHz / 120W) to ensure that the sample is completely dispersed. Then, the sample is measured according to GB / T19077-2016 / ISO 13320:2009 standard.

[0165] (4) Compaction density test of positive electrode active material

[0166] Measured according to GB / T5162-2006.

[0167] (5) pH test of positive electrode active material

[0168] At 25°C, 2 g of positive electrode active material was placed in a beaker, 18 mL of deionized water was added and stirred vigorously for 1 min. After standing for 30 min, the pH of the supernatant was tested with a pH meter.

[0169] (6) Phase and interlayer spacing test of positive electrode active materials

[0170] In a dry room or glove box, grind the sample to be tested in an agate mortar and pass it through a 350 mesh sieve. Take an appropriate amount of the sieved sample and place it in the middle of the sample rack groove so that the loose sample powder is slightly higher than the sample rack plane. Take a glass slide and gently press the sample surface to make the sample surface flat and consistent with the frame plane, and scrape off the excess powder. After the sample is prepared, use the Brucker D8A_A25 X-ray powder diffractometer from BruckerAxS, Germany, with CuKα radiation as the radiation source and the radiation wavelength The scanning 2θ angle range is 5°-60°, and the scanning rate is 4° / min. After the test is completed, the angle corresponding to the 003 crystal plane is used. According to the Bragg equation 2d·sinθ=λ, and each unit cell of the 003 crystal plane contains three layers of transition metal layers, the interlayer spacing d003 of the 003 crystal plane can be obtained. By comparing the XRD diffraction peak of the sample with the standard card of the XRD analysis software, the space group and phase of the sample can be confirmed.

[0171] (7) First cycle discharge capacity of positive electrode active materials and cycle performance test of button batteries

[0172] At 25°C, the obtained positive electrode active material was prepared into a button battery, which was then charged to 4.3 V at a 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 C0 of the button battery. Subsequently, the battery was charged and discharged at a current density of 10 mA / g for 50 cycles, and the discharge specific capacity C1 of the 50th cycle was taken. The capacity retention rate of the positive electrode active material after 50 cycles = C1 / C0.

[0173] The button cell is prepared according to the following steps:

[0174] Preparation of positive electrode sheets: The positive electrode active material, conductive agent carbon black (Super P), and binder polyvinylidene fluoride (PVDF) are thoroughly stirred and mixed in an appropriate amount of solvent NMP at a mass ratio of 80:15:5 to form a uniform positive electrode slurry; the positive electrode slurry is evenly coated on the surface of the positive electrode current collector aluminum foil, and after drying and cold pressing, it is punched into a disc with a diameter of 14 mm to obtain a positive electrode sheet.

[0175] Negative electrode: Sodium metal sheet.

[0176] Preparation of the electrolyte: Ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in equal volumes to obtain an organic solvent, and then NaClO4 was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0177] Isolation membrane: porous polyethylene membrane is used as the isolation membrane.

[0178] Preparation of button battery: stack the above-mentioned positive electrode sheet, separator, and negative electrode sheet in order, so that the separator is placed between the positive electrode sheet and the negative electrode sheet to play an isolating role, add the above-prepared electrolyte, and the preparation of the button battery is completed.

[0179] The structure diagram of the bulk phase selected area and surface selected area of ​​the positive electrode active material obtained in Example 1 is as follows Figure 8 As shown, the structure diagram of the bulk phase selected area and surface selected area of ​​the positive electrode active material obtained in Comparative Example 2 is shown in FIG. Figure 9 As shown, the SEM image of the positive electrode active material of the button battery obtained in Example 1 after 50 cycles is shown as follows: Figure 10 As shown, there are no obvious cracks on the particle surface, and it is not easy to produce serious side reactions when it comes into contact with the electrolyte, indicating that the difference between the molar ratio r of the Al element in the surface selected area of ​​the positive electrode active material of Example 1 and the molar ratio r of the Al element in the bulk selected area is 15%, which can reduce the surface activity of the positive electrode active material, thereby reducing the side reactions between the positive electrode active material and the electrolyte; the SEM image of the positive electrode active material disassembled after the button battery obtained in Comparative Example 2 is shown as follows: Figure 11 As shown, there are obvious cracks on the surface of the particles, and the side reaction with the electrolyte is serious. This shows that the difference between the molar ratio of Al element to other metal elements except Na in the surface selected area of ​​the positive electrode active material of Comparative Example 2 and the molar ratio of Al element to other metal elements except Na in the bulk selected area is 1%. The surface of the positive electrode active material still has high activity, which makes the side reaction between the positive electrode active material and the electrolyte serious. The sample of the positive electrode active material obtained in Example 2 after focused ion beam (FIB) thinning was vacuum transferred to a scanning transmission electron microscope (Talos F200S STEM) as shown below. Figure 11 As shown, the corresponding Figure 11 The EDS peaks of Ti elements at various positions are shown in the figure below. Figure 12 As shown, it can be seen that the Ti element content gradually decreases from the surface selection area to the bulk selection area of ​​the positive electrode active material.

[0180] Table 2

[0181]

[0182] Table 3

[0183]

[0184] Conclusion: It can be seen from Table 2 that the difference between the molar ratio of the R element content in the surface selected area of ​​the positive electrode active materials of Examples 1-5 to the molar ratio of the metal elements other than Na r and the molar ratio s of the R element in the bulk selected area to the metal elements other than Na is 5%-30%; the positive electrode active material of Comparative Example 1 is not doped with R element, and the difference between the molar ratio of the R element content in the surface selected area of ​​the positive electrode active material of Comparative Example 2 to the molar ratio of the R element in the bulk selected area to the metal elements other than Na r and the molar ratio s of the R element in the bulk selected area to the metal elements other than Na is 1%; the molar ratio of the R element content in the surface selected area of ​​the positive electrode active material of Comparative Example 3 to the molar ratio of the R element in the bulk selected area to the metal elements other than Na r is 2. The difference in the molar proportion s of the metal elements is 2%; the difference between the molar proportion r of the R element content in the surface selected area of ​​the positive electrode active material of comparative example 4 and the molar proportion s of the R element in the bulk selected area of ​​the metal elements other than Na is 4%; the difference between the molar proportion r of the R element content in the surface selected area of ​​the positive electrode active material of comparative example 5 and the molar proportion s of the R element in the bulk selected area of ​​the metal elements other than Na is 5%; the difference between the molar proportion r of the R element content in the surface selected area of ​​the positive electrode active material of comparative example 6 and the molar proportion s of the R element in the bulk selected area of ​​the metal elements other than Na is 33%. As can be seen from the data in Table 3, the cycle performance of the batteries of Examples 1-5 is better than that of Comparative Examples 1-6. At the same time, the specific capacity of the batteries of Examples 1-5 is higher than that of Comparative Examples 2-6. This shows that the positive electrode active material doped with the R element of the present application, and the molar ratio of the R element in the surface selected area of ​​the positive electrode active material to the metal elements other than Na is higher than the molar ratio of the R element in the bulk selected area to the metal elements other than Na, with a difference of 5% to 30%, can improve the specific capacity and cycle performance of the battery under high voltage. The reason why the specific capacity of the battery of Comparative Example 1 is higher is that there is no R element enrichment in its surface selected area, which does not reduce Na ion transport, thereby having a higher specific capacity.

[0185] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A sodium secondary battery, characterized in that The invention comprises a positive electrode active material, wherein the positive electrode active material comprises a sodium ion layered transition metal oxide, wherein the sodium ion layered transition metal oxide is doped with an R element, wherein the R element comprises at least one of Ti, Al, Mg, Zr or Zn, and the difference between the molar proportion r of the R element in the surface selected area to the metal elements other than Na and the molar proportion s of the R element in the bulk selected area to the metal elements other than Na is 5%-30%.

2. The sodium secondary battery according to claim 1, characterized in that The difference is 10%-25%.

3. The sodium secondary battery according to claim 1 or 2, characterized in that The value of r is 10%-30%.

4. The sodium secondary battery according to any one of claims 1 to 3, characterized in that The value of s is 0-10%.

5. The sodium secondary battery according to any one of claims 1 to 4, characterized in that The positive electrode active material includes Na x Mn a Q b O 2-c+δ F c , where a + b = 1, 0.5 ≤ x ≤ 1.1, 0 < a ≤ 0.8, 0 < b ≤ 0.8, 0 ≤ c ≤ 0.2, -0.1 ≤ δ ≤ 1, and Q includes R elements.

6. The sodium secondary battery according to claim 5, characterized in that The Q also includes 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 to 6, characterized in that The phase of the positive electrode active material includes an O3 phase, a space group includes R3m, and an interlayer spacing is 0.53nm-0.55nm.

8. The sodium secondary battery according to any one of claims 1 to 6, characterized in that The phase of the positive electrode active material includes a P2 phase, a space group includes P63 / mmc, and an interlayer spacing of 0.54 nm to 0.57 nm.

9. The sodium secondary battery according to any one of claims 1 to 8, characterized in that The pH value of the soaking solution 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 to 9, characterized in that The positive electrode active material satisfies at least one of the following conditions: The Dv50 of the positive electrode active material is 3 μm-30 μm; The BET specific surface area of ​​the positive electrode active material is 0.1 m 2 / g-5m 2 / g; The tap density of the positive electrode active material is 1 g / cm 3 -3g / cm 3 ; The compaction density of the positive electrode active material under a pressure of 300 MPa is 3.0 g / cm 3 -4.0g / cm 3 .

11. An electrical device, characterized in that: A sodium secondary battery comprising the sodium secondary battery according to any one of claims 1 to 10.

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