Positive electrode material, preparation method therefor and use thereof

Abstract

A positive electrode material, a preparation method therefor and a use thereof. The positive electrode material comprises a core and a first shell layer, a lattice in a near-surface region of the core having a layered structure is doped with at least one of Co, Ti and Zr, and the first shell layer comprises a material capable of improving the ionic conductivity of the positive electrode material, or a material capable of improving ionic conductivity and electronic conductivity.

Description

Positive electrode material and preparation method and application thereof

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on February 29, 2024, with application number 202410233358.2 and application name “Positive Electrode Materials, Preparation Methods, and Applications Thereof,” the entire contents of which are incorporated herein by reference. Technical Field

[0002] The embodiments of the present application relate to the field of battery technology, and in particular to positive electrode materials and preparation methods and applications thereof. Background Art

[0003] Sodium-ion batteries offer excellent safety and a high theoretical energy density, giving them broad application prospects in electric vehicles, energy storage, and renewable energy. A key difference between sodium-ion batteries and the widely used lithium-ion batteries is their cathode materials. Layered transition metal oxides have garnered significant attention due to their ability to balance high energy density with a long cycle life. However, these cathode materials often suffer from severe interfacial polarization during charge and discharge cycles, resulting in poor battery energy efficiency. Summary of the Invention

[0004] In view of this, the embodiments of the present application provide a positive electrode material and its preparation method and application. The positive electrode material can enable the battery to achieve both high energy density and good cycle performance while exhibiting excellent energy efficiency during the charge and discharge cycle.

[0005] In a first aspect, an embodiment of the present application provides a positive electrode material, comprising a core and a first shell layer coated on the surface of the core;

[0006] The core is a layered oxide;

[0007] The layered oxide includes Na a Ni 1-x-y-z Fe x Mn y E z O b , 0.5≤a≤1.2, 1.8≤b≤2.2, 0<x≤1, 0<y≤1, 0≤z≤0.5, and the values ​​of a, b, x, y, and z are such that the core is electrically neutral; the E element is selected from at least one of Zn, Al, Y, Ca, Li, Rb, Cs, W, Ce, Mo, Ba, Mg, Ta, Nb, V, Sc, Sr, B, F, P, and Cu;

[0008] The region of the core close to the first shell layer is doped with an M element, the M element is doped in the crystal lattice of the layered oxide, and the M element is selected from at least one of Co, Ti and Zr;

[0009] The material of the first shell layer includes an oxygen-containing sodium salt of M, or includes an oxygen-containing sodium salt of M and an oxide of M.

[0010] The above-mentioned positive electrode material is essentially a bulk doping of the near-surface of a sodium-based positive electrode material having a layered oxygen structure with one or more elements of the M element (Co, Ti and Zr), which can effectively inhibit the phase transition of the near-surface region of the core during the charge and discharge process, and effectively improve the electron and electron transmission capacity of the layered oxygen structure in the region doped with the M element, thereby effectively improving the interface dynamics of the core, reducing the interface impedance of the positive electrode material, and helping to improve the rate performance of the battery, reducing the DC internal resistance of the battery, and improving the energy efficiency of the battery. In addition, doping with the M element can also increase the charge and discharge platform voltage of the positive electrode material, thereby helping to improve the energy density of the battery. More importantly, doping with the M element only in the near-surface region of the core can ensure good returns while controlling the cost of the positive electrode material at a low level, which can greatly enhance the application prospects of the positive electrode material.

[0011] In addition, the first shell layer not only serves as a physical barrier to prevent the inner core layer oxygen material from directly contacting the electrolyte (e.g., electrolyte) in the battery, thereby reducing the occurrence of side reactions and facilitating the electrochemical performance of the inner core material, but also, the oxygen-containing sodium salt of M in the first shell layer has good ionic conductivity, and the oxide of M has good electronic conductivity. Therefore, regardless of whether the first shell layer contains the oxygen-containing sodium salt of M, or contains both the oxygen-containing sodium salt of M and the oxide of M, it can form a physical barrier while improving the interfacial dynamics of the positive electrode material 1.

[0012] In some embodiments of the present application, the doping depth of the M element is 0.1 nm to 20 nm, thereby achieving both cost and improvement of the interface dynamics of the positive electrode material.

[0013] In some embodiments of the present application, the doping depth of the M element is 1 nm to 10 nm. This can improve the cost-effectiveness of doping the M element and achieve both low cost and excellent electrochemical performance of the positive electrode material.

[0014] In some embodiments of the present application, the mass content of the M element in the positive electrode material is 1000ppm-25000ppm. This not only effectively improves the interfacial dynamics of the positive electrode material, but also avoids lattice deformation that may be caused by excessive doping, thereby improving the long-term cycling performance of the positive electrode material.

[0015] In some embodiments of the present application, in the first shell, the mass content of the oxygen-containing sodium salt of M is greater than the mass content of the oxide of M. This tends to improve the ionic conductivity of the first shell and further reduce the active sites on the surface of the positive electrode material, thereby reducing the possibility of side reactions with other components in the battery during use.

[0016] In some embodiments of the present application, the mass ratio of the oxygen-containing sodium salt of M to the oxide of M is ≥ 9:1. This can fully optimize the transport efficiency of active sodium ions at the cathode material interface while also taking into account electronic conductivity, further helping to reduce interfacial polarization during charge and discharge cycles and improving the energy efficiency of the cathode material.

[0017] In some embodiments of the present application, the first shell layer further comprises a first material; the oxygen-containing sodium salt of M, or the oxygen-containing sodium salt of M and an oxide of M, is dispersed in the first material; and the first material comprises at least one of a fast ion conductor material and a conductive material. This can improve the overall electrochemical performance of the positive electrode material.

[0018] In some embodiments of the present application, the thickness of the first shell layer is 1 nm to 20 nm. In this way, the first shell layer can fully play its role as a physical barrier, and the length of the active ion embedding / de-embedding path can be controlled within a suitable range.

[0019] In some embodiments of the present application, the positive electrode material further includes a second shell layer located on a surface of the first shell layer facing away from the core, and the second shell layer is composed of the first material. In this case, the electrochemical performance of the positive electrode material is further improved.

[0020] In some embodiments of the present application, the sum of the thicknesses of the first shell and the second shell is 1 nm to 20 nm. Similarly, in this way, not only do the first shell and the second shell have a good physical barrier effect, but the length of the active ion embedding / de-embedding path can also be controlled within a suitable range.

[0021] In some embodiments of the present application, when the M element is Co, the corresponding oxygen-containing sodium salt of M is sodium cobaltate, and the corresponding oxide of M is cobalt tetroxide;

[0022] When the M element is Ti, the corresponding oxygen-containing sodium salt of M is sodium titanate, and the corresponding oxide of M is titanium dioxide;

[0023] When the M element is Zr, the corresponding oxygen-containing sodium salt of M is sodium zirconate, and the corresponding oxide of M is zirconium dioxide.

[0024] In some embodiments of the present application, D50 of the positive electrode material is 0.5 μm-50 μm.

[0025] A second aspect of the present invention provides a method for preparing a positive electrode material, comprising:

[0026] The substrate is placed in a reaction chamber of an atomic layer deposition device, and a reaction layer is formed on the surface of the substrate using a first raw material to obtain a precursor; wherein the substrate is a layered oxide, and the layered oxide includes Na a Ni 1-x-y-z Fe x Mn y E z O b , 0.5≤a≤1.2, 1.8≤b≤2.2, 0<x≤1, 0<y≤1, 0≤z≤0.5, and the values ​​of a, b, x, y, and z are such that the core is electrically neutral; the E element is selected from at least one of Zn, Al, Y, Ca, Li, Rb, Cs, W, Ce, Mo, Ba, Mg, Ta, Nb, V, Sc, Sr, B, F, P, and Cu; the first raw material includes water and / or ozone, and a source of the M element;

[0027] The precursor is tempered in an oxygen-containing atmosphere to obtain the positive electrode material.

[0028] The preparation method has simple steps, strong process reliability, high production efficiency, and is suitable for large-scale industrial production.

[0029] In some embodiments of the present application, the temperature in the reaction chamber is 150°C-300°C.

[0030] In some embodiments of the present application, the tempering temperature is ≥850° C. Controlling the tempering temperature within the above range can promote the diffusion of the M element toward the center of the substrate and its doping into the lattice of the layered oxide material.

[0031] In some embodiments of the present application, the tempering time is 3-8 hours. This allows for a more complete reaction. When the tempering time is within the above range and is longer, the diffusion of the M element toward the center of the substrate can be promoted, thereby increasing the doping amount of the M element.

[0032] In some embodiments of the present application, the total mass of the M element in the M element source is 2000ppm-50000ppm of the total mass of the substrate. In this way, the mass content of the M element in the positive electrode material can be controlled within the range of 1000ppm-25000ppm.

[0033] In some embodiments of the present application, the surface of the substrate further comprises a coating layer, wherein the coating layer is made of at least one of a fast ion conductor material and a conductive material. Thus, in the prepared positive electrode material, the first shell layer further comprises the first material.

[0034] A third aspect of the present invention provides a positive electrode plate comprising a current collector and a positive electrode active material layer disposed on at least one side of the current collector; the positive electrode active material layer comprises the positive electrode material provided in the first aspect of the present invention, or a positive electrode material prepared according to the preparation method provided in the second aspect of the present invention. Due to the use of the positive electrode material provided in the present invention, the positive electrode plate can be used to provide a secondary battery that achieves high energy density, good cycle performance, and high energy efficiency.

[0035] A fourth aspect of the embodiment of the present application provides a secondary battery, which includes the positive electrode sheet and the negative electrode sheet provided in the third aspect of the embodiment of the present application, and an electrolyte located between the positive electrode sheet and the negative electrode sheet.

[0036] Due to the use of the positive electrode sheet provided in the embodiment of the present application, the secondary battery has a high first cycle efficiency and a high energy efficiency, and the secondary battery can also achieve a high energy density. In particular, the secondary battery has a high cost-effectiveness and a good market prospect.

[0037] A fifth aspect of the embodiments of the present application provides an electrical device, which includes the secondary battery provided in the fourth aspect of the embodiments of the present application.

[0038] Since the secondary battery provided in the embodiment of the present application is used to power the electrical equipment, the electrical equipment has a good market prospect.

[0039] A sixth aspect of the embodiments of the present application provides an energy storage system, characterized in that the energy storage system includes the secondary battery provided in the fourth aspect of the embodiments of the present application.

[0040] Due to the use of the secondary battery provided in the embodiment of the present application, the energy storage system can have a strong energy storage capacity and better energy efficiency, and is cost-effective. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG1 is a schematic structural diagram of a positive electrode material provided in one embodiment of the present application;

[0042] FIG2 is a schematic structural diagram of a positive electrode material provided in another embodiment of the present application;

[0043] FIG3A is a transmission electron microscope (TEM) photograph of Example 1 of the present application;

[0044] FIG3B is an energy spectrum result of the Co element based on FIG3A ;

[0045] FIG4A is a TEM photograph of Example 1 of the present application at a different magnification than FIG3A ;

[0046] FIG4B is an energy spectrum result of the Co element based on FIG4A ;

[0047] FIG5 is a fragment of the X-ray spectrum of the positive electrode material of Example 1;

[0048] FIG6 is a discharge platform voltage-cycle number curve of the soft-pack battery of Example 1 and Comparative Example 1;

[0049] FIG7 is a volume energy density-cycle number curve of the soft pack battery of Example 1 and Comparative Example 1;

[0050] FIG8 shows the capacity retention rates of the soft-pack batteries of Example 1 and Comparative Example 1 at different rates;

[0051] FIG9 is an AC impedance curve of the soft-pack battery of Example 1 and Comparative Example 1;

[0052] Description of the accompanying drawings: 1-positive electrode material; 10-core; 20-first shell; 30-second shell. DETAILED DESCRIPTION

[0053] Layered transition metal oxides have attracted widespread attention in the industry due to their high energy density, simple structure and easy synthesis. However, they will undergo phase transition in the initial stage of charge and discharge, especially layered transition metal materials for sodium batteries. + Changes in concentration can cause relative slippage of the transition metal layers, leading to structural rearrangements and complex phase transitions during charge and discharge. These structural changes in the material can cause additional overpotentials, resulting in excessive voltage polarization, increased energy loss in the battery, and consequently, low energy efficiency.

[0054] To solve the above technical problems, referring to FIG1 , an embodiment of the present application provides a positive electrode material 1 , comprising a core 10 and a first shell 20 coated on the surface of the core 10 ;

[0055] The core 10 is a layered oxide;

[0056] Layered oxides include Na a Ni 1-x-y-z Fe x Mn y E z O b , 0.5≤a≤1.2, 1.8≤b≤2.2, 0<x≤1, 0<y≤1, 0≤z≤0.5, and the values ​​of a, b, x, y, and z are selected so that the core 10 is electrically neutral; the E element is selected from at least one of Zn, Al, Y, Ca, Li, Rb, Cs, W, Ce, Mo, Ba, Mg, Ta, Nb, V, Sc, Sr, B, F, P, and Cu;

[0057] The region of the core 10 close to the first shell 20 is doped with an M element, the M element being doped in the crystal lattice of the material of the core 10 , and the M element being selected from at least one of Co, Ti and Zr;

[0058] The material of the first shell layer 20 includes an oxygen-containing sodium salt of M, or includes an oxygen-containing sodium salt of M and an oxide of M.

[0059] The above-mentioned positive electrode material 1 is essentially a bulk doping of the near-surface of the sodium-based positive electrode material 1 having a layered oxygen structure using one or more elements of the M element (Co, Ti and Zr), which can effectively inhibit the phase transition of the near-surface region of the core 10 during the charge and discharge process, especially in the initial stage of charge and discharge, and effectively improve the electron and electron transmission capacity of the layered oxygen structure in the region doped with the M element, thereby effectively improving the interface dynamics of the core 10, reducing the interface impedance of the positive electrode material, and helping to improve the rate performance of the battery, reducing the DC internal resistance of the battery, and improving the energy efficiency of the battery. In addition, doping with the M element can also increase the charge and discharge platform voltage of the positive electrode material, thereby helping to improve the energy density of the battery. More importantly, doping with the M element only in the near-surface region of the core 10 can ensure good returns while controlling the cost of the positive electrode material 1 at a low level, which can greatly enhance the application prospects of the positive electrode material 1. In addition, the surface of the core 10 is coated with a first shell 20. The first shell 20 not only serves as a physical barrier to prevent the layer oxygen material of the core 10 from directly contacting the electrolyte (e.g., electrolyte) in the battery, thereby reducing the occurrence of side reactions and facilitating the electrochemical performance of the core 10 material, but also, the oxygen-containing sodium salt of M in the first shell 20 has good ionic conductivity, and the oxide of M has good electronic conductivity. Therefore, regardless of whether the first shell 20 contains the oxygen-containing sodium salt of M, or contains both the oxygen-containing sodium salt of M and the oxide of M, it can form a physical barrier while improving the interfacial dynamics of the positive electrode material 1. Specifically, this is reflected in the better performance of the capacity, energy efficiency, cycle performance, and first coulombic efficiency of the positive electrode material 1. For example, doping with the Co element is more conducive to improving the energy efficiency and cycle performance of the positive electrode material 1; doping with the Ti element tends to improve the capacity and cycle performance of the positive electrode material 1; and doping with the Zr element tends to improve the cycle performance of the positive electrode material 1.

[0060] In the embodiment of the present application, the M element doped in the lattice of the material of the core 10 is specifically Co element, Ti element, Zr element doped in the Na a Ni 1-x-y-z Fe x Mn y E z O b The transition metal position can be replaced by one or more of Ni, Fe, Mn, or even E.

[0061] In the embodiments of the present application, time of flight secondary ion mass spectrometry (TOF-SIMS) can be used to characterize the doping of the M element, specifically to characterize the presence of M oxygen-containing sodium salts and M oxides.

[0062] In the embodiments of the present application, a scanning electron microscope (SEM) and a transmission electron microscope (TEM) can be used to characterize the coating thickness of the oxygen-containing sodium salt of M, the oxide of M, and the doping depth of the M element on the surface.

[0063] In the embodiments of the present application, TOF-SIMS or Auger electron spectroscopy (AES) can be used to characterize the presence of oxygen-containing sodium salts of M and oxides of M.

[0064] In the embodiment of the present application, the layered oxide may be Na a Ni 1-x-y Fe x Mn y O b , or Na doped with E element a Ni 1-x-y Fe x Mn y O b . When in Na a Ni 1-x-y Fe x Mn y O b When the E element is further doped on the basis of , it is more beneficial to the electrochemical performance of the positive electrode material 1, and the cost of the E element is low, and doping with the E element will hardly affect the cost of the positive electrode material 1. For example, when the E element is Al, the irreversible change of the layered structure of the core 10 material and the change of the lattice volume during the cycle process can be suppressed; when the E element is Zn element, the cycle stability and rate performance of the core 10 material can be improved; when the E element is Cu, the sodium ion content in the core 10 material can be increased, so that the layered phase structure can be stabilized under high voltage; when multiple of the above elements are further doped, there may be a synergistic effect between the multiple doping elements, thereby further improving the comprehensive electrochemical performance of the core 10 material. Ordinary technicians in this field can select doping elements and determine the doping amount of E element according to actual needs, which will not be repeated here.

[0065] In some embodiments of the present application, the layered oxide is an O3 phase or a P2 phase, or a stacked phase of O3 and P2. It should be noted that the above-mentioned O3 phase and P2 phase refer to the core 10 material that has not undergone any charging or discharging reaction. Among them, the P2 phase refers to the triangular prism coordination environment of the oxygen coordination of the sodium ions in the material lattice, and the O3 phase refers to the octahedral coordination of the oxygen coordination of the sodium ions in the material lattice. The core 10 material of the P2 phase can have higher ionic conductivity, good structural stability and relatively good environmental stability; the core 10 material of the O3 phase has a higher capacity; the core 10 material of the P2 and O3 stacked phases can compromise the various properties of the P2 phase and the O3 phase.

[0066] In order to take into account both the cost and the improvement effect of the interface dynamics of the positive electrode material 1, in some embodiments of the present application, the doping depth of the M element in the core 10 is 0.1nm-20nm. That is, the M element is doped in an area with a depth of 0.1nm-20nm from the surface of the core 10 to its center. However, for a specific core, whether there is E element doping, or when the specific selection of E element is different, the minimum doping depth of the M element may be different. Specifically, the doping depth of the M element in the core 10 can be, but is not limited to, 0.1nm, 0.2nm, 0.5nm, 0.8nm, 1nm, 2nm, 5nm, 8nm, 10nm, 12nm, 15nm, 18nm, 20nm, etc. In the embodiment of the present application, a transmission electron microscope (TEM) can be used to characterize the doping depth of the M element.

[0067] In some specific embodiments, in order to improve the cost-effectiveness of doping the M element and to balance the cost and the electrochemical performance of the positive electrode material 1 as much as possible, the embodiment of the present application controls the doping depth of the M element in the core 10 to be 1nm-10nm. Specifically, the doping depth of the M element can be, but is not limited to, 1nm, 2nm, 3nm, 4nm, 5nm, 6nm, 7nm, 8nm, 9nm, and 10nm. The doping depth of the M element ≥ 1nm can make the doped region have a certain thickness, which can not only effectively improve the interface dynamics of the positive electrode material 1 in the early stage of the charge and discharge cycle, but also, even if a part of the inner layer of the core 10 (the region not doped with the M element) undergoes a phase change during the charge and discharge cycle, the doped region can effectively isolate its influence on the interface of the core 10, thereby improving the energy efficiency of the positive electrode material 1 during the long cycle process. Controlling the doping depth of the M element ≤ 10nm can effectively suppress the irreversible change of the phase change of the material of the core 10 during the cycle, or in other words, can reduce the cost of the positive electrode material 1 while minimizing the degree of phase change of the material of the core 10. In addition, considering that in some cases, due to the preparation process, if the doping depth of the M element is increased, the thickness of the first shell layer 20 will increase: for example, when the doping depth is >10nm, it is beneficial to the cycle performance of the positive electrode material 1, but there is no gain in its capacity and first coulombic efficiency, and the benefits are limited.

[0068] On the basis of the doping depth, the doping amount of the M element will also affect the interface performance and production cost of the positive electrode material 1. In some embodiments of the present application, the mass content of the M element in the positive electrode material 1 is 1000ppm-25000ppm. It can be understood that although the characterization here is based on the total mass of the positive electrode material 1, because the M element is only doped in the near-surface area of ​​the core 10, the doping amount of the M element in the near-surface area of ​​the core 10 is also controlled within an appropriate range. In this way, not only can the interface dynamics of the positive electrode material 1 be effectively improved, but also the lattice deformation that may be caused by large-scale doping can be avoided, thereby making the long-cycle performance of the positive electrode material 1 better. In addition, the cost of the positive electrode material 1 can be further controlled within a lower range. Specifically, the mass content of the M element in the positive electrode material 1 can be, but is not limited to, 1000ppm, 1500ppm, 2000ppm, 2500ppm, 3000ppm, 3500ppm, 4000ppm, 4500ppm, 5000ppm, 5500ppm, 6000ppm, 6500ppm, 7000ppm, 7500ppm, 8000ppm, 8500ppm, 9000ppm, 9500ppm, 10000ppm, 12000ppm, 15000ppm, 18000ppm, 20000ppm, 22000ppm, 25000ppm, etc. In the embodiment of the present application, an X-ray fluorescence spectrometer (XRF) or an energy dispersive spectrometer (EDS) can be used to test the mass proportion of the M element in the positive electrode material 1.

[0069] In some embodiments of the present application, in the near-surface region of the core 10, the content of the M element gradually increases from the core 10 toward the first shell 20. In other embodiments, the content of the M element first increases and then decreases from the core 10 toward the first shell 20.

[0070] In some embodiments of the present application, in the first shell 20, the mass content of the oxygen-containing sodium salt of M is greater than the content of the oxide of M. Specifically, the mass ratio of the oxygen-containing sodium salt of M to the oxide of M in the first shell 20 can be, but is not limited to, 1.1:1, 1.2:1, 1.5:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 9.5:1. At this time, it is more inclined to improve the ionic conductivity of the first shell 20, and it can also further reduce the active sites on the surface of the positive electrode material 1, reducing the possibility of side reactions with other components in the battery (for example, electrolyte) during application, thereby further optimizing the electrochemical performance of the positive electrode material 1. In some specific embodiments, in the first shell 20, the ratio of the mass content of the oxygen-containing sodium salt of M to the content of the oxide of M is ≥9:1. In this way, the transmission efficiency of active sodium ions at the interface of the positive electrode material 1 can be fully optimized while taking into account the electronic conductivity, which is more conducive to reducing the interfacial polarization during the charge and discharge cycle and improving the energy efficiency of the positive electrode material 1. Specifically, the mass ratio of the oxygen-containing sodium salt of M to the oxide of M in the first shell 20 can be, but is not limited to, 9.1:1, 9.2:1, 9.3:1, 9.4:1, 9.5:1, 9.6:1, 9.7:1, 9.8:1, and 9.9:1. In the embodiment of the present application, TOF-SIMS or AES can be used to characterize the mass ratio of the oxygen-containing sodium salt of M to the oxide of M.

[0071] In some embodiments of the present application, the first shell 20 also includes a first material; an oxygen-containing sodium salt of M, or an oxygen-containing sodium salt of M and an oxide of M are dispersed in the first material; the first material includes at least one of a fast ion conductor material and a conductive material. The presence of the first material can increase the density of the first shell 20, thereby increasing the effect of the first shell 20 as a physical barrier; in addition, the first material can also optimize the ionic conductivity and / or electronic conductivity of the first shell 20, and can improve the comprehensive electrochemical performance of the positive electrode material 1. It can be understood that even if the oxygen-containing sodium salt of M has ionic conductivity and the oxide of M has electronic conductivity, the fast ion conductor material and the conductive material mentioned in the embodiments of the present application do not include the above two materials. Specifically, the fast ion conductor material can be a fast ion conductor material well known to those skilled in the art, for example, Na3Zr2Si2PO 12 , Na2O·Al2O3, etc. The conductive material may be a conductive material well known to those skilled in the art, including but not limited to conductive carbon, for example, at least one of amorphous carbon, graphene, carbon nanotubes, etc.

[0072] In the embodiment of the present application, the oxygen-containing sodium salt of M, or the oxygen-containing sodium salt of M and the oxide of M are uniformly dispersed in the first material. In the embodiment of the present application, the oxygen-containing sodium salt of M and the oxide of M can be dispersed or evenly distributed in a dotted manner in the first material.

[0073] In some embodiments of the present application, the thickness of the first shell layer 20 is 1nm-20nm. This not only forms a physical barrier of sufficient thickness on the surface of the core 10, substantially reducing the risk of direct contact between the core 10 material and other battery components such as the electrolyte and the atmospheric environment, but also controls the length of the active sodium ion extraction / intercalation path during the cycle within an appropriate range, while also benefiting the battery's long-cycle performance and rate performance. Specifically, the thickness of the first shell layer 20 can be, but is not limited to, 1nm, 2nm, 5nm, 8nm, 10nm, 12nm, 15nm, 18nm, 20nm, etc. In some specific embodiments, the thickness of the first shell layer 20 is 1nm-10nm. When the particle size of the core 10 material is constant, further controlling the thickness of the first shell layer 20 within the above range can result in a relatively large specific surface area of ​​the positive electrode material 1 and a shorter transport path for active ions. In the embodiments of the present application, the thickness of the first shell layer 20 can be characterized using TEM.

[0074] In the embodiment of the present application, the first shell 20 is evenly coated on the surface of the core 10 to form a thin and uniform coating layer. Specifically, in some cases, any 1 μm of the first shell 20 2 In the area of ​​the first shell layer 20, the thickness difference of the first shell layer 20 is ≤ 20%. Specifically, the thickness difference of the first shell layer 20 is defined as: 2 Within the area of ​​, the maximum and minimum thicknesses of the first shell layer are taken, the maximum thickness is a nm, the minimum thickness is b nm, and the thickness difference is (ab) / a×100%. For example, the thickness difference of the first shell layer 20 can be, but is not limited to, 0%, 1%, 2%, 5%, 8%, 10%, 12%, 15%, 18%, or 20%.

[0075] Please refer to Figure 2. In some embodiments of the present application, a second shell 30 is further provided on the surface of the first shell 20 facing away from the core 10. The second shell 30 is composed of the above-mentioned first material. There may be two situations: (1) The first material in the second shell 30 is exactly the same as the first material in the first shell 20. For example, if the first material in the first shell 20 is amorphous carbon, then the second shell 30 is also composed of amorphous carbon. (2) The first material in the second shell 30 is partially the same as or completely different from the first material in the first shell 20; for example, if the first material in the first shell 20 is amorphous carbon, then the second shell 30 is composed of amorphous carbon and Na3Zr2Si2PO 12 Alternatively, the second shell layer 30 is composed of amorphous carbon layers and Na3Zr2Si2PO 12 layer, the second shell 30 is composed of Na3Zr2Si2PO 12In the above case (1), the first shell 20 and the second shell 30 can be prepared simultaneously or in separate steps. In the above case (2), the first shell 20 and the second shell 30 are prepared in separate steps.

[0076] In some other embodiments of the present application, a second shell 30 is further provided on the surface of the first shell 20 facing away from the core 10. The second shell 30 is composed of at least one of a fast ion conductor material and a conductive material, and the first shell 20 does not contain any fast ion conductor material or conductive material. That is, the first shell 20 is composed of an oxyacid salt of M, or an oxyacid salt of M and an oxide of M, and the second shell 30 is composed of a fast ion conductor material and / or a conductive material.

[0077] When the positive electrode material 1 includes both a first shell layer 20 and a second shell layer 30, to achieve similar objectives, in some embodiments of the present application, the sum of the thicknesses of the first shell layer 20 and the second shell layer 30 is 1 nm to 20 nm. In some specific embodiments, the sum of the thicknesses of the first shell layer 20 and the second shell layer 30 is 1 nm to 20 nm. Similarly, the first shell layer 20 and the second shell layer 30 are both uniformly coated on the surface of the core 10, and analogy can be made to the aforementioned case of only having the first shell layer 20.

[0078] It is understandable that the thickness of the first shell layer 20, the presence of the second shell layer 30, and the presence of the first material in the first shell layer 20 will all affect the mass proportion of the M element in the positive electrode material 1. When the doping amount of the M element remains unchanged, but the mass of the fast ion conductor material and / or the conductive material is large, the mass proportion of the M element in the positive electrode material 1 may decrease slightly, but still be controlled within the range of 1000ppm-25000ppm.

[0079] It can be understood that the specific selection of the M element is different, and the specific types of the oxygen-containing sodium salt of M and the oxide of M are also different. In some embodiments of the present application, when the M element is Co, the corresponding oxygen-containing sodium salt of M is sodium cobaltate, and the corresponding oxide of M is cobalt trioxide. When the M element is Ti, the corresponding oxygen-containing sodium salt of M is sodium titanate, and the corresponding oxide of M is titanium dioxide. When the M element is Zr, the corresponding oxygen-containing sodium salt of M is sodium zirconate, and the corresponding oxide of M is zirconium dioxide. In some embodiments of the present application, the D50 of the positive electrode material 1 is 0.5μm-50μm. It can also be understood that the positive electrode material 1 provided in the embodiment of the present application can be prepared into any size commonly used in the industry, but no matter what the value of the D50 of the positive electrode material 1 is, the doping depth of the M element does not need to change according to the particle size of the particles of the positive electrode material 1. In the embodiment of the present application, the D50 of the positive electrode material 1 can be measured using a laser particle size analyzer. Specifically, D50 of the positive electrode material 1 can be, but is not limited to, 0.5 μm, 1 μm, 2 μm, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, or 50 μm.

[0080] The present embodiment accordingly provides a method for preparing the aforementioned positive electrode material 1, comprising:

[0081] S01, placing a substrate in a reaction chamber of an atomic layer deposition device, using a first raw material to form a reaction layer on the surface of the substrate to obtain a precursor; wherein the substrate is a layered oxide, and the layered oxide includes Na a Ni 1-x-y-z Fe x Mn y E z O b , 0.5≤a≤1.2, 1.8≤b≤2.2, 0<x≤1, 0<y≤1, 0≤z≤0.5, and the values ​​of a, b, x, y, and z are selected so that the core is electrically neutral; the E element is selected from at least one of Zn, Al, Y, Ca, Li, Rb, Cs, W, Ce, Mo, Ba, Mg, Ta, Nb, V, Sc, Sr, B, F, P, and Cu; the first raw material includes water and / or ozone, and a source of the M element;

[0082] S02. Tempering the precursor in an oxygen-containing atmosphere to obtain the aforementioned positive electrode material 1 provided in an embodiment of the present application.

[0083] It should be noted that, due to the influence of the preparation process, Na a Ni 1-x-y-z Fe x Mn y E z O bThere is residual sodium on the surface of the steel, which generally includes sodium carbonate, sodium hydroxide and sodium peroxide.

[0084] In step S01, since an atomic layer deposition (ALD) device is used to form a reaction layer on the substrate surface, the M source, water, and ozone all enter the reaction chamber in gaseous form, and the first raw material is adsorbed on the substrate surface. When the first raw material includes water, the water vapor not only serves as a carrier gas for the M source, but also, in this environment, the hydrogen ions in the water vapor can displace sodium ions near the substrate surface. In this case, the reaction layer formed by the first raw material includes sodium ions, oxygen ions, and the M source. During the tempering in step S02, these displaced sodium ions can react with the M source in an oxygen-containing atmosphere to produce an oxygen-containing sodium salt of M, forming the first shell layer. In some cases, the formation of the oxygen-containing sodium salt of M may also be accompanied by the formation of an oxide of M. When the first raw material includes ozone, ozone not only serves as a carrier gas but also, during the tempering in step S02, reacts with the residual sodium on the substrate surface in an oxygen-containing atmosphere to simultaneously produce the oxygen-containing sodium salt of M and an oxide of M, removing the residual sodium and forming the first shell layer. It is understandable that when the first raw material is only water, the oxygen ions formed in the reaction layer can also undergo an oxidation reaction with the residual sodium on the surface of the substrate in step S02 to remove the residual sodium.

[0085] In particular, during the tempering process in step S02, the M element also diffuses into the near-surface region of the substrate and is doped into the lattice of the layered oxide material in this region, transforming the substrate into a core, thereby obtaining the positive electrode material. In other words, the transformation of the substrate into the core is accompanied by the formation of the first shell. The resulting core and first shell of the positive electrode material are tightly bonded, further facilitating the first shell's ability to function as a physical barrier. Furthermore, the use of atomic layer deposition equipment enables atomic-level control of the thickness of the reaction layer, enabling precise nanometer-level control of the thickness of the first shell and the doping depth of the M element in the core.

[0086] The preparation method has simple steps, strong process reliability, high production efficiency, and is suitable for large-scale industrial production.

[0087] In some embodiments of the present application, the first raw material includes water. When the first raw material contains water, the proportion of the oxygen-containing sodium salt of M in the final first shell layer can be increased. In some specific embodiments, the first raw material comprises water and a source of the element M. This allows the mass of the oxygen-containing sodium salt of M in the first shell layer to be greater than the mass of the oxide of M.

[0088] In some embodiments of the present application, the above-mentioned M element source is a salt of the M element. Specifically, when the M element is a Co element, the M element source includes but is not limited to dihydroxycyclopentadienyl cobalt, N, N'-di-tert-butyl acetamidine cobalt, (3,3-dimethyl-1-butyne) hexacarbonyl cobalt, N, N'-diisopropyl acetamidine cobalt. When the M element is a Ti element, the M element source includes but is not limited to titanium tetrachloride, titanium tetraisopropoxide (dimethylamino) titanium, tetrakis (diethylamino) titanium, tris (dimethylamino) cyclopentadienyl titanium. When the M element is a Zr element, the M element source includes but is not limited to tetrakis (dimethylamino) zirconium, tetrakis (ethylmethylamino) zirconium, tris (dimethylamino) cyclopentadienyl zirconium.

[0089] In some embodiments of the present application, in step S01, a gaseous first raw material is introduced into a reaction chamber filled with a dry gas. In some specific embodiments, nitrogen is also used as a carrier gas to introduce the first raw material into the atomic layer deposition apparatus.

[0090] It is understandable that when using atomic layer deposition equipment for preparation, in order to achieve the expected thickness of the first shell and the doping amount of the M element, and to form a uniform reaction layer on the surface of the substrate, it is necessary to introduce the first raw material into the reaction chamber in multiple pulses. Set the reaction chamber temperature and substrate temperature of the ALD equipment to 100°C-300°C, set the carrier gas (H2O and / or O3) injection time to 0.005s-0.1s, and the M element source injection time to 0.005s-0.1s, and then perform inert gas purge, and the purge time is 1s-50s. The above steps are one deposition cycle, and those skilled in the art can determine the number of deposition cycles according to actual production requirements.

[0091] It can be understood that the number of deposition cycles depends on the amount of the first raw material. Among them, increasing the amount of the M element source can increase the thickness of the first shell and increase the doping amount of the M element to a certain extent. Taking into account the loss of raw materials, in some embodiments of the present application, the total mass of the M element in the M element source is 2000ppm-50000ppm of the mass of the substrate. In this way, the mass content of the M element in the positive electrode material can be controlled within the range of 1000ppm-25000ppm. Specifically, the total mass of the M element in the M element source can be, but is not limited to, 2000ppm, 3000ppm, 5000ppm, 8000ppm, 10000ppm, 15000ppm, 20000ppm, 25000ppm, 30000ppm, 35000ppm, 40000ppm, 45000ppm, 50000ppm of the total mass of the substrate. Those skilled in the art can determine the number of deposition cycles according to actual production conditions, for example, 1-100 cycles.

[0092] Since atomic layer deposition is achieved by alternately passing pulses of gaseous raw materials into the reaction chamber, this process is essentially accompanied by a temperature increase. Therefore, in some embodiments of the present application, in step S01, the temperature in the reaction chamber is 100°C-300°C. In particular, controlling the temperature in the reaction chamber within the above range is also conducive to the replacement of sodium ions on the surface of the substrate by hydrogen ions. Specifically, in step S01, the temperature in the reaction chamber can be, but is not limited to, 100°C, 120°C, 150°C, 180°C, 200°C, 220°C, 250°C, 280°C, and 300°C.

[0093] In some embodiments of the present application, in step S02, the tempering temperature is ≥850°C. Controlling the tempering temperature within the above range can promote the diffusion of the M element toward the center of the substrate and its doping into the lattice of the layered oxide material. In addition, at the above temperature, the residual sodium (mainly sodium carbonate) on the surface of the substrate can be decomposed, which can promote its reaction with the M element source. Specifically, the tempering temperature can be, but is not limited to, 850°C, 880°C, 900°C, 920°C, 950°C, 980°C, 1000°C, 1050°C, 1100°C, 1150°C, 1200°C, 1300°C, 1400°C, and 1500°C.

[0094] Further increasing the tempering temperature, for example, to 900°C or higher, increases the diffusion depth of the M element in the substrate. This increases the doping depth of the M element in the core of the cathode material and increases the doping level of the M element. This also promotes the formation of the oxygen-containing sodium salt of M, further increasing the mass fraction of the oxygen-containing sodium salt of M in the first shell.

[0095] In some embodiments of the present application, in step S02, the tempering duration is 3-8 hours. This allows for a more complete reaction. Furthermore, when the tempering time is within this range and is longer, it promotes diffusion of the M element toward the center of the substrate and increases the doping level of the M element. Specifically, the tempering duration can be, but is not limited to, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, or 8 hours.

[0096] In step S02 , the oxygen-containing atmosphere includes but is not limited to an oxygen atmosphere or an air atmosphere.

[0097] In some embodiments of the present application, the surface of the substrate further has a coating layer, and the material of the coating layer is selected from at least one of a fast ion conductor material and a conductive material. Because the coating layer is always looser than the substrate, in this case, in step S01, the first raw material can penetrate into the surface of the substrate through the coating layer. At this time, the reaction layer formed is dispersed in the coating layer. In the positive electrode material finally obtained, it is expressed as an oxygen-containing sodium salt of M, or an oxygen-containing sodium salt of M and an oxide of M dispersed in the first material. Of course, in other cases, the reaction layer is formed on the side of the coating layer close to the substrate. At this time, the positive electrode material includes a first shell and a second shell. The first shell includes an oxygen-containing sodium salt of M, or an oxygen-containing sodium salt of M and an oxide of M and the first material, and the second shell is on the surface of the first shell away from the substrate, and the second shell is composed of the first material, and the first material of the second shell is the same as the first material of the first shell.

[0098] Under extreme process conditions, it's possible to form a reaction layer solely on the substrate surface. In this case, the first shell of the resulting cathode material consists of an oxygen-containing sodium salt of M, or an oxygen-containing sodium salt of M and an oxide of M, while the second shell consists of the first material. In both cases, the second shell and the first shell can be considered to be formed simultaneously, resulting in strong interfacial bonding.

[0099] In other embodiments of the present application, the substrate surface does not have a coating layer. In this case, the process may further include step S03: coating the particles obtained after tempering in step S02 to obtain a positive electrode material having a second shell layer. The coating material is selected from at least one of a fast ion conductor material and a conductive material; the coating process includes, but is not limited to, solid-phase coating.

[0100] In yet other embodiments, the surface of the substrate already has a coating layer, and the particles obtained after tempering in step S02 include a core and a first shell layer, wherein the first shell layer includes a first material and an oxygen-containing sodium salt of M dispersed in the first material, or an oxygen-containing sodium salt of M and an oxide of M. In this case, step S03 may also be included: coating the particles to form a second shell layer on the surface of the first shell layer, wherein the material of the second shell layer is selected from at least one of a fast ion conductor material and a conductive material. In this case, the specific selection of the material of the second shell layer may be different from that of the first material.

[0101] The embodiments of the present application also provide a positive electrode plate, comprising a current collector and a positive electrode active material layer provided on at least one side of the current collector; the positive electrode active material layer comprises the positive electrode material provided in the embodiments of the present application, or comprises the positive electrode material prepared according to the preparation method provided in the embodiments of the present application. In some embodiments, the positive electrode active material layer comprises a positive electrode active material and a binder. In some specific embodiments, the positive electrode active material layer further comprises a conductive agent. Due to the use of the positive electrode material provided in the embodiments of the present application, the positive electrode plate can be used to provide a secondary battery that can achieve higher energy density, good cycle performance, and higher energy efficiency.

[0102] In the embodiments of the present application, the current collector can be selected from any current collector known in the art suitable for use in positive electrodes. Specifically, the current collector includes, but is not limited to, metal foil or alloy foil. Specifically, the metal foil can be, but is not limited to, aluminum foil or carbon-coated aluminum foil.

[0103] In the embodiments of the present application, there are no special restrictions on the binder and conductive agent in the positive electrode active material, and they can be any known binder and conductive agent for positive electrode sheets in the art. For example, the binder includes but is not limited to at least one of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polyacrylate, polyacrylamide (PAM), polyimide (PI), etc. The conductive agent can specifically include but is not limited to at least one of acetylene black (AB), Ketjen black, Super P conductive carbon black, graphite, graphene, carbon nanotubes, carbon fibers, etc.

[0104] The present invention also provides a secondary battery comprising a negative electrode, a positive electrode provided in the present invention, and an electrolyte located between the positive and negative electrodes. Furthermore, corresponding connecting components and a circuit are provided between the positive and negative electrodes.

[0105] In the embodiment of the present application, the negative electrode plate may include a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode current collector may be any negative electrode current collector known in the art that is suitable for sodium ion batteries.

[0106] In some embodiments of the present application, the negative electrode active material layer includes a negative electrode active material, a binder, and an optional conductive agent. The negative electrode active material can be any negative electrode material known in the art suitable for sodium ion batteries, such as carbon-based materials, titanium-based materials, metal oxide materials, metal sulfide materials, etc.

[0107] In the embodiment of the present application, the binder and the conductive agent in the negative electrode active material layer can be selected from materials known in the art to be suitable for the negative electrode of sodium ion batteries.

[0108] In some embodiments of the present application, the secondary battery is a liquid battery using a liquid electrolyte. That is, the electrolyte includes an electrolyte solution. Furthermore, a separator is provided between the negative electrode and the positive electrode. The separator can be any separator known to those skilled in the art.

[0109] In some embodiments of the present application, the secondary battery is a solid-state battery using a solid-state electrolyte. That is, the electrolyte is a solid-state electrolyte. In this case, the negative electrode and / or the positive electrode may also include a solid-state electrolyte.

[0110] In some other embodiments of the present application, the secondary battery may be a semi-solid-state battery. In this case, the electrolyte may include an electrolyte solution and a solid electrolyte, or the electrolyte may be a gel electrolyte.

[0111] Due to the use of the positive electrode sheet provided in the embodiment of the present application, the secondary battery has a high first cycle efficiency and a high energy efficiency, and the secondary battery can also achieve a high energy density. In particular, the secondary battery has a high cost-effectiveness and a good market prospect.

[0112] The present application also provides an electrical device comprising the secondary battery provided in the present application. The electrical device includes, but is not limited to, consumer electronics products such as cell phones, laptops, tablet computers, drones, and wearable electronic devices. The electrical device may also include powered vehicles such as new energy vehicles and electric bicycles.

[0113] Since the secondary battery provided in the embodiment of the present application is used to power the electrical equipment, the electrical equipment has a good market prospect.

[0114] The present invention also provides an energy storage system including the secondary battery provided in the embodiment of the present invention. Due to the use of the secondary battery provided in the embodiment of the present invention, the energy storage system can have a strong energy storage capacity and good energy efficiency, and is cost-effective.

[0115] In some embodiments of the present application, the energy storage system includes an electrically connected energy storage device and a power converter, the energy storage device includes a accommodating cavity and the secondary battery accommodated in the accommodating cavity, the power converter is used to perform power conversion processing on the voltage and / or current, and input the changed voltage and / or current into the energy storage device, so that the energy storage device can meet the power requirements of the electrical equipment.

[0116] The technical solution of this application is further illustrated below with multiple embodiments.

[0117] Example 1

[0118] (1) 100 g of substrate (specifically NaNi with a D50 of 5 μm) 0.33 Fe0.33 Mn 0.33 A precursor was obtained by placing a 50-cycle deposition cycle of H2O powder (presumably a 0.015-second H2O powder) in the reaction chamber of an ALD instrument. Water and an M source (specifically, N,N'-di-tert-butylacetamidinocobalt) were then pulsed in. Each H2O pulse spray cycle lasted 0.015 seconds, and each N,N'-di-tert-butylacetamidinocobalt pulse cycle lasted 0.015 seconds. The mixture was then left to rest for 10 seconds, and then purged with nitrogen for 15 seconds. The above deposition cycle was repeated 50 times to obtain the precursor. The total mass of cobalt in the M source was 0.4 g.

[0119] (2) The precursor was tempered in an air atmosphere at a tempering temperature of 850° C. for 5 h to obtain a positive electrode material.

[0120] Example 2

[0121] (1) 100 g of substrate (specifically NaNi with a D50 of 5 μm) 0.33 Fe 0.33 Mn 0.33 O2 powder was placed in the reaction chamber of the ALD instrument. Water and an M element source (specifically, N,N'-di-tert-butylacetamidinocobalt) were pulsed separately. The H2O pulse spraying time was 0.015s per cycle, and the N,N'-di-tert-butylacetamidinocobalt pulse spraying time was 0.015s per cycle. After each pulse cycle, the system was allowed to rest for 10s, followed by a 15s nitrogen purge. This constituted one deposition cycle. A total of 100 deposition cycles were performed to obtain the precursor. The total mass of cobalt in the M element source was 0.7g.

[0122] (2) The precursor was tempered in an air atmosphere at a tempering temperature of 900° C. for 8 h to obtain a positive electrode material.

[0123] Example 3

[0124] (1) 100 g of substrate (specifically NaNi with a D50 of 5 μm) 0.33 Fe 0.33 Mn 0.33 O2 powder was placed in the reaction chamber of the ALD instrument. Water and an M element source (specifically, N,N'-di-tert-butylacetamidinocobalt) were pulsed separately. The H2O pulse spraying time was 0.015s per cycle, and the N,N'-di-tert-butylacetamidinocobalt pulse spraying time was 0.015s per cycle. After each pulse cycle, the system was allowed to rest for 10s, followed by a 15s nitrogen purge. This constituted one deposition cycle. A total of 150 deposition cycles were performed to obtain the precursor. The total mass of cobalt in the M element source was 1.2g.

[0125] (2) The precursor was tempered in an air atmosphere at a tempering temperature of 950° C. for 10 h to obtain a positive electrode material.

[0126] Example 4

[0127] (1) 100 g of substrate (specifically NaNi with a D50 of 5 μm) 0.33 Fe 0.33 Mn 0.33 O2 powder was placed in the reaction chamber of the ALD equipment. Water and an M element source (specifically, titanium tetrakis(diethylamino)) were pulsed separately. The H2O pulse spraying time was 0.015 seconds per pulse, and the titanium tetrakis(diethylamino) pulse time was 0.050 seconds per pulse. After each pulse, a 10-second pause was followed by a 10-second nitrogen purge. This constituted one deposition cycle. A total of 80 deposition cycles were performed to obtain the precursor. The total mass of titanium in the M element source was 0.7g.

[0128] (2) The precursor was tempered in an air atmosphere at a tempering temperature of 900° C. for 8 h to obtain a positive electrode material.

[0129] Example 5

[0130] (1) 100 g of substrate (specifically NaNi with a D50 of 5 μm) 0.33 Fe 0.33 Mn 0.33 O2 powder was placed in the reaction chamber of the ALD equipment. Water and an M element source (specifically, tetrakis(ethylmethylamino)zirconium) were pulsed in separately. The H2O pulse spraying time was 0.015s per cycle, and the tetrakis(ethylmethylamino)zirconium pulse time was 0.015s per cycle. This was followed by a 10s rest and a 15s nitrogen purge. This constituted one deposition cycle. A total of 75 deposition cycles were performed to obtain the precursor. The total mass of zirconium in the M element source was 0.7g.

[0131] (2) The precursor was tempered in an air atmosphere at a tempering temperature of 900° C. for 8 h to obtain a positive electrode material.

[0132] Comparative Example 1

[0133] The same substrate as in Example 1 was calcined at 900° C. in air atmosphere for 5 h.

[0134] Material characterization

[0135] (1) The positive electrode material prepared in the embodiment was observed under a transmission electron microscope (TEM), and the energy spectrum scanning of the M element was performed to test the thickness of the first shell layer of each embodiment and the doping depth of the M element. The results are summarized in Table 1. The results of Example 1 are shown in Figures 3A to 4B. Figure 3B is the energy spectrum scanning result of the Co element corresponding to Figure 3A, wherein the gray bright spots represent the Co element, and it can be seen that the Co element is enriched in the first shell region. Figure 4B is the energy spectrum scanning result of the Co element corresponding to Figure 4A, wherein it can be seen that the first shell region and the near-surface region of the core both have the enrichment of the Co element. In addition, it should be noted that the vacuum region of Figure 3A and part of the core region both have the illusion of containing the Co element, which is caused by reflection; similarly, the vacuum region and the region not doped with the M element in Figure 4B also have the illusion of containing the Co element.

[0136] (2) X-ray photoelectron spectroscopy (XPS) analysis: The cathode material prepared in Example 1 was subjected to an X-ray photoelectron spectrometer. X-rays were used to excite the solid surface, and data were processed to qualitatively analyze the elements on the surface of the cathode material. As shown in FIG5 , the results show that the surface of the cathode material contains positive tetravalent cobalt and positive trivalent cobalt.

[0137] (3) Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) test: The cathode material prepared above was placed in a TOF-SIMS instrument, and the primary ions were used to excite the sample surface, producing a very small amount of secondary ions. The data was then processed, and the ion mass and elemental composition in the cathode material were determined based on the different times it took for the secondary ions to fly to the detector due to their different masses. The results showed that CoO2 - ion fragments.

[0138] (4) Characterization of electrochemical performance of button cells

[0139] A button cell was assembled using a lithium sheet as the negative electrode. The positive electrode sheet consisted of a current collector (Al foil) and a positive electrode material layer disposed on the surface of the current collector. The composition of the positive electrode material layer was m (active material): m (conductive agent SuperP): m (binder, polyvinylidene fluoride) = 90:5:5. At 25°C, with a test voltage of 2-4V, the battery was charged to 3.95V at a constant current and voltage of 0.1C, with a constant voltage cutoff current of 0.025C, and then discharged to 2V at a constant current of 0.1C. The cycle test condition was 1C, with a nominal specific capacity of 150mA / g. The charge capacity and initial coulombic efficiency of the button cell were calculated. The above represents one cycle. After 100 cycles, the battery capacity retention was recorded and the results are summarized in Table 1. Initial coulombic efficiency = initial discharge capacity / initial charge capacity * 100%.

[0140] Table 1

[0141] Electrochemical performance characterization of soft pack batteries

[0142] (1) Prepare positive electrode sheets using the positive electrode materials provided in Example 1 and Comparative Example 1 as the positive electrode active material. Specifically, the positive electrode material, a conductive agent (specifically, Super P), and a binder (specifically, polyvinylidene fluoride, PVDF) were mixed in a mass ratio of 90:5:5, and the resulting mixture was added to a dispersant (specifically, N-methyl-2-pyrrolidone, NMP) to obtain a positive electrode slurry. The positive electrode slurry was coated on a positive electrode current collector (specifically, aluminum foil), dried, and roll-pressed to obtain a positive electrode sheet.

[0143] The above-mentioned positive electrode sheets and negative electrode sheets were assembled into a soft-pack battery with a capacity of 1.5Ah, wherein the negative electrode sheet included a negative electrode current collector (specifically an Al current collector and a negative electrode active material layer arranged on the surface of the current collector, and the negative electrode active material layer was composed of hard carbon, a binder (specifically polyacrylic acid, PAA) and a conductive agent (specifically super P) in a mass ratio of 95:2.5:2.5).

[0144] (2) Each soft-pack battery assembled as above was subjected to a charge-discharge cycle test in a constant temperature box at 25°C. Specifically, the battery was charged to 3.95V at a constant current and constant voltage of 1C, with a constant voltage cutoff current of 0.05C, and then discharged to 2V at a constant current of 1C for charge-discharge cycles, and charged and discharged at 0.2C every 50 cycles.

[0145] In addition, the discharge curves of each soft-pack battery obtained by charging and discharging at the aforementioned 1C constant current were integrated and divided by the initial discharge capacity to obtain the average voltage of each battery, i.e., the charge-discharge platform voltage. The results are shown in Figure 6. Furthermore, the discharge platform voltage retention rate of the soft-pack battery in Example 1 after 150 cycles was measured to be 99.2%, which was higher than the 97.3% of the soft-pack battery in Comparative Example 1.

[0146] The volumetric energy density of the battery at different cycle numbers was measured using an average of four batteries per group. See Figure 7 for the results. The volumetric energy density of the battery = battery capacity × charge / discharge platform voltage / volume. The results show that the volumetric energy density of the soft-pack battery in Example 1 was significantly improved at all cycle numbers.

[0147] (3) The 5-week cycle capacity retention rate of each soft-pack battery was tested in a constant temperature box at 25°C at rates of 0.1C, 0.3C, 0.5C, 1.0C, 2.0C, and 3.0C, with a voltage range of 2.0V-3.95V. Four batteries were tested in each group and the average value was taken. The results are shown in Figure 8.

[0148] Direct Current Internal Resistance (DCIR) testing: The soft-pack batteries were charged and discharged at a current density of 1C for 20 seconds at 100% and 50% SOC, with a rest period of 15 minutes between charge and discharge. Two batteries were tested in each group, and the results are summarized in Table 2. The results show that the DCIR of the soft-pack battery in Example 1 at different states of charge was lower than that of the soft-pack battery in Comparative Example 1.

[0149] Energy efficiency test: Under the voltage range of 2.0-3.95V and constant power condition of 0.5C, the battery was charged and discharged for 100 cycles, and the ratio of discharge energy to charging energy was calculated. The results are summarized in Table 3. The results in Table 3 show that the electrical impedance spectroscopy (EIS) test of the soft-pack battery of Example 1: at 25°C, the frequency range is: 100kHz~0.1Hz, the amplitude is 10Hz, and the AC impedance of each battery is tested. Please see Figure 9 for the results. It can be seen from the results of Figure 9 that the charge transfer resistance Rct of the soft-pack battery of Example 1 is significantly smaller than that of the soft-pack battery of Comparative Example 1. It can be seen that the interface impedance of the positive electrode material provided in the embodiment of the present application is small, so the rate performance of the battery can be improved. Furthermore, the results of Figure 8 show that the charge and discharge platform voltage retention rate of the soft-pack battery of Example 1 at different rates is significantly better than that of the soft-pack battery of Comparative Example 1, and as the charge and discharge rate increases, the advantages of the soft-pack battery of Example 1 become more and more obvious.

[0150] Table 2

[0151] Table 3

[0152] In the positive electrode material provided in the embodiments of the present application, one or more of the M elements (Co, Ti and Zr) are bulk-doped near the surface of the sodium-based positive electrode material having a layered oxygen structure, which can effectively reduce the interfacial impedance of the positive electrode material, improve the interfacial dynamics, improve the energy effect of the positive electrode material, and improve the cycle performance, capacity and first effect of the positive electrode material.

[0153] In the embodiment of the present application, the range value represented by "1-10" includes endpoint values ​​1 and 10. For example, the range represented by "1nm-10nm" includes endpoint value 1nm and endpoint value 10nm.

[0154] The above is an exemplary embodiment of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made thereto without departing from the principles of the present application. These improvements and modifications are also considered to be within the scope of protection of the present application.

Claims

1. A positive electrode material, characterized in that comprising an inner core and a first shell layer covering the surface of the inner core; The core is a layered oxide; The layered oxide includes Na a Ni 1-x-y-z Fe x Mn y E z O b , 0.5≤a≤1.2, 1.8≤b≤2.2, 0<x≤1, 0<y≤1, 0≤z≤0.5, and the values ​​of a, b, x, y, and z are such that the core is electrically neutral; the E element is selected from at least one of Zn, Al, Y, Ca, Li, Rb, Cs, W, Ce, Mo, Ba, Mg, Ta, Nb, V, Sc, Sr, B, F, P, and Cu; The region of the core close to the first shell layer is doped with an M element, the M element is doped in the crystal lattice of the layered oxide, and the M element is selected from at least one of Co, Ti and Zr; The material of the first shell layer includes an oxygen-containing sodium salt of M, or includes an oxygen-containing sodium salt of M and an oxide of M.

2. The positive electrode material according to claim 1, characterized in that The doping depth of the M element is 0.1 nm-20 nm.

3. The positive electrode material according to claim 2, characterized in that The doping depth of the M element is 1 nm-10 nm.

4. The positive electrode material according to any one of claims 1 to 3, characterized in that The mass content of the M element in the positive electrode material is 1000ppm-25000ppm.

5. The positive electrode material according to any one of claims 1 to 4, characterized in that In the first shell layer, the mass content of the oxygen-containing sodium salt of M is greater than the mass content of the oxide of M.

6. The positive electrode material according to claim 5, characterized in that The mass ratio of the oxygen-containing sodium salt of M to the oxide of M is ≥9:

1.

7. The positive electrode material according to any one of claims 1 to 6, characterized in that The first shell layer further comprises a first material; the oxygen-containing sodium salt of M, or the oxygen-containing sodium salt of M and the oxide of M are dispersed in the first material; The first material includes at least one of a fast ion conductor material and a conductive material.

8. The positive electrode material according to any one of claims 1 to 7, characterized in that The thickness of the first shell layer is 1 nm-20 nm.

9. The positive electrode material according to claim 7, characterized in that The positive electrode material further includes a second shell layer located on a surface of the first shell layer facing away from the core, and the second shell layer is composed of the first material.

10. The positive electrode material according to claim 9, characterized in that The sum of the thicknesses of the first shell layer and the second shell layer is 1 nm to 20 nm.

11. The positive electrode material according to any one of claims 1 to 10, characterized in that When the M element is Co, the corresponding oxygen-containing sodium salt of M is sodium cobaltate, and the corresponding oxide of M is cobalt tetroxide; When the M element is Ti, the corresponding oxygen-containing sodium salt of M is sodium titanate, and the corresponding oxide of M is titanium dioxide; When the M element is Zr, the corresponding oxygen-containing sodium salt of M is sodium zirconate, and the corresponding oxide of M is zirconium dioxide.

12. The positive electrode material according to any one of claims 1 to 11, characterized in that The D50 of the positive electrode material is 0.5 μm-50 μm.

13. A method for preparing the positive electrode material according to any one of claims 1 to 12, characterized in that: include: The substrate is placed in a reaction chamber of an atomic layer deposition device, and a reaction layer is formed on the surface of the substrate using a first raw material to obtain a precursor; wherein the substrate is a layered oxide, and the layered oxide includes Na a Ni 1-x-y-z Fe x Mn y E z O b , 0.5≤a≤1.2, 1.8≤b≤2.2, 0<x≤1, 0<y≤1, 0≤z≤0.5, and the values ​​of a, b, x, y, and z are such that the core is electrically neutral; the E element is selected from at least one of Zn, Al, Y, Ca, Li, Rb, Cs, W, Ce, Mo, Ba, Mg, Ta, Nb, V, Sc, Sr, B, F, P, and Cu; the first raw material includes water and / or ozone, and a source of the M element; The precursor is tempered in an oxygen-containing atmosphere to obtain the positive electrode material.

14. The preparation method according to claim 13, characterized in that The temperature in the reaction chamber is 150°C-300°C.

15. The preparation method according to claim 13 or 14, characterized in that: The tempering temperature is ≥850°C.

16. The preparation method according to claims 13-15, characterized in that: The tempering time is 3h-8h.

17. The preparation method according to any one of claims 13 to 16, characterized in that: The total mass of the M element in the M element source is 2000ppm-50000ppm of the total mass of the substrate.

18. The preparation method according to any one of claims 13 to 17, characterized in that: The surface of the substrate further has a coating layer, and the material of the coating layer is selected from at least one of a fast ion conductor material and a conductive material.

19. A positive electrode plate, characterized in that: It comprises a current collector and a positive electrode active material layer arranged on at least one side of the current collector; the positive electrode active material layer comprises the positive electrode material according to any one of claims 1 to 12, or the positive electrode material prepared by the preparation method according to any one of claims 13 to 18.

20. A secondary battery, characterized in that: The secondary battery comprises the positive electrode sheet as claimed in claim 19, a negative electrode sheet, and an electrolyte located between the positive electrode sheet and the negative electrode sheet.

21. An electrical device, characterized in that: The electric device includes the secondary battery according to claim 20.

22. An energy storage system, characterized in that: The energy storage system includes the secondary battery according to claim 20.

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