Positive electrode active material and preparation method and application thereof

Abstract

The invention provides a positive electrode active material and a preparation method and application thereof.The positive electrode active material comprises twin crystals, in an electron backscatter diffraction pattern of the positive electrode active material, the area ratio of the twin crystals to the positive electrode active material is 40%-60%, lithium ion diffusion is promoted, transition metal migration is inhibited, and the performance of the positive electrode active material is improved. And the electrochemical performances such as cycle performance and rate capability of the battery are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a positive electrode active material and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries are currently a commonly used energy storage tool. Their working principle is that lithium ions shuttle back and forth between the positive electrode active material and the negative electrode material.

[0003] However, due to the limitations of lithium ion diffusion rate in lithium-ion batteries, compaction density of positive electrode active material layer, transition metal migration in positive electrode active materials, electrolyte side reactions and other issues, current batteries generally have defects such as low rate performance and poor cycle performance. Summary of the Invention

[0004] The present invention provides a positive electrode active material and a preparation method and application thereof. The positive electrode active material is beneficial to promoting lithium ion diffusion, inhibiting transition metal migration, and improving the electrochemical properties of the battery, such as cycle performance and rate performance.

[0005] One aspect of the present invention provides a positive electrode active material, comprising twin crystals, wherein in an electron backscatter diffraction pattern of the positive electrode active material, an area ratio of the twin crystals to the positive electrode active material is 40% to 60%.

[0006] In one possible implementation, the positive electrode active material includes a single crystal.

[0007] In a possible implementation, in an electron backscattered diffraction pattern of the positive electrode active material, an area ratio of the single crystal to the positive electrode active material is 40% to 60%.

[0008] In one possible implementation, the positive electrode active material includes a lithium-containing positive electrode active material doped with sodium.

[0009] In a possible embodiment, the molar ratio of lithium element to sodium element in the lithium-containing positive electrode active material doped with sodium element is (8-133):1.

[0010] In one possible embodiment, the positive electrode active material includes a core and a shell located on the surface of the core, the core includes the lithium-containing positive electrode active material doped with sodium; the shell includes a carbon material and / or a compound containing an M element, and the M element includes one or more of tungsten, boron, aluminum, zirconium, titanium, magnesium, molybdenum, cerium and strontium.

[0011] In a possible embodiment, the lithium-containing positive electrode active material doped with sodium element includes one or more of a ternary positive electrode active material doped with sodium element, a lithium iron phosphate material doped with sodium element, and a lithium manganese iron phosphate material doped with sodium element.

[0012] In one possible embodiment, the ternary positive electrode active material doped with sodium element includes Li x Na y Ni a Co b Mn c D z O2, wherein the D element includes one or more of zirconium, titanium, vanadium, aluminum, and tungsten, 0.9≤x≤1.2, 0.01≤y≤0.1, 0.6≤a≤0.8, 0.05≤b≤0.1, 0.1≤c≤0.4, and 0≤z≤0.006.

[0013] In one possible embodiment, the lithium ion diffusion coefficient of the positive electrode active material is 6×10 -13 cm 2 / s~9.5×10 -13 cm 2 / s.

[0014] Another aspect of the present invention provides a method for preparing the above-mentioned positive electrode active material, comprising the following steps: mixing a raw material system including a sodium-doped lithium-containing positive electrode active material and a first lithium source, and then performing ion exchange treatment to obtain the positive electrode active material.

[0015] In one possible embodiment, the process of mixing the sodium-doped lithium-containing positive electrode active material with the raw material system of the first lithium source and then performing ion exchange treatment includes: mixing the sodium-doped lithium-containing positive electrode active material with the raw material system of the first lithium source and then performing a first sintering treatment to obtain the positive electrode active material.

[0016] In one possible embodiment, the molar ratio of the lithium element of the first lithium source to the sodium element of the sodium-doped lithium-containing positive electrode active material is 1.05:1-1.2:1; and / or the temperature of the first sintering treatment is 400°C-700°C. o C; and / or, the first sintering treatment time is 6h~8h.

[0017] In one possible embodiment, the preparation process of the sodium-doped lithium-containing positive electrode active material includes: mixing a raw material system including a second lithium source, a sodium source, and a positive electrode precursor, and then performing heat treatment to obtain the sodium-doped lithium-containing positive electrode active material.

[0018] In one possible embodiment, the molar ratio of the second lithium source to the sodium source is 1:4 to 1:1; and / or the temperature of the heat treatment is 850° C. to 950° C.; and / or the time of the heat treatment is 8 h to 12 h.

[0019] In one possible embodiment, the process of mixing the sodium-doped lithium-containing positive electrode active material with the raw material system of the first lithium source and then performing a first sintering treatment includes: mixing the sodium-doped lithium-containing positive electrode active material with the raw material system of the first lithium source and then performing a first sintering treatment to obtain an intermediate product, and mixing the intermediate product with a coating agent and then performing a second sintering treatment to obtain the positive electrode active material.

[0020] In one possible embodiment, the coating agent includes a carbon source and / or a coating agent containing an M element, and the M element includes one or more of tungsten, boron, aluminum, zirconium, titanium, magnesium, molybdenum, cerium and strontium; and / or the temperature of the second sintering treatment is 400°C~800°C; and / or the time of the second sintering treatment is 6h~12h; and / or, the process of mixing the intermediate product with the coating agent and then subjecting it to a second sintering treatment includes: washing the intermediate product with a first solvent and drying it, and then wet ball milling it with the coating agent to obtain a mixed system, and the mixed system is spray dried and subjected to a second sintering treatment in sequence to obtain the positive electrode active material.

[0021] In one possible embodiment, the temperature of the first solvent is 80°C to 100°C; and / or the washing time is 0.5h to 1h; and / or the spray drying conditions are: inlet temperature is 200°C to 220°C, and outlet temperature is 90°C to 100°C.

[0022] Another aspect of the present invention provides a positive electrode sheet, comprising the above-mentioned positive electrode active material or the positive electrode active material prepared according to the above-mentioned method for preparing the positive electrode active material.

[0023] Another aspect of the present invention provides a battery comprising the above-mentioned positive electrode sheet.

[0024] The positive electrode active material in the present invention includes twins. In the electron backscatter diffraction pattern of the positive electrode active material, the area ratio of the twins to the positive electrode active material is 40% to 60%. The positive electrode active material includes a lithium-containing positive electrode active material doped with sodium, which is beneficial to improving the structural stability of the positive electrode active material, promoting lithium ion diffusion, and inhibiting transition metal migration, thereby improving the battery's electrochemical properties such as capacity, cycle performance, and rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a scanning electron microscope image of the precursor in Example 1 of the present invention;

[0026] Figure 2 This is a scanning electron microscope image of the positive electrode active material in Example 1 of the present invention;

[0027] Figure 3This is a scanning electron microscope image of the positive electrode active material in Comparative Example 1;

[0028] Figure 4 This is a scanning electron microscope image of a cross section of the positive electrode active material in Example 1 of the present invention;

[0029] Figure 5 This is the cross-sectional electron backscatter diffraction pattern of the positive electrode active material in Example 1 of the present invention;

[0030] Figure 6 This is a high-resolution transmission electron microscopy image of the positive electrode active material in Example 1 of the present invention;

[0031] Figure 7 This is a cross-sectional scanning electron microscope image of the positive electrode active material in Comparative Example 1;

[0032] Figure 8 This is the cross-sectional electron backscatter diffraction pattern of the positive electrode active material in Comparative Example 1;

[0033] Figure 9 This is a high-resolution transmission electron microscopy image of the positive electrode active material in Comparative Example 1;

[0034] Figure 10 Graph showing rate performance of batteries in embodiments of the present invention and comparative examples;

[0035] Figure 11 Graph showing the cycle performance of batteries in the embodiments of the present invention and the comparative example. DETAILED DESCRIPTION

[0036] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0037] In the existing technology, measures such as preparing nano-sized positive electrode active materials, doping or using positive electrode active materials with larger specific surface areas (such as porous positive electrode active materials) are often adopted to increase the contact surface area between the electrolyte and the positive electrode active materials, and change the unit cell parameters of the positive electrode active materials, thereby reducing the diffusion distance of lithium ions and increasing the lithium ion transmission speed, thereby improving the battery's rate performance. However, the practical application of nano-sized positive electrode active materials or porous positive electrode active materials in high-rate batteries is hindered by problems such as electrolyte side reactions and low compaction density. These problems accelerate the dissolution of transition metals and oxygen loss in the positive electrode active materials, resulting in deterioration of battery performance such as Coulombic efficiency and cycle performance.

[0038] Based on this, an embodiment of the present invention provides a positive electrode active material including twin crystals, wherein in an electron backscatter diffraction pattern of the positive electrode active material, an area ratio of the twin crystals to the positive electrode active material is 40% to 60%.

[0039] According to the inventors' research, adjusting the unit cell parameters and crystal structure of cathode active materials can effectively improve battery rate performance. Twins (or symmetry defects within the crystal) enhance battery rate and cycling performance by optimizing ion diffusion, distributing mechanical stress, and stabilizing the structure. Specifically, the presence of twin boundaries within twins serves as a fast pathway for active ion migration, reducing the energy barrier for active ion diffusion, thereby enhancing the ionic conductivity of the cathode active material. Furthermore, twins optimize the lattice arrangement of the cathode active material, shortening the lithium ion diffusion path and improving the battery's high-rate charge and discharge capabilities. During charge and discharge, the insertion and extraction of active ions causes volume changes in the cathode active material, generating stress. Twins can adjust the grain orientation within the cathode active material, distributing local stress, reducing crack formation within the cathode active material, inhibiting particle breakage, thereby slowing capacity decay and improving battery cycling performance. The presence of twins also helps pin lattice distortion, limiting structural phase transitions in the cathode active material during cycling, maintaining structural integrity, and ultimately improving battery cycling performance. Twin boundaries can introduce localized electron states, altering the electronic structure of cathode active materials, optimizing electron transport pathways, and improving the conductivity of cathode active materials, reducing battery resistance, and enhancing rate and cycling performance. Furthermore, twins act as structural stabilizing units within cathode active materials, acting as a "breakwater" and suppressing the migration and loss of transition metals within them.

[0040] Among them, if the proportion of twins in the positive electrode active material is too high, stress concentration will easily occur at the twin boundaries, becoming the starting point of cracks and accelerating structural failure. The excessive presence of twins will weaken the overall structural integrity. Therefore, by controlling the area ratio of twins to positive electrode active materials within the range of 40%~60%, it is possible to effectively improve the ion transport performance and structural stability of the positive electrode active material, and improve the battery's rate performance and cyclability.

[0041] Illustratively, in the electron backscatter diffraction pattern of the positive electrode active material, the area ratio of the twin crystals to the positive electrode active material is 40%, 45%, 50%, 55%, 60% or a range between any two thereof.

[0042] In the embodiment of the present invention, the electron backscatter diffraction pattern of the positive electrode active material can be measured by conventional methods to obtain the area ratio of the twin crystals to the positive electrode active material. In specific implementation, the area ratio of the twin crystals to the positive electrode active material can be measured by the following process:

[0043] The TSL OIM Analysis software converts crystal orientation into Red, Green, and Blue (RGB) values by using the intensity of electron backscattered diffraction (EBSD) to determine the crystal planes of a single crystal. Different crystal planes of a single crystal correspond to different backscattered electron intensities, resulting in different colors appearing on different planes of the single crystal. EBSD also acquires orientation data and separates twin boundaries through grain boundary type analysis (defined as grain boundaries with an orientation difference of 5° or more). The corresponding grain boundary type is selected to generate a twin region mask. The TSL OIM Analysis software automatically calculates the pixel ratio of the twin region and converts it into an area fraction.

[0044] The positive electrode active material in the present application is in a particle form, that is, the positive electrode active material particles include twin crystals.

[0045] In some embodiments, the positive electrode active material includes a single crystal. The single crystal structure has no grain boundaries, which can avoid grain boundary cracking caused by anisotropic volume changes during battery charging and discharging, reduce side reactions caused by electrolyte penetration along the grain boundaries, and help further improve the cycle life and other performance of the positive electrode active material.

[0046] In some embodiments, in the electron backscatter diffraction pattern of the positive electrode active material, the area ratio of the single crystal to the positive electrode active material is 40% to 60%. For example, it can be 40%, 45%, 50%, 55%, 60% or any range therebetween, which is beneficial to further improve the ion transport performance and structural stability of the positive electrode active material.

[0047] In some embodiments, the positive electrode active material includes a lithium-containing positive electrode active material doped with sodium. In the above system, through the synergistic effect of sodium doping and twinning in the lithium-containing positive electrode active material doped with sodium, a "bridge" is constructed between different lithium ion diffusion channels (including two-dimensional diffusion channels of transition metal layers and twin lattice diffusion channels, etc.), so that the positive electrode active material has multi-dimensional lithium ion diffusion channels, which is beneficial to increase the transmission speed of lithium ions and thereby improve the rate performance of the battery. The area ratio of twins to positive electrode active materials is 40% to 60%, which is beneficial to improve the structural stability of the positive electrode active material and improve the cycle performance and other properties of the battery.

[0048] In some embodiments, the molar ratio of lithium to sodium in the lithium-containing positive electrode active material doped with sodium is (8~133):1, for example, 8:1, 10:1, 20:1, 30:1, 50:1, 70:1, 90:1, 100:1, 120:1, 133:1 or a range consisting of any two thereof, which is beneficial to further improve the lithium ion diffusion coefficient of the positive electrode active material, further inhibit the migration and loss of transition metals in the positive electrode active material, improve the structural stability of the positive electrode material, and further take into account the improvement of the battery's performance such as rate and cycle performance.

[0049] In some embodiments, the positive electrode active material includes a core and a shell located on the surface of the core, the core includes a lithium-containing positive electrode active material doped with sodium; the shell includes a carbon material and / or a compound containing the element M, and the element M includes one or more of tungsten (W), boron (B), aluminum (Al), zirconium (Zr), titanium (Ti), magnesium (Mg), molybdenum (Mo), cerium (Ce) and strontium (Sr), which is beneficial to further improve the electrical conductivity and structural stability of the positive electrode active material, reduce the dissolution of transition metals, and further improve the battery's first coulombic efficiency, rate and cycle performance.

[0050] Specifically, when the M element includes one or more of tungsten, aluminum, zirconium, titanium, magnesium, molybdenum, cerium and strontium, the compound containing the M element includes an oxide containing the M element, and the oxide containing the M element includes one or more of tungsten oxide, aluminum oxide, zirconium oxide, titanium oxide, magnesium oxide, molybdenum oxide, cerium oxide and strontium oxide.

[0051] Specifically, when the M element includes boron, the compound containing the M element includes lithium borate.

[0052] In some embodiments, the lithium-containing positive electrode active material doped with sodium element includes one or more of a ternary positive electrode active material doped with sodium element, a lithium iron phosphate material doped with sodium element, and a lithium manganese iron phosphate material doped with sodium element.

[0053] In some embodiments, the ternary positive electrode active material doped with sodium element includes Li x Na y Ni a Co b Mn c D z O2, wherein the D element may include one or more of zirconium, titanium, vanadium, aluminum, and tungsten, 0.9≤x≤1.2, 0.01≤y≤0.1, 0.6≤a≤0.8, 0.05≤b≤0.1, 0.1≤c≤0.4, and 0≤z≤0.006, which is beneficial to further improve the diffusion rate of lithium ions and improve the structural stability of the positive electrode active material, thereby improving the battery's performance such as rate and cycle stability.

[0054] Specifically, x can be 0.9, 0.95, 1.0, 1.05, 1.1, 1.15, 1.2 or a range consisting of any two thereof, y can be 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1 or a range consisting of any two thereof, a can be 0.6, 0.65, 0.7, 0.75, 0.8 or a range consisting of any two thereof, b can be 0.05, 0.06, 0.07, 0.08, 0.09, 0.1 or a range consisting of any two thereof, c can be 0.1, 0.2, 0.3, 0.4 or a range consisting of any two thereof, and z can be 0, 0.001, 0.002, 0.003, 0.004, 0.005, 0.006 or a range consisting of any two thereof.

[0055] In some embodiments, the ternary positive electrode active material doped with sodium element includes Li 1.045 Na 0.131 Ni 0.688 Co 0.090 Mn 0.222 O2、Li 1.075 Na 0.027 Ni 0.688 Co 0.091 Mn 0.221 O2、Li 1.070 Na 0.016 Ni 0.688 Co 0.091 Mn 0.221 O2、Li 1.07 9Na 0.031 Ni0. 688 Co 0.090 Mn 0.222 O2、Li 1.074 Na 0.025 Ni 0.688 Co 0.089 Mn 0.223 O2、Li 1.065 Na 0.008 Ni 0.688 Co 0.089 Mn 0.223 O2、Li 1.078 Na 0.009 Ni 0.688 Co 0.090 Mn 0.222 One or more of O2.

[0056] In some other embodiments, the core may be doped with D element, which includes transition metal elements. For example, D element may be one or more of zirconium, titanium, vanadium, aluminum, and tungsten. The core of the ternary positive electrode active material doped with sodium element includes Li 1.075 Na 0.027 Ni 0.688 Co 0.091 Mn 0.221 Zr 0.004 O2.

[0057] In some embodiments, the lithium ion diffusion coefficient of the positive electrode active material is 6×10 -13 cm 2 / s~9.5×10 - 13 cm 2 / s, for example, it can be 6×10 -13 cm 2 / s, 7×10 -13 cm 2 / s, 8×10 -13 cm 2 / s, 9×10 -13 cm 2 / s, 9.5×10 -13 cm 2 / s or any range between them. When the lithium ion diffusion coefficient is in the above range, it is beneficial to promote the diffusion speed of lithium ions and improve the battery's rate performance and other performance.

[0058] In the present embodiment, the lithium ion diffusion coefficient of the positive electrode active material is measured using the galvanostatic intermittent titration technique (GITT). The overall GITT test process is composed of a series of "pulse-galvanostatic-relaxation" cycles. Each "pulse-galvanostatic-relaxation" cycle applies a constant current to charge / discharge the battery for a certain period of time, then disconnects the current while recording the voltage change throughout the process. The corresponding diffusion coefficient can be further calculated from the GITT test data using the following formula:

[0059]

[0060] Where D is the lithium ion diffusion coefficient, m B is the mass of active substance, V m is the molar volume of the electrode material, M B is the relative molecular mass of the material, S is the effective surface area of the electrode in contact with the electrolyte, τ is the relaxation time, ΔE t is the change in battery voltage during the charge / discharge process, ΔE s Voltage change during the relaxation phase, t is the pulse time, and L is the thickness of the electrode.

[0061] In some embodiments, the alkali content of the positive electrode active material is 2000 ppm to 3000 ppm, for example, 2000 ppm, 2200 ppm, 2400 ppm, 2600 ppm, 2800 ppm, 3000 ppm, or any range therebetween.

[0062] The alkali content of the positive electrode active material is the ratio of the mass of the alkali in the positive electrode active material to the total mass of the positive electrode active material. Specifically, the mass of the alkali in the positive electrode active material includes the mass of lithium carbonate and lithium hydroxide.

[0063] An embodiment of the present invention further provides a method for preparing the above-mentioned positive electrode active material, comprising the following steps: mixing a raw material system including a sodium-doped lithium-containing positive electrode active material and a first lithium source, and then performing ion exchange treatment to obtain the positive electrode active material.

[0064] Specifically, in the above system, the lithium element in the first lithium source undergoes ion exchange with the sodium element in the sodium-doped lithium-containing positive electrode active material, generating twins while introducing sodium. This dual-functional structural modification of twins and sodium doping is beneficial for improving the lithium ion diffusion coefficient of the positive electrode active material, enhancing the battery's rate and cycle performance, and other performance. For example, when the sodium-doped lithium-containing positive electrode active material is a sodium-doped lithium-containing ternary positive electrode active material, the sodium-doped lithium-containing ternary positive electrode active material contains a NaEO2 (E is selected from one or more of Ni, Co, and Mn) structure. The lithium element in the first lithium source will replace the sodium element in the NaEO2, forming twins that exist between multiple agglomerated single crystals.

[0065] In some embodiments, the process of mixing a sodium-doped lithium-containing positive electrode active material with a raw material system of a first lithium source and then performing an ion exchange treatment includes: mixing the sodium-doped lithium-containing positive electrode active material with a raw material system of a first lithium source and then performing a first sintering treatment to obtain a positive electrode active material.

[0066] In the above system, the twins generated after the sodium-doped lithium-containing positive electrode active material is mixed with the first lithium source and subjected to the first sintering treatment may be annealing twins rather than deformation twins. Specifically, during the first sintering treatment, the temperature reaches above the recrystallization temperature. The twins generated at this time are twins formed by recrystallization or grain growth, and the twins are annealing twins.

[0067] Specifically, the positive electrode active material can be prepared by grinding and mixing a raw material system including a sodium-doped lithium-containing positive electrode active material and a first lithium source under an inert gas atmosphere, followed by a first sintering process. Exemplarily, the inert gas includes argon and / or nitrogen.

[0068] In some embodiments, the first lithium source may include a lithium salt and / or a lithium-containing hydroxide. The lithium salt may include one or more of lithium chloride, lithium nitrate, and lithium dihydrogen phosphate. The lithium-containing hydroxide may include lithium hydroxide.

[0069] In some embodiments, the molar ratio of the lithium element of the first lithium source to the sodium element of the sodium-doped lithium-containing positive electrode active material is 1.05:1~1.2:1, for example, 1.05:1, 1.08:1, 1.10:1, 1.13:1, 1.15:1, 1.18:1, 1.2:1 or a range composed of any two of them, which is beneficial for increasing the lithium element content in the positive electrode active material while retaining the sodium element doping content, thereby improving the lithium ion diffusion coefficient of the positive electrode active material, and improving the electrochemical performance of the battery, such as the rate capability.

[0070] In some embodiments, the first sintering treatment is performed at a temperature of 400° C. to 700° C., such as 400° C., 450° C., 500° C., 550° C., 600° C., 650° C., 700° C., or any two thereof, for 6 hours.

[0071] Specifically, the temperature of the first sintering process can be 650°C to 700°C, for example, 650 o C, 660°C, 670°C, 680°C, 690°C, 700°C or a range consisting of any two of them. When the temperature of the first sintering treatment is within the above range, the first lithium source is in a molten state, which is more conducive to ion exchange between the lithium element in the first lithium source and the sodium element in the sodium-doped lithium-containing positive electrode active material, and is more conducive to the formation of twins.

[0072] In some embodiments, the preparation process of the sodium-doped lithium-containing positive electrode active material includes: mixing a raw material system including a second lithium source, a sodium source, and a positive electrode precursor, and then performing heat treatment to obtain the sodium-doped lithium-containing positive electrode active material.

[0073] Specifically, the atmosphere for the heat treatment may be an oxygen atmosphere, the sodium source may include sodium carbonate, and the second lithium source may include lithium carbonate.

[0074] In some embodiments, the positive electrode precursor includes a ternary precursor, the ternary precursor includes a nickel-cobalt-manganese ternary precursor, and the nickel-cobalt-manganese ternary precursor may include Ni 0.68 Co 0.08 Mn 0.24 (OH)2.

[0075] The average particle size D of the sodium-doped lithium-containing positive electrode active material in the embodiment of the present invention is 50 In a specific implementation, the product obtained by the above heat treatment can be crushed (such as air flow crushing) to obtain a sodium-doped lithium-containing positive electrode active material with a preset particle size.

[0076] In the embodiment of the present invention, the average particle size D of the sample is 50 Indicates the particle size distribution based on volume, the particle size at which the volume accumulation reaches 50% from the smallest particle size side, i.e. Dv 50 For example, the average particle size D of the sodium-doped lithium-containing positive electrode active material is 50 It indicates the particle size of the sodium-doped lithium-containing positive electrode active material particles reaching 50% of the cumulative volume from the smallest particle size side in the volume-based particle size distribution.

[0077] In some embodiments, the molar ratio of the second lithium source to the sodium source is 1:4~1:1, for example, 1:4, 1:3.5, 1:3, 1:2.5, 1:2, 1:1.5, 1:1 or a range consisting of any two of them. This ratio range helps to form a positive electrode active material with a twin structure area accounting for 40%~60% and sodium doping to varying degrees, thereby improving the ion transport performance and structural stability of the positive electrode active material.

[0078] In some embodiments, the heat treatment temperature is 850 o C-950 o C, for example, can be 850 o C. 870 o C.890 o C. 910 o C. 930 o C. 950 o C or the range between any two thereof; the heat treatment time is 8h~12h, for example, it can be 8h, 9h, 10h, 11h, 12h or the range between any two thereof.

[0079] In specific implementation, the heat treatment process includes: heating to the heat treatment temperature at a heating rate of 1°C / min to 5°C / min, and then performing heat treatment at the heat treatment temperature, wherein the heating rate can be 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min or a range between any two of them.

[0080] In some embodiments, the process of mixing a sodium-doped lithium-containing positive electrode active material with a raw material system of a first lithium source and then performing a first sintering treatment includes: mixing a sodium-doped lithium-containing positive electrode active material with a raw material system of a first lithium source and then performing a first sintering treatment to obtain an intermediate product, mixing the intermediate product with a coating agent and then performing a second sintering treatment to obtain a positive electrode active material.

[0081] In the above system, the intermediate product is used to form the core of the positive electrode active material, and the coating agent forms a shell located on the surface of the core, which is beneficial to further improve the electrical conductivity and structural stability of the positive electrode active material, reduce the dissolution of transition metals, and further improve the battery's first coulombic efficiency, rate and cycle performance.

[0082] In some embodiments, the coating agent includes a flexible coating agent, which includes a carbon source and / or a coating agent containing an M element, and the M element includes one or more of tungsten, boron, aluminum, zirconium, titanium, magnesium, molybdenum, cerium and strontium, which is beneficial to further improve the electrical conductivity and structural stability of the positive electrode active material, while taking into account the improvement of the battery's sub-coulombic efficiency, rate and cycle performance.

[0083] Specifically, when the M element includes one or more of tungsten, aluminum, zirconium, titanium, magnesium, molybdenum, cerium and strontium, the coating agent containing the M element includes an oxide coating agent containing the M element and / or a hydroxide coating agent containing the M element, the oxide coating agent containing the M element includes one or more of tungsten oxide, aluminum oxide, zirconium oxide, titanium oxide, magnesium oxide, molybdenum oxide, cerium oxide and strontium oxide, and the hydroxide coating agent containing the M element includes one or more of tungsten hydroxide, aluminum hydroxide, zirconium hydroxide, titanium hydroxide, magnesium hydroxide, molybdenum hydroxide, cerium hydroxide and strontium hydroxide.

[0084] Specifically, when the M element includes boron, the coating agent containing the M element includes lithium borate.

[0085] In some embodiments, the carbon source may include one or more of monosaccharides, disaccharides, and polysaccharides, and the polysaccharides may include one or more of sucrose, glucose, starch, and cellulose.

[0086] In some embodiments, the process of mixing the intermediate product with the coating agent and then subjecting it to a second sintering treatment includes: washing the intermediate product with a first solvent and drying it, wet ball milling it with the coating agent to obtain a mixed system, and spray drying and subjecting the mixed system to a second sintering treatment in sequence to obtain a positive electrode active material.

[0087] Specifically, the first solvent may include water.

[0088] In a specific implementation, the intermediate product is washed with a first solvent, and the resulting precipitate is dried to obtain a dried material. The dried material is then wet-ball milled with a coating agent to obtain a mixed system. The mixed system is then spray-dried and subjected to a second sintering treatment to obtain the positive electrode active material. The washing of the intermediate product with the first solvent can be performed by conventional methods in the art, such as placing the intermediate product in the first solvent and then ultrasonically treating it. The drying of the resulting precipitate can also be performed by conventional methods in the art, such as drying in an oven.

[0089] Generally, the process of wet ball milling the dried material and the coating agent includes dispersing the dried material and the coating agent in a second solvent and wet ball milling. The second solvent may include water. For example, when the coating agent is a carbon source, the carbon source may be dispersed in water to obtain a carbon source solution, and the dried material is then added to the solution, mixed, and wet ball milled.

[0090] In some embodiments, the temperature of the first solvent is 80°C~100°C, for example, 80°C, 85°C, 90°C, 95°C, 100°C or a range consisting of any two thereof, and the washing time is 0.5h~1h, for example, 0.5h, 0.6h, 0.7h, 0.8h, 0.9h, 1.0h or a range consisting of any two thereof. In the aforementioned system, it is beneficial to remove the lithium remaining on the surface of the intermediate product and the sodium element in the first lithium source that undergoes ion exchange with the sodium element in the sodium-doped lithium-containing positive electrode active material.

[0091] In some embodiments, the spray drying conditions are: an inlet temperature of 200°C to 220°C, for example, 200°C, 205°C, 210°C, 215°C, 220°C, or a range consisting of any two thereof, and an outlet temperature of 90°C to 100°C, for example, 90°C, 92°C, 95°C, 98°C, 100°C, or a range consisting of any two thereof, which can remove moisture from the mixed system, help improve the uniformity of the particle size of the material after the mixed system is spray-dried, further improve the lithium ion diffusion coefficient of the positive electrode active material, and improve the battery's performance such as rate.

[0092] In some embodiments, the temperature of the second sintering treatment is 400°C~800°C, for example, 400°C, 500°C, 600°C, 700°C, 800°C or a range composed of any two of them, which is conducive to uniform coating of the coating agent, improves the coating effect, and further improves the structural stability of the positive electrode active material and the cycle performance of the battery.

[0093] In addition, the second sintering treatment time is 6 hours to 12 hours, for example, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours or a range consisting of any two thereof.

[0094] An embodiment of the present invention further provides a positive electrode sheet, comprising the above-mentioned positive electrode active material or the positive electrode active material prepared according to the above-mentioned preparation method of the positive electrode active material, which has the same advantages as the above-mentioned positive electrode active material and will not be elaborated here.

[0095] Specifically, the positive electrode sheet includes a positive electrode active material layer, and the positive electrode active material layer includes the positive electrode active material (positive electrode active material) or the positive electrode active material prepared according to the preparation method of the positive electrode active material.

[0096] Specifically, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer (or positive electrode active material layer) located on at least one side surface of the positive electrode current collector. Specifically, the positive electrode active material layer can be provided on one side surface of the positive electrode current collector, or the positive electrode active material layer can be provided on the surfaces of the opposite sides in the thickness direction of the positive electrode current collector.

[0097] Specifically, the positive electrode active material layer may also include a positive electrode conductor and a positive electrode binder, both of which may be conventional materials in the art. For example, the positive electrode conductor may include one or more of conductive carbon black, conductive graphite, carbon nanotubes (CNTs), acetylene black, Ketjen black, and carbon fibers; the positive electrode binder may include one or more of polyvinylidene fluoride (PVDF), polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, polyvinyl chloride, carboxylated polyvinyl chloride, polymers containing ethylene oxide, polyvinyl pyrrolidone, and polyurethane.

[0098] The positive electrode current collector in the embodiment of the present invention may be a conventional positive electrode current collector in the art. For example, the positive electrode current collector may include aluminum foil.

[0099] In embodiments of the present invention, the positive electrode sheet can be prepared by conventional methods in the art, such as a coating method. Specifically, the components used to form the positive electrode active material layer, such as the positive electrode active material, positive electrode conductive agent, and positive electrode binder, can be dispersed in a positive electrode solvent to prepare a positive electrode slurry, which is then coated on the surface of the positive electrode current collector. After drying and roller pressing, the positive electrode sheet is prepared. The positive electrode solvent includes, for example, N-methylpyrrolidone (NMP). The coating, drying, and roller pressing steps involved are conventional operations for preparing positive electrode sheets using a coating method and are not particularly limited thereto.

[0100] An embodiment of the present invention further provides a battery, including the above-mentioned positive electrode sheet, which has corresponding advantages to the above-mentioned positive electrode sheet and will not be described in detail here.

[0101] The battery of the present invention may be a lithium ion battery.

[0102] Generally speaking, a battery consists of a cell, an electrolyte, and a casing that encapsulates the cell. The electrolyte is injected into the cell within the casing. The cell includes a positive electrode sheet, a negative electrode sheet, and a separator located between the positive and negative electrodes. The cell can be a wound cell and / or a stacked cell.

[0103] The battery in the embodiment of the present invention can be prepared by conventional methods in the field. For example, the positive electrode sheets, separators, and negative electrode sheets can be stacked in an alternating manner to produce a laminated battery cell; or the positive electrode sheets, separators, and negative electrode sheets can be wound in sequence to produce a wound battery cell. The battery cell is then placed in a casing and subjected to conventional processes such as liquid injection (i.e., injection of electrolyte), packaging, standing, formation, and capacity separation to produce a battery.

[0104] Generally, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer (or negative electrode active material layer) located on at least one side surface of the negative electrode current collector. Specifically, the negative electrode active material layer can be provided on one side surface of the negative electrode current collector, or the negative electrode active material layer can be provided on both sides of the negative electrode current collector in the thickness direction.

[0105] Specifically, the negative electrode active material layer may include a negative electrode active material, a negative electrode conductor and a negative electrode binder, all of which may be conventional materials in the art. For example, the negative electrode active material may include graphite; the negative electrode binder may include one or more of sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyvinyl alcohol, and sodium polyacrylate; the negative electrode conductor may include one or more of conductive carbon black, carbon nanotubes (CNTs), acetylene black, graphene, Ketjen black, and carbon fibers.

[0106] The negative electrode current collector in the embodiment of the present invention may be a conventional negative electrode current collector in the art, for example, the negative electrode current collector includes copper foil.

[0107] The negative electrode sheet of the present invention can be produced by conventional methods in the art, such as coating. Specifically, the components used to form the negative electrode active material layer, such as the negative electrode active material, negative electrode conductive agent, and negative electrode binder, are dispersed in a negative electrode solvent to prepare a negative electrode slurry, which is then coated on the surface of the negative electrode current collector. After drying and roller pressing, the negative electrode sheet is produced. The negative electrode solvent includes, for example, water. The coating, drying, and roller pressing steps involved are conventional operations for producing negative electrode sheets using a coating method and are not particularly limited thereto.

[0108] In an embodiment of the present invention, the diaphragm is located between the positive electrode sheet and the negative electrode sheet, and is used to separate the positive electrode sheet and the negative electrode sheet to prevent the positive electrode sheet and the negative electrode sheet from direct contact and short circuit. The diaphragm in the embodiment of the present invention can adopt conventional diaphragms in the field. For example, the diaphragm includes one or more of polypropylene film (PP film) and polyethylene (PE), but is not limited thereto.

[0109] In the embodiment of the present invention, the electrolyte may be a non-aqueous electrolyte, which generally includes a lithium salt, a carbonate solvent, and an additive. The carbonate solvent may include one or more of a cyclic carbonate solvent and a chain carbonate solvent, specifically including one or more of ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), propylene carbonate (PC), ethyl propionate (EP), and fluoroethylene carbonate (FEC). The lithium salt may include one or more of lithium hexafluorophosphate (LiPF6) and lithium perchlorate. The additive may include one or more of fluoroethylene carbonate, difluoroethylene carbonate, fluorocyclotriphosphazene, hexafluorocyclotriphosphazene, and vinylene carbonate (VC).

[0110] The shell for encapsulating the battery cell in the embodiment of the present invention can be made of conventional shell materials in the art to encapsulate the battery cell. The shell includes, for example, soft packaging materials such as aluminum plastic film (the battery in this case is a soft packaging battery), but is not limited thereto.

[0111] The present invention is further described below through specific examples.

[0112] The average particle size D50 of the sodium-doped lithium-containing positive electrode active materials in the following examples and comparative examples is 3.6±0.2 μm.

[0113] Example 1

[0114] (1) Synthesis of positive electrode active materials

[0115] Ni 0.68 Co 0.08 Mn 0.24 The (OH)2 precursor, lithium carbonate, and sodium carbonate were mixed in a molar ratio of 1:0.15:0.4 and then heated to 930°C at a heating rate of 2°C / min for 12 hours. The mixture was then naturally cooled and pulverized by jet milling to obtain a sodium-doped lithium-containing cathode active material. The average particle size D50 of the sodium-doped lithium-containing cathode active material was 3.6±0.2 μm. The mixing speed was 800 rpm / min and the mixing time was 40 minutes.

[0116] Under an argon inert atmosphere, the sodium-doped lithium-containing positive electrode active material and lithium chloride are fully ground and then subjected to a first sintering treatment to obtain an intermediate product, wherein the molar ratio of the lithium element in the lithium chloride to the sodium element in the sodium-doped lithium-containing positive electrode active material is 1.1:1, and the temperature of the first sintering treatment is 700°C and the time is 6 hours.

[0117] The intermediate product was placed in 80°C water and ultrasonicated for 0.5h, and the precipitate was collected and dried to obtain a dried material; the dried material was mixed with 1000 ppm tungsten oxide, 1000 ppm titanium oxide, and 1000 ppm strontium oxide (i.e., the mass ratio of the dried material to tungsten oxide, titanium oxide, and strontium oxide was 1:0.001:0.001:0.001) and then wet-milled for 1h to obtain a mixed system (i.e., the material after wet milling). The average particle size of the particles in the mixed system was 3.6±0.2 μm; the mixed system was spray-dried and subjected to a second sintering treatment to obtain a positive electrode active material. The spray-drying treatment conditions were an inlet temperature of 200°C, an outlet temperature of 90°C, a second sintering treatment temperature of 800°C, and a second sintering treatment time of 12h.

[0118] (2) Preparation of positive electrode

[0119] The positive electrode active material, positive electrode conductive agent SUP-P, and positive electrode binder PVDF in (1) are mixed in a mass ratio of 94.5:2.5:3, and NMP is added and stirred evenly to prepare a positive electrode slurry;

[0120] The positive electrode slurry is coated on the front and back surfaces of the aluminum foil. After drying and roller pressing, a positive electrode coating is formed on the front and back surfaces of the aluminum foil to produce a positive electrode sheet.

[0121] (3) Preparation of negative electrode sheet

[0122] Graphite, negative electrode conductive agent SUP-P, negative electrode binder CMC and SBR are mixed in a mass ratio of 95.5:1:1.5:2.0, and water is added and stirred evenly to prepare a negative electrode slurry; the negative electrode slurry is coated on the front and back surfaces of the copper foil, and after drying and rolling, a negative electrode coating is formed on the front and back surfaces of the copper foil to prepare a negative electrode sheet.

[0123] (4) Battery assembly

[0124] The positive electrode sheets, PP separators and negative electrode sheets are stacked in an alternating manner to produce a laminated battery cell. The battery cell is then placed in a shell and undergoes conventional processes such as packaging, standing, formation, and capacity separation to produce a battery.

[0125] Example 2

[0126] The difference from Example 1 is that Ni 0.68 Co 0.08 Mn 0.24 The (OH)2 precursor, lithium carbonate and sodium carbonate were mixed in a molar ratio of 1:0.275:0.275, and the other steps and conditions were consistent with those in Example 1.

[0127] Example 3

[0128] The difference from Example 1 is that lithium hydroxide is used instead of lithium chloride, and the other steps and conditions are consistent with Example 1.

[0129] Example 4

[0130] The difference from Example 1 is that tungsten oxide, titanium oxide and strontium oxide are not added, the dried material and 1000 ppm glucose are mixed and wet-milled for 1 hour to obtain a mixed system, and the remaining steps and conditions are consistent with Example 1.

[0131] Example 5

[0132] The difference from Example 1 is that tungsten oxide, titanium oxide and strontium oxide are not added, and the dried material is directly wet-milled for 1 hour to obtain a mixed system. The remaining steps and conditions are consistent with Example 1.

[0133] Example 6

[0134] The difference from Example 1 is that Ni 0.68 Co 0.08 Mn 0.24 The (OH)2 precursor, lithium carbonate and sodium carbonate were mixed in a molar ratio of 1:0.4:0.15, and the remaining steps and conditions were consistent with those in Example 1.

[0135] Example 7

[0136] The difference from Example 1 is that Ni 0.68 Co 0.08 Mn 0.24 The (OH)2 precursor, lithium carbonate and sodium carbonate were mixed in a molar ratio of 1:0.05:0.5, and the remaining steps and conditions were consistent with those in Example 1.

[0137] Example 8

[0138] The difference from Example 1 is that Ni 0.68 Co 0.08 Mn 0.24 After the (OH)2 precursor, lithium carbonate and sodium carbonate were mixed in a molar ratio of 1:0.15:0.4, 4000 ppm of zirconium oxide was added, and the remaining steps and conditions were consistent with Example 1.

[0139] Comparative Example 1

[0140] Ni 0.68 Co 0.08 Mn 0.24The (OH)2 precursor and lithium carbonate were mixed at a molar ratio of 1:0.55 and then heated to 930°C at a heating rate of 2°C / min for 12 hours. The mixture was then naturally cooled and pulverized by jet milling to obtain a lithium-containing positive electrode active material with an average particle size D50 of 3.6±0.2μm. The mixing speed was 800 rpm / min and the mixing time was 40 minutes.

[0141] The lithium-containing positive electrode active material was placed in 80°C water and ultrasonicated for 0.5h, and the precipitate was collected and dried to obtain a dried material; the dried material was mixed with 1000 ppm tungsten oxide, 1000 ppm titanium oxide, and 1000 ppm strontium oxide and wet-milled for 1h to obtain a mixed system (i.e., the material after wet-milling), and the average particle size of the mixed system was 3.6±0.2 μm; the mixed system was spray-dried and subjected to a second sintering treatment to obtain a positive electrode active material. The spray-drying treatment conditions were an inlet temperature of 200°C, an outlet temperature of 90°C, a second sintering treatment temperature of 800°C, and a second sintering treatment time of 12h.

[0142] The remaining conditions and steps remained the same as in Example 1.

[0143] The performance tests of the precursors, positive electrode active materials and batteries in the examples and comparative examples were carried out through the following processes. The results are shown in Table 1, Table 2, Table 3, Figures 1 to 11 :

[0144] (1) Scanning electron microscope (SEM) test: Prepare a sample on the conductive adhesive, find the appropriate sample particles by SEM test, adjust the magnification to about 100K, first make a rough adjustment, then make a fine adjustment, after the clarity is achieved, adjust to 50K and adjust the brightness, and test after the contrast meets the requirements, and test at 50K, 30K, 10K, 5K, 2K, and 1K in sequence. The scanning electron microscope image of the precursor in Example 1 is shown in FIG. Figure 1 (The scanning electron microscope images of the precursors of other embodiments are similar to those of embodiment 1); the scanning electron microscope images of the positive electrode active material in embodiment 1 are shown in FIG. Figure 2 (The scanning electron microscope images of the positive electrode active materials of other embodiments are similar to those of Example 1); the scanning electron microscope images of the positive electrode active materials in Comparative Example 1 are shown in FIG. Figure 3 .

[0145] (2) Cross-sectional SEM and cross-sectional EBSD test: stick the conductive adhesive to the dedicated sample stage of the ion milling instrument, click on the ion beam energy on the milling interface, input the ion beam energy (0.1 keV ~ 8 keV), click on the milling interface, the milling speed is generally set to 1.0 RPM, rotate the angles of the left and right ion guns, and start cross-sectional processing. After the cross-sectional processing is completed, perform cross-sectional SEM and cross-sectional EBSD tests. EBSD test: set the appropriate voltage / current in the SEM, adjust the focal length to obtain a clear image, scan the sample area through the Mapping function, generate an orientation distribution map, use EBSD to obtain orientation data, separate the twin boundaries through grain boundary type analysis (such as setting an orientation difference of 5° or more as a grain boundary), and use TSL OIM Analysis software to convert the crystal orientation into RGB values. The cross-sectional scanning electron microscope image of the positive active material in Example 1 is shown in FIG. Figure 4 ; The cross-sectional electron backscatter diffraction pattern of the positive electrode active material in Example 1 is shown in Figure 5 ; Comparative Example 1, a cross-sectional scanning electron microscope image of the positive electrode active material is shown Figure 7 ; Comparative Example 1, the cross-section electron backscatter diffraction pattern of the positive electrode active material is shown Figure 8 .

[0146] (3) High-resolution transmission electron microscopy test: The positive electrode active material is dispersed on the ultra-thin carbon film, the current and voltage center of the objective lens are adjusted to optimize the focus, the target electron microscopy morphology is found, and different magnification tests are performed. The high-resolution transmission electron microscopy image of the positive electrode active material in Example 1 is shown in FIG. Figure 6 ; Comparative Example 1 high resolution transmission electron microscopy of the positive electrode active material is shown in Figure 9 .

[0147] (4) Rate performance (0.1C, 0.33C, 0.5C, 1C, 2C and 3C discharge capacity) test: voltage range 4.4-3.0V, 0.1C charge / 0.1C discharge, 0.33C charge / 0.33C discharge, 0.33C charge / 0.5C discharge, 0.33C charge / 1.0C discharge, 0.33C charge / 2.0C discharge, 0.33C charge / 3.0C discharge, discharge capacity test, the results are shown in Table 3, among which, the curves of capacity retention rate (Rate) of Examples 1 to 4 and Comparative Example 1 as a function of discharge rate are shown in Table 3. Figure 10 For example, when discharging at a 0.33C rate, the capacity retention rate = 0.33C discharge capacity / 0.33C discharge capacity; when discharging at a 0.5C rate, the capacity retention rate = 0.5C discharge capacity / 0.33C discharge capacity.

[0148] (5) Cyclic performance (capacity retention) test: 400 cycles of 1C charge / 1C discharge at a voltage range of 4.4-3.0V and 45°C. The discharge capacity at the first week is recorded as Q1, and the discharge capacity at the 400th week is recorded as Q2. The capacity retention is calculated as Q2 / Q1. 100%, the results are shown in Table 3, wherein the curves of the cycle capacity retention rate (CapacityRetention) of Examples 1 to 4 and Comparative Example 1 as a function of the cycle number (Cycle Number) are shown in Table 3. Figure 11 .

[0149] (6) Elemental composition (the molar ratio of lithium to sodium in the lithium-containing positive electrode active material doped with sodium, and the molar content of Li, Na, Ni, Co and Mn elements) test: Use a 1 / 10,000 balance to weigh 0.1000g±0.0100g (accurate to 0.0001g) of the positive electrode active material (i.e., sample) into a beaker. First, add 7.5mL of high-grade pure hydrochloric acid and 2.5mL of high-grade pure nitric acid into the beaker. Rinse the inner wall of the beaker with pure water and cover it with a watch glass. Place it on a 195℃ heating plate for digestion for 30min. Remove it and let it cool slightly, then rinse the watch glass with pure water. After cooling, transfer it to a 500mL volumetric flask. Use a pipette to add 2.5mL of 1g / L yttrium standard into the volumetric flask, and then dilute to 500mL with pure water. The inlet tube was placed in the sample solution and analyzed using inductively coupled plasma mass spectrometry (ICP). After the analysis, the inlet tube was placed in pure water for cleaning for more than 20 seconds. The spectrum was processed offline, and the required results for the corresponding elements were recorded after processing. The results are shown in Table 2.

[0150] (7) Particle size (D50 of positive electrode active material) test: Take the corresponding mass of positive electrode active material (i.e., sample) in Table 1 and place it in a 50 mL beaker. Add one dropper of 3% sodium hexametaphosphate. Shake the beaker to fully soak the sample. Then, rinse with pure water using a washing bottle and slowly transfer it to the sample pool. Transfer completely to a light shielding value between 8% and 12%. After the light shielding value stabilizes, click Start Measurement to enter the 10-second ultrasonic countdown to allow the sample to be evenly dispersed in the measuring medium (usually pure water). After the ultrasonic wave ends, the software automatically tests three times, takes the average value, and outputs the result. The results are shown in Table 2.

[0151] (8) Specific surface area (BET) test: The BET of the positive electrode active material was tested using a Micromeritics TriStar II 3020 specific surface area analyzer. The positive electrode active material was first degassed, the sample tube was weighed (empty tube), and about 0.1–0.5 g of positive electrode active material powder (i.e., sample) was added. The sample tube was placed in a degassing station and vacuum degassed at 150–300 °C for ≥3 hours to completely remove adsorbed water and gas impurities on the sample surface. After cooling to room temperature, the sample tube was weighed again to calculate the actual sample mass. The degassed sample tube was installed in the analysis station and an isothermal clamp was placed on it to ensure temperature uniformity. The relative pressure (P / P0) range was set to 0.05–0.35 (BET linear range), and the instrument automatically recorded the adsorption / desorption curve. The software automatically fitted the BET equation and generated a specific surface area (m² / g) report. The results are shown in Table 2.

[0152] Table 1

[0153]

[0154] Table 2

[0155]

[0156] Table 3

[0157]

[0158] According to Table 1, Table 2, and Table 3, compared with Comparative Example 1, in Examples 1 to 8, the area ratio of twin crystals to the positive electrode active material in the electron backscattered diffraction patterns of the positive electrode active materials is 40% to 60%. Figure 4 、 Figure 5 、 Figure 7 and Figure 8 As shown, the positions of single crystals and twin crystals can be seen. The area ratio of twin crystals to positive electrode active materials is calculated through the cross-sectional electron backscatter diffraction pattern of the positive electrode active material, which is beneficial to improving the structural stability of the positive electrode active material, promoting the diffusion of lithium ions, and inhibiting the migration of transition metals, thereby improving the capacity, cycle performance, rate performance and other electrochemical properties of the battery. Furthermore, as the area ratio of twin crystals to positive electrode active materials increases, the diffusion of lithium ions is further promoted, and the migration of transition metals is inhibited, and the capacity, cycle performance, rate performance and other electrochemical properties of the battery are further enhanced.

[0159] In addition, from Examples 1 to 8, it can be seen that when the molar ratio of lithium element to sodium element in the lithium-containing positive electrode active material doped with sodium element is (8~133):1, it is beneficial to improve the lithium ion diffusion coefficient of the positive electrode active material, inhibit the migration and loss of transition metals in the positive electrode active material, improve the structural stability of the positive electrode material, and further take into account the improvement of battery performance such as rate and cycle performance.

[0160] In addition, the molecular formula of the lithium-containing positive electrode active material doped with sodium in Example 1 in Table 2 is Li 1.075 Na 0.027 Ni 0.688 Co 0.091 Mn 0.221 O2, such as Figure 6 As shown in the figure, due to the high atomic numbers of Ni, Co, and Mn atoms, the atomic contrast under the transmission electron microscope is strong. Figure 5 The white highlights in the middle correspond to the positions of Ni, Co, and Mn atoms, such as Figure 5 As shown, there are many twin structures inside the crystal. Figure 9 The positive electrode active material of Comparative Example 1 has a relatively complete single crystal structure and has fewer twin structures. The area ratio of twins to positive electrode active materials is only 10%. It can be seen from Table 3 that the electrochemical properties such as capacity, cycle performance, and rate performance of Comparative Example 1 are worse than those of the embodiment. The area ratio of twins to positive electrode active materials in the embodiment is 40% to 60%, which is beneficial to improving the structural stability of the positive electrode active material, promoting lithium ion diffusion, and inhibiting transition metal migration, thereby improving the capacity, cycle performance, rate performance and other electrochemical properties of the battery.

[0161] like Figure 1 and Figure 2 As shown in the figure, it can be seen that in step (1) of Example 1, polycrystalline Ni 0.68 Co 0.08 Mn 0.24 (OH)2 precursor sintered into single crystal morphology, compared Figure 3 (Comparative example), you can see Figure 2 The surface granularity of the positive electrode active material is weakened, and there is an obvious coating layer, which is conducive to further improving the conductivity and structural stability of the positive electrode active material, while taking into account the improvement of the battery's sub-coulombic efficiency, rate and cycle performance.

[0162] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A positive electrode active material, characterized in that The positive electrode active material includes twin crystals, and in an electron backscattered diffraction pattern, an area ratio of the twin crystals to the positive electrode active material is 40% to 60%.

2. The positive electrode active material according to claim 1, characterized in that The positive electrode active material includes a single crystal.

3. The positive electrode active material according to claim 2, characterized in that In an electron backscattered diffraction pattern of the positive electrode active material, an area ratio of the single crystal to the positive electrode active material is 40% to 60%.

4. The positive electrode active material according to claim 1, characterized in that The positive electrode active material includes a lithium-containing positive electrode active material doped with sodium.

5. The positive electrode active material according to claim 4, characterized in that The molar ratio of lithium element to sodium element in the lithium-containing positive electrode active material doped with sodium element is (8-133):

1.

6. The positive electrode active material according to claim 4, characterized in that The positive electrode active material includes a core and a shell located on the surface of the core, wherein the core includes the lithium-containing positive electrode active material doped with sodium; the shell includes a carbon material and / or a compound containing an M element, and the M element includes one or more of tungsten, boron, aluminum, zirconium, titanium, magnesium, molybdenum, cerium and strontium.

7. The positive electrode active material according to any one of claims 4 to 6, characterized in that The lithium-containing positive electrode active material doped with sodium element includes one or more of a ternary positive electrode active material doped with sodium element, a lithium iron phosphate material doped with sodium element, and a lithium manganese iron phosphate material doped with sodium element.

8. The positive electrode active material according to claim 7, characterized in that The ternary positive electrode active material doped with sodium element includes Li x Na y Ni a Co b Mn c D z O2, wherein the D element includes one or more of zirconium, titanium, vanadium, aluminum, and tungsten, 0.9≤x≤1.2, 0.01≤y≤0.1, 0.6≤a≤0.8, 0.05≤b≤0.1, 0.1≤c≤0.4, and 0≤z≤0.

006.

9. The positive electrode active material according to claim 1, characterized in that The lithium ion diffusion coefficient of the positive electrode active material is 6×10 -13 cm 2 / s~9.5×10 -13 cm 2 / s.

10. A method for preparing the positive electrode active material according to any one of claims 1 to 9, characterized in that: The following steps are involved: The positive electrode active material is prepared by mixing a raw material system including a sodium-doped lithium-containing positive electrode active material and a first lithium source and then performing an ion exchange treatment.

11. The method for preparing a positive electrode active material according to claim 10, characterized in that: The process of mixing the sodium-doped lithium-containing positive electrode active material with a raw material system of a first lithium source and then performing an ion exchange treatment includes: mixing the sodium-doped lithium-containing positive electrode active material with the raw material system of the first lithium source and then performing a first sintering treatment to obtain the positive electrode active material.

12. The method for preparing a positive electrode active material according to claim 11, wherein: The molar ratio of the lithium element of the first lithium source to the sodium element of the sodium-doped lithium-containing positive electrode active material is 1.05:1 to 1.2:1; And / or, the temperature of the first sintering treatment is 400° C. to 700° C.; And / or, the first sintering treatment time is 6h~8h.

13. The method for preparing a positive electrode active material according to claim 10, characterized in that: The preparation process of the sodium-doped lithium-containing positive electrode active material comprises: mixing a raw material system including a second lithium source, a sodium source, and a positive electrode precursor, and then performing heat treatment to obtain the sodium-doped lithium-containing positive electrode active material.

14. The method for preparing a positive electrode active material according to claim 13, wherein: The molar ratio of the second lithium source to the sodium source is 1:4 to 1:1; And / or, the heat treatment temperature is 850 o C~950 o C; And / or, the heat treatment time is 8h~12h.

15. The method for preparing a positive electrode active material according to claim 11, wherein: The process of mixing the sodium-doped lithium-containing positive electrode active material with the raw material system of the first lithium source and then performing a first sintering treatment includes: mixing the sodium-doped lithium-containing positive electrode active material with the raw material system of the first lithium source and then performing a first sintering treatment to obtain an intermediate product; mixing the intermediate product with a coating agent and then performing a second sintering treatment to obtain the positive electrode active material.

16. The method for preparing a positive electrode active material according to claim 15, characterized in that: The coating agent includes a carbon source and / or a coating agent containing an M element, wherein the M element includes one or more of tungsten, boron, aluminum, zirconium, titanium, magnesium, molybdenum, cerium and strontium; And / or, the temperature of the second sintering treatment is 400° C. to 800° C.; And / or, the second sintering treatment time is 6h~12h; And / or, the process of mixing the intermediate product with a coating agent and then subjecting it to a second sintering treatment includes: washing the intermediate product with a first solvent and drying it, and then wet-ball milling it with the coating agent to obtain a mixed system, and the mixed system is spray-dried and subjected to a second sintering treatment in sequence to obtain the positive electrode active material.

17. The method for preparing a positive electrode active material according to claim 16, wherein: The temperature of the first solvent is 80° C. to 100° C.; And / or, the washing time is 0.5h~1h; And / or, the spray drying conditions are: inlet temperature is 200°C~220°C, and outlet temperature is 90°C~100°C.

18. A positive electrode sheet, characterized in that: The positive electrode active material comprises the positive electrode active material according to any one of claims 1 to 9 or the positive electrode active material prepared according to the preparation method of the positive electrode active material according to any one of claims 10 to 17.

19. A battery, characterized in that: Including the positive electrode sheet according to claim 18.

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