Cathode material, preparation method thereof, cathode sheet, secondary battery, and electric device

Abstract

The application provides a positive electrode material, a preparation method thereof, a positive electrode sheet, a secondary battery and an electric device. The positive electrode material comprises an inner core, a first coating layer covering at least part of the outer surface of the inner core, and a second coating layer covering at least part of the outer surface of the inner core with the first coating layer. The material of the first coating layer comprises lithium and an X element. The material of the second coating layer comprises the X element and a Z element. By forming the first coating layer, residual lithium on the surface of the inner core can be eliminated. The first coating layer has a high oxygen vacancy formation energy, which can reduce or even avoid oxygen dissolution in the inner core, reduce or even avoid the side reaction between oxygen and electrolyte due to oxygen dissolution, and improve the cycle life and safety of the secondary battery. By forming a uniform protective film on the surface of the inner core with the first coating layer, the overflow of oxygen from the position without the first coating layer can be reduced or even avoided.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a positive electrode material, a preparation method thereof, a positive electrode sheet, a secondary battery and an electric device. BACKGROUND

[0002] Secondary batteries are widely used in various consumer electronic products and electric vehicles due to their light weight, no pollution and no memory effect. With the development of secondary battery technology, people have higher and higher requirements for the energy density of secondary batteries. In order to improve the energy density of the battery, a nickel-containing positive electrode material is often used. However, in the related art, the secondary battery using the nickel-containing positive electrode material has the problem of cycle life deterioration. SUMMARY

[0003] Therefore, it is necessary to provide a positive electrode material, a preparation method thereof, a positive electrode sheet, a secondary battery and an electric device to improve the cycle life and safety of the secondary battery.

[0004] To achieve the above-mentioned purpose, the first aspect of the present application provides a positive electrode material, comprising:

[0005] a core, the material of the core comprising a lithium-nickel-cobalt metal composite oxide; in the lithium-nickel-cobalt metal composite oxide, the molar percentage of nickel element in the total metal elements other than lithium element is 30%-95%;

[0006] a first coating layer coated on at least part of the outer surface of the core, the material of the first coating layer comprising lithium element and X element; and

[0007] a second coating layer coated on at least part of the outer surface of the core with the first coating layer, the material of the second coating layer comprising X element and Z element;

[0008] wherein the X element comprises one or more of fluorine element, chlorine element and bromine element, and the Z element comprises one or more of iron element, titanium element, aluminum element, zinc element and magnesium element.

[0009] The positive electrode material of the present application can eliminate residual lithium on the surface of the core by forming a first coating layer containing lithium and X elements on at least part of the surface of the core, and can reduce or even avoid oxygen dissolution in the core, which is beneficial to improving the structural stability of the core during the cycle process, reducing or even avoiding the side reaction between oxygen and electrolyte due to oxygen dissolution, and improving the cycle life and safety of the secondary battery. By forming a second coating layer on at least part of the surface of the core with the first coating layer, the problem of uneven coating of the first coating layer caused by too low residual lithium on the surface of the core can be reduced or even avoided, and a uniform protective film is formed on the surface of the first coating layer to reduce or even avoid oxygen overflow from the position without the first coating layer.

[0010] In some embodiments, the second coating layer coats the entire outer surface of the core with the first coating layer.

[0011] In some embodiments, the lithium nickel cobalt metal composite oxide has a structural formula of Li x (Ni a Co b Mn c ) 1-d M d O 2- y A y , wherein 0.2≤x≤1.2, a+b+c=1, M elements include one or more of Al, Mg, Zr, W, Ti and Y, and A elements include one or more of B, P and F.

[0012] In some embodiments, the first coating layer has at least one of the following characteristics:

[0013] (1) The lithium element contained in the first coating layer accounts for 0.1%-0.3% of the mass percentage of the positive electrode material;

[0014] (2) The material of the first coating layer includes a salt containing both the lithium element and the X element;

[0015] (3) The thickness of the first coating layer is 3nm-10nm.

[0016] In some embodiments, the second coating layer has at least one of the following characteristics:

[0017] (1) The Z element contained in the second coating layer accounts for 0.2%-0.6% of the mass percentage of the positive electrode material;

[0018] (2) The material of the second coating layer includes a salt containing both the Z element and the X element;

[0019] (3) the thickness of the second coating layer is 5-10 nm.

[0020] The second aspect of the present application provides a preparation method of the positive electrode material of the first aspect of the present application, comprising the following steps:

[0021] dispersing the core in a solvent, adding a precursor containing X element at 40-70°C to perform a first reaction, and preparing the first coating layer on at least part of the surface of the core;

[0022] adding a precursor containing Z element to the reaction solution of the first reaction at 40-70°C to perform a second reaction, and preparing the second coating layer on at least part of the surface of the core with the first coating layer.

[0023] In some embodiments, the first reaction comprises at least one of the following conditions:

[0024] (1) the precursor containing X element comprises a salt containing the X element and being acidic;

[0025] Optionally, the precursor containing X element comprises one or more of ammonium fluoride, ammonium chloride and ammonium bromide;

[0026] (2) the molar ratio of the X element contained in the precursor containing X element to the lithium element remaining on the surface of the core is (1.5-6):1; optionally (2-4):1; more optionally 3:1;

[0027] (3) the time for reaction after adding the precursor containing X element is 20-40 min;

[0028] (4) the solvent comprises a fatty alcohol;

[0029] Optionally, the solvent comprises one or more of ethanol, ethylene glycol, propanol, butanol and butylene glycol.

[0030] In some embodiments, the second reaction comprises at least one of the following conditions:

[0031] (1) the precursor containing Z element comprises one or more of nitrate, carbonate, sulfate and sulfite containing Z element; the Z element comprises one or more of iron element, titanium element, aluminum element, zinc element and magnesium element;

[0032] (2) the molar ratio of the Z element contained in the precursor containing Z element to the lithium element remaining on the surface of the core is (0.125-2.5):1; optionally (0.25-1.5):1; more preferably 0.66:1;

[0033] (3) the time for the reaction after the addition of the Z element-containing precursor is 20 min to 40 min.

[0034] In some embodiments, the preparation method further comprises the following steps:

[0035] The reaction solution obtained in the second reaction is filtered, the filter residue is dried, and the filter residue is subjected to a heating treatment in a protective atmosphere to prepare the positive electrode material.

[0036] In some embodiments, the heating treatment comprises at least one of the following conditions:

[0037] (1) the protective gas used comprises one or more of nitrogen and inert gas;

[0038] Optionally, the protective gas used comprises one or more of nitrogen and argon;

[0039] (2) the reaction temperature is 300°C to 400°C, and the reaction time is 3 h to 5 h.

[0040] A third aspect of the present application provides a positive electrode tab, comprising:

[0041] a positive electrode current collector; and

[0042] a positive electrode active material layer located on at least one side of the positive electrode current collector, the positive electrode active material layer comprising the positive electrode material of the first aspect of the present application.

[0043] A fourth aspect of the present application provides a secondary battery comprising the positive electrode tab of the third aspect of the present application.

[0044] A fifth aspect of the present application provides an electric device comprising the secondary battery of the fourth aspect of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0045] Figure 1 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0046] Figure 2 is a schematic diagram of a secondary battery according to an embodiment of the present application. Figure 1 is an exploded view of a secondary battery according to an embodiment of the present application.

[0047] Figure 3 is a schematic diagram of an electric device using a secondary battery according to an embodiment of the present application as a power source.

[0048] REFERENCE SIGNS

[0049] 1 secondary battery; 11 housing; 12 electrode assembly; 13 cover plate; 2 electric device. DETAILED DESCRIPTION

[0050] For the purpose of promoting an understanding of the application, the application will now be described in greater detail with reference to the figures. The preferred embodiments of the application are illustrated in the figures. However, the application can be implemented in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the application to those skilled in the art.

[0051] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description of the application herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0052] In this application, the technical features described in an open way include both the closed technical solution consisting of the listed features and the open technical solution containing the listed features.

[0053] In this application, when referring to a numerical interval, unless otherwise specified, the numerical interval is considered to be continuous and includes the minimum value and the maximum value of the range, as well as every value between the minimum value and the maximum value. Further, when the range refers to integers, every integer between the minimum value and the maximum value of the range is included. In addition, when multiple ranges are provided to describe a feature or a characteristic, the ranges can be combined. In other words, unless otherwise indicated, all ranges disclosed herein are to be understood as including any and all sub-ranges subsumed therein.

[0054] In this application, when referring to a data range, if only the unit is provided after the right endpoint, it means that the units of the left endpoint and the right endpoint are the same. For example, 10-1000 nm means that the units of the left endpoint "10" and the right endpoint "1000" are both nm (nanometer).

[0055] In this application, when referring to "a plurality of", "a plurality of kinds", "a plurality of times", etc., unless otherwise specified, it means more than two or equal to two in quantity. For example, "a plurality of kinds" means more than or equal to two kinds. Only some numerical ranges are specifically disclosed herein. However, any lower limit can be combined with any upper limit to form a range that is not explicitly recited; and any lower limit can be combined with other lower limits to form a range that is not explicitly recited, and similarly any upper limit can be combined with any other upper limit to form a range that is not explicitly recited. In addition, each individually disclosed point or single numerical value can itself be combined as a lower limit or an upper limit with any other point or single numerical value or with other lower limits or upper limits to form a range that is not explicitly recited.

[0056] The "range" disclosed in the present application is defined in the form of a lower limit and an upper limit, and the given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of the specific range. The range defined in this way can include the end value or not include the end value.

[0057] The temperature parameter in the present application, if not particularly limited, allows for constant temperature treatment, and also allows for treatment within a certain temperature range. The constant temperature treatment allows the temperature to fluctuate within the accuracy range controlled by the instrument.

[0058] In the related art, the secondary battery using the nickel-containing positive electrode material has the problem of cycle life deterioration. The skilled person found through research that lithium-containing compounds, such as lithium oxides, lithium hydroxides or lithium salts, etc., are easily left on the surface of the material during the preparation process of the nickel-containing positive electrode material, and the lithium-containing compounds are mainly concentrated on the surface of the nickel-containing positive electrode material particles, affecting the electronic conductivity of the nickel-containing positive electrode material, causing greater polarization during the battery cycle process; at the same time, the lithium-containing compounds will also have a side reaction with the electrolyte, causing the consumption of effective components in the electrolyte, and thus leading to the deterioration of the cycle life of the battery; in addition, the lithium carbonate on the surface of the nickel-containing positive electrode material particles will also cause serious outgassing at high temperature, causing the internal pressure of the battery to rise, and causing safety problems.

[0059] The skilled person also found during the research process that the nickel-containing positive electrode material is accompanied by the release of lithium ions during the charging process, and the content of Ni 4+ gradually increases, and since Ni 4+ is easily reduced, the transformation of Ni 4+ → Ni 3+ is easy to occur, in order to maintain electrical neutrality, oxygen will be released from the nickel-containing positive electrode material, forming oxygen vacancies, the existence of oxygen vacancies makes the transition metal elements in the nickel-containing positive electrode material more prone to displacement, causing irreversible changes in the crystal structure of the nickel-containing positive electrode material, thereby affecting the electrical performance of the nickel-containing positive electrode material; in addition, the dissolved oxygen will further react with the electrolyte, accelerating the occurrence of side reactions inside the battery, not only deteriorating the electrical performance of the battery, but also leading to a decrease in the safety performance of the battery.

[0060] Based on the above problems, the present application prepares a first coating layer on at least part of the outer surface of the core, and a second coating layer on at least part of the outer surface of the core having the first coating layer, reducing or even avoiding the dissolution of oxygen inside the core, which is conducive to improving the structural stability of the core during the cycle process, reducing or even avoiding the side reaction between oxygen and the electrolyte, and improving the cycle life and safety of the secondary battery.

[0061] The first aspect of the present application provides a positive electrode material, comprising: a core, a first coating layer coated on at least part of the outer surface of the core, and a second coating layer coated on at least part of the outer surface of the core with the first coating layer; the material of the core comprises lithium nickel cobalt metal composite oxide, in the lithium nickel cobalt metal composite oxide, the molar percentage of nickel element in the total metal elements other than lithium element is 30%-95%; the material of the first coating layer comprises lithium element and X element; the material of the second coating layer comprises X element and Z element; X element comprises one or more of fluorine element, chlorine element and bromine element, and Z element comprises one or more of iron element, titanium element, aluminum element, zinc element and magnesium element.

[0062] In the lithium nickel cobalt metal composite oxide, the molar percentage of nickel element in the total metal elements other than lithium element is 30%-95%. As an example, in the lithium nickel cobalt metal composite oxide, the molar percentage of nickel element in the total metal elements other than lithium element can be, but is not limited to, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or a range between any two of the above values.

[0063] As an example, in the above-mentioned lithium nickel cobalt metal composite oxide, the molar percentage of nickel element in the total metal elements other than lithium element can be determined by the following method: using inductively coupled plasma atomic emission spectrometry, the mass percentages of nickel element, cobalt element, manganese element and doped metal elements in the lithium nickel cobalt metal composite oxide are obtained respectively, the molar ratio between the metal elements is obtained after conversion, normalized processing is performed, and the molar percentage of nickel element in the total metal elements other than lithium element is calculated.

[0064] It should be noted that the first coating layer can be coated on part of the outer surface of the core, or the outer surface of the core can be completely coated, and the coating degree of the first coating layer on the core is related to the amount of lithium-containing compounds remaining on the surface of the core. The second coating layer can be coated on part of the outer surface of the core with the first coating layer, or the outer surface of the core with the first coating layer can be completely coated.

[0065] It should be noted that the specific type of X element contained in the first coating layer and the specific type of X element contained in the second coating layer can be the same or different.

[0066] Understandably, by forming the first cladding layer containing lithium element and X element on at least part of the surface of the core, residual lithium on the surface of the core can be eliminated, and meanwhile the first cladding layer has a high oxygen vacancy formation energy, which can reduce or even avoid oxygen dissolution in the core, thereby improving the structural stability of the core during the cycle process, reducing or even avoiding the side reaction between oxygen and electrolyte due to oxygen dissolution, and improving the cycle life and safety of the secondary battery. By forming the second cladding layer on at least part of the outer surface of the core with the first cladding layer, the problem of uneven cladding of the first cladding layer that may be caused by too low residual lithium on the surface of the core can be reduced or even avoided, and a uniform protective film is formed on the surface of the first cladding layer, thereby reducing or even avoiding the overflow of oxygen from the position where the first cladding layer is not cladded.

[0067] As a possible implementation, the first cladding layer is cladded on the entire outer surface of the core; the probability of oxygen dissolution in the core can be greatly reduced.

[0068] In some possible implementations, the second cladding layer is cladded on the entire outer surface of the core with the first cladding layer; the second cladding layer uniformly covers the material surface to form a film-shaped cladding, which can further avoid the overflow of oxygen from the position where the core surface is not cladded, and the effect is better.

[0069] In some implementations, the structural formula of the lithium nickel cobalt metal composite oxide is Li x (Ni a Co b Mn c ) 1-d M d O 2-y A y , wherein 0.2≤x≤1.2, a+b+c=1, the M element includes one or more of Al, Mg, Zr, W, Ti and Y, and the A element includes one or more of B, P and F.

[0070] It should be noted that the secondary battery will be accompanied by lithium ion deintercalation and consumption during the charging and discharging process, and the molar content of lithium ions is different when the secondary battery is discharged to different states. The above x value range includes the molar content of lithium element when the secondary battery is in different charging and discharging states under the voltage condition of 2V-5V.

[0071] In some embodiments, the lithium element contained in the first coating layer accounts for 0.1%-0.3% of the mass percentage of the positive electrode material; when the lithium element contained in the first coating layer accounts for the mass percentage of the positive electrode material within the above range, the residual lithium on the surface of the inner core can be fully converted into the coating product, and the side reaction caused by the residual lithium can be reduced. For example, the lithium element contained in the first coating layer can account for 0.1%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, 0.2%, 0.21%, 0.22%, 0.23%, 0.24%, 0.25%, 0.26%, 0.27%, 0.28%, 0.29%, 0.3% or a range between any two of the above values of the mass percentage of the positive electrode material, but not limited to.

[0072] For example, the mass percentage of the lithium element contained in the first coating layer mentioned above can be determined by the following method: the types and contents of the X element and the Z element contained in the coating layer are determined by ion chromatography, and the remaining part X is obtained by deducting the part of the X element used to form a compound with the Z element, which is used to form the first coating layer together with the lithium element. Based on the mass of the remaining part X element, the mass of the lithium element contained in the first coating layer can be calculated, and then the mass percentage of the lithium element contained in the first coating layer to the positive electrode material can be calculated.

[0073] In some embodiments, the material of the first coating layer includes a salt containing both lithium element and X element; the X element includes one or more of fluorine element, chlorine element and bromine element, and the material of the first coating layer includes one or more of LiF, LiCl and LiBr.

[0074] For example, the composition of the first coating layer mentioned above can be determined by the following method: the types of the X element and the Z element contained in the coating layer are determined by ion chromatography, and then it can be determined that the composition of the first coating layer includes a salt formed by the lithium element and the X element.

[0075] In some embodiments, the thickness of the first coating layer is 3 nm-10 nm. When the thickness of the first coating layer is within the above range, the residual lithium on the surface of the inner core can be removed to a large extent. For example, the thickness of the first coating layer can be 3 nm, 3.5 nm, 4 nm, 4.5 nm, 5 nm, 5.5 nm, 6 nm, 6.5 nm, 7 nm, 7.5 nm, 8 nm, 8.5 nm, 9 nm, 9.5 nm, 10 nm or a range between any two of the above values, but not limited to.

[0076] As an example, the thickness of the first coating layer can be determined by the following method: using a transmission electron microscope to perform phase analysis on the surface of the positive electrode material, and simultaneously obtaining the thickness information of the first coating layer.

[0077] In some embodiments, the Z element contained in the second coating layer accounts for 0.2%-0.6% of the mass percentage of the positive electrode material. When the mass percentage of the Z element contained in the second coating layer accounts for the mass percentage of the positive electrode material within the above range, a uniform second coating layer can be formed, and the surface area that the first coating layer fails to coat can be filled and coated, preventing the electrolyte from directly contacting the surface of the ternary material. As an example, the mass percentage of the Z element contained in the second coating layer can be, but is not limited to, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, or a range between any two of the above values.

[0078] As an example, the mass percentage of the Z element contained in the second coating layer mentioned above can be determined by the following method: using ion chromatography to determine the types and contents of the X element and the Z element contained in the coating layer, and further calculating the mass percentage of the Z element in the positive electrode material.

[0079] As one possible embodiment, the material of the second coating layer includes a salt containing both the Z element and the X element; the Z element includes one or more of iron, titanium, aluminum, zinc, and magnesium, and the X element includes one or more of fluorine, chlorine, and bromine; and the material of the second coating layer includes one or more of iron fluoride, titanium fluoride, aluminum fluoride, zinc fluoride, magnesium fluoride, iron chloride, titanium chloride, aluminum chloride, zinc chloride, magnesium chloride, iron bromide, titanium bromide, aluminum bromide, zinc bromide, and magnesium bromide.

[0080] As an example, the composition of the second coating layer mentioned above can be determined by the following method: using ion chromatography to determine the types of the X element and the Z element contained in the coating layer, and further determining the salt formed by the Z element and the X element in the second coating layer.

[0081] In some embodiments, the thickness of the second coating layer is 5-10 nm; when the thickness of the second coating layer is within the above range, a uniform second coating layer can be formed, and the surface area that the first coating layer fails to coat can be filled and coated, preventing the electrolyte from directly contacting the surface of the ternary material. As an example, the thickness of the second coating layer can be, but is not limited to, 5 nm, 5.5 nm, 6 nm, 6.5 nm, 7 nm, 7.5 nm, 8 nm, 8.5 nm, 9 nm, 9.5 nm, 10 nm, or a range between any two of the above values.

[0082] As an example, the thickness of the second coating layer mentioned above can be determined by the following method: using a transmission electron microscope to perform phase analysis on the surface of the positive electrode material, and synchronously obtaining the thickness information of the second coating layer.

[0083] In some embodiments, the positive electrode material comprises:

[0084] The core, the material of the core comprising lithium nickel cobalt metal composite oxide; in the lithium nickel cobalt metal composite oxide, the molar percentage of the nickel element in the total metal elements other than the lithium element is 30%-95%; the structural formula of the lithium nickel cobalt metal composite oxide is Li x (Ni a Co b Mn c ) 1-d M d O 2-y A y , wherein 0.2≤x≤1.2, the M element comprises one or more of Al, Mg, Zr, W, Ti and Y, and the A element comprises one or more of B, P and F;

[0085] The first coating layer, coated on at least part of the outer surface of the core, the material of the first coating layer comprising lithium element and X element, the X element comprising one or more of fluorine element, chlorine element and bromine element; the lithium element contained in the first coating layer accounts for 0.1%-0.3% of the mass percentage of the positive electrode material; the thickness of the first coating layer is 3nm-10nm; and

[0086] The second coating layer, coated on at least part of the outer surface of the first coating layer, the material of the second coating layer comprising X element and Z element, the Z element comprising one or more of iron element, titanium element, aluminum element, zinc element and magnesium element; the Z element contained in the second coating layer accounts for 0.2%-0.6% of the mass percentage of the positive electrode material; the thickness of the second coating layer is 5nm-10nm.

[0087] The second aspect of the present application provides a preparation method of the positive electrode material of the first aspect of the present application, comprising the following steps:

[0088] Dispersing the core in a solvent, adding a precursor containing X element at 40℃-70℃ to perform a first reaction, and preparing a first coating layer on at least part of the surface of the core;

[0089] Adding a precursor containing Z element to the reaction solution of the first reaction at 40℃-70℃ to perform a second reaction, and preparing a second coating layer on at least part of the surface of the core with the first coating layer.

[0090] The positive electrode material preparation method of the present application performs in-situ coating treatment on the surface of the inner core at a lower temperature, consumes the lithium-containing compounds remaining on the surface of the inner core, and simultaneously prevents oxygen dissolution from the inner core due to the high oxygen vacancy formation energy of the generated coating layer.

[0091] During the preparation process, first, the precursor containing the X element reacts with the lithium-containing compounds remaining on the surface of the inner core to form a salt containing both lithium and X elements, which is coated on the outer surface of the inner core as a first coating layer; then, a precursor containing the Z element is added to the reaction solution, so that the Z element reacts with the excess X element to form a salt containing both Z and X elements, which is coated on the outer surface of the inner core with the first coating layer as a second coating layer.

[0092] In some embodiments, the precursor containing the X element used in the first reaction includes a salt containing the X element and being acidic; since the lithium-containing compounds remaining on the surface of the inner core are alkaline, the addition of the precursor containing the X element which is acidic can neutralize the lithium-containing compounds and consume the residual lithium.

[0093] In some embodiments, the precursor containing the X element used in the first reaction includes one or more of ammonium fluoride, ammonium chloride, and ammonium bromide.

[0094] In some embodiments, the molar ratio of the X element contained in the precursor containing the X element to the lithium element remaining on the surface of the inner core is (1.5-6):1; when the molar ratio of the X element contained in the precursor containing the X element to the lithium element remaining on the surface of the inner core is within the above range, the consumption effect of the residual lithium on the surface of the inner core is better. As an example, the molar ratio of the X element contained in the precursor containing the X element to the lithium element remaining on the surface of the inner core can be, but is not limited to, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 5:1, 5.5:1, 6:1, or a range between any two of the above ratios. Alternatively, the molar ratio of the X element contained in the precursor containing the X element to the lithium element remaining on the surface of the inner core is (2-4):1. Further alternatively, the molar ratio of the X element contained in the precursor containing the X element to the lithium element remaining on the surface of the inner core is 3:1.

[0095] As one possible implementation, the time for the reaction after the addition of the precursor containing the X element is 20 min-40 min; for example, it can be, but is not limited to, 20 min, 25 min, 30 min, 35 min, 40 min, or a range between any two of the above values.

[0096] In some embodiments, the solvent of the first reaction includes a fatty alcohol; alternatively, the solvent of the first reaction includes one or more of ethanol, ethylene glycol, propanol, butanol, and butanediol.

[0097] In some embodiments, the Z-element-containing precursor in the second reaction comprises one or more of a nitrate, a carbonate, a sulfate, and a sulfite of the Z-element; the Z-element comprises one or more of an iron element, a titanium element, an aluminum element, a zinc element, and a magnesium element.

[0098] In some embodiments, the molar ratio of the Z-element contained in the Z-element-containing precursor in the second reaction to the lithium element remaining on the surface of the core is (0.125-2.5):1. When the molar ratio of the Z-element contained in the Z-element-containing precursor in the second reaction to the lithium element remaining on the surface of the core is within the above range, the excess X-element-containing precursor can be consumed. As an example, the molar ratio of the Z-element contained in the Z-element-containing precursor in the second reaction to the lithium element remaining on the surface of the core can be, but is not limited to, 0.125:1, 0.15:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1, 2.5:1, or a range between any two of the above ratios.

[0099] In some possible embodiments, the molar ratio of the Z-element contained in the Z-element-containing precursor in the second reaction to the lithium element remaining on the surface of the core is (0.25-1.5):1.

[0100] In some possible embodiments, the molar ratio of the Z-element contained in the Z-element-containing precursor in the second reaction to the lithium element remaining on the surface of the core is 0.66:1.

[0101] In some embodiments, after the Z-element-containing precursor is added, the reaction is performed for a time of 20 min-40 min; for example, it can be, but is not limited to, 20 min, 25 min, 30 min, 35 min, 40 min, or a range between any two of the above values.

[0102] In some embodiments, the method for preparing the positive electrode material further comprises the following steps:

[0103] The reaction solution obtained in the second reaction is filtered, the filter residue is dried, and the filter residue is subjected to a heating treatment under a protective atmosphere to prepare the positive electrode material.

[0104] Through the heating treatment, the coating layer can be strengthened, and the coating force can be improved, so that the coating layer is not easy to fall off.

[0105] In some embodiments, the protective gas employed in the heating treatment includes one or more of nitrogen and inert gas. Alternatively, the protective gas employed in the heating treatment includes one or more of nitrogen and argon.

[0106] In some embodiments, the reaction temperature of the heating treatment is 300-400℃; for example, it can be, but is not limited to, 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, 390℃, 400℃, or a range between any two of the above values. The reaction time of the heating treatment is 3-5h; for example, it can be 3h, 3.5h, 4h, 4.5h, 5h, or a range between any two of the above values.

[0107] In some embodiments, the method for preparing the positive electrode material includes the following steps:

[0108] The core is dispersed in a solvent, and then a precursor containing an X element is slowly added under the condition of a water bath at 40-70℃ to perform a first reaction to prepare a first coating layer on at least part of the surface of the core; wherein the precursor containing the X element includes one or more of ammonium fluoride, ammonium chloride, and ammonium bromide; the molar ratio of the X element contained in the precursor containing the X element to the lithium element remaining on the surface of the core is (1.5-6):1; the time for the reaction after the addition of the precursor containing the X element is 20-40min; the solvent of the first reaction includes a fatty alcohol;

[0109] A precursor containing a Z element is slowly added to the reaction solution of the first reaction under the condition of a water bath at 40-70℃ to perform a second reaction to prepare a second coating layer on at least part of the surface of the core with the first coating layer; wherein the precursor containing the Z element includes one or more of a nitrate salt, a carbonate salt, a sulfate salt, and a sulfite salt containing the Z element, and the Z element includes one or more of iron element, titanium element, aluminum element, zinc element, and magnesium element; the molar ratio of the Z element contained in the precursor containing the Z element to the lithium element remaining on the surface of the core is (0.125-2.5):1; the time for the reaction after the addition of the precursor containing the Z element is 20-40min.

[0110] The third aspect of the present application provides a positive electrode tab, including a positive electrode current collector and a positive electrode active material layer located on at least one side of the positive electrode current collector, wherein the positive electrode active material layer includes the positive electrode material of the first aspect of the present application. The positive electrode tab prepared by using the positive electrode material has a longer cycle life and higher safety.

[0111] For example, the positive electrode current collector has two opposite surfaces in the thickness direction of the positive electrode current collector, and the positive electrode film layer is arranged on any one or both of the two opposite surfaces of the positive electrode current collector.

[0112] In some embodiments, the positive current collector can employ a metal foil or a composite current collector. For example, as a metal foil, an aluminum foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material on a polymer material base material. The metal material includes, but is not limited to, aluminum, an aluminum alloy, nickel, a nickel alloy, titanium, a titanium alloy, silver, a silver alloy, and the like. The polymer material base material includes, but is not limited to, one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0113] In some embodiments, the lithium ion positive active material can further include a positive active material for a battery known in the art. As an example, the positive active material can further include at least one of a lithium-containing phosphate of an olivine structure, a lithium transition metal oxide, and a modified compound of each thereof. However, the present application is not limited to these materials, and other conventional materials that can be used as a positive active material for a battery can also be used. These positive active materials can be used alone only one or two or more can be used in combination. Examples of the lithium transition metal oxide can include, but are not limited to, at least one of a lithium cobalt oxide (e.g., LiCoO2), a lithium nickel oxide (e.g., LiNiO2), a lithium manganese oxide (e.g., LiMnO2, LiMn2O4), a lithium manganese cobalt oxide, a lithium nickel manganese oxide, a lithium nickel cobalt aluminum oxide (e.g., LiNi1 / 3Co1 / 3Al1 / 3O2), and a modified compound thereof. Examples of the lithium-containing phosphate of an olivine structure can include, but are not limited to, at least one of lithium iron phosphate (e.g., LiFePO4 (which can also be referred to as LFP)), a composite of lithium iron phosphate and carbon, lithium manganese phosphate (e.g., LiMnPO4), a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, and a composite of lithium manganese iron phosphate and carbon. The weight ratio of the positive active material in the positive electrode film layer is 80-100% by weight, based on the total weight of the positive electrode film layer. 0.85 Co 0.15 Al 0.05 O2) and a modified compound thereof. Examples of the lithium-containing phosphate of an olivine structure can include, but are not limited to, at least one of lithium iron phosphate (e.g., LiFePO4 (which can also be referred to as LFP)), a composite of lithium iron phosphate and carbon, lithium manganese phosphate (e.g., LiMnPO4), a composite of lithium manganese phosphate and carbon, lithium manganese iron phosphate, and a composite of lithium manganese iron phosphate and carbon. The weight ratio of the positive active material in the positive electrode film layer is 80-100% by weight, based on the total weight of the positive electrode film layer.

[0114] In some embodiments, the positive electrode film layer can further optionally include a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylic ester resin. The weight ratio of the binder in the positive electrode film layer is 0-20% by weight, based on the total weight of the positive electrode film layer.

[0115] In some embodiments, the positive electrode film layer can further optionally include a conductive agent. As an example, the conductive agent can include at least one of super P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The weight ratio of the conductive agent in the positive electrode film layer is 0-20 wt%, based on the total weight of the positive electrode film layer.

[0116] In some embodiments, the positive electrode tab can be prepared by dispersing the above-mentioned components for preparing the positive electrode tab, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., N-methyl pyrrolidone) to form a positive electrode slurry, wherein the solid content of the positive electrode slurry is 40-80 wt%, the viscosity at room temperature is adjusted to 5000-25000 mPa·s, the positive electrode slurry is coated on both sides of the positive electrode current collector, and the positive electrode tab is formed after drying and cold pressing by a cold rolling mill; the unit area density of the positive electrode powder single-sided coating is 12-20 mg / cm 2 , and the positive electrode tab has a compacted density of 3.0-3.75 g / cm 3 , which can be 3.4-3.65 g / cm 3 . The formula for calculating the compacted density is

[0117] Compacted density = coating area density / (thickness of the tab after extrusion - thickness of the current collector).

[0118] Generally, a secondary battery includes a positive electrode tab, a negative electrode tab, an electrolyte, and a separator. During the charging and discharging of the battery, active ions are inserted and extracted between the positive electrode tab and the negative electrode tab. The electrolyte serves to conduct ions between the positive electrode tab and the negative electrode tab. The separator is disposed between the positive electrode tab and the negative electrode tab, mainly serving to prevent short circuiting between the positive and negative electrodes, while allowing ions to pass through.

[0119] Negative electrode tab

[0120] The negative electrode tab includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector, the negative electrode film layer including a negative electrode active material.

[0121] As an example, the negative electrode current collector has two opposite surfaces in the thickness direction thereof, and the negative electrode film layer is disposed on either one or both of the two opposite surfaces of the negative electrode current collector.

[0122] In some embodiments, the negative current collector can employ a metal foil or a composite current collector. For example, as a metal foil, a copper foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material on a polymer material base layer. Herein, the metal material includes, but is not limited to, copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc., and the polymer material base layer includes, but is not limited to, a polypropylene (PP) base layer, a polyethylene terephthalate (PET) base layer, a polybutylene terephthalate (PBT) base layer, a polystyrene (PS) base layer, a polyethylene (PE) base layer, etc.

[0123] In some embodiments, the negative active material can employ a negative active material for a battery known in the art. As an example, the negative active material can include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, a silicon-based material, a tin-based material, and lithium titanate, etc. The silicon-based material can be selected from at least one of elemental silicon, a silicon oxide compound, a silicon-carbon composite, a silicon-nitrogen composite, and a silicon alloy. The tin-based material can be selected from at least one of elemental tin, a tin oxide compound, and a tin alloy. However, the present application is not limited to these materials, and other conventional materials that can be used as a negative active material for a battery can also be used. These negative active materials can be used alone or in combination of two or more. The weight ratio of the negative active material in the negative electrode film layer is 70-100% by weight, based on the total weight of the negative electrode film layer.

[0124] In some embodiments, the negative electrode film layer can further optionally include a binder. The binder can be selected from at least one of styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS). The weight ratio of the binder in the negative electrode film layer is 0-30% by weight, based on the total weight of the negative electrode film layer.

[0125] In some embodiments, the negative electrode film layer can further optionally include a conductive agent. The conductive agent can be selected from at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The weight ratio of the conductive agent in the negative electrode film layer is 0-20% by weight, based on the total weight of the negative electrode film layer.

[0126] In some embodiments, the negative electrode film layer can further optionally include other auxiliary agents, such as a thickening agent (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like. The weight ratio of the other auxiliary agents in the negative electrode film layer is 0-15% by weight, based on the total weight of the negative electrode film layer.

[0127] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-mentioned components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry, wherein the negative electrode slurry has a solid content of 30-70 wt%, and the viscosity at room temperature is adjusted to 2000-10000 mPa·s; coating the obtained negative electrode slurry on both sides of the negative electrode current collector, and performing a drying process, e.g., calendering, to obtain the negative electrode sheet. The unit area density of the negative electrode powder on one side is 7-13 mg / cm 2 , and the compaction density of the negative electrode sheet is 1.2-2.0 g / m 3 .

[0128] Electrolyte

[0129] The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The type of the electrolyte is not particularly limited in the present application, and can be selected as needed. For example, the electrolyte can be in a liquid state, a gel state, or a full solid state.

[0130] In some embodiments, the electrolyte employs an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0131] In some embodiments, the electrolyte salt can be selected from one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bisfluorosulfonylimide (LiFSI), lithium bis-trifluoromethanesulfonylimide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluoro-oxalato-borate (LiDFOB), lithium bis-oxalato-borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluoro-di-oxalato-phosphate (LiDFOP), and lithium tetrafluoro-oxalato-phosphate (LiTFOP). The concentration of the electrolyte salt is generally 0.5-5 mol / L.

[0132] In some embodiments, the solvent can be selected from one or more of fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0133] In some embodiments, the electrolyte solution can optionally further include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can further include an additive capable of improving certain performance of the battery, such as an additive capable of improving overcharge performance of the battery, an additive capable of improving high-temperature or low-temperature performance of the battery, and the like.

[0134] Separation film

[0135] In some embodiments, a separation film is further included in the secondary battery. The type of the separation film is not particularly limited in the present application, and any known porous structure separation film having good chemical stability and mechanical stability can be used.

[0136] In some embodiments, the material of the separation film can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separation film can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separation film is a multi-layer composite film, the materials of the respective layers can be the same or different, and are not particularly limited.

[0137] In some embodiments, the thickness of the separation film is 3-20 μm, and can be 5-12 μm.

[0138] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separation film can be used to form an electrode assembly by a winding process or a stacking process; and the electrode assembly and the electrolyte solution are packaged by using the packaging material of the first aspect of the present application.

[0139] The shape of the secondary battery is not particularly limited in the present application, and can be cylindrical, square, or any other shape. For example, Figure 1 is a square structure secondary battery 1 as an example.

[0140] In some embodiments, with reference to Figure 2 , the outer package can include a housing 11 and a cover plate 13. The housing 11 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The housing 11 has an opening communicating with the receiving cavity, and the cover plate 13 can be provided on the opening to close the receiving cavity.

[0141] The positive electrode sheet, the negative electrode sheet, and the separation film can be used to form an electrode assembly 12 by a winding process or a stacking process. The electrode assembly 12 is packaged in the receiving cavity. The electrolyte solution is impregnated in the electrode assembly 12. The number of electrode assemblies 12 contained in the lithium ion battery 1 can be one or more, which can be adjusted according to the requirements.

[0142] In some embodiments, the secondary battery can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module.

[0143] In the battery module, the plurality of secondary batteries can be arranged in sequence along the length direction of the battery module. Of course, they can also be arranged in any other manner. Further, the plurality of lithium ion batteries can be fixed by fasteners.

[0144] Optionally, the battery module can further include a housing having an accommodation space, and the plurality of secondary batteries are accommodated in the accommodation space.

[0145] In some embodiments, the above-mentioned battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0146] In the battery pack, a battery box and a plurality of battery modules arranged in the battery box can be included. The battery box includes an upper box body and a lower box body, and the upper box body can be arranged on the lower box body to form a closed space for accommodating the battery modules. The plurality of battery modules can be arranged in the battery box in any manner.

[0147] Electric device

[0148] The fourth aspect of the present application provides an electric device, which includes at least one of the secondary battery, the battery module or the battery pack of the third aspect of the present application. The secondary battery, the battery module or the battery pack can be used as a power source of the device, or as an energy storage unit of the device. The device can be, but is not limited to, a mobile device, an electric vehicle, an electric train, a ship and a satellite, an energy storage system, etc.; wherein the mobile device can include, but is not limited to, at least one of a mobile phone, a notebook computer, etc.; the electric vehicle can include, but is not limited to, at least one of a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.

[0149] The device can select a secondary battery, a battery module or a battery pack according to its use requirements.

[0150] Figure 3 The device 2 is an example of an electric device. The device 2 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the high power and high energy density requirements of the device for secondary batteries, a battery pack or a battery module can be used.

[0151] Another example of the device can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thinning, and a lithium ion battery can be used as a power source.

[0152] The beneficial effects of the present application are further illustrated below in conjunction with embodiments.

[0153] In order to make the technical problems, technical solutions and beneficial effects solved in the present application clearer, further detailed description will be made in combination with embodiments and drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The description of the at least one exemplary embodiment below is actually only illustrative, but not as any limitation on the present application and its application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0154] Unless otherwise specified in the embodiments, the techniques or conditions are carried out according to the techniques or conditions described in the literature in the art or according to the product instructions. Unless otherwise specified, the reagents or instruments used are all conventional products that can be obtained by purchase in the market.

[0155] I. Preparation of the positive electrode material

[0156] Embodiment 1

[0157] The ternary material (as the inner core) is dispersed in anhydrous ethanol, and (NH4)F (as the precursor containing the X element) is slowly added. The solution is subjected to a 60℃ water bath during the addition of (NH4)F, and slowly stirred for 30 min after the addition is completed. The structural formula of the ternary material is Li(Ni 92 Co6Mn2)O2, and the molar ratio of F contained in (NH4)F to the lithium element remaining on the surface of the ternary material is 1.5:1;

[0158] Fe(NO3)3 (as the precursor containing the Z element) is slowly added to the above-mentioned mixed solution after reaction. The solution is subjected to a 60℃ water bath during the addition of Fe(NO3)3, and slowly stirred for 30 min after the addition is completed. The molar ratio of Fe contained in Fe(NO3)3 to the lithium element remaining on the surface of the ternary material is 1.5:1;

[0159] The reaction solution after reaction is filtered, and the filter residue is dried. The filter residue is heated at 350℃ for 4h in an argon atmosphere to obtain the positive electrode material.

[0160] Embodiments 2-17

[0161] The preparation method of the positive electrode material in embodiments 2-17 and the preparation method of the positive electrode material in embodiment 1 are basically similar, and the main difference lies in that at least one of the structural formula of the inner core, the type and / or amount of the precursor containing the X element, the type and / or amount of the precursor containing the Z element, the reaction temperature and / or time, and the heating treatment temperature and / or time is different.

[0162] The positive electrode material prepared in each of the above examples and comparative examples was measured for the following items: the mass percentage of lithium element contained in the first coating layer with respect to the positive electrode material, the thickness of the first coating layer, the mass percentage of Z element contained in the second coating layer with respect to the positive electrode material, and the thickness of the second coating layer.

[0163] Comparative Example 1

[0164] The difference between Comparative Example 1 and Example 1 is that neither the first coating layer nor the second coating layer was prepared.

[0165] Comparative Example 2

[0166] The difference between Comparative Example 2 and Example 1 is that only the first coating layer was prepared, and the second coating layer was not prepared.

[0167] Comparative Example 3

[0168] The difference between Comparative Example 3 and Example 1 is that the Al2O3 coating layer was prepared by the co-deposition method when preparing the first coating layer, and the second coating layer was not prepared; the specific preparation process is as follows:

[0169] Al (NO3) ·9H2O was added in an alcohol-water solution with a volume ratio of 1:1, and stirred until dissolved. The ternary material (as the inner core) was added, and stirred thoroughly to obtain a suspension. The pH of the suspension was adjusted to 5 with ammonia water, and stirred for 30 min. After standing, the Al2O3 coating layer was formed on the surface of the ternary material after filtration, drying, and calcination (400°C).

[0170] Comparative Example 4

[0171] The difference between Comparative Example 4 and Example 1 is that the Al2O3 coating layer was prepared by the co-precipitation method when preparing the second coating layer; the specific preparation process is as follows: the suspension after the first reaction was filtered to obtain powder particles containing the first coating layer; Al (NO3) ·9H2O was added in an alcohol-water solution with a volume ratio of 1:1, and stirred until dissolved. The above powder particles were added, and stirred thoroughly to obtain a suspension. The pH of the suspension was adjusted to 5 with ammonia water, and stirred for 30 min. After standing, the Al2O3 coating layer was formed on the surface of the powder particles containing the first coating layer after filtration, drying, and calcination (400°C).

[0172] Comparative Example 5

[0173] The difference between Comparative Example 5 and Example 1 is that the molar ratio of X element in the X element-containing precursor to the residual lithium element on the surface of the inner core is different, and other conditions are the same; see Table 1 for details. The parameter settings of the above examples and comparative examples are shown in Table 1.

[0174] Table 1

[0175]

[0176] wherein, in Table 1, n1 represents the molar ratio of X element in the precursor containing X element to the lithium element remaining on the surface of the core, n2 represents the molar ratio of Z element in the precursor containing Z element to the lithium element remaining on the surface of the core; T2 represents the reaction temperature of the first reaction, T3 represents the reaction temperature of the second reaction, T3 represents the temperature of the heating treatment; w1 represents the mass percentage of lithium element contained in the first coating layer to the positive electrode material, w2 represents the mass percentage of Z element contained in the second coating layer to the positive electrode material; L1 represents the thickness of the first coating layer, L2 represents the thickness of the second coating layer; LiX represents a salt containing both lithium element and X element, and ZX represents a salt containing both Z element and X element.

[0177] In Table 1, the mass percentage of Z element contained in the second coating layer to the positive electrode material in Comparative Example 5, the thickness of the second coating layer, and the salt containing both Z element and X element in the second coating layer could not be detected, indicating that the second coating layer could not be formed; (NH4)F had been completely used to react with the lithium element remaining on the surface of the core in the first reaction.

[0178] In Table 1, “~” represents approximately; for example, “~2” in Comparative Example 5 represents that the thickness of the first coating layer is approximately 2 nm.

[0179] It should be noted that the mass percentage of lithium element contained in the first coating layer to the positive electrode material mentioned above is determined by the following method: the types and contents of X element and Z element contained in the coating layer are determined by ion chromatography, and the remaining part X is obtained by deducting the part of X element used to form a compound with Z element, which is used to form the first coating layer with lithium element. The mass of lithium element contained in the first coating layer can be calculated based on the mass of the remaining part X element, and then the mass percentage of lithium element contained in the first coating layer to the positive electrode material is calculated.

[0180] The thickness of the first coating layer mentioned above is determined by the following method: the phase analysis of the surface of the positive electrode material is performed by transmission electron microscopy, and the thickness information of the first coating layer is obtained synchronously.

[0181] The mass percentage of Z element contained in the second coating layer to the positive electrode material mentioned above is determined by the following method: the types and contents of X element and Z element contained in the coating layer are determined by ion chromatography, and the mass percentage of Z element to the positive electrode material is further calculated.

[0182] The thickness of the second coating layer mentioned above is determined by the following method: the phase analysis of the surface of the positive electrode material is performed by transmission electron microscopy, and the thickness information of the second coating layer is obtained synchronously.

[0183] The molar ratio of X element in the X element-containing precursor in Example 6 to the residual lithium element on the surface of the core is as high as 7:1, the thickness of the second coating layer is as high as 13 nm, and the Z element contained in the second coating layer accounts for as high as 0.92% of the mass percentage of the positive electrode material; the skilled person analyzes the reason that the Z element contained in the second coating layer accounts for as high as 0.92% of the mass percentage of the positive electrode material may be that: the X element-containing precursor is added in excess, and after the residual lithium and the X element-containing precursor form the first coating layer, there may still be too much X element-containing precursor remaining, the X element-containing precursor and the Z element-containing precursor continue to react to form the second coating layer, and the thickness of the second coating layer is too large.

[0184] In Example 10, the molar ratio of X element in the X element-containing precursor to the residual lithium element on the surface of the core is 1.5:1, the molar ratio of Z element in the Z element-containing precursor to the residual lithium element on the surface of the core is 0.125:1, and the Z element contained in the second coating layer accounts for only about 0.11% of the mass percentage of the positive electrode material; the skilled person analyzes the reason that the Z element contained in the second coating layer accounts for only about 0.11% of the mass percentage of the positive electrode material may be that: the amount of X element added in the X element-containing precursor and the amount of element-containing precursor added are both small, resulting in too low generation amount of the second coating layer, and the mass percentage of the Z element contained in the second coating layer in the positive electrode material is also reduced.

[0185] In Example 17, the thickness of the first coating layer is as high as 10 nm, and the lithium element contained in the first coating layer accounts for 0.25% of the mass percentage of the positive electrode material; indicating that the residual lithium content on the surface of the core in Example 17 is relatively high.

[0186] Preparation of a secondary battery

[0187] 1. The positive electrode material, acetylene black (as a conductive agent), and polyvinylidene fluoride (as a binder) are dissolved in N-methylpyrrolidone (as a solvent) according to a weight ratio of 96.5:1.5:2, and after being fully stirred and mixed uniformly, a positive electrode slurry is obtained; then the positive electrode slurry is uniformly coated on the double-sided surface of an aluminum foil, dried, cold-pressed, and cut to obtain a positive electrode sheet.

[0188] 2. The negative electrode active material graphite, acetylene black (as a conductive agent), carboxymethyl cellulose sodium (as a thickening agent), and butadiene-styrene rubber (as a binder) are dissolved in deionized water (as a solvent) according to a weight ratio of 95:1:2:1, and after being fully stirred and mixed uniformly, a negative electrode slurry is obtained; then the negative electrode slurry is uniformly coated on the double-sided surface of a copper foil, dried, cold-pressed, and cut to obtain a negative electrode sheet.

[0189] 3. PE film is used as a separator.

[0190] 4. Preparation of an electrolyte

[0191] Ethylene carbonate (EC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1, and then LiPF6 was uniformly dissolved in the above solution to obtain an electrolyte. In the electrolyte, the concentration of LiPF6 was 1 mol / L.

[0192] 5. The positive electrode sheet, the separator, and the negative electrode sheet were stacked in order with the separator between the positive and negative electrode sheets to play a role of isolation, and then were wound to obtain a bare battery cell. The bare battery cell was welded with tabs, and was put into an aluminum shell. The bare battery cell was baked at 80°C to remove water, and then was injected with electrolyte and sealed to obtain a non-charged battery. The non-charged battery was sequentially subjected to processes of standing, hot and cold pressing, formation, shaping, capacity testing, and the like to obtain a secondary battery.

[0193] Three, secondary battery performance test

[0194] 1. Cycle life test

[0195] At 25°C, the battery was charged at a rate of 0.5C to 80% SOC, and then was charged at a rate of 0.33C to 4.25V. After standing for 5 min, the battery was discharged at a rate of 0.5C to 2.8V to complete one charge-discharge cycle. The cycle number when the battery capacity was attenuated to 80% SOC was recorded.

[0196] 2. High-temperature swelling test

[0197] A gas production nail was welded at the electrolyte injection and sealing nail of the battery cell, and a gas pressure gauge was connected through an oil pipe. The cycle test was carried out according to the above cycle life test process at 45°C, and the gas pressure value was monitored synchronously. The cycle number when the gas pressure value reached 0.35Mpa was taken as the swelling evaluation parameter.

[0198] The performance test structure of each of the above examples and comparative examples is shown in Table 2.

[0199] Table 2

[0200]

[0201]

[0202] In the above, “~” represents approximately. For example, the cycle life result “~3500” in Example 1 means that the cycle is approximately 3500 times.

[0203] As can be seen from the results of Examples 1-17 and Comparative Examples 1-5, the use of the positive electrode material provided in the present application can improve the cycle life and safety of the secondary battery.

[0204] The differences between Examples 1-6 and Comparative Example 5 mainly lie in that the molar ratio of X element in the X element-containing precursor to the residual lithium element on the surface of the core and the molar ratio of Z element in the Z element-containing precursor to the residual lithium element on the surface of the core are different; the molar ratio of X element in the X element-containing precursor to the residual lithium element on the surface of the core and the molar ratio of Z element in the Z element-containing precursor to the residual lithium element on the surface of the core in Example 6 are the largest, and the molar ratio of X element in the X element-containing precursor to the residual lithium element on the surface of the core and the molar ratio of Z element in the Z element-containing precursor to the residual lithium element on the surface of the core in Comparative Example 5 are the smallest; it can be known from the results of Examples 1-6 and Comparative Example 5 that when the molar ratio of X element in the X element-containing precursor to the residual lithium element on the surface of the core is (1.5-6):1 and the molar ratio of Z element in the Z element-containing precursor to the residual lithium element on the surface of the core is (0.125-2.5):1, the prepared positive electrode material can improve the cycle life and safety of the secondary battery.

[0205] In addition, when the molar ratio of X element in the X element-containing precursor to the residual lithium element on the surface of the core is less than 3:1, the residual lithium consumption is small, the thickness of the first coating layer is small, and the high-temperature swelling test result is poor; at the same time, due to the small amount of the X element-containing precursor added, the coating amount of the second coating layer is also small, which affects the cycle effect. When the molar ratio of X element in the X element-containing precursor to the residual lithium element on the surface of the core is more than 3:1, the residual lithium on the surface of the core is completely consumed, but the overall coating amount increases, which will lead to the increase of the surface impedance of the positive electrode material and the increase of the polarization, which is not conducive to the improvement of the life.

[0206] In Example 6, the molar ratio of X element in the X element-containing precursor to the residual lithium element on the surface of the core is as high as 7:1, and the molar ratio of Z element in the Z element-containing precursor to the residual lithium element on the surface of the core is as high as 2:1; Examples 1-5 are better than Example 6 in cycle performance, and Examples 2-6 are better than Example 6 in high-temperature swelling result; the skilled person may analyze the reason as follows: in Example 6, the residual lithium on the surface of the core can be completely consumed, but due to the introduction of too much X element, the second layer coating amount may be too thick, which may affect the long-term performance.

[0207] The difference between Example 3 and Example 17 mainly lies in that the thickness of the first coating layer is different; Example 3 is better than Example 17 in cycle performance and safety; the skilled person may analyze the reason as follows: in Example 17, the residual lithium content on the surface of the core may be high, which leads to the high thickness of the first coating layer, thereby possibly deteriorating the performance of the positive electrode material itself.

[0208] Any combination of the technical features in the above-described embodiments can be made, and for the sake of brevity, not all possible combinations are described, however, it is to be understood that the application embraces all such possible combinations.

[0209] The above-described embodiments only express several implementation manners of the present application, and the description is relatively specific and detailed, but it should not be understood as a limitation on the patent scope of the application. It should be pointed out that for ordinary skilled persons in the art, some modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the patent protection scope of the present application should be subject to the appended claims.

Claims

1. A positive electrode material, characterized by, Comprise: a core, a material of the core comprising lithium nickel cobalt metal composite oxide; in the lithium nickel cobalt metal composite oxide, the molar percentage of nickel element in the total metal elements other than lithium element is 30%-95%; a first coating layer, coated on at least part of the outer surface of the core, a material of the first coating layer comprising lithium element and X element; and a second coating layer, coated on at least part of the outer surface of the core with the first coating layer, a material of the second coating layer comprising X element and Z element; wherein the X element comprises one or more of fluorine element, chlorine element and bromine element, and the Z element comprises one or more of iron element, titanium element, aluminum element, zinc element and magnesium element.

2. The positive electrode material of claim 1, wherein, The second coating layer is coated on all the outer surface of the core with the first coating layer.

3. The positive electrode material of claim 1, wherein, The chemical formula of the lithium nickel cobalt metal composite oxide is Li x (Ni a Co b Mn c ) 1-d M d O 2-y A y , wherein 0.2≤x≤1.2, a+b+c=1, the M element includes one or more of Al, Mg, Zr, W, Ti and Y, and the A element includes one or more of B, P and F.

4. The positive electrode material according to any one of claims 1 to 3, wherein The first coating layer has at least one of the following characteristics: (1) the lithium element contained in the first coating layer accounts for 0.1%-0.3% of the mass percentage of the positive electrode material; (2) the material of the first coating layer comprises a salt containing both the lithium element and the X element; (3) the thickness of the first coating layer is 3nm-10nm.

5. The positive electrode material according to any one of claims 1 to 3, wherein The second coating layer has at least one of the following characteristics: (1) the Z element contained in the second coating layer accounts for 0.2%-0.6% of the mass percentage of the positive electrode material; (2) the material of the second coating layer comprises a salt containing both the Z element and the X element; (3) the thickness of the second coating layer is 5nm-10nm.

6. A method for producing the positive electrode material according to any one of claims 1 to 5, characterized by, Comprise the following steps: Disperse the core in a solvent, add an X element-containing precursor at 40℃-70℃ to carry out a first reaction to prepare the first coating layer on at least part of the surface of the core; Add a Z element-containing precursor to the reaction solution of the first reaction at 40℃-70℃ to carry out a second reaction to prepare the second coating layer on at least part of the surface of the core with the first coating layer.

7. The method for preparing the cathode material as described in claim 6, characterized in that, The first reaction comprises at least one of the following conditions: (1) the X element-containing precursor comprises a salt containing the X element and being acidic; (2) the molar ratio of the X element contained in the X element-containing precursor to the lithium element remaining on the surface of the core is (1.5-6):1; (3) after adding the X element-containing precursor, the reaction is carried out for 20min-40min; (4) the solvent comprises a fatty alcohol.

8. The method for preparing the cathode material as described in claim 7, characterized in that, The first reaction comprises at least one of the following conditions: (1) the X element-containing precursor comprises one or more of ammonium fluoride, ammonium chloride and ammonium bromide; (2) the molar ratio of the X element contained in the X element-containing precursor to the lithium element remaining on the surface of the core is (2-4):1; (3) the solvent comprises one or more of ethanol, ethylene glycol, propanol, butanol and butanediol.

9. The method for preparing the cathode material as described in claim 8, characterized in that, The molar ratio of the X element contained in the X element-containing precursor to the lithium element remaining on the surface of the core is 3:

1.

10. The method of claim 6, wherein the positive electrode material is prepared by the steps of: preparing a mixture of Li2CO3, LiOH, and Li2O; and mixing the mixture with a transition metal compound. The second reaction comprises at least one of the following conditions: (1) the Z element-containing precursor comprises one or more of nitrate, carbonate, sulfate and sulfite containing the Z element; the Z element comprises one or more of iron element, titanium element, aluminum element, zinc element and magnesium element; (2) the molar ratio of the Z element contained in the Z element-containing precursor to the lithium element remaining on the surface of the core is (0.125-2.5):1; (3) the time for reaction after adding the Z element-containing precursor is 20 min-40 min.

11. The method of claim 10, wherein the positive electrode material is prepared by the steps of: mixing the lithium metal oxide, the conductive material, and the binder to form a mixture; and compressing the mixture to form the positive electrode material. The molar ratio of the Z element contained in the Z element-containing precursor to the lithium element remaining on the surface of the core is (0.25-1.5):

1.

12. The method of claim 11, wherein the positive electrode material is prepared by the steps of: mixing the lithium metal oxide, the conductive material, and the binder to form a mixture; and compressing the mixture to form the positive electrode material. The molar ratio of the Z element contained in the Z element-containing precursor to the lithium element remaining on the surface of the core is 0.66:

1.

13. The method of producing a positive electrode material according to any one of claims 6 to 12, wherein Further comprising the following steps: filtering the reaction solution obtained in the second reaction, drying the filter residue, and performing heating treatment on the filter residue in a protective atmosphere to prepare the positive electrode material.

14. The method of claim 13, wherein the positive electrode material is prepared by the steps of: mixing the lithium metal oxide, the conductive material, and the binder to form a mixture; and compressing the mixture to form the positive electrode material. The heating treatment comprises at least one of the following conditions: (1) the protective gas used comprises one or more of nitrogen and inert gas; (2) the reaction temperature is 300℃-400℃, and the reaction time is 3h-5h.

15. The method of claim 14, wherein the positive electrode material is prepared by the steps of: mixing the lithium metal oxide, the conductive material, and the binder to form a mixture; and compressing the mixture to form the positive electrode material. The protective gas used in the heating treatment comprises one or more of nitrogen and argon.

16. A positive electrode sheet characterized by comprising: Comprise: a positive electrode current collector; and a positive electrode active material layer located on at least one side of the positive electrode current collector, the positive electrode active material layer comprising the positive electrode material according to any one of claims 1 to 5.

17. A secondary battery characterized by comprising: Comprise the positive electrode sheet according to claim 16.

18. An electrical device, comprising: Comprise the secondary battery according to claim 17.

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