Positive electrode active material and preparation method thereof, positive plate and battery

Abstract

The invention provides a positive active material and a preparation method thereof, a positive plate and a battery. The positive electrode material comprises an active material inner core, and a first coating layer and a second coating layer which are sequentially stacked on at least part of the surface of the active material inner core, wherein the first coating layer comprises lithium carbonate, and the second coating layer comprises an ion conductor. According to the positive electrode active material provided by the invention, through a specific multi-layer coating strategy, the stability of the positive electrode active material can be improved, and meanwhile, the specific discharge capacity of the positive electrode active material is remarkably improved, so that the cycle performance and the overall electrical performance of the battery are ensured.

Description

Technical Field

[0001] The present invention relates to the field of battery materials, and particularly to a cathode active material, a preparation method thereof, a cathode sheet and a battery. Background Art

[0002] The commonly used cathode active materials for lithium-ion batteries usually contain transition metals, which play a crucial role in these cathode active materials and can store and release lithium ions through reversible redox reactions during charge and discharge processes. Different transition metals have different chemical stabilities in the cathode active materials. For example, cobalt and manganese ions may undergo metal dissolution under high voltage and high temperature conditions, and certain components of the electrolyte may also react with the cathode active materials, further promoting the dissolution of cobalt and manganese ions, which will cause structural changes in the cathode materials and thus affect the capacity and cycle life of the battery.

[0003] Surface coating of the cathode active material can slow down the dissolution of transition metal ions and prevent direct contact between the active material and the electrolyte, thus effectively improving the cycle performance of the lithium-ion battery to a certain extent. However, the existing coating materials are non-active substances themselves. Although the cycle performance of the battery is improved, there is a certain loss in the capacity of the battery. Summary of the Invention

[0004] The present invention provides a cathode active material. By introducing a first coating layer and a second coating layer which are sequentially stacked on the surface, the cathode active material can effectively improve the cycle performance of the battery while keeping the discharge specific capacity of the cathode active material in a relatively high range.

[0005] The present invention also provides a preparation method of the above-mentioned cathode material. The preparation method can prepare the above-mentioned cathode material and has a simple process.

[0006] The present invention also provides a cathode sheet. Since the cathode sheet includes the above-mentioned cathode active material, the cathode sheet is used in a battery and helps to improve the discharge specific capacity and cycle performance of the battery.

[0007] The present invention also provides a battery. Since the battery includes the above-mentioned cathode sheet, the battery has a relatively high discharge specific capacity and cycle performance.

[0008] In a first aspect, the present invention provides a cathode active material, comprising: an active material core, and a first coating layer and a second coating layer which are sequentially stacked on at least a part of the surface of the active material core; wherein, the first coating layer includes lithium carbonate, and the second coating layer includes an ion conductor.

[0009] In an optional embodiment, in the first coating layer, the mass ratio of the lithium carbonate is not less than 90wt%.

[0010] In an optional embodiment, the thickness of the second coating layer is greater than that of the first coating layer.

[0011] In an optional embodiment, the thickness of the second coating layer is n, and the thickness of the first coating layer is m, where 1.5 ≤ n / m ≤ 4;

[0012] And / or, the thickness of the first coating layer is 5 - 15 nm.

[0013] In an optional embodiment, the ionic conductor includes at least one of metal lithium compounds, boron lithium compounds, sulfur lithium compounds, and phosphorus lithium compounds.

[0014] In an optional embodiment, the molecular formula of the active material core is Li 1+a Ni x Co y M z N b O 2±c A d , where 0.6 ≤ x < 1, 0 < y < 0.3, 0 < z < 0.3, 0 < a < 0.2, 0 < b < 0.2, c < 0.02, 0 ≤ d ≤ 0.05, and x + y + z + b = 1; M is Mn and / or Al, N is at least one of Zr, Sr, La, Mg, Y, Ti, Ca, Mo, Ce, W, and A is at least one of F, Cl, S.

[0015] In a second aspect, the present invention provides a method for preparing the above-mentioned positive electrode active material, comprising the following steps:

[0016] Stir a first mixed system containing an active material and a carbonic acid solution, and perform a drying treatment until the water content of the first mixed system is not higher than 5 wt%, to obtain an intermediate;

[0017] At 280 - 700 °C, heat-treat a second mixed system containing the intermediate and an ionic conductor to obtain the positive electrode active material.

[0018] In an optional embodiment, the carbonic acid concentration of the carbonic acid solution is 0.01 mol / L - 0.033 mol / L;

[0019] And / or, the atmosphere of the drying treatment contains carbon dioxide;

[0020] And / or, the temperature of the drying treatment is 150 - 200 °C;

[0021] And / or, in the second mixed system, the concentration of the ionic conductor is 500 - 10000 ppm;

[0022] And / or, the time of the stirring treatment is 1-30 min.

[0023] In a third aspect, the present invention provides a positive electrode sheet, comprising a current collector and a positive electrode active layer provided on at least one functional surface of the current collector; the positive electrode active layer comprises the positive electrode active material described in the first aspect.

[0024] In a fourth aspect, the present invention provides a battery, comprising the above positive electrode sheet.

[0025] The positive electrode active material provided by the present invention can improve the stability of the positive electrode active material and significantly increase the discharge specific capacity of the positive electrode active material through a specific multi-layer coating strategy, thereby ensuring the cycle performance and overall electrical performance of the battery. Description of the Drawings

[0026] The drawings here are incorporated into the specification and form a part of this specification, showing embodiments consistent with the present invention, and are used together with the specification to explain the principles of the present invention.

[0027] Figure 1 It is a schematic structural diagram of the positive electrode active material in a specific embodiment of the present invention;

[0028] In the figure, 01 - active material core, 02 - first coating layer, 03 - second coating layer;

[0029] Figure 2 It is a TEM picture of the positive electrode active material in a specific embodiment of the present invention;

[0030] In the figure, 01 - active material core, 02 - first coating layer, 03 - second coating layer. Detailed Embodiments

[0031] To enable those skilled in the art to better understand the solution of the present invention, the present invention will be further described in detail below. The specific embodiments listed below only describe the principles and features of the present invention, and the examples given are only used to explain the present invention and do not limit the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0032] In order to improve the stability of lithium cathode active materials, coating modification can be carried out using a coating layer. For example, coating with metal oxides, ion conductors, etc. However, although coating with metal oxides can improve the cycle performance of the battery, there is a certain loss in the discharge specific capacity of the cathode active material. Moreover, when directly coated with metal oxides, it is prone to peeling off after a long charge-discharge cycle, resulting in a sudden drop in the battery capacity, which is not conducive to the stable cycle of the battery. Therefore, in order to effectively improve the cycle stability of the cathode active material and ensure its discharge specific capacity, the present invention adopts the following technical solutions:

[0033] In a first aspect, the present invention provides a cathode active material, see Figure 1 , including: an active material core 01, and a first coating layer 02 and a second coating layer 03 that are sequentially stacked on at least a part of the surface of the active material core; wherein, the first coating layer 02 includes lithium carbonate, and the second coating layer 03 includes an ion conductor.

[0034] Through a specific multi-layer coating strategy, the present invention can significantly improve the discharge specific capacity of the cathode active material while improving the stability of the cathode active material, thereby ensuring the cycle performance and overall electrical performance of the battery. The main reasons include: As the first layer of coating, lithium carbonate can form a chemically stable protective film on the surface of the active material core, reduce the dissolution of transition metals under high-temperature conditions, and avoid direct contact between the electrolyte and the cathode material, thereby reducing the occurrence of side reactions. As the second coating layer, the ion conductor can further stabilize the interface between the first coating layer and the active material core, stabilize the transition metal ions in the active material, reduce their dissolution and migration, thereby further improving the high-temperature stability of the battery. At the same time, the ion conductor can also significantly improve the lithium-ion transmission ability of the overall coating layer, reduce the interface impedance, thereby improving the overall ionic conductivity of the cathode active material and increasing the discharge specific capacity of the cathode active material.

[0035] In some embodiments, the positional relationship and composition of the active material, the first coating layer, and the second coating layer can be tested in the following manner:

[0036] Obtain the positional relationship: Use a focused ion beam (FIB) to perform ion beam shearing on the cathode active material sample to be tested, expose its cross-section, and then transfer the tested sample with the exposed cross-section into a transmission electron microscope (TEM) test device under vacuum. Operate the electron microscope at a high magnification and use the objective aperture to obtain a high-resolution image of the particle micro-region. Then, use an image acquisition system to measure and analyze the morphological image data of the particles, and measure the coating layer of the selected micro-region of the particles (see Figure 1 ) to obtain the positional relationship between the active material, the first coating layer, and the second coating layer;

[0037] Obtain the composition of the coating layer: Instruments used: Energy Dispersive X-ray Spectrometer (EDS). Transfer the sample to an environment with a humidity < 2%, obtain a fresh cross-section of the sample by ion polishing, and quickly transfer it to a scanning electron microscope. Perform EDS elemental analysis on the cut fresh cross-section, take points for elemental analysis according to the approximate thickness of the coating layer, and test at least 10 points in each area to obtain the composition of the active material core, the first coating layer, and the second coating layer.

[0038] In a specific embodiment, in the first coating layer, the mass ratio of lithium carbonate is not less than 90 wt%.

[0039] Among them, when the proportion of lithium carbonate in the first coating layer is within the above range, the chemical stability and thermal stability of the first coating layer can be further improved, which is beneficial to further improving the high-temperature cycle stability of the battery.

[0040] Exemplarily, in the first coating layer, the mass ratio of lithium carbonate is any value among 90 wt%, 91 wt%, 92 wt%, 93 wt%, 94 wt%, 95 wt%, 96 wt%, 97 wt%, 98 wt%, 99 wt%, 100 wt% times, etc., or the range composed of the two.

[0041] In some embodiments, in addition to lithium carbonate, the first coating layer further includes at least one of lithium hydroxide and lithium oxide.

[0042] In some embodiments, the mass ratio of lithium carbonate in the first coating layer can be tested by the following method:

[0043] Adopt X-ray Photoelectron Spectroscopy (XPS) etching. The etching time is based on the element signal of the second coating layer where lithium carbonate cannot be detected. Test the O1s signal peak and decompose O1s to obtain the content of lithium carbonate in the first coating layer.

[0044] In a specific embodiment, the thickness of the second coating layer is greater than the thickness of the first coating layer.

[0045] Among them, the above-described embodiments can, while improving the stability of the cathode active material, more significantly improve the lithium ion transport ability of the overall coating layer, thereby significantly enhancing the discharge specific capacity of the cathode active material.

[0046] In a specific embodiment, the thickness of the second coating layer is n, and the thickness of the first coating layer is m, where 1.5 ≤ n / m ≤ 4;

[0047] And / or, the thickness of the first coating layer is 5 - 15 nm.

[0048] Among them, the above-described embodiments can further improve the chemical stability, thermal stability, and discharge specific capacity of the positive electrode active material.

[0049] Exemplarily, the thickness of the second coating layer is at least 1.5 times, 1.6 times, 1.7 times, 1.8 times, 1.9 times, 2.0 times, 2.1 times, 2.2 times, 2.3 times, 2.4 times, 2.5 times, 2.6 times, 2.7 times, 2.8 times, 2.9 times, 3.0 times, 3.1 times, 3.2 times, 3.3 times, 3.4 times, 3.5 times, 3.6 times, 3.7 times, 3.8 times, 3.9 times, 4.0 times, etc. of the thickness of the first coating layer, or any value within the range formed by the two.

[0050] Exemplarily, the thicknesses of the first and second coating layers can be measured using conventional testing instruments according to conventional testing methods. For example, an ion beam shearing is performed on a positive electrode active material sample to be tested using a focused ion beam (FIB) to expose its cross-section, and then the sample with the exposed cross-section is transferred into a transmission electron microscope (TEM) testing device under vacuum, and at a high magnification, the objective aperture of the electron microscope is used to obtain a high-resolution image of the particle micro-region, and then an image acquisition system is used to measure and analyze the morphological image data of the particle to measure the thicknesses of the active material core, the first coating layer, and the second coating layer; by way of example and not limitation, the thickness of the first coating layer is any value among 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, etc. or the range formed by the two.

[0051] In a specific embodiment, the ion conductor includes at least one of metal lithium compounds, boron lithium compounds, sulfur lithium compounds, and phosphorus lithium compounds.

[0052] The ion conductor as described above can further improve the lithium ion transport ability of the second coating layer, thereby enhancing the overall lithium ion transport ability of the positive electrode active material and ensuring the exertion of the discharge specific capacity.

[0053] The metal lithium compounds include but are not limited to: at least one of lithium cobaltate, nickel cobalt manganese oxide materials (molar content of nickel < 60%), lithium manganate, lithium nickelate, lithium titanate, lithium aluminum titanium phosphate, lithium lanthanum titanate, lithium lanthanum tantalate, lithium germanium aluminum phosphate, lithium lanthanum zirconium oxide, lanthanum zirconium aluminum lithium oxide, niobium-doped lithium lanthanum zirconium oxide, tantalum-doped lithium lanthanum zirconium oxide, etc.;

[0054] The boron lithium compounds include but are not limited to: at least one of lithium borohydride (LiBH4), lithium tetrafluoroborate (LiBF4), lithium tetraborate (Li2B4O7), lithium metaborate (Li3BO3), and lithium metaborite (LiBO2);

[0055] The sulfur lithium compounds include but are not limited to at least one of lithium sulfide, lithium aluminum sulfide, lithium titanium sulfide, lithium molybdenum sulfide, etc.;

[0056] The phosphorus lithium compounds include but are not limited to at least one of lithium phosphide, lithium aluminum phosphide, lithium titanium phosphide, etc.

[0057] In a specific embodiment, the molecular formula of the active material core is Li 1+a Ni x Co y M z N b O 2±c A d , where 0.6 ≤ x < 1, 0 < y < 0.3, 0 < z < 0.3, 0 < a < 0.2, 0 < b < 0.2, c < 0.02, 0 ≤ d ≤ 0.05, and x + y + z + b = 1; M is Mn and / or Al, N is at least one of Zr, Sr, La, Mg, Y, Ti, Ca, Mo, Ce, W, and A is at least one of F, Cl, S.

[0058] Among them, the stability of the active material core as described above is relatively high, which is more conducive to the stable cycling of the battery.

[0059] In a specific embodiment, the average diameter of the active material core is 8 - 12 μm. The active material core of the above size can further ensure the kinetic performance and specific capacity of the positive electrode active material.

[0060] In a second aspect, the present invention provides a method for preparing the above positive electrode active material, comprising the following steps:

[0061] Stir a first mixed system containing an active material and a carbonic acid solution, and perform a drying treatment until the water content of the first mixed system is not higher than 5 wt% to obtain an intermediate;

[0062] At 280 - 700 °C, heat-treat a second mixed system containing the intermediate and an ion conductor source to obtain the positive electrode active material.

[0063] Among them, in the above preparation method, the active material and the carbonic acid solution are first mixed and stirred, so that the flaky lithium hydroxide on the surface of the active material can be converted into a uniformly thick and dense lithium carbonate film, that is, the first coating layer. Since the solubility of lithium carbonate in water is relatively low, the situation of over-washing can be avoided. Subsequently, the intermediate is obtained by drying. After the intermediate and the ion conductor are mixed and heat-treated, an ion conductor coating layer, that is, the second coating layer, can be formed on the surface of the lithium carbonate coating layer. The combination of the two coating structures can improve the stability of the positive electrode active material while significantly increasing the discharge specific capacity of the positive electrode active material.

[0064] In a specific embodiment, the carbonic acid concentration of the carbonic acid solution is 0.01 mol / L - 0.033 mol / L. By limiting the concentration of the carbonic acid solution in this embodiment, more lithium carbonate can be contained in the first coating layer, and the formation of an uneven or too thin first coating layer can be avoided, thereby affecting the performance.

[0065] In a specific embodiment, the atmosphere of the drying treatment contains carbon dioxide.

[0066] Among them, the removal of water vapor on the surface of the material during the drying process will cause damage to the coating layer. By introducing carbon dioxide as a protective gas, a dense lithium carbonate coating layer can be formed on the surface again. In addition, due to the surface tension of water, in the stirring process, actually some of the active material surfaces do not come into contact with the carbonic acid solution. Therefore, by introducing carbon dioxide during drying, it can ensure that the lithium hydroxide on the active material is also converted into lithium carbonate during the drying process, thereby further ensuring the density and integrity of the lithium carbonate coating layer.

[0067] In a specific embodiment, the temperature of the drying treatment is 150 - 200 °C.

[0068] Among them, in the above-described embodiment, by controlling the drying temperature, the removal of water vapor can be accelerated, and at the same time, part of the lithium source can be carried out, promoting its corresponding conversion into lithium carbonate, thereby improving the density of the first coating layer and being beneficial to further improving the cycle stability of the positive electrode active material.

[0069] In a specific embodiment, in the second mixing system, the concentration of the ionic conductor source is 500 - 10000 ppm.

[0070] Among them, in the above-described embodiment, by controlling the concentration of the ionic conductor, the thickness of the second coating layer can be adjusted, so that while improving the stability of the positive electrode active material, the lithium ion transmission ability of the overall coating layer can be more significantly improved, thereby further enhancing the discharge specific capacity of the positive electrode active material.

[0071] In a specific embodiment, the time of the stirring treatment is 1 - 30 min.

[0072] Among them, the time of the stirring treatment is not less than 1 min, which can ensure that most of the lithium hydroxide is converted into lithium carbonate, thereby forming a uniform first coating layer on the surface of the active material; the time of the stirring treatment is not more than 30 min, which can avoid the dissolution of the active lithium of the active material, thereby ensuring the performance of the active material itself.

[0073] In a specific embodiment, it further includes the step of pulverizing the positive electrode active material, and the purpose of this step is only to disperse the agglomerates sintered into large blocks.

[0074] The ionic conductor source is a raw material that can form an ionic conductor through heat treatment and intermediate reactions. In some embodiments, the ionic conductor source is boric acid, tetrafluoroboric acid, etc.

[0075] In a third aspect, the present invention provides a positive electrode sheet, which includes a current collector and a positive electrode active layer disposed on at least one functional surface of the current collector; the positive electrode active layer includes the positive electrode active material described in the first aspect.

[0076] Exemplarily, the material of the current collector can be at least one of aluminum foil and nickel foil; the positive electrode active layer includes the positive electrode active material, a conductive agent, and a binder; the binder can be at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, a polymer containing ethylene oxide, polyvinylpyrrolidone, and polyurethane, and the conductive agent can be at least one of carbon black, acetylene black, graphene, Ketjen black, and carbon fiber.

[0077] In a fourth aspect, the present invention provides a battery, which includes the positive electrode sheet described above.

[0078] It can be understood that the above battery further includes a negative electrode sheet, a separator, and an electrolyte.

[0079] Exemplarily, the negative electrode sheet includes a negative electrode active material layer, and the negative electrode active material layer includes a negative electrode active material, a conductive agent, a binder, and optionally a dispersant; wherein, the negative electrode active material includes at least one of graphite, a tin-based material (such as SnO2), lithium titanate, black phosphorus, and tin sulfide (SnS); the conductive agent can be selected from at least one of carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanotubes, metal powder, and graphene; the binder can be selected from at least one of carboxymethyl cellulose, styrene-butadiene rubber, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyvinyl alcohol, and sodium polyacrylate; the dispersant can be selected from at least one of sodium carboxymethyl cellulose, triethylhexyl phosphate, and sodium dodecyl sulfate.

[0080] The present invention has no particular limitation on the above separator, and any publicly known porous structure separator with electrochemical stability and chemical stability can be selected. For example, it can be at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, or polyvinylidene fluoride. The separator can be single-layer or multi-layer.

[0081] The above electrolyte includes an organic solvent and an electrolyte salt. The organic solvent serves as a medium for transporting ions in the electrochemical reaction, and an organic solvent known in the art for battery electrolytes can be used.

[0082] Exemplarily, the organic solvent may be at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), 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), ethyl methyl sulfone (EMS), diethyl sulfone (ESE). In a specific embodiment, two or more of the above organic solvents may be selected.

[0083] As a source of ions, the electrolyte salt may be an electrolyte salt known in the art for battery electrolytes. Exemplarily, the electrolyte salt may be at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium hexafluoroantimonate (LiSbF6), lithium difluorophosphate (LiPF2O2), 4,5-dicyano-2-trifluoromethylimidazole lithium (LiDTI), lithium bis(oxalato)borate (LiBOB), lithium trifluoromethanesulfonate (LiTFS), lithium bis(malonato)borate (LiBMB), lithium difluorooxalate borate (LiDFOB), lithium bis(difluoromalonato)borate (LiBDFMB), (malonatooxalato)borate lithium (LiMOB), (difluoromalonatooxalato)borate lithium (LiDFMOB), tris(oxalato)phosphate lithium (LiTOP), tris(difluoromalonato)phosphate lithium (LiTDFMP), tetrafluorooxalate phosphate lithium (LiTFOP), difluorodioxalate phosphate lithium (LiDFOP), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), (fluorosulfonyl)(trifluoromethanesulfonyl)imide lithium (LiN(SO2F)(SO2CF3)), lithium nitrate (LiNO3), lithium fluoride (LiF).

[0084] In some embodiments, the above positive electrode sheet, separator, and negative electrode sheet may be stacked in sequence to obtain an electrode core, or the above positive electrode sheet, separator, and negative electrode sheet may be stacked in sequence and then wound to obtain an electrode core; the electrode core is placed in a packaging battery film shell (such as an aluminum-plastic film shell), electrolyte is injected into the outer package and sealed to prepare the battery of the present invention.

[0085] The present invention will be further described below in conjunction with specific embodiments:

[0086] Example 1

[0087] This example provides a positive electrode active material, comprising: an active material core and a first coating layer (with a thickness of 5 nm) and a second coating layer (with a thickness of 7.5 nm) that are sequentially stacked on at least a part of the surface of the active material core; wherein, the first coating layer comprises Li2CO3, and the second coating layer comprises LiBO3; the molecular formula of the active material core is LiNi 0.8 Co 0.1 Mn 0.1 O2; the average diameter of the active material core is 10.57 μm.

[0088] Its preparation method comprises the following steps:

[0089] Mix the active material and a carbonic acid solution with a washing liquid concentration of 0.015 mol / L according to a solid-liquid ratio of 1:1, perform stirring for 1 min, and under a carbon dioxide atmosphere, conduct drying treatment at 150 °C until the water content of the first mixed system is not higher than 2 wt% to obtain an intermediate;

[0090] Mix the intermediate and the ionic conductor source H3BO3, the concentration of H3BO3 in the mixed system is 1000 ppm, heat up from room temperature at a heating rate of 3 °C to 310 °C, and keep warm for 10 h to obtain the positive electrode active material.

[0091] Examples 2 - 9

[0092] This example provides a positive electrode active material that is basically the same as Example 1, except that the conditions shown in Table 1 are changed.

[0093] Example 10

[0094] This example provides a positive electrode active material that is basically the same as Example 1, except that the molecular formula of the active material core is LiNi 0.78 Co 0.1 Mn 0.1 Zr 0.01 Y 0.01 O2.

[0095] Examples 11 - 12

[0096] This example provides a positive electrode active material that is basically the same as Example 1, except that the conditions shown in Table 1 are changed.

[0097] Comparative Example 1

[0098] This example provides a positive electrode active material, comprising: an active material core and a coating layer (with a thickness of 19 nm) provided on at least a part of the surface of the active material core; wherein, the coating layer is LiBO3.

[0099] The molecular formula of the active material core is LiNi 0.8 Co0.1 Mn 0.1 O₂。

[0100] Its preparation method includes the following steps:

[0101] According to the solid-liquid ratio of 1:1, mix the active material with ice-water washing liquid at 0 °C, perform stirring treatment for 1 min, and under an oxygen atmosphere, dry at 120 °C until the water content of the first mixed system is not higher than 2 wt%, to obtain an intermediate;

[0102] Mix the intermediate with H₃BO₃, the concentration of H₃BO₃ in the mixed system is 1000 ppm, heat up from room temperature at a heating rate of 3 °C to 310 °C, and keep warm for 10 h to obtain the positive electrode active material.

[0103] Comparative Example 2

[0104] This example provides a positive electrode active material, including: an active material core and a coating layer provided on at least part of the surface of the active material core, and the coating layer includes Li₂CO₃;

[0105] The molecular formula of the active material core is LiNi 0.8 Co 0.1 Mn 0.1 O₂。

[0106] Its preparation method includes the following steps:

[0107] According to the solid-liquid ratio of 1:1, mix the active material with a carbonic acid solution with a concentration of 0.015 mol / L, perform stirring treatment for 1 min, and under a carbon dioxide atmosphere, dry at 150 °C until the water content of the first mixed system is not higher than 2 wt% to obtain the positive electrode active material.

[0108] Comparative Example 3

[0109] The positive electrode active material provided in this example has the same active material core as that in Example 1.

[0110] Test Example 1

[0111] Using the positive electrode active materials of the above examples and comparative examples to prepare positive electrode sheets, including the following steps:

[0112] Preparation of the positive electrode sheet: Mix the positive electrode active material, conductive carbon black, and binder polyvinylidene fluoride (PVDF) evenly by mass ratio of 95:3:2 in an N-methylpyrrolidone solvent system, then coat it on an aluminum foil to obtain a positive electrode active layer, where the thickness of the positive electrode active material layer is 100 μm, and then dry and cold press to obtain a positive electrode plate.

[0113] Test Example 2

[0114] Prepare a battery using the above-mentioned positive electrode plates, including the following steps:

[0115] Preparation of the negative electrode plate: Mix artificial graphite as the negative active material, styrene-butadiene rubber as the binder, and carboxymethyl cellulose as the dispersant in a mass ratio of 96:2:2, and then disperse the mixture in deionized water to form a slurry. After stirring evenly, coat the slurry on a copper foil to obtain a negative active layer. The thickness of the negative active material layer is 120 μm. Dry it to form a negative active material layer, and then cold press and slit it to obtain a negative electrode plate;

[0116] Prepare the electrolyte: Mix ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 to obtain a solvent, and add LiPF6 so that its mass percentage content in the electrolyte is 12.5% to obtain the electrolyte;

[0117] Assembly of the battery: At 25 °C and normal pressure (0.1 MPa), in a button cell box, stack the positive electrode plate, separator, and lithium sheet in sequence to obtain an electrode assembly. Place the electrode assembly in an outer packaging aluminum-plastic film, and inject the electrolyte into the aluminum-plastic film to obtain a button cell.

[0118] Test examples

[0119] Perform the following tests on the positive active materials of the examples and comparative examples:

[0120] 1. Use a focused ion beam (FIB) to perform ion beam shearing on the sample of the positive active material to be tested, expose its cross-section, and then transfer the tested sample with the exposed cross-section into a transmission electron microscope (TEM) test device under vacuum. Operate the electron microscope at a high magnification and use the objective aperture to obtain a high-resolution image of the particle micro-region. Then, use an image acquisition system to measure and analyze the morphological image data of the particles, and measure the thicknesses of the active material core, the first coating layer, and the second coating layer of each example; among them, see the test results of Example 1 in Figure 2 .

[0121] 2. Test the lithium carbonate proportion in the first coating layer: Use X-ray photoelectron spectroscopy (XPS) etching. The etching time is based on the element signal of the second coating layer where lithium carbonate cannot be detected, test the O1s signal peak, and decompose O1s to obtain the lithium carbonate content in the first coating layer;

[0122] 3. Test the discharge specific capacity and high-temperature capacity retention rate of each battery:

[0123] Button cell capacity: At 25°C and normal pressure (0.1 MPa), the positive electrode sheet, lithium sheet, separator, and electrolyte are assembled into a button cell in a button cell box. Constant current charging is carried out at a rate of 0.2C until the voltage reaches the cut-off voltage (4.25V for eight-series and above, 4.3V for seven-series), and then constant voltage charging is carried out at the cut-off voltage until the current is less than 0.05C. The charging capacity at this time is recorded as the first-cycle charging capacity. After standing for 5 minutes, constant current discharging is carried out at a rate of 0.2C until the voltage reaches 2.5V. The discharging capacity this time is recorded as the first-cycle discharging specific capacity of the battery, which is also the initial capacity.

[0124] High-temperature cycling performance:

[0125] Test method: At 45°C, the lithium-ion battery is charged and discharged at 1C. After 300 cycles of testing, the capacity Q2 of the battery is recorded. The initial capacity of the battery is Q1, and the capacity retention rate = Q2 / Q1×100%.

[0126] Summarize at least some of the above test results in Table 1 and Table 2.

[0127] Table 1:

[0128]

[0129]

[0130] In the table, "-" represents that the result is not recorded.

[0131]

[0132]

[0133] From the above test results, it can be seen that compared with the comparative example, the positive active material of the example helps to further improve the button cell capacity and high-temperature cycling performance of the assembled battery.

[0134] Furthermore, by comparing Examples 2 and 3, it can be seen that introducing a CO2 atmosphere during the drying process and simultaneously extending the water washing time can increase the coating thickness of lithium carbonate, which is beneficial to further improving the button cell capacity performance and high-temperature cycling stability of the battery;

[0135] Furthermore, by comparing Examples 3, 6, and 7, it can be seen that extending the water washing time can increase the coating thickness of lithium carbonate, but when the coating layer reaches a certain thickness, the improvement of the high-temperature cycling performance of the battery is limited;

[0136] Furthermore, by comparing Examples 1 and 8, it can be seen that increasing the concentration of the carbonic acid solution can also increase the coating thickness of lithium carbonate, which is beneficial to further improving the button cell capacity performance and high-temperature cycling stability of the battery;

[0137] Furthermore, by comparing Examples 4 and 5, it can be seen that increasing the temperature of the drying treatment can promote the conversion of lithium carbonate, thereby increasing the coating thickness of lithium carbonate, which is beneficial to further improving the discharge capacity performance and high-temperature cycle stability of the battery.

[0138] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A positive electrode active material, characterized in that, Comprising: An active material core, and a first coating layer and a second coating layer sequentially stacked on at least a part of the surface of the active material core; wherein, the first coating layer comprises lithium carbonate, and the second coating layer comprises an ion conductor.

2. The positive electrode active material according to claim 1, wherein, In the first coating layer, the mass ratio of the lithium carbonate is not less than 90 wt%.

3. The cathode active material according to claim 1 or 2, characterized in that, The thickness of the second coating layer is greater than the thickness of the first coating layer.

4. The cathode active material according to claim 3, characterized in that, The thickness of the second coating layer is n, and the thickness of the first coating layer is m, wherein, 1.5 ≤ n / m ≤ 4; And / or, the thickness of the first coating layer is 5 - 15 nm.

5. The cathode active material according to any one of claims 1-4, characterized in that, The ion conductor comprises at least one of metal lithium compounds, boron lithium compounds, sulfur lithium compounds, and phosphorus lithium compounds.

6. The positive electrode active material according to any one of claims 1-5, characterized in that, The molecular formula of the active material core is Li 1+a Ni x Co y M z N b O 2±c A d , where 0.6 ≤ x < 1, 0 < y < 0.3, 0 < z < 0.3, 0 < a < 0.2, 0 < b < 0.2, c < 0.02, 0 ≤ d ≤ 0.05, and x + y + z + b = 1; M is Mn and / or Al, N is at least one of Zr, Sr, La, Mg, Y, Ti, Ca, Mo, Ce, W, and A is at least one of F, Cl, S.

7. A method for preparing a positive electrode active material according to any one of claims 1-6, characterized in that, Comprising the following steps: Stirring a first mixed system containing an active material and a carbonic acid solution, and drying it until the water content of the first mixed system is not higher than 5 wt% to obtain an intermediate; Performing a heat treatment on a second mixed system containing the intermediate and an ion conductor source at 280 - 700 °C to obtain the positive electrode active material.

8. The preparation method according to claim 7, wherein The carbonic acid concentration of the carbonic acid solution is 0.01 mol / L - 0.033 mol / L; And / or, the atmosphere of the drying treatment contains carbon dioxide; And / or, the temperature of the drying treatment is 150 - 200 °C; And / or, in the second mixed system, the concentration of the ion conductor source is 500 - 10000 ppm; And / or, the time of the stirring treatment is 1 - 30 min.

9. A positive electrode sheet, characterized in that, Comprising a current collector and a positive electrode active layer provided on at least one functional surface of the current collector; the positive electrode active layer comprises the positive electrode active material according to any one of claims 1 - 6.

10. A battery, characterized in that, Comprising the positive electrode sheet according to claim 9.

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