A tungsten-niobium co-doped nickel-rich ternary cathode material and its preparation method

Abstract

The present invention belongs to the technical field of cathode materials for lithium-ion batteries, a new energy material, and specifically relates to a tungsten-niobium co-doped nickel-rich ternary cathode material and a preparation method thereof. The present invention obtains a dual-element co-doped nickel-rich ternary cathode material by calcining a precursor with tungsten and niobium mixed and pretreated. Compared with undoped and single-element doped samples, the prepared co-doped sample can effectively enhance the structural stability of the material, reduce the polarization of the electrode, and improve the diffusion kinetics of lithium ions, thus simultaneously improving the cycle stability and rate performance of the battery.

Description

Technical Field

[0001] The present invention belongs to the technical field of cathode materials for lithium-ion batteries in new energy materials, and specifically relates to a tungsten-niobium co-doped nickel-rich ternary cathode material and a preparation method thereof. Background Art

[0002] With the development of society, humans have paid increasing attention to issues such as environmental pollution and energy crises. The solution to these problems still requires the continuous promotion of the application of green and renewable energy and storage devices. The new generation of lithium-ion batteries has rapidly dominated the fields of 3C digital and new energy vehicles due to their excellent performance, mature technology, and environmental friendliness. With the rapid development of new energy vehicles, people's demands for the cruising range and service life of electric vehicles are constantly increasing. To solve the problems of long cruising range and safety, the nickel-rich ternary cathode material LiNi x Co y Mn 1-x-y O2 (0.6 ≤ x < 1, 0 < y ≤ 0.20) has become a research hotspot closely concerned by researchers as one of the most promising candidate materials to solve the above problems due to its advantages such as higher specific capacity, low cost, and environmental friendliness.

[0003] The nickel-cobalt-manganese ternary system, like other cathode materials on the market (such as lithium cobaltate, lithium manganate, lithium iron phosphate, etc.), has its own problems. For example: the cobalt in lithium cobaltate is expensive and the cost is high; the structure of lithium manganate is unstable during discharge and the battery safety performance is poor; the lithium iron phosphate has low conductivity and poor high-temperature performance. Although the nickel-cobalt-manganese ternary system battery can solve some problems to a certain extent through the combination of nickel, cobalt, and manganese, on the way to pursue high energy density of the battery, the nickel-rich ternary cathode material still has the defects of limited stability and safety.

[0004] Taking the nickel-rich ternary cathode material NCM811 as an example, due to its advantages such as high specific capacity, it has become a popular material for the cathode of lithium-ion batteries, but there are still many problems to be solved, generally divided into problems inside the material crystal and on the material surface, including: (1) Li / Ni cation mixing, Li + and Ni 2+ have similar ionic radii. During the charge and discharge process, Ni in the transition metal layer 2+It will migrate to the lithium layer, occupy lithium vacancies, and form cation mixing. (2) The surface structure of the material is unstable, and the layered structure on the surface will transform into a spinel phase structure and an inert rock salt phase structure during cycling. (3) Side reactions occur between the transition metals on the material surface and the electrolyte, resulting in an increase in battery impedance and capacity decay. (4) Microcracks are prone to occur in the secondary particles of the nickel-rich ternary cathode material. With the volume expansion and contraction of the ternary material caused by the insertion / extraction of lithium ions, cracks are formed near the grain boundaries inside the particles, and the crack surfaces will continue to react with the electrolyte. (5) Residual alkali on the surface will exacerbate the side reactions between the active material and the electrolyte during the reaction process and release gases, affecting the battery life and safety performance. Therefore, in-depth research on modification technologies for nickel-rich ternary cathode materials is required. Modifying the nickel-rich ternary cathode material to further improve its physical and electrochemical properties has important scientific significance and practical application value.

[0005] In response to the above problems faced by the nickel-rich ternary cathode material, current modification methods such as coating, doping, constructing a concentration gradient cathode, and synthesizing single-crystal materials have been proposed to improve the physical and chemical properties of the nickel-rich ternary cathode material. Among them, the two modification methods of coating and doping are more widely used. Coating modification is a simple and efficient modification method. Generally, a protective layer can be "covered" evenly on the surface of the nickel-rich cathode particles through methods such as wet chemical method or atomic layer deposition, thereby isolating the direct contact between the electrolyte and the active electrode material and playing a protective role for both the particle surface and the internal grain boundaries. In addition, some coating materials themselves have good electrical conductivity, which can reduce the material impedance while stabilizing the material structure and enhance the ion diffusion rate, etc. Doping modification generally involves adding other metal cations or halogen anions to the crystal structure of the material to change the lattice constant or element valence state. Ion doping usually has an inhibitory effect on Li + / Ni 2+ mixing. Moreover, the added ions often form more stable chemical bonds in the lattice, thereby stabilizing the crystal structure, weakening the structural collapse and phase transformation of the material, and improving the cycle stability. In addition, doping with some materials with larger ionic radii can also broaden the diffusion channels of lithium ions and improve the kinetic performance of the material.

[0006] However, in the existing modification technology methods for nickel-rich ternary cathode materials, most are single-element coating, doping, or single-element doping and coating integration. In the dual-element modification scheme for nickel-rich ternary cathode materials, usually one element is doped into the material and the other element plays a coating role. There is no case of simultaneous doping of high-valence dual elements in nickel-rich ternary cathode materials.

[0007] Therefore, by reasonably designing a preparation method for high-valence dual-element doped nickel-rich ternary cathode materials, and the two high-valence elements have a synergistic effect, which has a greater modification effect than single-element doping. Thus, guiding the synergistic doping of dual-elements or multi-elements is one of the key exploration directions for the modification of nickel-rich ternary cathode materials currently and in the future. Summary of the Invention

[0008] In view of this, the purpose of the present invention is to provide a method for co-doping and modifying nickel-rich ternary cathode materials with tungsten and niobium dual-elements playing a synergistic role, aiming at the problems existing in the prior art.

[0009] In order to achieve the above purpose, the first object of the present invention is to provide a preparation method for tungsten-niobium co-doped nickel-rich ternary cathode materials, adopting the following technical solutions:

[0010] A preparation method for tungsten-niobium co-doped nickel-rich ternary cathode materials, by calcining the precursor pretreated with tungsten and niobium, a nickel-rich ternary cathode material co-doped with dual-elements is obtained.

[0011] It is worth noting that the present invention realizes the dual-element doping of tungsten and niobium in nickel-rich ternary cathode materials only under the same calcination process conditions through a relatively simple process method, thereby effectively improving the rate performance and cycle stability performance of nickel-rich ternary cathode materials. In addition, the preparation method disclosed in the present invention requires a short preparation time, a simple operation process, low-cost and easily obtainable raw materials, and the performance of the obtained materials is better than that of unmodified nickel-rich ternary cathode materials.

[0012] Further, the specific steps of the preparation method include:

[0013] (1) Mix the nickel-rich ternary precursor with a tungsten source and a niobium source through an ethanol solution, heat and evaporate, and dry to obtain a nickel-rich ternary precursor pretreated with tungsten and niobium;

[0014] (2) Mix the nickel-rich ternary precursor pretreated with tungsten and niobium with a lithium source, grind, calcine, and cool to obtain a tungsten-niobium co-doped nickel-rich ternary cathode material.

[0015] Compared with the dual-element modification scheme in the prior art where one element dopes the material and the other element plays a coating role on the material, the present invention significantly reduces the system energy of single-element doping through dual-element co-doping, and the dual-element synergistic effect makes doping easier to obtain, realizing the dual-element co-doping of nickel-rich ternary cathode materials.

[0016] Furthermore, in the step (1), the nickel-rich ternary precursor is Ni x Co y Mn 1-x-y(OH)2, where 0.6 ≤ x < 1 and 0 < y ≤ 0.20; the tungsten source is one or more of tungsten trioxide, lithium tungstate, sodium tungstate, and ammonium tungstate; the niobium source is one or more of niobium pentoxide, lithium niobate, and ammonium oxalate niobate.

[0017] It should be noted that the nickel-cobalt-manganese ternary precursor used in the present invention can determine the specific ternary ratio for the nickel-rich ternary cathode material. The tungsten source and niobium source adopted can stabilize the crystal structure of the cathode material, effectively inhibit the occurrence of side reactions, and improve the electrochemical performance of the cathode material.

[0018] Furthermore, the mass ratio of the tungsten source and niobium source is 0.1% - 5% of the mass of the nickel-rich ternary cathode material.

[0019] Furthermore, in the step (2), the lithium source is one or a combination of lithium hydroxide and lithium carbonate.

[0020] Furthermore, the molar ratio of Li in the lithium source to Ni + Co + Mn in the nickel-rich ternary precursor is 1:(1.05 - 1.10).

[0021] It should be noted that excessive lithium can compensate for the loss of lithium during the calcination process. Appropriate excess lithium will improve the crystallization degree of the material, reduce the lithium-nickel mixing, and obtain a nickel-rich ternary cathode material with a better layered structure. However, too high lithium content will increase the particle size, make the surface rough, reduce the insertion and extraction rate of lithium ions, reduce the conductivity of the cathode material, and thus reduce the rate capacity. Moreover, too high lithium content will cause the residual alkali to react with the electrolyte to release gas, which will affect the safety and cycle life of the battery.

[0022] Furthermore, in the step (2), the calcination is divided into two-step calcination. Among them, the heating rate of the calcination is 2 - 6 °C / min, and the calcination atmosphere is air and / or oxygen.

[0023] Furthermore, the temperature range of the first-step calcination is 450 - 550 °C, and the time is 4 - 8 h; the temperature range of the second-step calcination is 750 - 850 °C, and the time is 13 - 18 h.

[0024] Furthermore, in the step (2), the grinding time is 0.5 - 2 h.

[0025] It should be noted that suitable grinding conditions can effectively disperse the secondary particles of the nickel-rich ternary cathode material, and mix it fully and evenly with the lithium source to obtain an ideal nickel-rich ternary cathode material.

[0026] The second object of the present invention is to provide a tungsten-niobium co-doped nickel-rich ternary cathode material prepared by the preparation method as described above.

[0027] A tungsten-niobium co-doped nickel-rich ternary cathode material, with the chemical formula: Li a Ni x Co y Mn 1-x-y (WNb) z O2, where 0.6 ≤ x < 1, 0 < y ≤ 0.20, 0.002 ≤ z ≤ 0.10; 1.0 ≤ a ≤ 1.10.

[0028] It should be noted that the dual-element co-doped nickel-rich ternary cathode material obtained in the present invention changes the elemental composition of the nickel-rich ternary precursor. With only a small amount of dopants, it effectively changes the bonding strength and lattice parameters of the traditional nickel-rich ternary cathode material. In particular, a protective layer only a few nanometers thick is formed on the surface of the initial sample, thereby achieving a significant improvement in material performance, making the dual-element co-doped nickel-rich ternary cathode material have excellent structural stability, cycle stability, and thermal stability.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] 1. Compared with the traditional method of improving the cycle stability of nickel-rich ternary cathode materials by single-element coating, doping, or doping-coating integration, the present invention realizes the modification of nickel-rich ternary cathode materials by dual-doping of tungsten and niobium elements with synergistic effects. The co-doping of tungsten and niobium significantly reduces the system energy of single-tungsten doping or single-niobium doping. The dual-doping can further significantly enhance the realization of the doping effect, effectively strengthen the crystal structure of the nickel-rich ternary cathode material, and inhibit the anisotropic volume change caused by phase transformation, thereby maintaining the integrity of spherical secondary particles and preventing the formation of microcracks. The passivation layer formed by co-doping can prevent the occurrence of side reactions and also prevent the formation of microcracks, which is beneficial to inhibiting the formation of internal oxygen vacancies, surface Li2CO3, and harmful gases, reducing the polarization of the electrode, and improving the diffusion kinetics of lithium ions. Therefore, the cycle stability and rate performance of the battery are improved simultaneously.

[0031] 2. The present invention can achieve the synergistic optimization modification of dual-element doping with better effects than undoped, single-tungsten, and single-niobium doping by simply calcining the pre-treated precursor of tungsten-niobium mixture. This is of great significance for the development of dual-element co-doped and multi-element co-doped nickel-rich ternary cathode materials.

[0032] 3. The preparation method of the tungsten-niobium co-doped nickel-rich ternary cathode material disclosed in the present invention is simple in operation. Only by calcining the pre-treated precursor can the required material be obtained. The process is environmentally friendly, economically applicable, and suitable for large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention or in the prior art, the following will briefly introduce the accompanying drawings required for the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on the provided drawings.

[0034] Figure 1 This is the system energy diagram of the single tungsten doping, single niobium doping, and tungsten-niobium co-doping of the present invention, as well as the XRD diagrams of Example 1 and Comparative Examples 1-3.

[0035] Figure 2 This is the scanning electron microscope diagram of Comparative Examples 1-3 of the present invention.

[0036] Figure 3 This is the scanning electron microscope diagram of Example 1 of the present invention.

[0037] Figure 4 This is the HRTEM diagram of Comparative Example 1 of the present invention and the Fourier transform diagram of the selected area.

[0038] Figure 5 This is the HRTEM diagram of Comparative Example 2 of the present invention and the Fourier transform diagram of the selected area.

[0039] Figure 6 This is the HRTEM diagram of Comparative Example 3 of the present invention and the Fourier transform diagram of the selected area.

[0040] Figure 7 This is the HRTEM diagram of Example 1 of the present invention and the Fourier transform diagram of the selected area.

[0041] Figure 8 This is the cycling performance diagram of Comparative Examples 1-3 and Example 1 of the present invention at different rates (1.0C, 5.0C, 10.0C).

[0042] Figure 9 This is the rate performance diagram of Comparative Examples 1-3 and Example 1 of the present invention. Detailed implementation manners

[0043] The following will clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0044] As used herein, the term "embodiment" refers to any embodiment described as "exemplary", which does not necessarily have to be construed as superior to or better than other embodiments. For the performance index tests in the embodiments of the present application, unless otherwise specified, conventional test methods in the art are adopted. It should be understood that the terms described in the present application are only used to describe specific embodiments and are not intended to limit the content disclosed in the present application.

[0045] Unless otherwise specified, the technical and scientific terms used herein have the same meanings as commonly understood by those of ordinary skill in the technical field to which this application belongs; other test methods and technical means not specifically noted in this application refer to the experimental methods and technical means commonly adopted by those of ordinary skill in the art.

[0046] To better illustrate the content of this application, numerous specific details are given in the following specific embodiments. Those skilled in the art should understand that this application can still be implemented without some specific details. In the embodiments, some methods, means, instruments, equipment, etc. well-known to those skilled in the art are not described in detail in order to highlight the gist of this application.

[0047] On the premise of no conflict, the technical features disclosed in the embodiments of this application can be combined arbitrarily, and the obtained technical solutions belong to the content disclosed in the embodiments of this application.

[0048] The present invention discloses a preparation method of a tungsten-niobium co-doped nickel-rich ternary cathode material, belonging to the technical field of cathode materials for lithium-ion batteries in new energy materials. The present invention obtains a dual-element co-doped nickel-rich ternary cathode material by calcining a precursor subjected to tungsten-niobium mixed pretreatment. The co-doping of tungsten and niobium in the material significantly reduces the system energy of single tungsten doping or single niobium doping. And the co-doping makes the material have a more stable structure and inhibits the anisotropic volume change caused by phase transformation, thereby maintaining the integrity of spherical secondary particles and preventing the formation of microcracks. In addition, the passivation layer formed by co-doping can prevent the occurrence of side reactions and also prevent the formation of microcracks. The synergistic effect of tungsten and niobium is also beneficial to inhibiting the formation of internal oxygen vacancies and surface Li2CO3. Compared with undoped and single-element doped samples, the prepared co-doped samples can effectively enhance the structural stability of the material, reduce the polarization of the electrode, improve the diffusion kinetics of lithium ions, and thus improve the cycle stability and rate performance of the battery at the same time.

[0049] To better understand the present invention, the following embodiments are used to further specifically illustrate the present invention, but it should not be construed as a limitation of the present invention. For those skilled in the art, some non-essential improvements and adjustments made according to the above-mentioned invention content are also considered to fall within the protection scope of the present invention.

[0050] Example 1

[0051] Preparation method of tungsten-niobium co-doped nickel-rich ternary cathode material, specifically including the following steps:

[0052] (1) Add 2.0 g of nickel-cobalt-manganese ternary precursor Ni 0.8 Co 0.1 Mn 0.1 (OH)2, 0.04 g of ammonium paratungstate and 0.04 g of ammonium niobium oxalate into the ethanol dispersant and stir for 3 h in sequence. Then, heat the above dispersion under stirring in a water bath at 80 °C until the ethanol is completely evaporated, and then dry it in a forced-air drying oven at 80 °C for 6 h to obtain the precursor of the high-nickel ternary cathode material pretreated with tungsten and niobium.

[0053] Among them, the masses of ammonium paratungstate and ammonium niobium oxalate are respectively 2% of the mass of the nickel-rich ternary cathode material precursor.

[0054] (2) Mix 1.8458 g of the nickel-rich ternary cathode precursor material pretreated with tungsten and niobium with 0.8895 g of LiOH·H2O and grind for 2 h, then put it into a calcination furnace and perform two-step calcination under an oxygen atmosphere. The heating rate is 5 °C / min, heat up to 500 °C and hold for 6 h, then continue to heat up to 800 °C and calcine for 15 h. After cooling to 500 °C at a cooling rate of 5 °C / min, cool with the furnace. Finally, obtain the tungsten-niobium co-modified nickel-rich ternary cathode material.

[0055] Among them, the molar ratio of Li in lithium hydroxide to Ni 0.8 Co 0.1 Mn 0.1 (OH)2 in Ni+Co+Mn is 1:1.06.

[0056] In order to further prove the beneficial effects of the present invention and better understand the present invention, the properties and application performance of the preparation method of the tungsten-niobium co-doped nickel-rich ternary cathode material disclosed in the present invention are further clarified through the following comparative examples and performance tests. However, it should not be understood as a limitation to the present invention. For the method properties obtained by other determination experiments made by those skilled in the art according to the above invention content and the applications based on the above properties, they are also considered to fall within the protection scope of the present invention.

[0057] Comparative Example 1

[0058] Preparation method of the original nickel-rich ternary cathode material, specifically including the following steps:

[0059] Add 1.8458 g of the untreated nickel-cobalt-manganese ternary precursor Ni 0.8 Co 0.1 Mn 0.1(OH)2 was mixed and ground with 0.8895 g of LiOH·H2O for 2 h, then placed in a calcination furnace and calcined in two steps under an oxygen atmosphere. The heating rate was 5 °C / min, heated to 500 °C and held for 6 h, then continued to be heated to 800 °C and calcined for 15 h. After cooling to 500 °C at a cooling rate of 5 °C / min, it was cooled with the furnace. Finally, the original sample of nickel-rich ternary cathode material was obtained.

[0060] Among them, Li in lithium hydroxide and the nickel-cobalt-manganese ternary precursor Ni 0.8 Co 0.1 Mn 0.1 The molar ratio of Ni+Co+Mn in (OH)2 is 1:1.06.

[0061] Comparative Example 2

[0062] Preparation method of tungsten single-element doped nickel-rich ternary cathode material, specifically including the following steps:

[0063] (1) 2.0 g of nickel-cobalt-manganese ternary precursor Ni 0.8 Co 0.1 Mn 0.1 (OH)2 and 0.04 g of ammonium paratungstate were successively and uniformly added to an ethanol dispersant and stirred for 3 h. Then, the above dispersion was stirred and heated in a water bath at 80 °C until the ethanol was completely evaporated, and then dried in a blast drying oven at 80 °C for 6 h to obtain a precursor of a high-nickel ternary cathode material pretreated with tungsten element.

[0064] Among them, the mass of ammonium paratungstate is 2% of the mass of the precursor of the nickel-rich ternary cathode material.

[0065] (2) 1.8458 g of the tungsten element-pretreated nickel-rich ternary cathode precursor material was mixed and ground with 0.8895 g of LiOH·H2O for 2 h, then placed in a calcination furnace and calcined in two steps under an oxygen atmosphere. The heating rate was 5 °C / min, heated to 500 °C and held for 6 h, then continued to be heated to 800 °C and calcined for 15 h. After cooling to 500 °C at a cooling rate of 5 °C / min, it was cooled with the furnace. Finally, a single-tungsten modified nickel-rich ternary cathode material was obtained.

[0066] Among them, Li in lithium hydroxide and the nickel-cobalt-manganese ternary precursor Ni 0.8 Co 0.1 Mn 0.1 The molar ratio of Ni+Co+Mn in (OH)2 is 1:1.06.

[0067] Comparative Example 3

[0068] Preparation method of niobium single-element doped nickel-rich ternary cathode material, specifically including the following steps:

[0069] (1) 2.0 g of nickel-cobalt-manganese ternary precursor Ni0.8 Co 0.1 Mn 0.1 (OH)2 and 0.04 g of ammonium niobium oxalate are successively and uniformly added to an ethanol dispersant and stirred for 3 h. Then, the above-mentioned dispersion is stirred and heated in a water bath at 80 °C until the ethanol is completely evaporated, and then dried in a forced-air drying oven at 80 °C for 6 h to obtain a high-nickel ternary cathode material precursor pretreated with niobium element.

[0070] Among them, the mass of ammonium niobium oxalate is 2% of the mass of the nickel-rich ternary cathode material precursor.

[0071] (2) 1.8458 g of the nickel-rich ternary cathode precursor material pretreated with niobium element is mixed and ground with 0.8895 g of LiOH·H2O for 2 h, and then put into a calcination furnace and calcined in two steps under an oxygen atmosphere. The heating rate is 5 °C / min, heated to 500 °C and kept warm for 6 h, and then continuously heated to 800 °C and calcined for 15 h. After cooling to 500 °C at a cooling rate of 5 °C / min, it is cooled with the furnace. Finally, a nickel-rich ternary cathode material modified with single niobium is obtained.

[0072] Among them, the molar ratio of Li in lithium hydroxide to Ni in the nickel-cobalt-manganese ternary precursor 0.8 Co 0.1 Mn 0.1 (OH)2 is 1:1.06.

[0073] Performance test

[0074] 1. X-ray diffraction (XRD) test

[0075] Figure 1 It is the system energy diagram of single tungsten doping, single niobium doping, and tungsten-niobium co-doping, as well as the XRD patterns of Example 1 and Comparative Examples 1-3.

[0076] From Figure 1 it can be seen that taking the crystal structure surface of Comparative Example 1 as the energy 0 point, theoretical calculations show that the addition of single tungsten and single niobium both show that the added elements are easy to enter the body, resulting in a decrease in the system energy, indicating that the addition of single tungsten and single niobium are both beneficial to achieving doping. Particularly, the simultaneous addition of tungsten and niobium further significantly reduces the system energy, indicating that the synergistic effect of tungsten and niobium is more conducive to the realization of doping, forming a co-doped nickel-rich ternary cathode material. The samples of Comparative Examples 1-3 and Example 1 are all typical α-NaFeO2 structures and belong to Space group. In Comparative Examples 1-3 and Example 1, the two main diffraction peaks (003) and (104) are sharp, the clear splitting of the peaks (006) / (012) and (018) / (110) is observed, and no obvious impurity phase is detected, indicating that the addition of tungsten and niobium does not change the crystal structure of the whole phase. However, the main peak (003) is shifted, and the effects of tungsten and niobium elements on the nickel-rich ternary cathode material are different. The (003) peak of the tungsten-modified sample is shifted to a lower angle, the position change of the (003) peak of the niobium-modified sample is not obvious, and the position movement of the (003) peak of the tungsten-niobium double-doped sample is between that of the tungsten-modified sample and the niobium-modified sample, showing the synergistic effect of the two elements. Overall, the doping brings about a change in the lattice parameter, increasing the layer spacing, which is beneficial to the diffusion of lithium ions during the charge and discharge process, thereby improving the electrochemical performance of the battery.

[0077] 2. Scanning Electron Microscope (SEM) Test

[0078] Figure 2 The scanning electron microscope images of Comparative Examples 1-3 are shown respectively. Figure 3 The scanning electron microscope image of Example 1 is shown.

[0079] All samples are formed by the aggregation of nanoscale primary particles into spherical or quasi-spherical secondary particles, and the size of the secondary particles is about 5-10 μm. The modification of tungsten and niobium elements does not significantly change the morphology of the secondary particles of the samples. However, it can be seen from the scanning electron microscope images that the primary particles of the single-tungsten and single-niobium modified samples are smaller than those of the unmodified sample, and the primary particles of the modified samples are more uniform and compact, making the secondary particles aggregate more closely. This further shows that the addition of the modified material has a certain influence on the crystallization and growth during the calcination process of the material. In particular, whether compared with the unmodified sample or with the single-tungsten and single-niobium modified samples, the primary particles of the tungsten-niobium double-element doped and synergistically modified sample are more uniform, and the secondary particles are more smoothly and compactly aggregated, which is more conducive to preventing the structural collapse of the secondary particles during the charge and discharge process of the battery.

[0080] 3. High-Resolution Transmission Electron Microscope (HRTEM) Test and Selected Area Fourier Transform Results

[0081] Figure 4 、 Figure 5 、 Figure 6 、 Figure 7HRTEM images and Fourier transform images of selected regions of Comparative Examples 1-3 and Example 1, respectively. It can be seen from the selected area Fourier transform analysis that there is a residual lithium layer on the surface of Comparative Example 1, and the internal is a standard layered phase; due to the doping effect of tungsten element in Comparative Example 2, the surface of the nickel-rich ternary cathode material undergoes a phase transformation caused by doping and turns into a spinel phase, and the inner part is a transition phase mixed with spinel phase and layered phase; due to the doping effect of niobium element in Comparative Example 3, the surface of the nickel-rich ternary cathode material undergoes a phase transformation caused by doping and turns into a rock salt phase, and the inner part is also a transition phase mixed with rock salt phase and layered phase; while in Example 1, due to the co-doping achieved by the synergistic effect of tungsten and niobium, a rock salt phase layer thicker than that of single-element modification is formed on the surface of the nickel-rich cathode material, and the inner part is a transition phase mixed with rock salt phase and layered phase, and the inside of the particles is still a layered phase. The foreign phases (such as spinel phase and rock salt phase) formed by modification are different from the foreign phases that appear during the battery cycling process. The foreign phases are derived from the partial mixing of Li and Ni ions, and the spinel phase represents an intermediate state between the layered phase and the rock salt phase. For the layered phase, Li only occupies the 3a position, and TM occupies the 3b position, while for the rock salt phase, Li and TM randomly occupy the 3a and 3b positions. Different from the direct transformation of LiNiO2 (LNO) from the layered phase to the rock salt phase during the doping process, due to the presence of Co and Mn, the transformation of NCM from the layered phase to the rock salt phase caused by the complete mixing of Li and TM is prevented, resulting in NCM possibly undergoing an intermediate transformation through the spinel phase. The foreign phases formed by surface modification rather than structural degradation are beneficial to the cycling stability of the material. Due to the inhibition of the mixing of Li and TM by Co and Mn, W doping forms a spinel phase. Nb doping destroys the inhibition of the mixing of Li and TM by Co and Mn, resulting in the direct formation of a rock salt phase. However, increasing the mixing of Li and Ni by niobium modification is disadvantageous. The synergistic effect of tungsten and niobium not only forms a protective layer of rock salt phase, but also reduces the serious mixing of Li and Ni. The doped samples form a stable structure beneficial to the material during the subsequent cycling process before battery charge and discharge, thereby enhancing the cycling stability of the material. Compared with the tungsten and niobium single-element doped samples, the tungsten-niobium co-doped samples have a more stable protective layer, so they can more significantly enhance the cycling stability of the material.

[0082] 4. Cycling and rate performance tests

[0083] Take Comparative Examples 1-3 and Example 1 as the positive electrode respectively, and assemble them into coin cells with metallic lithium sheets as the negative electrode, and test their cycling and rate performance in the voltage range of 2.8-4.4V. The results are shown in Table 1 and Figures 8-9 as follows. Among them, Figure 8 are the cycling performance diagrams of Comparative Examples 1-3 and Example 1 at different rates (1.0C, 5.0C, 10.0C), Figure 9 are the rate performance diagrams of Comparative Examples 1-3 and Example 1.

[0084] Table 1 Test Results of Coin Cell Performance

[0085]

[0086] As can be seen from Table 1 and Figure 8 it can be known that in the voltage range of 2.8 - 4.4 V, except for the loss of initial capacity at a low current of 0.2 C, the capacity retention rates of the tungsten, niobium single-element doped and tungsten-niobium co-doped nickel-rich ternary cathode materials prepared in Comparative Examples 2 - 3 and Example 1 are all higher than that of Comparative Example 1 after 300 cycles at current densities of 1.0 C and 5.0 C and after 350 cycles at a current density of 10.0 C. The loss of initial capacity is due to the fact that the nanoscale protective layer formed by doping does not contribute to the capacity, but the capacity loss brought is limited. The above results show that the tungsten-niobium dual-element co-doped sample in Example 1 is the best example, and the capacity retention rate after 350 cycles at a high current of 10.0 C is increased by 38.1% compared with that of Comparative Example 1 after cycling.

[0087] The above tests show that the cycle stability of the tungsten-niobium co-doped nickel-rich ternary cathode material prepared by the present invention has been significantly improved.

[0088] From Figure 9 it can be known that Comparative Examples 2 - 3 and Example 1 have more advantages in high-rate discharge capacity. Among them, the 10.0 C high-rate capacity of Example 1 is as high as 142.0 mAh / g. Compared with 115.0 mAh / g of Comparative Example 1, its discharge specific capacity at a high rate of 10.0 C is increased by 27.0 mAh / g.

[0089] Both the cycle stability and the rate performance indicate that the method of the present invention has significant effects and improves the electrochemical performance of the cathode material.

[0090] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but will be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A preparation method of a tungsten-niobium co-doped nickel-rich ternary cathode material, characterized in that, A nickel-rich ternary cathode material co-doped with two elements is obtained by calcining a tungsten-niobium mixed pretreated precursor. The specific steps include: (1) Mix the nickel-rich ternary precursor with a tungsten source and a niobium source through an ethanol solution, heat and evaporate, and dry to obtain a tungsten-niobium pretreated nickel-rich ternary precursor. (2) Mix the tungsten-niobium pretreated nickel-rich ternary precursor with a lithium source, grind, calcine, and cool to obtain a tungsten-niobium co-doped nickel-rich ternary cathode material. The molar ratio of Li in the lithium source to Ni + Co + Mn in the nickel-rich ternary precursor is 1:(1.05 - 1.10). The calcination is divided into two steps. Among them, the heating rate of calcination is 2 - 6 °C / min, and the calcination atmosphere is air and / or oxygen; the temperature range of the first-step calcination is 450 - 550 °C, and the time is 4 - 8 h; the temperature range of the second-step calcination is 750 - 850 °C, and the time is 13 - 18 h. The chemical formula of the tungsten-niobium co-doped nickel-rich ternary cathode material is: Li a Ni x Co y Mn 1-x-y (WNb) z O2, where 0.6 ≤ x < 1, 0 < y ≤ 0.20, 0.002 ≤ z ≤ 0.10; 1.0 ≤ a ≤ 1.

10.

2. The preparation method according to claim 1, wherein In the step (1), the nickel-rich ternary precursor is Ni x Co y Mn 1-x-y (OH)2, where 0.6 ≤ x < 1 and 0 < y ≤ 0.20; the tungsten source is one or more of tungsten trioxide, lithium tungstate, sodium tungstate, ammonium tungstate; the niobium source is one or more of niobium pentoxide, lithium niobate, ammonium oxalate niobate.

3. The preparation method according to claim 2, wherein, The mass ratio of the tungsten source and the niobium source accounts for 0.1% - 5% of the mass of the nickel-rich ternary cathode material.

4. The preparation method according to claim 1, characterized in that, In the step (2), the lithium source is one or a combination of lithium hydroxide and lithium carbonate.

5. The preparation method according to claim 1, characterized in that, In the step (2), the grinding time is 0.5 - 2 h.

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