A cathode material with a coating layer, a preparation method thereof, and a lithium-ion battery

Abstract

The present invention discloses a cathode material with a coating layer, a preparation method thereof, and a lithium-ion battery. The cathode material with the coating layer includes a cathode core and a coating layer coated on the surface of the cathode core. The chemical composition of the cathode core is Li α Ni x Co y Al z A m O2; the coating layer contains Co element and X element, and the mass ratio of the X element to the Co element in the coating layer is 50% - 100%; X is a metal element different from Co, and more than 50 wt.% of X is Al element; the coating layer includes lithium cobalt oxide, and the lithium cobalt oxide includes specific contents of Li a Co 1‑a Co2O4, Li b Co 1‑b O and LiCoO2. Based on the total mass of the cathode core, the content of the Co element in the coating layer is 1000 ppm - 4000 ppm. The present invention can enable the cathode material to balance low heat generation and gas generation.

Description

Technical Field

[0001] The present invention belongs to the technical field of batteries, and relates to a positive electrode material with a coating layer, a preparation method thereof, and a lithium-ion battery. Background Art

[0002] Currently, lithium-ion batteries are widely used in various industries as environmentally friendly green energy. High-rate tool-type electronic products have increasingly high requirements for the electrical abuse and safety characteristics of lithium-ion batteries under high-temperature conditions. Considering the actual application scenarios of rate-type battery cells, it is required that the battery cells do not catch fire or explode during the process of constant-current charging from the open-circuit voltage to the cut-off voltage. Generally, the cut-off voltage exceeds the upper limit voltage of normal application. In order to meet customer requirements, continuous innovation has been carried out in the design of the battery cell structure. However, the intrinsic safety improvement of each component of the battery cell remains the focus. Among them, within the range from the open-circuit voltage to the normal application voltage (<4.3V) of the positive electrode material, due to its intrinsic impedance, Ohmic heat is generated following Ohm's law, and almost no gas is generated; under high-voltage conditions (such as ≥4.3V), as the amount of lithium deintercalation increases, the crystal structure becomes increasingly unstable, lattice oxygen loss and an increase in the Ni valence state are prone to occur, reacting with the electrolyte to generate a large amount of gas and heat, thereby triggering thermal runaway of the battery cell and catching fire and explosion.

[0003] Generally, the phenomena of heat generation and gas generation "complement each other". If the heat accumulates too quickly, it will cause the metastable structure to become unstable, resulting in self-structural decline or reaction with the electrolyte; if the gas generation amount suddenly increases and the reaction is intense, it will also cause a sharp increase in the heat generation amount. The gas generation and heat generation cycle viciously. Even though the battery cell structure is designed to release and cut off power in a timely manner, the phenomenon of catching fire and explosion will still occur. Therefore, how to simultaneously reduce the heat generation and gas generation of the positive electrode material between 4.3V and 5V is crucial.

[0004] Unfortunately, in the prior art, for the positive electrode of a lithium-ion battery, the heat generation and gas generation of the positive electrode material between 4.3V and 5V usually cannot be taken into account simultaneously. Generally, doping or coating elements such as Al, Co, Zr, Mg, etc. on the bulk phase and surface of the positive electrode material can stabilize the bulk-phase lattice or surface structure and improve the crystal structure stability at high voltages. However, not all coating elements or coating amounts can achieve the improvement purpose. The reason is that for the coating of non-active elements, it will significantly deteriorate the impedance of the material in the bulk, greatly increase the heat generation, increase the temperature of the battery cell monomer, promote the gas generation reaction, and then cause thermal runaway; while for the coating of active elements, although it will reduce the Ohmic heat, it will increase the surface reaction activity. Under high-voltage conditions, the structure is unstable, prone to structural decomposition and side reactions with the electrolyte, greatly increasing the gas generation and heat generation.

[0005] Therefore, how to select the coating element and amount to balance the heat generation and gas generation of the positive electrode material between 4.3V and 5V has become one of the problems that the industry urgently needs to solve. Summary of the Invention

[0006] In view of the above technical problems existing in the prior art, the purpose of the present invention is to provide a cathode material with a coating layer, a preparation method thereof, and a lithium-ion battery.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] In a first aspect, the present invention provides a cathode material with a coating layer, the cathode material with a coating layer includes a cathode core and a coating layer coated on the surface of the cathode core, and the chemical composition of the cathode core is Li α Ni x Co y Al z A m O2, 1≤α≤1.05, 0.80≤x≤0.95, 0.04≤y≤0.15, 0.005≤z≤0.06, 0≤m≤0.02, x + y + z + m = 1, wherein A includes one or more of Zr, Mg, Sr, Y, La, Nb, Ba, and Ti;

[0009] The coating layer contains Co element and X element, and the mass ratio of X element to Co element in the coating layer is 50% - 100%; X is a metal element different from Co, and more than 50wt.% of X is Al element; the coating layer includes a lithium cobalt compound, and the lithium cobalt compound includes Li a Co 1-a Co2O4, Li b Co 1-b O, and LiCoO2, wherein, 0 < a < 1, 0 < b < 1, and the contents of Li a Co 1-a Co2O4, Li b Co 1-b O, and LiCoO2 in the total mass of the three (i.e., Li a Co 1-a Co2O4, Li b Co 1-b O, and LiCoO2) are 1wt.% - 15wt.%, 10wt.% - 25wt.%, and 65wt.% - 76wt.%, respectively, and the content of Li b Co 1-b O is greater than the content of Li a Co 1-a Co2O4;

[0010] Based on the total mass of the cathode core, the content of Co element in the coating layer is 1000ppm - 4000ppm.

[0011] In one embodiment, the mass ratio of element X to Co element in the coating layer is greater than or equal to 50% and less than 100%. At this time, their contents are different.

[0012] In another embodiment, the mass ratio of element X to Co element in the coating layer is 100%, that is, their contents are the same.

[0013] Preferably, the mass fraction of element X in the coating layer is less than that of Co element in the coating layer.

[0014] The following are the preferred technical solutions of the present invention, but do not limit the technical solutions provided by the present invention. Through the following preferred technical solutions, the technical objectives and beneficial effects of the present invention can be better achieved.

[0015] In one embodiment, X is 100 wt.% of Al element. Thus, the coating layer contains Co element and Al element, and the mass ratio of Al element to Co element in the coating layer is 50% - 100%.

[0016] Preferably, the total mass of Co element and X element in the coating layer is ω1, and the total mass of metal elements (excluding lithium metal element) in the coating layer is ω2, then ω1 / ω2 ≥ 90%, preferably ≥ 95%. That is, the mass ratio of Co element and X element in the coating layer to the total mass of metal elements (excluding lithium metal element) in the coating layer is more than 90%, more preferably 95%.

[0017] In the present invention, if other metals and other trace metal doping are not considered, Co element and X element in the coating layer of the present invention can account for nearly 100% of the total of metal elements (excluding lithium metal element) in the coating layer.

[0018] Preferably, when X is selected as 100% of Al element, the mass ratio of Co element and Al element in the coating layer to the total mass of metal elements (excluding lithium metal element) in the coating layer is more than 90%, more preferably 95%. In the present invention, if other metals and other trace metal doping are not considered, Co element and Al element in the coating layer of the present invention can account for nearly 100% of the total of metal elements (excluding lithium metal element) in the coating layer.

[0019] In another embodiment, X further includes other coating elements, and the other coating elements are one or more of W, Ti, and Ce. Typical but non-limiting combinations are: the combination of W and Ce, the combination of W and Ti, the combination of Ti and Ce, and the combination of W, Ti, and Ce.

[0020] In the present invention, in element X, while ensuring that the Al element accounts for more than 50 wt.%, other cooperating elements can be introduced as appropriate, such as one or more of W, Ti, and Ce.

[0021] In a second aspect, the present invention provides a method for preparing a positive electrode material with a coating layer as described in the first aspect, and the preparation method includes the following steps:

[0022] Mix the positive electrode core with a coating source and then sinter at a temperature of 350 °C to 500 °C to obtain a positive electrode material with a coating layer;

[0023] Among them, the coating source includes a cobalt source and an X source, the X source includes an aluminum source, and more than 50 wt.% of X is the Al element.

[0024] Preferably, the cobalt source includes at least one of cobalt oxide, cobalt hydroxide, and cobalt oxyhydroxide.

[0025] Preferably, the aluminum source includes at least one of aluminum oxide, aluminum hydroxide, and aluminum oxyhydroxide.

[0026] Preferably, the X source further includes other coating sources, and the elements in the other coating sources include at least one of W, Ti, and Ce.

[0027] Preferably, the other coating source is at least one of nitrates, chlorides, hydroxides, or oxides.

[0028] Preferably, the sintering is carried out in an oxygen-containing atmosphere. Exemplarily, the oxygen-containing atmosphere can be an air atmosphere or an oxygen atmosphere.

[0029] Preferably, the sintering time is 6 h to 24 h.

[0030] Preferably, after the sintering is completed, the temperature is decreased at a rate of 0.01 °C / min to 3 °C / min.

[0031] As a preferred technical solution of the method for preparing a positive electrode material with a coating layer as described in the present invention, the method for preparing the positive electrode core includes the following steps:

[0032] Mix nickel cobalt hydroxide, a doping source, and a lithium source and then sinter to obtain a positive electrode core;

[0033] Among them, the doping elements in the doping source include the Al element and the A element, where A includes one or more of Zr, Mg, Sr, Y, La, Nb, Ba, and Ti.

[0034] Preferably, the lithium source is lithium hydroxide and / or lithium carbonate.

[0035] Preferably, in the preparation process of the positive electrode core, the molar ratio of each element satisfies that Li / (Ni + Co + Al + A) is 1 to 1.05.

[0036] Preferably, in the preparation process of the positive electrode core, the sintering temperature is 400°C to 1000°C.

[0037] Preferably, in the preparation process of the positive electrode core, the sintering time is 6h to 20h.

[0038] In a third aspect, the present invention provides a lithium-ion battery, including a positive electrode, a negative electrode, and a separator, wherein the positive electrode includes the positive electrode material with a coating layer described in the first aspect.

[0039] The numerical ranges described in the present invention not only include the above-listed point values, but also include any point values between the above numerical ranges that are not listed. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the ranges.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] The positive electrode material of the present invention uses a high-nickel nickel-cobalt-aluminum-based ternary positive electrode material as the core, which has good rate performance at high voltages. Further, three different chemically composed lithium cobalt compounds are coated on it and Al element is introduced into the coating layer. Since the morphologies of the lithium cobalt compounds with different chemical compositions are different, it will affect the stability of the interface structure of the material itself. Therefore, by using spinel-type LiCoO2 as the main body of the coating layer and cooperating with a specific content of Li a Co 1-a Co2O4, Li b Co 1-b O and the synergistic effect of the Al element, the interface structure can be stabilized and the ohmic heat can be reduced, so as to be able to balance low heat generation and gas generation, and improve the stability of the material and the safety of the lithium-ion battery. Among them, the control of the following parameters is very critical: First, the mass ratio of the X element to the Co element in the coating layer is 50% to 100%. This is because the introduction of the X element may deteriorate the ohmic heat. Therefore, its coating amount cannot exceed the Co element. At the same time, in order to achieve the purpose of completely stabilizing the interface structure, its coating amount cannot be lower than 50% of the Co coating amount. Second, the content of the Co element in the coating layer is 1000ppm to 4000ppm. If the content of the Co element is too small, it will cause an incomplete coating layer to be formed, and only dispersed island-like coatings can be formed, which completely fails to achieve the technical effect of coating; if the content of the Co element is too large, it will cause Li without positive electrode activity a Co 1-a Co2O4, Li b Co 1-bIf the content of O is too large, it will significantly reduce the proportion of the positive electrode active material, thereby reducing the specific capacity of the entire material. Moreover, it will significantly deteriorate the impedance of the positive electrode material, thereby affecting the product performance. Description of the Drawings

[0042] Figure 1 It is a scanning electron microscope image of the positive electrode material with a coating layer provided in Example 1. Detailed Embodiments

[0043] The technical solution of the present invention will be further described below in conjunction with the drawings and through specific embodiments.

[0044] The specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0045] In a cylindrical lithium-ion battery, the wound core is arranged in a laminated and wound manner with a positive electrode sheet, a separator, a negative electrode sheet, and a separator in sequence, and is accommodated in a battery case in a state where the electrolyte is infiltrated.

[0046] In one embodiment, a positive electrode active material layer is coated on both sides of a positive electrode foil to form a positive electrode sheet. The positive electrode foil can be a metal foil made of aluminum or an aluminum alloy.

[0047] In one embodiment, a negative electrode active material layer is coated on both sides of a negative electrode foil to form a negative electrode sheet. The material of the negative electrode foil can be a metal foil made of copper or a copper alloy.

[0048] In one embodiment, the separator can be any separator known in the prior art of lithium-ion batteries.

[0049] In one embodiment, the negative electrode material can be a carbon material, such as a graphite material. Among them, the graphite material can be artificial graphite and / or natural graphite, and is preferably a graphite-based composite negative electrode material doped with a certain amount of silicon oxide or silicon carbide.

[0050] In one embodiment, the separator can be a PE film, a PP film, or a composite film. Among them, the composite film includes at least two of a PP film, a PE film, and a PP film. Of course, in one embodiment, the separator can also be a porous film coated with ceramics.

[0051] In one embodiment, the electrolyte includes a solvent and an electrolyte salt.

[0052] In one embodiment, the aforementioned solvent includes any one or two or more of non-aqueous solvents such as organic solvents. Among them, the electrolyte of the non-aqueous solvent is a so-called non-aqueous electrolyte. The non-aqueous solvent can be, for example, a cyclic carbonate, a chain carbonate, a lactone, a chain carboxylic acid ester, a nitrile, etc. In addition, the electrolyte can further include any one or two or more of other materials such as additives.

[0053] In one embodiment, the foregoing electrolyte salt may include, for example, any one or more of salts such as lithium salts. In addition, the electrolyte salt may also include salts other than lithium salts. The salts other than lithium salts may be, for example, light metal salts other than lithium.

[0054] In one example, the battery housing is a metal case, and the metal case may be a steel case or an aluminum case, and more preferably a steel case.

[0055] In the present invention, the dimensions of the cylindrical battery may be models such as 18650 and 21700.

[0056] Preferably, the following tests of the present invention are carried out on 18650-type cylindrical battery cells.

[0057] In one embodiment, the present embodiment provides a positive electrode material having a coating layer. The positive electrode material having a coating layer includes a positive electrode core and a coating layer coated on the surface of the positive electrode core. The chemical composition of the positive electrode core is Li α Ni x Co y Al z A m O2, 1≤α≤1.05, 0.80≤x≤0.95, 0.04≤y≤0.15, 0.005≤z≤0.06, 0≤m≤0.02, x + y + z + m = 1, where A includes one or more of Zr, Mg, Sr, Y, La, Nb, Ba, and Ti;

[0058] The coating layer contains Co element and X element, and the mass ratio of X element to Co element in the coating layer is 50% - 100%; X is a metal element different from Co, and more than 50 wt.% of X is Al element;

[0059] The coating layer includes a lithium cobalt compound, and the lithium cobalt compound includes Li a Co 1-a Co2O4, Li b Co 1-b O, and LiCoO2, where 0 < a < 1, 0 < b < 1. The contents of Li a Co 1-a Co2O4, Li b Co 1-b O, and LiCoO2 in the total mass of the three are 1 wt.% - 15 wt.%, 10 wt.% - 25 wt.%, and 65 wt.% - 76 wt.% respectively, and the content of Li b Co 1-b O is greater than Li a Co1-a Content of Co2O4;

[0060] Based on the total mass of the positive electrode core, the content of Co element in the coating layer is 1000 ppm to 4000 ppm.

[0061] In one embodiment of the present invention, 1 ≤ α ≤ 1.05, for example, it can be 1, 1.01, 1.02, 1.03, 1.04 or 1.05, etc.; 0.80 ≤ x ≤ 0.95, for example, it can be 0.80, 0.82, 0.83, 0.85, 0.88, 0.89, 0.90, 0.92, 0.93, 0.94 or 0.95, etc.; 0.04 ≤ y ≤ 0.15, for example, it can be 0.04, 0.06, 0.08, 0.10, 0.12, 0.13, 0.14 or 0.15, etc.; 0.005 ≤ z ≤ 0.06, for example, it can be 0.01, 0.02, 0.03, 0.04, 0.05 or 0.06, etc.; 0 ≤ m ≤ 0.02, for example, it can be 0, 0.001, 0.003, 0.005, 0.007, 0.008, 0.01, 0.012, 0.015, 0.017 or 0.02, etc. Wherein, when m is 0, it means that the chemical composition does not contain element A.

[0062] In one embodiment of the present invention, Li a Co 1-a For a in Co2O4, 0 < a < 1. Exemplarily, a can be 0.01, 0.03, 0.05, 0.07, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85 or 0.9, etc.

[0063] In one embodiment of the present invention, Li b Co 1-b For b in O, 0 < b < 1. Exemplarily, b can be 0.01, 0.03, 0.05, 0.07, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85 or 0.9, etc.

[0064] In one embodiment of the present invention, Li a Co 1-a The content of Co2O4 is 1 wt.% to 15 wt.%, for example, it can be 1 wt.%, 2 wt.%, 4 wt.%, 5 wt.%, 7 wt.%, 8 wt.%, 10 wt.%, 12 wt.%, 13 wt.% or 15 wt.%, etc.

[0065] In one embodiment of the present invention, the content of Li b Co 1-b O is 10 wt.% to 25 wt.%, for example, it can be 10 wt.%, 12 wt.%, 13 wt.%, 15 wt.%, 16 wt.%, 18 wt.%, 19 wt.%, 20 wt.%, 21 wt.%, 23 wt.% or 25 wt.% etc.

[0066] In one embodiment of the present invention, the content of LiCoO2 is 65 wt.% to 76 wt.%, for example, it can be 65 wt.%, 66 wt.%, 68 wt.%, 70 wt.%, 72 wt.%, 74 wt.%, 75 wt.% or 76 wt.% etc.

[0067] In one embodiment of the present invention, the mass ratio of element X to Co element in the coating layer is 50% to 100%, for example, it can be 50%, 52%, 55%, 58%, 60%, 62.5%, 65%, 67%, 70%, 73%, 76%, 78%, 80%, 83%, 85%, 88%, 90%, 92%, 94%, 95%, 96%, 98% or 100% etc.

[0068] In one embodiment of the present invention, the content of Co element in the coating layer is 1000 ppm to 4000 ppm, for example, it can be 1000 ppm, 1100 ppm, 1200 ppm, 1300 ppm, 1400 ppm, 1500 ppm, 1600 ppm, 1800 ppm, 2000 ppm, 2200 ppm, 2300 ppm, 2500 ppm, 2600 ppm, 2800 ppm, 3000 ppm, 3150 ppm, 3300 ppm, 3400 ppm, 3500 ppm, 3600 ppm, 3800 ppm or 4000 ppm etc.

[0069] In one embodiment of the present invention, the cathode material uses a high-nickel nickel-cobalt-aluminum-based ternary cathode material as the core, which has good rate performance at high voltages. Further, it is coated with three lithium cobalt compounds with different chemical compositions and Al element is introduced into the coating layer. Since the morphologies of the lithium cobalt compounds with different chemical compositions are different, it will affect the stability of the interface structure of the material itself. Therefore, by using spinel-type LiCoO2 as the main body of the coating layer, and cooperating with a specific content of Li a Co 1- a Co2O4, Li b Co 1-bThe synergistic effect of O and Al elements can stabilize the interface structure and reduce the ohmic heat, thus being able to balance low heat generation and gas generation, and enhancing the stability of the material and the safety of the lithium-ion battery. Among them, the control of the following parameters is very crucial: First, the mass ratio of element X to Co element in the coating layer is 50% - 100%. This is because the introduction of element X may deteriorate the ohmic heat. Therefore, its coating amount cannot exceed that of the Co element. At the same time, in order to achieve the purpose of completely stabilizing the interface structure, its coating amount cannot be less than 50% of the Co coating amount. Second, the content of Co element in the coating layer is 1000 ppm - 4000 ppm. If the content of Co element is too small, less than 1000 ppm, it will lead to the inability to form a complete coating layer, and only dispersed island-like coatings can be formed, completely failing to achieve the technical effect of coating. If the content of Co element is too large, it will lead to the lack of positive electrode active Li a Co 1-a Co2O4, Li b Co 1-b The excessive content of O will significantly reduce the proportion of the positive electrode active material, thereby reducing the specific capacity of the entire material. Moreover, it will significantly deteriorate the impedance of the positive electrode material, thereby affecting the product performance.

[0070] In one embodiment, the mass ratio of element X in the coating layer to Co element in the coating layer is greater than or equal to 50% and less than 100%. At this time, their contents are different.

[0071] In another embodiment, the mass ratio of element X in the coating layer to Co element in the coating layer is 100%, that is, their contents are the same. In one embodiment, X is 100 wt.% of Al element. Therefore, the coating layer contains Co element and Al element, and the mass ratio of Al element in the coating layer to Co element in the coating layer is 50% - 100%.

[0072] In one embodiment, the total mass of Co element and X element in the coating layer is ω1, and the total mass of metal elements (excluding lithium metal elements) in the coating layer is ω2, then ω1 / ω2 ≥ 90%, for example, it can be 90%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or more than 99.5%, preferably ≥ 95%. That is, the Co element and X element in the coating layer account for more than 90% of the total mass ratio of metal elements (excluding lithium metal elements) in the coating layer, and more preferably 95%.

[0073] In the present invention, if other metals and other trace metal doping are not considered, the Co element and X element in the coating layer of the present invention can account for nearly 100% of the total of metal elements (excluding lithium metal elements) in the coating layer.

[0074] In one embodiment, when X selects 100% of Al element, the mass ratio of Co element and Al element in the coating layer accounts for more than 90% of the total mass of the metal elements (excluding lithium metal element) in the coating layer, and more preferably 95%. In the present invention, without considering other metals and other trace metal doping, the Co element and Al element in the coating layer of the present invention can account for nearly 100% of the total metal elements (excluding lithium metal element) in the coating layer.

[0075] In one embodiment, X further includes other coating elements, and the other coating elements are one or more of W, Ti, and Ce. Typical but non-limiting combinations include: the combination of W and Ce, the combination of W and Ti, the combination of Ti and Ce, and the combination of W, Ti, and Ce.

[0076] In one embodiment, in the X element, when ensuring that the Al element accounts for more than 50 wt.%, other cooperating elements can be introduced as appropriate, such as one or more of W, Ti, and Ce.

[0077] For the introduction of W and Ce, at this time, X includes Al, W, and Ce at the same time. Al cooperates with W and Ce, and W and Ce are used to further cooperate with Al to reduce the surface impedance of the coating layer, which is beneficial to further reducing the DC internal resistance of the battery cell; at the same time, if the content of W and Ce is too large, it will affect the performance of the battery cell in high-temperature storage, high-rate charge and discharge, etc. Therefore, it should be ensured that the Al element accounts for more than 50 wt.% in the X element.

[0078] For the introduction of Ti, at this time, X includes Al and Ti at the same time. Al and Ti cooperate, and Ti can further reduce the surface impedance of the coating layer, which is beneficial to further reducing the DC internal resistance of the battery cell; at the same time, if the content of Ti is too large, it will reduce the capacity of the battery cell, etc. Therefore, it should be ensured that the Al element accounts for more than 50 wt.% in the X element.

[0079] In one embodiment of the present invention, there may also be a certain amount of Co source or cobalt compound after the reaction of the Co source in the coating layer. The Co source or cobalt compound may include, for example, cobalt oxide, cobalt hydroxide, cobalt oxyhydroxide, etc. The Co source is generally the unreacted Co source during the preparation of the positive electrode material, and the cobalt compound is generally the substance formed by the reaction of the unreacted Co source during the preparation of the positive electrode material during the second sintering process. In one embodiment, the unreacted Co source accounts for 12% or less of the total mass of the Co source used in the coating process, and more preferably 5% or less. Obviously, the unreacted or Co source, etc. have no obvious benefit or harm to the coating, and are only generated depending on the sintering temperature in the preparation method, and belong to the possible unreacted substances when the secondary sintering uses a low temperature.

[0080] In another embodiment, the present invention provides a method for preparing the positive electrode material having a coating layer as described above, the preparation method comprising the following steps:

[0081] The positive electrode core and the coating source are mixed and sintered at a temperature of 350° C. to 500° C. to obtain a positive electrode material with a coating layer;

[0082] The coating source includes a cobalt source and an X source, the X source includes an aluminum source, and more than 50 wt.% of X is Al element.

[0083] In the preparation method of the positive electrode material with a coating layer provided by one embodiment of the present invention, the sintering temperature is 350°C to 500°C, for example, it can be 350°C, 370°C, 400°C, 425°C, 450°C, 475°C or 500°C. The preparation process of the positive electrode core is generally prepared by sintering a lithium source and other raw materials, and its surface generally has residual Li substances. The method of the present invention uses a coating source including a cobalt source and an aluminum source to mix and sinter the positive electrode core at a low temperature. The Co element mainly reacts with the residual Li substance on the surface of the positive electrode core to form three lithium cobalt compounds with different chemical compositions within a specific temperature, achieving synergistic synergy of multiple morphologies. At the same time, due to the presence of the aluminum source, the Al element can improve the interface stability, so that the positive electrode material can take into account low gas production and heat production.

[0084] When the inventors completed the technical solution, they found that in the temperature range of 350°C to 500°C, the cobalt source was basically reacted into lithium cobalt oxide during the secondary sintering process (the sintering involved in the preparation process of the positive electrode core is generally defined as primary sintering, so the sintering here is defined as secondary sintering), and the mass of the unreacted cobalt source only accounted for less than 12% of the mass of the cobalt source used in the coating process, preferably less than 5%.

[0085] Furthermore, the cobalt source of the reaction is basically converted into Li a Co 1-a Co2O4(0 <a<1)、Li b Co 1-b O(0 <b<1)和LiCoO2这三种锂钴氧化物,且,所述Li a Co 1-a Co2O4、Li b Co 1-b The contents of O and LiCoO2 in the lithium cobalt oxide are 1wt.%~15wt.%, 10wt.%~25wt.% and 65wt.%~76wt.%, respectively.

[0086] From 350℃ to 500℃, we can roughly observe the following rules: Li a Co1-a The content of Co2O4 rapidly decays from the upper limit of 15 wt.% at 350 °C to about 1 wt.%. Although the amount of Li a Co 1-a Co2O4 is already very small at 500 °C, we can still observe that Li a Co 1-a Co2O4 still exists in the coating layer. We can also observe that Li b Co 1-b O increases from 10 wt.% at 350 °C to 25 wt.% at 500 °C, while the content of LiCoO2 is roughly stable within the temperature range from 350 °C to 500 °C, approximately in the range from 65 wt.% to 76 wt.%, showing a slightly upward trend.

[0087] In one embodiment, the total mass of Co element and X element in the coating layer is ω1, and the total mass of metal elements in the coating layer is ω2, then ω1 / ω2 ≥ 90%, preferably ≥ 98%.

[0088] In one embodiment, ω1 / ω2 is 100%.

[0089] In one embodiment, the cobalt source includes at least one of cobalt oxide, cobalt hydroxide, and cobalt oxyhydroxide.

[0090] In one embodiment, the aluminum source includes at least one of aluminum oxide, aluminum hydroxide, and aluminum oxyhydroxide.

[0091] In one embodiment, the X source further includes other coating sources, and the elements in the other coating sources are at least one of W, Ti, and Ce.

[0092] In one embodiment, the other coating source is at least one of nitrate, chloride, hydroxide, or oxide.

[0093] Exemplarily, the other coating source can be one or more of tungsten nitrate, tungsten chloride, tungsten hydroxide, tungsten oxide, titanium nitrate, titanium chloride, titanium hydroxide, titanium oxide, cerium nitrate, cerium chloride, cerium hydroxide, or cerium oxide.

[0094] In one embodiment, the sintering is carried out in an oxygen-containing atmosphere. Exemplarily, the oxygen-containing atmosphere can be an air atmosphere or an oxygen atmosphere.

[0095] In one embodiment, the sintering time is 6 h to 24 h, for example, it can be 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, 16 h, 17 h, 18 h, 19 h, 20 h, 21 h, 22 h, 23 h, or 24 h, etc.

[0096] In one embodiment, after sintering is completed, the temperature is decreased at a rate of 0.01 °C / min to 3 °C / min. Exemplarily, the temperature decreasing rate can be 0.01 °C / min, 0.05 °C / min, 0.1 °C / min, 0.2 °C / min, 0.5 °C / min, 0.7 °C / min, 1 °C / min, 1.5 °C / min, 2 °C / min, 2.5 °C / min, 3 °C / min, etc.

[0097] In one embodiment, the method for preparing the positive electrode core includes the following steps:

[0098] Mix nickel cobalt hydroxide, doping source and lithium source and then sinter them to obtain a positive electrode core;

[0099] Among them, the doping elements in the doping source include Al element and A element, where A includes one or more of Zr, Mg, Sr, Y, La, Nb, Ba and Ti.

[0100] In one embodiment, the lithium source is lithium hydroxide and / or lithium carbonate.

[0101] In one embodiment, during the preparation of the positive electrode core, the molar ratio of each element satisfies that Li / (Ni + Co + Al + A) is 1 to 1.05. For example, it can be 1, 1.01, 1.02, 1.03, 1.04 or 1.05, etc.

[0102] In one embodiment, during the preparation of the positive electrode core, the sintering temperature is 400 °C to 1000 °C. For example, it can be 400 °C, 450 °C, 500 °C, 550 °C, 600 °C, 650 °C, 700 °C, 750 °C, 800 °C, 850 °C, 900 °C, 950 °C or 1000 °C, etc.

[0103] In one embodiment, during the preparation of the positive electrode core, the sintering time is 6 h to 20 h. For example, it can be 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, 16 h, 17 h, 18 h, 19 h or 20 h, etc.

[0104] In another embodiment of the present invention, a lithium ion battery is provided, which includes a positive electrode, a negative electrode and a separator, and the positive electrode includes the above positive electrode material with a coating layer.

[0105] Based on the above embodiments, the following typical but non-limiting examples are provided:

[0106] Example 1

[0107] This embodiment provides a cathode material with a coating layer. The cathode material with a coating layer includes a cathode core and a coating layer coated on the surface of the cathode core. The cathode core is a nickel-cobalt-aluminum ternary cathode material (chemical formula: Li 1.03 (Ni 0.88 Co 0.10 Al 0.02 )O2), and the coating layer includes a lithium cobalt compound. The lithium cobalt compound includes Li a Co 1-a Co2O4, Li b Co 1-b O, and LiCoO2, where 0 < a < 1 and 0 < b < 1. Based on the total mass of Li a Co 1-a Co2O4, Li b Co 1-b O, and LiCoO2 being 100 wt.%, the contents of Li a Co 1-a Co2O4, Li b Co 1-b O, and LiCoO2 are 3 wt.%, 23 wt.%, and 74 wt.%, respectively;

[0108] The coating layer further includes Al element, and the Al element exists in the form of Al2O3;

[0109] Based on the total mass of the cathode core, the content of Co element in the coating layer is 1000 ppm, and the content of Al element is 800 ppm. Then, the mass ratio of Al element to Co element in the coating layer is 80%.

[0110] Figure 1 is the scanning electron microscope image of the cathode material with a coating layer provided by this embodiment.

[0111] This embodiment also provides a preparation method of the above-mentioned cathode material with a coating layer, including the following steps:

[0112] (1) Prepare the cathode core:

[0113] Mix nickel cobalt hydroxide, a doping source, and a lithium source, and then perform a first sintering to obtain the cathode core. Among them, the doping source is aluminum oxide, the lithium source is LiOH, the molar ratio of Li / (Ni + Co + Al) is 1.03, the sintering temperature is 700 °C, and the sintering time is 16 h.

[0114] (2) Coating process:

[0115] Mix the positive electrode core with the coating source and grind them uniformly. After uniform grinding, perform secondary sintering. The sintering is carried out in an air atmosphere. The sintering temperature is 485 °C, and the sintering time is 8 h. After sintering is completed, cool down at a cooling rate of 2 °C / min to obtain a positive electrode material with a coating layer; wherein, the coating source is cobalt hydroxide and aluminum hydroxide.

[0116] Example 2

[0117] This example provides a positive electrode material with a coating layer. The positive electrode material with a coating layer includes a positive electrode core and a coating layer coated on the surface of the positive electrode core. The positive electrode core is a nickel-cobalt-aluminum ternary positive electrode material (chemical formula: Li 1.01 (Ni 0.85 Co 0.12 Al 0.025 Zr 0.005 )O2), the coating layer includes a lithium cobalt compound, and the lithium cobalt compound includes Li a Co 1-a Co2O4, Li b Co 1-b O and LiCoO2, where 0 < a < 1, 0 < b < 1. Based on the total mass of the Li a Co 1-a Co2O4, Li b Co 1-b O and LiCoO2 being 100 wt.%, the contents of the Li a Co 1-a Co2O4, Li b Co 1-b O and LiCoO2 are 8 wt.%, 18 wt.% and 74 wt.% respectively; the coating layer also includes a cobalt source, the cobalt source is cobalt oxide, and the mass of the cobalt source accounts for 13 wt.% of the total mass of the cobalt source used in the coating process;

[0118] The coating layer also includes Al element, and the Al element exists in the form of Al2O3;

[0119] Based on the total mass of the positive electrode core, the content of Co element in the coating layer is 3000 ppm, and the content of Al element is 2000 ppm. Then the mass ratio of Al element to Co element in the coating layer is 66.7%.

[0120] This example also provides a preparation method of the above positive electrode material with a coating layer, including the following steps:

[0121] (1) Prepare the positive electrode core:

[0122] Mix nickel cobalt hydroxide, doping source and lithium source, and then conduct a primary sintering to obtain a cathode core; wherein, the doping source is aluminum oxide and zirconium oxide, the lithium source is Li2CO3, the molar ratio of Li / (Ni+Co+Al) is 1.01, the sintering temperature is 750°C, and the sintering time is 20h.

[0123] (2)Coating process:

[0124] Mix the cathode core and the coating source and grind them evenly, then conduct a secondary sintering. The sintering is carried out in an oxygen atmosphere, the sintering temperature is 400°C, the sintering time is 12h, and after sintering, the temperature is decreased at a rate of 0.5°C / min to obtain a cathode material with a coating layer; wherein, the coating source is cobalt oxide and aluminum oxide.

[0125] Example 3

[0126] This example provides a cathode material with a coating layer. The cathode material with a coating layer includes a cathode core and a coating layer coated on the surface of the cathode core. The cathode core is a nickel cobalt aluminum ternary cathode material (chemical formula: Li 1.05 (Ni 0.90 Co 0.09 Al 0.009 Mg 0.001 )O2), the coating layer includes a lithium cobalt compound, and the lithium cobalt compound includes Li a Co 1-a Co2O4, Li b Co 1-b O and LiCoO2, where 0 < a < 1, 0 < b < 1. Based on the total mass of the Li a Co 1-a Co2O4, Li b Co 1-b O and LiCoO2 being 100wt.%, the contents of the Li a Co 1-a Co2O4, Li b Co 1-b O and LiCoO2 are 14wt.%, 15wt.% and 71wt.% respectively; the coating layer also includes cobalt oxide, and the mass of cobalt oxide accounts for 11wt.% of the total mass of the cobalt source used in the coating process;

[0127] The coating layer also includes Al element, and the Al element exists in the form of Al2O3;

[0128] Based on the total mass of the cathode core, the content of Co element in the coating layer is 4000ppm, and the content of Al element is 2000ppm, so the mass ratio of Al element to Co element in the coating layer is 50%.

[0129] This embodiment also provides a method for preparing the above-mentioned cathode material with a coating layer, including the following steps:

[0130] (1) Prepare the cathode core:

[0131] Mix nickel cobalt hydroxide, doping source and lithium source and then carry out primary sintering to obtain the cathode core; wherein, the doping source is alumina and magnesia, the lithium source is Li2CO3, the molar ratio of Li / (Ni + Co + Al) is 1.05, the sintering temperature is 690 °C, and the sintering time is 14 h.

[0132] (2) Coating process:

[0133] Mix and grind the cathode core with the coating source, and then carry out secondary sintering after uniform grinding. The sintering is carried out in an oxygen atmosphere, the sintering temperature is 350 °C, the sintering time is 9 h, and after sintering, the temperature is decreased at a rate of 0.5 °C / min to obtain the cathode material with a coating layer; wherein, the coating source is cobalt hydroxide and alumina.

[0134] Example 4

[0135] This embodiment provides a cathode material with a coating layer. The cathode material with a coating layer includes a cathode core and a coating layer coated on the surface of the cathode core. The cathode core is a nickel cobalt aluminum ternary cathode material (chemical formula: Li(Ni 0.83 Co 0.11 Al 0.05 Zr 0.002 Mg 0.002 La 0.002 Sr 0.004 )O2), and the coating layer includes lithium cobalt compounds. The lithium cobalt compounds include Li a Co 1-a Co2O4, Li b Co 1-b O and LiCoO2, where 0 < a < 1, 0 < b < 1. Based on the total mass of the Li a Co 1- a Co2O4, Li b Co 1-b O and LiCoO2 being 100 wt.%, the Li a Co 1-a Co2O4, Li b Co 1-bThe contents of O and LiCoO2 are 4 wt.%, 22 wt.% and 74 wt.% respectively; the coating layer further includes a cobalt source, the cobalt source is cobalt oxide, and the mass of the cobalt source accounts for 10 wt.% of the total mass of the cobalt source used in the coating process;

[0136] The coating layer further includes Al element, and the Al element exists in the form of Al2O3;

[0137] Based on the total mass of the positive electrode core, the content of Co element in the coating layer is 1500 ppm, and the content of Al element is 1500 ppm, then the mass ratio of Al element to Co element in the coating layer is 100%.

[0138] This embodiment also provides a preparation method of the positive electrode material with the coating layer as described above, including the following steps:

[0139] (1) Prepare the positive electrode core:

[0140] Mix nickel-cobalt hydroxide, doping source and lithium source and conduct primary sintering to obtain the positive electrode core; wherein, the doping source is aluminum oxide, zirconium oxide, magnesium hydroxide, lanthanum oxide and strontium oxide, the lithium source is Li2CO3, the molar ratio of Li / (Ni + Co + Al) is 1, the sintering temperature is 690 °C, and the sintering time is 14 h.

[0141] (2) Coating process:

[0142] Mix and grind the positive electrode core and the coating source, and conduct secondary sintering after grinding evenly. The sintering is carried out in an air atmosphere, the sintering temperature is 450 °C, the sintering time is 6 h, and after sintering, the temperature is reduced at a rate of 0.6 °C / min to obtain the positive electrode material with the coating layer; wherein, the coating source is cobalt oxide and aluminum oxide.

[0143] Example 5

[0144] This embodiment provides a positive electrode material with a coating layer. The positive electrode material with the coating layer includes a positive electrode core and a coating layer coated on the surface of the positive electrode core. The positive electrode core is a nickel-cobalt-aluminum ternary positive electrode material (chemical formula: Li 1.04 (Ni 0.92 Co 0.07 Al 0.01 )O2), the coating layer includes a lithium cobalt compound, and the lithium cobalt compound includes Li a Co 1-a Co2O4, Li b Co 1-b O and LiCoO2, where 0 < a < 1, 0 < b < 1, based on the Li a Co 1-a Co2O4, Lib Co 1-b The total mass of Co, O and LiCoO₂ is 100 wt.%, and the contents of Li a Co 1-a Co₂O₄, Li b Co 1-b O and LiCoO₂ are 2 wt.%, 23 wt.% and 75 wt.%, respectively;

[0145] The coating layer further includes Al element, and the Al element exists in the form of Al₂O₃;

[0146] Based on the total mass of the positive electrode core, the content of Co element in the coating layer is 2000 ppm, the content of Al element is 1000 ppm, and the content of W element is 1000 ppm. Then, the mass ratio of the total content of Al element and W element in the coating layer to Co element is 100%.

[0147] This embodiment also provides a preparation method of the positive electrode material with the coating layer as described above, including the following steps:

[0148] (1) Prepare the positive electrode core:

[0149] Mix nickel-cobalt hydroxide, doping source and lithium source and then perform a first sintering to obtain the positive electrode core; wherein, the doping source is aluminum oxide, the lithium source is Li₂CO₃, the molar ratio of Li / (Ni + Co + Al) is 1.04, the sintering temperature is 600 °C, and the sintering time is 14 h.

[0150] (2) Coating process:

[0151] Mix the positive electrode core and the coating source and grind them evenly, then perform a second sintering. The sintering is carried out in an oxygen atmosphere, the sintering temperature is 500 °C, the sintering time is 10 h, and after sintering, the temperature is decreased at a rate of 2 °C / min to obtain the positive electrode material with the coating layer; wherein, the coating source is cobalt oxide, aluminum oxide and tungsten oxide.

[0152] Example 6

[0153] This embodiment provides a positive electrode material with a coating layer. The positive electrode material with the coating layer includes a positive electrode core and a coating layer coated on the surface of the positive electrode core. The positive electrode core is a nickel-cobalt-aluminum ternary positive electrode material (chemical formula is Li 1.02 (Ni 0.90 Co 0.07 Al 0.02 La 0.005 )O₂), the coating layer includes a lithium cobalt compound, and the lithium cobalt compound includes Li a Co 1-a Co₂O₄, Lib Co 1-b O and LiCoO2, where 0 < a < 1, 0 < b < 1, taking the total mass of said Li a Co 1-a Co2O4, Li b Co 1-b O and LiCoO2 as 100 wt.%, the contents of said Li a Co 1-a Co2O4, Li b Co 1-b O and LiCoO2 are 2 wt.%, 23 wt.% and 75 wt.% respectively;

[0154] The coating layer further includes Al element, and the Al element exists in the form of Al2O3;

[0155] Based on the total mass of the positive electrode core, the content of Co element in the coating layer is 2000 ppm, the content of Al element is 1000 ppm, and the content of W element is 1000 ppm, then the mass ratio of Al element to Co element in the coating layer is 100%.

[0156] This embodiment also provides a preparation method of the above positive electrode material with a coating layer, including the following steps:

[0157] (1) Prepare the positive electrode core:

[0158] Mix nickel-cobalt hydroxide, doping source and lithium source and then perform primary sintering to obtain the positive electrode core; wherein, the doping source is aluminum oxide and lanthanum oxide, the lithium source is Li2CO3, the Li / (Ni + Co + Al) molar ratio is 1.02, the sintering temperature is 600 °C, and the sintering time is 14 h.

[0159] (2) Coating process:

[0160] Mix the positive electrode core and the coating source and grind them evenly, then perform secondary sintering. The sintering is carried out in an oxygen atmosphere, the sintering temperature is 500 °C, the sintering time is 10 h, and after sintering, the temperature is decreased at a rate of 2 °C / min to obtain the positive electrode material with a coating layer; wherein, the coating source is cobalt oxide, aluminum oxide and tungsten oxide.

[0161] Example 7

[0162] This embodiment provides a positive electrode material with a coating layer. The positive electrode material with a coating layer includes a positive electrode core and a coating layer coated on the surface of the positive electrode core. The positive electrode core is a nickel-cobalt-aluminum ternary positive electrode material (chemical formula Li 1.02 (Ni 0.90 Co 0.07 Al0.02 La 0.005 )O2), the coating layer includes a lithium cobalt compound, and the lithium cobalt compound includes Li a Co 1-a Co2O4, Li b Co 1-b O and LiCoO2, where 0 < a < 1, 0 < b < 1. Based on the total mass of the Li a Co 1-a Co2O4, Li b Co 1-b O and LiCoO2 being 100 wt.%, the contents of the Li a Co 1-a Co2O4, Li b Co 1-b O and LiCoO2 are 8 wt.%, 18 wt.% and 74 wt.% respectively;

[0163] The coating layer also includes Al element, and the Al element exists in the form of Al2O3;

[0164] Based on the total mass of the positive electrode core, the content of Co element in the coating layer is 2000 ppm, the content of Al element is 1000 ppm, and the content of Ti element is 500 ppm. Then the mass ratio of Al + Ti element to Co element in the coating layer is 75%.

[0165] This embodiment also provides a preparation method of the above positive electrode material with a coating layer, including the following steps:

[0166] (1) Prepare the positive electrode core:

[0167] Mix nickel cobalt hydroxide, doping source and lithium source and then conduct a primary sintering to obtain the positive electrode core; among them, the doping source is aluminum oxide and lanthanum oxide, the lithium source is Li2CO3, the Li / (Ni + Co + Al) molar ratio is 1.02, the sintering temperature is 600 °C, and the sintering time is 14 h.

[0168] (2) Coating process:

[0169] Mix the positive electrode core and the coating source and grind them evenly, then conduct a secondary sintering. The sintering is carried out in an oxygen atmosphere, the sintering temperature is 500 °C, the sintering time is 10 h, and after sintering, the temperature is decreased at a rate of 2 °C / min to obtain the positive electrode material with a coating layer; among them, the coating source is cobalt oxide, aluminum oxide and tungsten oxide.

[0170] Comparative Example 1

[0171] This comparative example provides a cathode material with a coating layer. Compared with Example 1, the difference is that based on the total mass of the cathode core, the content of Co element in the coating layer is 1000 ppm, and the content of Al element is 3000 ppm, so the mass ratio of Al element to Co element in the coating layer is 300%; for the types and contents of Co element and X element in the coating layer, refer to Table 1.

[0172] Comparative Example 2

[0173] This comparative example provides a cathode material with a coating layer. Compared with Example 1, the only difference is that based on the total mass of the cathode core, the content of Co element in the coating layer is 1000 ppm, and the content of Al element is 400 ppm, so the mass ratio of Al element to Co element in the coating layer is 40%; for the types and contents of Co element and X element in the coating layer, refer to Table 1.

[0174] Comparative Example 3

[0175] This comparative example provides a cathode material with a coating layer. Compared with Example 1, the only difference is that by changing the temperature of the secondary sintering to 300 °C, for the types and contents of Co element and X element in the coating layer, refer to Table 1.

[0176] Comparative Example 4

[0177] This comparative example provides a cathode material with a coating layer. Compared with Example 1, the only difference is that by changing the temperature of the secondary sintering to 600 °C, for the types and contents of Co element and X element in the coating layer, refer to Table 1.

[0178] Comparative Example 5

[0179] This comparative example provides a cathode material with a coating layer. Compared with Example 1, the only difference is that the content of Co element in the coating layer is replaced from 1000 ppm to 800 ppm.

[0180] Comparative Example 6

[0181] This comparative example provides a cathode material with a coating layer. Compared with Example 1, the only difference is that the content of Co element in the coating layer is replaced from 1000 ppm to 4200 ppm.

[0182] For the parameters and preparation parameters of the cathode materials provided in Examples 1 - 6 and Comparative Examples 1 - 6, refer to Table 1, where Li x Co 1-x Co2O4, Li b Co 1-b O and the contents of LiCoO2 are based on Li x Co 1-xCo2O4, Li b Co 1-b The total mass of O and LiCoO2 is 100 wt.%.

[0183] Table 1

[0184]

[0185] Prepare the battery:

[0186] (1) Prepare the positive electrode sheet:

[0187] Take the positive electrode materials of Examples 1-7 and Comparative Examples 1-6 as the active substances, mix the active substances with the conductive agent Super P and the binder PVDF in a mass ratio of 95:2:3 in NMP to obtain a positive electrode slurry, coat the positive electrode slurry on the surface of the aluminum foil, and after drying, obtain the positive electrode sheet.

[0188] (2) Prepare the negative electrode sheet:

[0189] Mix artificial graphite negative electrode with conductive agent Super P, binder CMC, and thickener SBR in a mass ratio of 96:0.5:1.5:2 in water to obtain a negative electrode slurry, coat the negative electrode slurry on the surface of the copper foil, and after drying, obtain the negative electrode sheet.

[0190] (3) Provide a separator and an electrolyte. Among them, the separator is a multi-layer composite separator, the base material is a polyolefin polymer, and the composite material is a high heat-resistant inorganic ceramic; the electrolyte is obtained by dissolving LiPF6 in a mixed solvent of EC, DMC, and FEC, where EC:DMC:FEC:LiPF6 (mass ratio) = 20:60:5:15.

[0191] (4) After winding the above positive electrode sheet, negative electrode sheet and separator, put them into the battery case and inject the electrolyte to obtain the battery.

[0192] Conduct performance tests on the battery:

[0193] (1) Test the cycle performance: At room temperature of 25 °C, charge at 2C and discharge at 10C for 600 cycles, calculate the capacity retention rate, and the capacity retention rate = discharge capacity of the 600th cycle / discharge capacity of the first cycle × 100%.

[0194] (2) Test the highest temperature of the fully charged battery cell: The test is carried out in a hot box at a temperature of 133 °C.

[0195] (3) Test the maximum temperature of a single cell when overcharged at 5C to 6V, the opening time of CID, and check whether the cell catches fire or explodes. Among them, the maximum temperature of a single cell overcharged at 5C to 6V reflects the heat generation situation, and the opening time of CID reflects the gas generation situation. A delayed opening time of CID indicates less gas generation. At the same time, the present invention hopes that CID opens when overcharged to nearly 5V. Therefore, the opening time of CID should not be too early or too late. If it opens too early, it shows that the CID has opened before the battery is charged to 5V, and the cell has disconnected, prematurely causing the battery and even the battery pack to fail. On the other hand, if it opens too late, it is easy for the battery to overheat internally and pose risks such as explosion. At this time, the overcharge voltage of the battery has obviously exceeded 5V seriously, and the battery will be in a very critical state.

[0196] The results are shown in Table 2.

[0197] Table 2

[0198]

[0199] As can be seen from the above, when the secondary sintering temperature exceeds 500 °C, for example, the secondary sintering temperature in Comparative Example 4 is 600 °C, the relative content of LiCo2O4, LiCoO and LiCoO2 among the three lithium cobalt oxides, the LiCoO2 will exceed 76 wt.%, and rise to more than 80 wt.%. LiCoO2 gradually dominates and gradually transforms into a layered oxide. Its lattice oxygen is extremely easy to escape under high voltage conditions, resulting in the collapse of the cathode material structure, and then reacting with the electrolyte to cause thermal runaway of the cell. a Co 1-a Co2O4, Li b Co 1-b O and LiCoO2 will exceed 76 wt.%, and rise to more than 80 wt.%. LiCoO2 gradually dominates and gradually transforms into a layered oxide. Its lattice oxygen is extremely easy to escape under high voltage conditions, resulting in the collapse of the cathode material structure, and then reacting with the electrolyte to cause thermal runaway of the cell.

[0200] When the secondary sintering temperature is lower than 350 °C, for example, the secondary sintering temperature in Comparative Example 3 is 300 °C, although we found that the relative contents of LiCo2O4, LiCoO and LiCoO2 are 16 wt.%, 14 wt.% and 70 wt.%. At this time, LiCo2O4, LiCoO and LiCoO2 are 16 wt.%, 14 wt.% and 70 wt.%. a Co 1-a Co2O4, Li b Co 1-b O and LiCoO2 are 16 wt.%, 14 wt.% and 70 wt.%. At this time, Li a Co 1-aThe content of Co2O4 is less than 15 wt.%, its capacity retention rate decreases, and an explosion also occurred during the experiment. Therefore, the technical solution of the present invention cannot be realized, nor can the corresponding technical effects be achieved. As can be seen from Table 2, by regulating the type and content of lithium cobalt compounds, the content of Co element, and the mass ratio of Co element to other elements in the surface coating layer of the positive electrode material, the positive electrode material exhibits the following properties: enhanced interface stability and reduced gas production. The capacity retention rate of the 18650 cylindrical battery cells assembled with the positive electrode materials of Examples 1-6 after 600 cycles at 2C / 10C is > 80%, the highest temperature of the fully charged battery cells is above 128 °C, and there will be no release due to gas production. Moreover, even when a single battery cell is overcharged to 5V at 5C, the highest temperature exceeds 100 °C, and there will be no fire or explosion. Among them, the single battery cell of Example 3 has the highest temperature during overcharging, indicating that under this design, the interface protection effect is the best, and even when the highest temperature of the battery cell exceeds 120 °C, the battery cell will not catch fire or explode.

[0201] As can be seen from Examples 1-7, the CID opening time is between 700 s and 820 s, which is a more suitable design range for the present invention, indicating that the CID opens when overcharged to nearly 5V, indicating that the battery has a good operating state. If it opens too early, it shows that the CID has opened before the battery is charged to 5V, and the battery cell has disconnected, prematurely rendering the battery and even the battery pack ineffective. Moreover, if it opens too late, it does not belong to the technical solution required by the present invention. When the CID opening time is too long, it is easy to cause risks such as overheating and explosion inside the battery. At this time, the overcharge voltage of the battery has clearly exceeded 5V seriously, and the battery will be in a very critical state.

[0202] In Comparative Example 1, the mass ratio of Al element to Co element is greater than 100%. Although the CID opening time is delayed, indicating less gas production, the surface impedance deteriorates significantly, and the battery cell catches fire and explodes; in Comparative Example 2, the mass ratio of Al element to Co element is less than 50%, resulting in a decline in cycle performance, and there are phenomena of fire and explosion during the test of the highest temperature of the fully charged battery cell. It can be seen that the other elements in the coating layer and Co element need to satisfy 50% - 100%, otherwise it will be impossible to balance low gas production and heat generation.

[0203] In Comparative Example 5, the content of Co element in the coating layer is too low, and in Comparative Example 6, the content of Co element in the coating layer is too high, both of which result in an inability to balance less gas production and heat generation.

[0204] The applicant declares that the detailed method of the present invention is illustrated by the above embodiments, but the present invention is not limited to the above detailed method, that is, it does not mean that the present invention must rely on the above detailed method to be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent substitution of each raw material of the product of the present invention, the addition of auxiliary components, the selection of specific methods, etc. all fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A positive electrode material having a coating layer, characterized in that: The positive electrode material with a coating layer comprises a positive electrode core and a coating layer coated on the surface of the positive electrode core, and the chemical composition of the positive electrode core is Li α Ni x Co y Al z A m O2, 1≤α≤1.05, 0.80≤x≤0.95, 0.04≤y≤0.15, 0.005≤z≤0.06, 0≤m≤0.02, x+y+z+m=1, wherein A includes one or more of Zr, Mg, Sr, Y, La, Nb, Ba and Ti; The coating layer contains Co element and X element, and the mass ratio of the X element in the coating layer to the Co element in the coating layer is 50% to 100%; X is 100 wt.% of Al; or, X includes Al and other coating elements, wherein the other coating elements are one or more of W, Ti and Ce, and more than 50 wt.% and not including 100 wt.% of X is Al; The coating layer includes lithium cobalt oxide, and the lithium cobalt oxide includes Li a Co 1-a Co2O4, Li b Co 1-b O, and LiCoO2, where 0 < a < 1, 0 < b < 1. The contents of Li a Co 1-a Co2O4, Li b Co 1-b O, and LiCoO2 in the total mass of the three are 1 wt.% - 15 wt.%, 10 wt.% - 25 wt.%, and 65 wt.% - 76 wt.% respectively, and the content of Li b Co 1-b O is greater than the content of Li a Co 1-a Co2O4; Based on the total mass of the positive electrode core, the content of the Co element in the coating layer is 1000ppm~4000ppm.

2. A method for preparing a positive electrode material having a coating layer as claimed in claim 1, characterized in that: The preparation method comprises the following steps: The positive electrode core and the coating source are mixed and sintered at a temperature of 350° C. to 500° C. to obtain a positive electrode material with a coating layer; The coating source includes a cobalt source and an X source, the X source includes an aluminum source, and more than 50 wt.% of X is Al element.

3. The preparation method according to claim 2, characterized in that: The cobalt source comprises at least one of cobalt oxide, cobalt hydroxide and cobalt oxyhydroxide; and / or, The aluminum source includes at least one of aluminum oxide, aluminum hydroxide, and aluminum oxyhydroxide.

4. The preparation method according to claim 2, characterized in that: The X source also includes other coating sources, and the elements in the other coating sources are at least one of W, Ti and Ce; and / or, The other coating source is at least one of nitrate, chloride, hydroxide or oxide.

5. The preparation method according to claim 2, characterized in that: The sintering is performed in an oxygen-containing atmosphere; and / or, The sintering time is 6h~24h; and / or, After the sintering is completed, the temperature is lowered at a cooling rate of 0.01° C. / min to 3° C. / min.

6. The preparation method according to claim 2, characterized in that: The method for preparing the positive electrode core comprises the following steps: The nickel-cobalt hydroxide, the doping source and the lithium source are mixed and sintered to obtain a positive electrode core; The doping elements in the doping source include Al and A, wherein A includes one or more of Zr, Mg, Sr, Y, La, Nb, Ba and Ti.

7. The preparation method according to claim 6, characterized in that: During the preparation of the positive electrode core, the molar ratio of each element satisfies Li / (Ni+Co+Al+A) of 1 to 1.05; and / or, During the preparation of the positive electrode core, the sintering temperature is 400° C. to 1000° C.; and / or, During the preparation of the positive electrode core, the sintering time is 6h~20h.

8. A lithium ion battery comprising a positive electrode, a negative electrode and a separator, characterized in that: The positive electrode comprises the positive electrode material with a coating layer as claimed in claim 1.

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