Ternary positive electrode material with core-shell structure as well as preparation method and application of ternary positive electrode material

Abstract

The invention relates to a ternary positive electrode material with a core-shell structure and a preparation method and application thereof, and the method comprises the following steps: S1, mixing a lithium source with a nickel-cobalt-manganese hydroxide ternary precursor, and carrying out first heat treatment on the obtained mixture in a first oxygen-containing atmosphere to obtain a first material; s2, enabling the first material to be in contact with a solution containing ammonium salt, performing impregnation, and performing second heat treatment on an obtained solid in a second oxygen-containing atmosphere; the temperature of the first heat treatment is lower than that of the second heat treatment. The ternary positive electrode material prepared by adopting the method disclosed by the invention is relatively low in residual alkali content, and relatively good thermal stability and electrochemical performance can be obtained when the ternary positive electrode material is used as a positive electrode material of a lithium ion battery.

Description

Technical Field

[0001] The present disclosure relates to the technical field of lithium-ion battery positive electrode materials, and in particular to a ternary positive electrode material with a core-shell structure, a preparation method thereof, and applications thereof. Background Art

[0002] Nickel cobalt manganese oxide ternary cathode material (NCM) has the outstanding advantage of high energy density and is one of the important choices for cathode materials for electric vehicle power batteries. As users' requirements for the driving range of electric vehicles are getting higher and higher, high energy density power batteries are also receiving great attention. In ternary cathode materials, the higher the nickel content, the greater the discharge capacity and the higher the energy density of the battery. Therefore, high nickel (Ni ≥ 0.8) or ultra-high nickel ternary cathode materials (Ni ≥ 0.9) have become a hot spot for research and development. As the nickel content increases, although the discharge capacity of the ternary cathode material continues to increase, the starting thermal runaway temperature of the material decreases and the heat release increases, resulting in a decrease in the safety performance of the battery.

[0003] As is known to all, high nickel content is beneficial to improve discharge capacity, and high manganese content is beneficial to improve safety performance. 0.8 Co 0.1 Mn 0.1 O2 is the core (accounting for 80%), with LiNi having excellent thermal stability 0.5 Mn 0.5 O2 is the shell (accounting for 20%), and a high nickel-based ternary cathode material Li[(Ni 0.8 Co 0.1 Mn 0.1 ) 0.8 (Ni 0.5 Mn 0.5 O2) 0.2 ]O2, a material that combines high discharge capacity with high thermal stability. Subsequently, a large number of core-shell structured ternary cathode materials were extensively studied. The synthesis method of core-shell structured ternary cathode materials can be summarized as follows: first, a composite hydroxide with a high nickel and low manganese component is synthesized, and then a layer of manganese-rich, low-nickel composite hydroxide component is coated in an "additive" manner to form a ternary precursor with a "core-shell structure". The ternary cathode material is then mixed with a lithium salt and calcined at a high temperature, generally between 700-950°C.

[0004] There are three problems with the above method for synthesizing core-shell structured ternary cathode materials:

[0005] First, the electrochemical properties between the core layer and the shell layer are difficult to reconcile. The core material is a high-nickel, low-manganese cathode material with a high discharge capacity, and the shell material is a manganese-rich, low-nickel cathode material with high thermal stability. The optimal synthesis temperature of cathode materials with different compositions is quite different. The high-nickel, low-manganese cathode material requires a low calcination temperature (650-850°C), and the manganese-rich, low-nickel cathode material requires a high calcination temperature (900-1000°C). The temperature difference is even as high as 100°C or more. For high-nickel ternary materials, the electrochemical performance will be quite different if the calcination temperature differs by 10-30°C. If the calcination temperature is consistent with that of the high-nickel, low-manganese core material, it will be difficult for the manganese-rich, low-nickel shell layer to form a good crystal structure, resulting in a high residual alkali content and poor rate performance of the material. If the calcination temperature is consistent with that of the manganese-rich, low-nickel shell layer, the high-nickel, low-manganese core material will be overheated, resulting in poor cycle performance of the material.

[0006] Second, the core-shell structure is difficult to maintain. Research in the literature (Electrochimica Acta, 2018, 270:319-329) shows that during high-temperature calcination (temperature ≥ 600°C), the core-shell structure cannot be well maintained due to the rapid diffusion between the metals. This makes it impossible to achieve the desired structure of a manganese-rich, low-nickel cathode material surface, and it is difficult to effectively improve the safety performance of high-nickel ternary power batteries.

[0007] Third, the material has poor uniformity. If the shell is thin, that is, the shell material accounts for a small proportion of the main structure, then it cannot be guaranteed that each particle can be evenly coated with the shell material, or even a single particle may only be coated in a spotty manner. Summary of the Invention

[0008] The purpose of the present disclosure is to provide a ternary cathode material with a core-shell structure, a preparation method thereof, and applications thereof.

[0009] In order to achieve the above object, the present disclosure provides a first aspect of a method for preparing a ternary cathode material having a core-shell structure, the method comprising the following steps:

[0010] S1. Mixing a lithium source with a nickel-cobalt-manganese hydroxide ternary precursor, and subjecting the obtained mixture to a first heat treatment in a first oxygen-containing atmosphere to obtain a first material;

[0011] S2. Contacting the first material with a solution containing an ammonium salt and impregnating the solution, and subjecting the obtained solid to a second heat treatment in a second oxygen-containing atmosphere.

[0012] Optionally, in step S1, the chemical formula of the nickel-cobalt-manganese hydroxide ternary precursor is Ni x Co y Mn z(OH)2, wherein x+y+z=1, 0.7≤x≤0.9, 0.05≤y≤0.2, 0.05≤z≤0.1.

[0013] Optionally, the lithium source includes one or more of lithium hydroxide, lithium carbonate and lithium nitrate;

[0014] Optionally, in step S1, the molar ratio of the lithium source calculated as lithium element to the nickel-cobalt-manganese hydroxide ternary precursor calculated as metal element is (1.2-1.3):1, preferably (1.2-1.25):1.

[0015] Optionally, in step S1, the first heat treatment is performed in two stages;

[0016] The conditions of the first heat treatment include: time of 3-5h, temperature of 400-500℃;

[0017] The conditions of the second heat treatment include: time of 10-20h, temperature of 650-850℃;

[0018] The oxygen content in the first oxygen-containing atmosphere is greater than 90 volume %.

[0019] Optionally, in step S2, in the solution containing ammonium salt, the concentration of ammonium salt is 0.5-1.0 mol / L;

[0020] The ammonium salt includes one or more of NH4HCO3, (NH4)2CO3, (NH4)2SO4 and NH4NO3.

[0021] Optionally, step S2 further comprises: adjusting the pH value of the solution containing the ammonium salt to 2.0-4.0; or,

[0022] Step S2 further includes: adjusting the pH value of the mixture obtained by contacting the first material with the solution containing ammonium salt to 2.0-4.0.

[0023] Optionally, in step S2, the weight ratio of the first material to the solution containing the ammonium salt is 1:(3-5).

[0024] Optionally, in step S2, the immersion conditions include: time of 1-3 hours, temperature of 5-30°C;

[0025] Optionally, the conditions of the second heat treatment include: a time of 5-10 hours, a temperature of 500-600° C., and an oxygen content in the second oxygen-containing atmosphere of more than 90% by volume.

[0026] A second aspect of the present disclosure provides a ternary positive electrode material having a core-shell structure prepared by the method described in the first aspect of the present disclosure.

[0027] Optionally, the ternary positive electrode material comprises a nickel-cobalt-manganese ternary material core and a nickel-cobalt-manganese ternary material shell;

[0028] The thickness of the nickel-cobalt-manganese ternary material shell is 5-30 nm, preferably 10-20 nm.

[0029] Optionally, the content of the Mn element is 15-25 wt% relative to the total weight of the nickel-cobalt-manganese ternary material shell.

[0030] A third aspect of the present disclosure provides a lithium-ion battery, comprising a positive electrode, an electrolyte, and a negative electrode, wherein the positive electrode comprises the ternary positive electrode material described in the second aspect of the present disclosure.

[0031] Through the above technical scheme, the method disclosed in the present invention first synthesizes a high-nickel ternary positive electrode material by heat treating a lithium source and a nickel-cobalt-manganese hydroxide ternary precursor; then, by contacting with a solution containing an ammonium salt, part of the nickel element is removed, and part of the crystalline lithium element is removed at the same time, and the Co element and the Mn element cannot be dissociated; lithium rearrangement and crystal repair between the crystalline phases are then carried out by heat treatment to form a ternary positive electrode material with a core-shell structure; the method disclosed in the present invention avoids the contradiction between the electrochemical properties and the heat treatment temperature between the core material and the shell material in the traditional method, as well as the problems of disappearance of the core-shell structure and poor uniformity of particle coating at high temperatures (>600°C); in the prepared ternary positive electrode material, the core material has an elemental composition of high nickel and low manganese, the shell material has an elemental composition of rich manganese and low nickel, and the above-mentioned ternary positive electrode material has low residual alkali, and when used as a positive electrode material for a lithium-ion battery, it has high electrochemical properties and good thermal stability.

[0032] Other features and advantages of the present disclosure will be described in detail in the following detailed description. DETAILED DESCRIPTION

[0033] The following describes the specific embodiments of the present disclosure in detail. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present disclosure and are not intended to limit the present disclosure.

[0034] The first aspect of the present disclosure provides a method for preparing a ternary cathode material having a core-shell structure, the method comprising the following steps:

[0035] S1. Mixing a lithium source with a nickel-cobalt-manganese hydroxide ternary precursor, and subjecting the obtained mixture to a first heat treatment in a first oxygen-containing atmosphere to obtain a first material;

[0036] S2. Contacting the first material with a solution containing an ammonium salt and impregnating the solution, and subjecting the obtained solid to a second heat treatment in a second oxygen-containing atmosphere.

[0037] The method disclosed herein synthesizes a high-nickel ternary positive electrode material by heat treatment, and then removes part of the surface nickel element and part of the crystalline lithium element by contact and complexation with a solution containing an ammonium salt, while avoiding the dissociation of the Co element and the Mn element, thereby forming a structure with high manganese and low nickel in the shell and high nickel and low manganese in the core; then, lithium rearrangement and crystal repair between the crystalline phases are carried out by heat treatment, and finally a ternary positive electrode material with a core-shell structure is formed.

[0038] According to one embodiment of the present disclosure, in step S1, the chemical formula of the nickel-cobalt-manganese hydroxide ternary precursor is Ni x Co y Mn z (OH)2, wherein x+y+z=1, 0.7≤x≤0.9, 0.05≤y≤0.2, 0.05≤z≤0.1.

[0039] In the present disclosure, the types of lithium sources are conventional in the art, for example, may include one or more of lithium hydroxide, lithium carbonate and lithium nitrate; when there are two or more types of lithium sources, the present disclosure does not impose specific restrictions on their ratio.

[0040] In order to obtain a higher material discharge specific capacity, according to one embodiment of the present disclosure, in step S1, the molar ratio of the lithium source calculated as lithium element to the nickel cobalt manganese hydroxide ternary precursor calculated as metal element is (1.2-1.3):1, preferably (1.2-1.25):1.

[0041] In the present disclosure, in step S1, the method of mixing the lithium source and the nickel-cobalt-manganese hydroxide ternary precursor is conventional in the art, and the lithium source and the nickel-cobalt-manganese hydroxide ternary precursor can be mixed evenly.

[0042] According to one embodiment of the present disclosure, in step S1, the first heat treatment is carried out in two stages; the conditions of the first heat treatment include: time is 3-5 hours, temperature is 400-500°C; the conditions of the second heat treatment include: time is 10-20 hours, temperature is 650-850°C, and the oxygen content in the first oxygen-containing atmosphere is greater than 90 volume%, and the first oxygen-containing atmosphere can be, for example, an oxygen atmosphere.

[0043] According to an embodiment of the present disclosure, the method further includes: crushing the first material using an air flow crusher, and then performing step S2.

[0044] According to one embodiment of the present disclosure, in step S2, in the solution containing ammonium salt, the concentration of ammonium salt is 0.5-1.0 mol / L. The ammonium salt solution of this concentration can complex part of the Ni element out of the bulk structure, thereby forming a low-nickel shell.

[0045] In the present disclosure, the types of ammonium salts are conventional in the art, for example, they may include one or more of NH4HCO3, (NH4)2CO3, (NH4)2SO4 and NH4NO3. When there are more than two ammonium salts, the present disclosure does not impose specific restrictions on their ratio.

[0046] In order to further improve the removal effect of the surface Ni element, according to one embodiment of the present disclosure, the method further includes: adjusting the pH value of the solution containing ammonium salt to 2.0-4.0; or, the method includes: adjusting the pH value of the mixed material obtained by contacting the first material with the solution containing ammonium salt to 2.0-4.0; the method for adjusting the pH value can be adding an acid solution, such as one or more of sulfuric acid solution, hydrochloric acid solution and citric acid, and the concentration of the acid solution is conventional in the art.

[0047] In the present disclosure, unless otherwise specified, pH values ​​are all measured at 25° C. and normal pressure.

[0048] According to one embodiment of the present disclosure, the weight ratio of the first material to the solution containing the ammonium salt is 1:(3-5).

[0049] In order to make the ternary material have a more suitable shell thickness and higher thermal stability, and avoid the transitional separation of lithium ions, so that the material has a higher discharge specific capacity, according to one embodiment of the present disclosure, the impregnation conditions include: time of 1-3h, temperature of 5-30°C, and the impregnation can be carried out under stirring conditions.

[0050] According to one embodiment of the present disclosure, step S2 also includes: performing solid-liquid separation on the solid-liquid mixture obtained by immersion, washing and drying the obtained solid, and performing a second heat treatment on the dried solid under an oxygen atmosphere; wherein the solid-liquid separation method can be, for example, filtration, and the washing and drying methods and conditions are conventional in the art, and the present disclosure does not impose specific limitations.

[0051] In order to further increase the initial thermal runaway temperature of the material, reduce the heat release of the material, and make it have better cycle stability, according to one embodiment of the present disclosure, in step S2, the conditions of the second heat treatment include: time is 5-10h, temperature is 500-600℃, preferably 550-600℃, and the oxygen content in the second oxygen-containing atmosphere is more than 90% by volume. The second oxygen-containing atmosphere can be, for example, an oxygen atmosphere.

[0052] In the present disclosure, the heat treatment method may be calcination. The specific method is conventional in the art and is not specifically required by the present disclosure.

[0053] A second aspect of the present disclosure provides a ternary positive electrode material having a core-shell structure prepared by the method described in the first aspect of the present disclosure.

[0054] According to one embodiment of the present disclosure, the ternary positive electrode material includes a nickel-cobalt-manganese ternary material core and a nickel-cobalt-manganese ternary material shell. The core and the shell have the same element types, both containing Li, Ni, Co, Mn and O elements. The Mn element content in the shell is greater than the Mn element content in the core, and the Ni element content in the core is greater than the Ni element content in the shell.

[0055] According to one embodiment of the present disclosure, the thickness of the shell of the nickel-cobalt-manganese ternary material is 5-30nm, preferably 10-20nm. The shell thickness is relatively thin, that is, the proportion of the shell weight to the total weight is very low, and the discharge specific capacity of the material will not be significantly reduced; the shell thickness can be tested by argon ion sputtering. When the content of the Ni element and the Mn element suddenly changes, it is the boundary between the shell and the core; at different positions of the shell, the content of each metal element is uniform.

[0056] According to one embodiment of the present disclosure, the content of the Mn element is 15-25 wt% relative to the total weight of the shell. Although the shell thickness is relatively thin, the higher Mn content in the shell can significantly reduce the side reactions between the material and the electrolyte, thereby improving the thermal stability of the material and the safety performance of the battery.

[0057] According to one embodiment of the present disclosure, the chemical formula of the ternary cathode material is Li 1+u Ni x Co y Mn z O2, where 0≤u≤0.2, x+y+z=1, 0.69≤x≤0.89, 0.05≤y≤0.2, 0.06≤z≤0.11.

[0058] A third aspect of the present disclosure provides a lithium-ion battery, comprising a positive electrode, an electrolyte, and a negative electrode, wherein the positive electrode comprises the ternary positive electrode material described in the second aspect of the present disclosure.

[0059] According to one embodiment of the present disclosure, the negative electrode is conventional in the art, and for example, may include one or more of a lithium sheet, a carbon material, and a silicon-carbon composite material.

[0060] According to one embodiment of the present disclosure, the electrolyte is conventional in the art, for example, the solute is lithium hexafluorophosphate, and the solvent may include one or more of ethylene carbonate, propylene carbonate, ethyl methyl carbonate and dimethyl carbonate.

[0061] The present disclosure is further illustrated below by way of examples, but the present disclosure is not limited thereby. Unless otherwise specified, the raw materials, reagents, instruments and equipment designed in the examples of the present disclosure can all be purchased.

[0062] XPS test conditions and instrument model: The nickel, cobalt and manganese metal contents on the sample surface were determined using a Thermo Fisher Thermo ESCALAB 250 X-ray photoelectron spectrometer (XPS) from the United States. An Al Kα X-ray emission source with a power of 300 W was used, and charge correction was performed using C1s (284.8 eV) as the reference peak.

[0063] ICP test conditions and instrument model: The contents of Li, Ni, Co, Mn and other elements in the sample materials were determined using a Shimadzu ICP-7500 inductively coupled plasma spectrometer (ICP).

[0064] Shell thickness test method: Ar ion sputtering etching method is used to measure the shell thickness of the material;

[0065] The test method for total residual lithium content is as follows: weigh 15g of sample material and add it to 60mL of deionized water. Stir vigorously for 15 minutes, then filter with a sand core funnel and add a small amount of deionized water to wash the filter cake. The total volume of the filtrate is recorded as V 总 Use a pipette to transfer 25mL of the filtrate to a conical flask, add a small amount of colorless phenolphthalein indicator, and titrate with 0.05-0.1mol / L dilute hydrochloric acid. When the red color of the filtrate disappears, the volume of hydrochloric acid used is recorded as V1 (at this time, OH - All are neutralized, and CO3 2- Neutralized only to HCO3 - ). Then add a few drops of methyl red-bromocresol green indicator to the filtrate. The solution will turn green. Continue titrating with dilute hydrochloric acid until the filtrate turns red. Boil for 2 minutes to drive out CO2. Cool and continue titrating until the filtrate suddenly turns dark red. Note the volume of hydrochloric acid used at this time (V2). The formula for calculating the residual lithium content is as follows:

[0066]

[0067]

[0068]

[0069] Where:

[0070] C HCl is the concentration of the hydrochloric acid standard titration solution (mol / L);

[0071] V1 and V2 are the volumes of the hydrochloric acid titration solution for the first and second times (mL);

[0072] m is the mass of the sample (g);

[0073] V 总 is the total volume of the filtrate (mL);

[0074] M Li2CO3 is the molar mass of lithium carbonate (g / mol);

[0075] M LiOH is the molar mass of lithium hydroxide (g / mol);

[0076] M Li is the molar mass of lithium (g / mol).

[0077] Example 1

[0078] The following steps were used to prepare the core-shell structured ternary cathode material A1:

[0079] (1) Lithium hydroxide and nickel-cobalt-manganese composite hydroxide ternary precursor Ni were weighed at a molar ratio of Li / metal element = 1.25. 0.88 Co 0.05 Mn 0.07 (OH)2, mix the two evenly;

[0080] (2) calcining the obtained mixture at 500°C for 5 h in an oxygen atmosphere and then calcining at 720°C for 15 h, and crushing the sintered material using a jet crusher to obtain a ternary basic powder;

[0081] (3) adding the ternary basic powder to a 0.5 mol / L ammonium carbonate solution, adjusting the pH value of the mixture to 3.0 with a 0.01 mol / L sulfuric acid solution, stirring and soaking the mixture at 25°C for 2 hours, then filtering, washing, and drying; the weight ratio of the ternary basic powder to the ammonium carbonate solution is 1:3;

[0082] (4) The dried material was calcined in an oxygen atmosphere at a temperature of 550°C for 5 h to obtain the ternary cathode material A1. The parameters are listed in Table 1.

[0083] Example 2

[0084] The method of Example 1 was used to prepare a ternary positive electrode material A2 having a core-shell structure, except that Li / metal element=1.2. The parameters of the ternary positive electrode material A2 are listed in Table 1.

[0085] Example 3

[0086] The method of Example 1 was used to prepare a ternary positive electrode material A3 having a core-shell structure, except that Li / metal element=1.3. The parameters of the ternary positive electrode material A3 are listed in Table 1.

[0087] Example 4

[0088] The method of Example 1 was used to prepare a ternary positive electrode material A4 having a core-shell structure, except that Li / metal element=1.1. The parameters of the ternary positive electrode material A4 are listed in Table 1.

[0089] Example 5

[0090] The method of Example 1 was used to prepare a ternary positive electrode material A5 having a core-shell structure, except that Li / metal element=1.35. The parameters of the ternary positive electrode material A5 are listed in Table 1.

[0091] Example 6

[0092] The method of Example 1 was used to prepare a ternary cathode material A6 having a core-shell structure, except that the concentration of the ammonium carbonate solution was 1.0 mol / L and the immersion time was 1 h. The parameters of the ternary cathode material A6 are listed in Table 1.

[0093] Example 7

[0094] The method of Example 1 was used to prepare a ternary cathode material A7 having a core-shell structure, except that the concentration of the ammonium carbonate solution was 0.5 mol / L and the immersion time was 3 h. The parameters of the ternary cathode material A7 are listed in Table 1.

[0095] Example 8

[0096] The method of Example 1 was used to prepare a ternary positive electrode material A8 having a core-shell structure, except that the immersion time was 4 h. The parameters of the ternary positive electrode material A8 are listed in Table 1.

[0097] Example 9

[0098] The method of Example 1 was used to prepare a ternary positive electrode material A9 having a core-shell structure, except that the immersion time was 0.5 h. The parameters of the ternary positive electrode material A9 are listed in Table 1.

[0099] Example 10

[0100] The method of Example 1 was used to prepare a ternary cathode material A10 having a core-shell structure, except that the ammonium carbonate solution was replaced with an ammonium sulfate solution of equal concentration. The parameters of the ternary cathode material A10 are listed in Table 1.

[0101] Example 11

[0102] The method of Example 1 was used to prepare a ternary positive electrode material A11 having a core-shell structure, except that the calcination temperature in step (4) was 400° C. The parameters of the ternary positive electrode material A11 are listed in Table 1.

[0103] Comparative Example 1

[0104] The following steps were used to prepare the ternary cathode material D1:

[0105] (1) Lithium hydroxide and nickel-cobalt-manganese composite hydroxide ternary precursor Ni were weighed at a molar ratio of Li / metal element = 1.05. 0.88 Co 0.05 Mn 0.07 (OH)2, mix the two evenly;

[0106] (2) calcining the obtained mixture at 500°C for 5 h in an oxygen atmosphere and then calcining at 720°C for 15 h, and crushing the sintered material using a jet crusher to obtain a ternary basic powder;

[0107] (3) The ternary basic powder was calcined at 600 °C for 5 h to obtain the ternary positive electrode material D1. The parameters are listed in Table 1.

[0108] Comparative Example 2

[0109] The following steps are used to prepare the ternary cathode material D2:

[0110] (1) preparing a 2 mol / L nickel-cobalt-manganese metal sulfate mixed solution A, wherein the ratio of n(Ni):n(Co):n(Mn) in solution A is 0.88:0.05:0.07; preparing a 2 mol / L nickel-cobalt-manganese metal sulfate mixed solution B, wherein the ratio of n(Ni):n(Co):n(Mn) in solution B is 0.4:0.25:0.35; preparing a 6 mol / L sodium hydroxide solution C as a precipitant; and preparing a 10% by mass ammonia solution D as a complexing agent;

[0111] (2) Under nitrogen protection, solution A, solution C, and solution D were added to a strongly stirred reactor in parallel to carry out precipitation reaction, wherein the pH value was maintained at 11.0 (50°C as the reference temperature), the reaction temperature was 60°C, and the reaction time was 100h. When the medium particle size D50 of the material was 10.0μm, the addition of solution A was stopped, and solution B, solution C, and solution D were added to the strongly stirred reactor in parallel to continue the precipitation reaction, the reaction temperature was 60°C, the reaction pH value was 10.5 (50°C as the reference temperature), and the reaction time was 15min. After stopping the addition, stirring and aging were continued for 3h;

[0112] (3) The reaction-completed material was filtered, washed, and then dried at 120°C for 24 h and passed through a 325-mesh sieve to obtain a ternary precursor [(Ni 0.88 Co 0.05 Mn 0.07 ) 0.9975 (Ni 0.40 Co 0.25 Mn 0.35 ) 0.0025 ](OH)2;

[0113] (4) lithium hydroxide and the nickel-cobalt-manganese composite hydroxide ternary precursor with a core-shell structure synthesized in step (3) were weighed at a molar ratio of Li / metal element = 1.05, and the two were mixed evenly;

[0114] (5) The obtained mixture was calcined at 500°C for 5 h in an oxygen atmosphere and then calcined at 720°C for 15 h. The sintered material was crushed using an air flow crusher to obtain a ternary basic powder;

[0115] (6) The ternary basic powder was calcined at 600 °C for 5 h to obtain the ternary positive electrode material D2. The parameters are listed in Table 1.

[0116] Table 1

[0117]

[0118]

[0119] In Table 1, the Co element content, Mn element content and Ni element content of the shell are tested by XPS, and the total Co element content, total Mn element content and total Ni element content of the ternary material are tested by ICP.

[0120] Test Case

[0121] The electrochemical tests were performed on the ternary cathode materials A1-A11 and D1-D2 prepared in the above examples and comparative examples. The specific steps are as follows:

[0122] Accurately weigh 0.8g of active material and 0.1g of acetylene black conductive agent and grind them in an agate mortar for 0.5h. Place the ground material into a 25x40mm weighing bottle, add 2.5g of a 4% by weight PVDF solution in NMP, place a magnetic stirrer, and magnetically stir for 5h. Use a wet film applicator with a 200μm slit to coat the stirred slurry onto 16μm-thick aluminum foil and dry at 105°C for 1h. Roll the dried electrode and punch it into 12mm thick sheets using a sheet puncher. Dry the sheet in a vacuum oven at 120°C for 12h before use.

[0123] In an argon-filled glove box, a CR2032 button-type half-cell was fabricated using a lithium sheet as the negative electrode, a 1 mol / L LiPF6 solution in EC / EMC / DMC (1:1:1, mass ratio) as the electrolyte, Celgard 2300 as the separator, and the prepared electrode sheet as the positive electrode. The resulting cell was left to rest for 6 hours before electrochemical performance testing.

[0124] The battery's charge and discharge voltage was 2.75-4.3V. It was first cycled five times at a charge and discharge rate of 0.1C (1C = 220mA / g), and then subjected to a long-term cycling performance test at a charge and discharge rate of 0.5C at a test temperature of 25°C. The formed battery was charged to 4.2V, then disassembled and the positive electrode removed. A DSC instrument was used to test the initial thermal runaway temperature and heat release of the positive electrode.

[0125] The electrochemical test parameters and thermal performance parameter test results are listed in Table 2.

[0126] Table 2

[0127]

[0128] According to the above data, the method of the present invention can be used to prepare a ternary material with a core-shell structure, the core material has an elemental composition of high nickel and low manganese, and the shell material has an elemental composition of rich manganese and low nickel. The residual alkali of the above-mentioned ternary positive electrode material is low, and it is used as a positive electrode material for lithium-ion batteries. It has a high discharge specific capacity and cycle stability, as well as good thermal stability; specifically, according to the comparison of Examples 1-3 and Examples 4-5, when the ratio of the lithium source calculated as lithium element to the nickel-cobalt-manganese hydroxide ternary precursor calculated as metal element is in the range of (1.2-1.3):1, a higher discharge specific capacity can be obtained. ; Further, according to the comparison between Example 1-2 and Example 3, it can be seen that when the ratio of the lithium source calculated as lithium element to the nickel-cobalt-manganese hydroxide ternary precursor calculated as metal element is in the preferred range of (1.2-1.25):1, the amount of residual lithium can be further reduced; According to the comparison between Example 1 and Examples 8-9, when the immersion time is 1-3h, a higher discharge specific capacity, initial thermal runaway temperature and total heat release can be obtained at the same time; According to the comparison between Example 1 and Example 11, when the temperature of the second heat treatment is in the range of 500-600℃, a higher discharge specific capacity and cycle stability can be obtained at the same time.

[0129] The preferred embodiments of the present disclosure are described in detail above. However, the present disclosure is not limited to the specific details of the above embodiments. Within the technical concept of the present disclosure, various simple modifications can be made to the technical solutions of the present disclosure, and these simple modifications all fall within the scope of protection of the present disclosure.

[0130] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present disclosure will not further describe various possible combinations.

[0131] In addition, the various embodiments of the present disclosure may be arbitrarily combined, and as long as they do not violate the concept of the present disclosure, they should also be regarded as the contents disclosed by the present disclosure.

Claims

1. A method for preparing a ternary cathode material having a core-shell structure, characterized in that: The method comprises the following steps: S1. Mixing a lithium source with a nickel-cobalt-manganese hydroxide ternary precursor, and subjecting the obtained mixture to a first heat treatment in a first oxygen-containing atmosphere to obtain a first material; S2. Contacting the first material with a solution containing an ammonium salt and impregnating the solution, and subjecting the obtained solid to a second heat treatment in a second oxygen-containing atmosphere.

2. The method according to claim 1, wherein In step S1, the chemical formula of the nickel-cobalt-manganese hydroxide ternary precursor is Ni x Co y Mn z (OH)2, wherein x+y+z=1, 0.7≤x≤0.9, 0.05≤y≤0.2, 0.05≤z≤0.

1.

3. The method according to claim 1, wherein The lithium source includes one or more of lithium hydroxide, lithium carbonate and lithium nitrate; Optionally, in step S1, the molar ratio of the lithium source calculated as lithium element to the nickel-cobalt-manganese hydroxide ternary precursor calculated as metal element is (1.2-1.3):1, preferably (1.2-1.25):

1.

4. The method according to claim 1, wherein In step S1, the first heat treatment is performed in two stages; The conditions of the first heat treatment include: time of 3-5h, temperature of 400-500℃; The conditions of the second heat treatment include: time of 10-20h, temperature of 650-850℃; The oxygen content in the first oxygen-containing atmosphere is greater than 90 volume %.

5. The method according to claim 1, wherein In step S2, in the solution containing ammonium salt, the concentration of ammonium salt is 0.5-1.0 mol / L; The ammonium salt includes one or more of NH4HCO3, (NH4)2CO3, (NH4)2SO4 and NH4NO3.

6. The method according to claim 1, wherein Step S2 further comprises: adjusting the pH value of the solution containing the ammonium salt to 2.0-4.0; or, Step S2 further includes: adjusting the pH value of the mixture obtained by contacting the first material with the solution containing ammonium salt to 2.0-4.

0.

7. The method according to claim 1, wherein In step S2, the weight ratio of the first material to the solution containing the ammonium salt is 1:(3-5).

8. The method according to claim 1, wherein In step S2, the immersion conditions include: time of 1-3 hours, temperature of 5-30°C; Optionally, the conditions of the second heat treatment include: a time of 5-10 hours, a temperature of 500-600° C., and an oxygen content in the second oxygen-containing atmosphere of more than 90% by volume.

9. A ternary cathode material having a core-shell structure prepared by the method according to any one of claims 1 to 8.

10. The ternary cathode material according to claim 9, wherein: The ternary positive electrode material comprises a nickel-cobalt-manganese ternary material core and a nickel-cobalt-manganese ternary material shell; The thickness of the nickel-cobalt-manganese ternary material shell is 5-30 nm, preferably 10-20 nm.

11. The ternary cathode material according to claim 10, wherein: Relative to the total weight of the nickel-cobalt-manganese ternary material shell, the content of Mn element is 15-25wt%.

12. A lithium-ion battery comprising a positive electrode, an electrolyte and a negative electrode, characterized in that: The positive electrode comprises the ternary positive electrode material according to any one of claims 9 to 11.

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