Positive electrode active material, electrochemical device, and electronic apparatus

Abstract

The invention discloses a positive electrode active material, an electrochemical device and electronic equipment. The positive electrode active material comprises lithium cobalt oxide particles; each lithium cobalt oxide particle comprises a matrix and a coating layer positioned on the surface of the matrix; the coating layer comprises a first region and a second region; the first region has a spinel structure; the second region contains an S element. The co-coating layer containing the first region and the second region is arranged on the surface of the substrate, so that the interface stability of the positive electrode active material is enhanced, the interface impedance of the positive electrode active material is reduced, and the cycle performance and the rate capability of an electrochemical device are improved.

Description

Positive electrode active material, electrochemical device, and electronic device Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a positive electrode active material, an electrochemical device, and an electronic device. Background Art

[0002] Lithium cobalt oxide, as the positive electrode material for lithium-ion batteries, is widely used in everyday consumer electronics. As demand for lithium-ion battery capacity continues to increase, the operating voltage of lithium-ion batteries has gradually increased. However, at high voltages, the structure of lithium cobalt oxide is unstable, making it increasingly difficult to meet cycle life requirements. Based on the degradation mechanism of lithium cobalt oxide, improvements are currently being made through doping and coating. Doping is typically performed using metal elements such as Al, Mg, Ti, Ni, and Mn, as well as the non-metallic element F. However, at high voltages, it is still difficult to effectively suppress the irreversible phase transition of lithium cobalt oxide during cycling. Coating is typically performed using oxides such as Al2O3, MgO, ZnO, ZrO2, and TiO2, as well as fluorides such as AlF3. However, this can easily increase interfacial impedance and affect the dynamics of lithium-ion batteries.

[0003] Summary of the Invention

[0004] In view of this, the present application provides a positive electrode active material, an electrochemical device, and an electronic device to improve the interfacial stability of the positive electrode active material and reduce the interfacial impedance, thereby improving the cycle performance and rate performance of the electrochemical device at high voltage.

[0005] In the first aspect, the present application provides a positive electrode active material, which includes lithium cobalt oxide particles, the lithium cobalt oxide particles including a matrix and a coating layer located on the surface of the matrix, the coating layer including a first region and a second region, the first region having a spinel structure, and the second region containing the S element.

[0006] The present application provides a coating layer on the surface of lithium cobalt oxide particles, which contains a first region with a spinel structure and a second region containing an S element. The first region with a spinel structure has good lattice adaptability with the matrix, which is beneficial to the improvement of the material interface stability. The first region with a spinel structure is also a Li ion conductor, which can promote the diffusion of Li ions. At the same time, the S element in the second region can form SO4 with the O element. 2-The polyanion conductor has good Li-ion conductivity and excellent stability at high voltages. Therefore, the co-coating of the first and second regions forms a stable interface, which helps suppress the irreversible phase transition of lithium cobalt oxide and reduces interfacial side reactions between the electrolyte and lithium cobalt oxide during cycling, thereby improving the cycling performance of the electrochemical device at high voltages. Furthermore, the co-coating of the first and second regions can increase Li-ion conductivity and reduce interfacial impedance, thereby improving the rate performance of the electrochemical device.

[0007] In some embodiments, the X-ray photoelectron spectrum of the positive electrode active material has a characteristic peak in the range of 167eV to 170eV. This indicates that SO4 2- Polyanion conductor, which is beneficial to improve the Li ion conductivity and interface stability of lithium cobalt oxide particles, thereby improving the cycle performance and rate performance of electrochemical devices at high voltage.

[0008] In some embodiments, the second region has a monoclinic crystal structure.

[0009] In some embodiments, the coating layer further includes a third region having a layered structure, wherein both the second region and the third region are located on the surface of the substrate, and the third region contains the element S. Based on the molar amount of the metal elements other than Li in the third region, the molar percentage of the element S in the third region is D', and D' ranges from 0.01% to 1.0%. The third region covers the portion of the substrate surface not covered by the first and second regions. Further doping the third region with the element S in the layered structure, on the one hand, increases the Li-O interlayer spacing, promotes Li ion intercalation and deintercalation, thereby reducing the interfacial impedance of the lithium cobalt oxide particles and improving the rate performance of the electrochemical device. It also reduces the degree of cation mixing, improves the crystallinity of the material, and enhances its structural stability at high voltages. Furthermore, it enhances the interfacial compatibility between the second and third regions, thereby further improving the stability of the lithium cobalt oxide particle interface and, in turn, improving the cycling performance of the electrochemical device at high voltages.

[0010] In some embodiments, in the XRD spectrum of the positive electrode active material, the peak intensity of the (003) crystal plane of the lithium cobalt oxide particle is 1 A The peak intensity of the crystal surface of the lithium cobalt oxide particle (104) is I B , satisfying: 1.75≤I A / I B≤1.95. Among them, the peak position of the (003) crystal plane is in the range of 18°~20°, and the peak position of the (104) crystal plane is in the range of 44°~46°. The peak intensity ratio of the (003) crystal plane and the (104) crystal plane of lithium cobalt oxide particles can reflect the degree of internal cation mixing, I A / I B Within the above range, the degree of cation mixing is low, the structure is more stable, and the crystallinity is better, thereby further improving the cycle performance of the electrochemical device. In some embodiments, the following conditions are satisfied: 1.8≤I A / I B ≤1.95.

[0011] In some embodiments, the C-axis of the lithium cobalt oxide particles is This indicates that the Li-O interlayer spacing on the surface of lithium cobalt oxide particles is large, which is conducive to the deintercalation of Li ions, thereby reducing the interfacial impedance of lithium cobalt oxide particles and improving the rate performance of electrochemical devices.

[0012] In some embodiments, the Li ion diffusion coefficient of the positive electrode active material is greater than or equal to 2×10 -13 cm 2 s -1 This indicates that the positive electrode active material of the present application has lower impedance, thereby improving the rate performance of the electrochemical device.

[0013] In some embodiments, the microscopic internal stress of the lithium cobalt oxide particles is 3×10 -3 ~4×10 -1 The lithium cobalt oxide particles of the present application have low microstress, indicating that the material has fewer defects, which is beneficial for suppressing cracking of the particles, thereby improving the cycle performance of the electrochemical device.

[0014] In some embodiments, the coating layer has a thickness of 5 nm to 65 nm. By controlling the thickness of the coating layer within the above range, the interfacial stability of the lithium cobalt oxide particles can be improved while reducing the impact on the capacity of the positive electrode active material.

[0015] In some embodiments, the first region is lithium cobalt oxide having a spinel structure.

[0016] In some embodiments, the matrix and the first region have a eutectic lattice structure. The eutectic lattice structure refers to atoms at the interface between the two phases being simultaneously located at the junctions of the two phase lattices, the lattices of the two phases being connected, and the atoms at the interface being shared by both phases. This facilitates the intercalation and deintercalation of lithium ions between the matrix and the first region, reducing the interfacial impedance of the lithium cobalt oxide particles. It also enhances the bonding between the matrix and the first region, thereby improving the interfacial stability of the lithium cobalt oxide particles.

[0017] In some embodiments, the mass percentage of the S element in the positive electrode active material is 0.05% to 1% based on the mass of the positive electrode active material. In this case, the interfacial stability of the lithium cobalt oxide particles is improved and the interfacial impedance is reduced, while the impact on the capacity can be reduced.

[0018] In some embodiments, the thickness of the first region is 5 to 65 nm. In some embodiments, the thickness of the second region is 5 to 65 nm. The present application regulates the thickness of the first region and the second region within an appropriate range, and the first region and the second region cooperate with each other to improve the interface stability of the particles. The appropriate region thickness is also more conducive to Li ion conduction, thereby improving the rate performance of the electrochemical device.

[0019] In some embodiments, based on the molar amount of the metal elements other than Li in the lithium cobalt oxide particles, the molar percentage of the Co element in the lithium cobalt oxide particles is 90% to 100%.

[0020] In some embodiments, the lithium cobalt oxide particles further contain an R element, wherein the R element includes at least one of Al, Mg, Ti, Ca, La, Y, Zr, Fe, Mn, Ni, Nb, Mo, W, Ta, Zn, Cr, Sn, V, Ce, Sr, Ge, Ga, Pb, Na, K, Ba, or Cu. The molar percentage of the R element in the lithium cobalt oxide particles is 0.1% to 10% based on the molar amount of the metal elements other than Li in the lithium cobalt oxide particles.

[0021] In some embodiments, the particle size Dv50 of the lithium cobalt oxide particles satisfies: 6 μm ≤ Dv50 ≤ 30 μm.

[0022] In a second aspect, the present application provides an electrochemical device, comprising a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode active layer, and the positive electrode active layer comprises any one of the above-mentioned positive electrode active materials.

[0023] In a third aspect, the present application provides an electronic device comprising the above-mentioned electrochemical device. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For those skilled in the art, other drawings can also be obtained based on these drawings.

[0025] FIG1 is a comparison chart of the cycle performance of lithium-ion batteries prepared in Example 1 and Comparative Example 1 of the present application;

[0026] FIG2 is a comparison of the rate performance of the lithium-ion batteries prepared in Example 1 and Comparative Example 1 of the present application;

[0027] FIG3 is a SEM image of the morphology of the coated particles in Example 1 of the present application;

[0028] FIG4 is a SEM image of the morphology of the uncoated particles of Comparative Example 1 of the present application;

[0029] FIG5 is an XRD test spectrum of lithium cobalt oxide particles prepared in Example 1 and Comparative Example 1 of the present application;

[0030] FIG6 is an XPS test spectrum of the lithium cobalt oxide particles prepared in Example 1 and Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0031] In order to make the purpose, technical solutions and advantages of this application more clearly understood, the present application is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0032] electrochemical devices

[0033] The present application provides an electrochemical device, which includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte, wherein the separator is arranged between the positive electrode sheet and the negative electrode sheet.

[0034] Positive electrode

[0035] The positive electrode sheet includes a positive electrode collector and a positive electrode active layer arranged on the surface of the positive electrode collector. As an example, the positive electrode collector includes two opposite surfaces in its thickness direction, and the positive electrode active layer covers any one or both of the two surfaces of the positive electrode collector.

[0036] The positive electrode active layer includes a positive electrode active material, the positive electrode active material includes lithium cobalt oxide particles, the lithium cobalt oxide particles include a substrate and a coating layer located on the surface of the substrate, the coating layer includes a first region and a second region, the first region has a spinel structure, and the second region contains an S element. The present application provides a coating layer on the surface of the lithium cobalt oxide particles, which contains both a first region with a spinel structure and a second region containing an S element. The first region with a spinel structure has good lattice compatibility with the substrate, which is beneficial to improving the stability of the material interface. The first region with a spinel structure is also a Li ion conductor, which can promote the diffusion of Li ions. At the same time, the S element in the second region can form SO4 with the O element. 2-The polyanion conductor has good Li-ion conductivity and excellent stability at high voltages. Therefore, the co-coating of the first and second regions forms a stable interface, which helps suppress the irreversible phase transition of lithium cobalt oxide and reduces interfacial side reactions between the electrolyte and lithium cobalt oxide during cycling, thereby improving the cycling performance of the electrochemical device at high voltages. Furthermore, the co-coating of the first and second regions can increase Li-ion conductivity and reduce interfacial impedance, thereby improving the rate performance of the electrochemical device.

[0037] In some embodiments, the positive electrode active material is characterized by X-ray photoelectron spectroscopy (XPS), and a characteristic peak exists in the range of 167eV to 170eV in the X-ray photoelectron spectrum. This indicates that SO4 2- Polyanion conductor, which is beneficial to improve the Li ion conductivity and interface stability of lithium cobalt oxide particles, thereby improving the cycle performance and rate performance of electrochemical devices at high voltage. For example, as shown in Figure 6, the X-ray photoelectron spectrum of the positive electrode active material of one embodiment of the present application shows SO4 at 168.2eV. 2- Characteristic peak.

[0038] In some embodiments, the second region has a monoclinic crystal structure.

[0039] In some embodiments, the coating layer further includes a third region having a layered structure, the second region and the third region are both located on the surface of the substrate, the third region contains S element, and based on the molar amount of the metal element other than Li in the third region, the molar percentage content D' of the S element in the third region is 0.01% to 1.0%. Exemplarily, based on the molar amount of the metal element other than Li in the third region, the molar percentage content D' of the S element in the third region is 0.01%, 0.02%, 0.03%, 0.05%, 0.08%, 0.1%, 0.2%, 0.3%, 0.5%, 0.8%, 1.0%, or a range consisting of any two of the above values.

[0040] In some embodiments, in the XRD spectrum of the positive electrode active material, the peak intensity of the (003) crystal plane of the lithium cobalt oxide particle is 1 A The peak intensity of the crystal surface of the lithium cobalt oxide particle (104) is I B , satisfying: 1.75≤I A / I B ≤1.95. For example, I A / I Bis 1.75, 1.78, 1.80, 1.82, 1.84, 1.86, 1.88, 1.90, 1.92, 1.94, 1.95, or a range consisting of any two of the above values. Further, in some embodiments, 1.80≤I A / I B ≤1.95.

[0041] In some embodiments, the C-axis of the lithium cobalt oxide particles is For example, the C axis of the lithium cobalt oxide particles is Or a range consisting of any two of the above values. Further, in some embodiments, the C axis of the lithium cobalt oxide particles is

[0042] In some embodiments, the Li ion diffusion coefficient of the positive electrode active material is greater than or equal to 2×10 -13 cm 2 s -1 For example, the Li ion diffusion coefficient of the positive electrode active material is 2×10 -13 cm 2 s -1 , 1×10 -12 cm 2 s -1 , 2×10 -12 cm 2 s -1 , 1×10 -11 cm 2 s -1 , 1×10 -10 cm 2 s -1 , 1×10 -9 cm 2 s -1 , 1×10 -8 cm 2 s -1 Or a range consisting of any two of the above values. Further, in some embodiments, the Li ion diffusion coefficient of the positive electrode active material is greater than or equal to 2×10 -12 cm 2 s -1 .

[0043] In some embodiments, the microscopic internal stress of the lithium cobalt oxide particles is 3×10 -3 ~4×10 -1 For example, the microscopic internal stress of the lithium cobalt oxide particles is 3×10 -3 , 1×10 -2 , 6×10 -2 , 8×10 -2, 1.0×10 -1 , 1.5×10 -1 , 1.8×10 -1 , 2.0×10 -1 , 2.5×10 -1 , 2.8×10 -1 , 3.0×10 -1 , 3.2×10 -1 , 3.6×10 -1 , 4×10 -1 Or a range consisting of any two of the above values. Further, the microscopic internal stress of the lithium cobalt oxide particles is 3×10 -3 ~1×10 -1 .

[0044] In some embodiments, the coating layer has a thickness of 5 nm to 65 nm. For example, the coating layer has a thickness of 5 nm, 8 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, or a range consisting of any two of the foregoing values. Furthermore, in some embodiments, the coating layer has a thickness of 10 to 55 nm.

[0045] In some embodiments, the first region is lithium cobalt oxide having a spinel structure.

[0046] In some embodiments, the matrix and the first region have a eutectic lattice structure.

[0047] In some embodiments, the mass percentage D of the S element in the positive electrode active material is 0.05% to 1% based on the mass of the positive electrode active material. For example, the mass percentage D of the M element in the positive electrode active material is 0.05%, 0.1%, 0.3%, 0.5%, 0.7%, 0.9%, 1%, or a range consisting of any two of the foregoing values, based on the mass of the positive electrode active material.

[0048] In some embodiments, the thickness of the first region is 5 to 65 nm. In some embodiments, the thickness of the second region is 5 to 65 nm. Exemplarily, the thickness of the first region is 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, or a range consisting of any two of the above values. Exemplarily, the thickness of the second region is 5 nm, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, or a range consisting of any two of the above values.

[0049] In some embodiments, based on the molar amount of the metal elements other than Li in the lithium cobalt oxide particles, the molar percentage of the Co element in the lithium cobalt oxide particles is 90% to 100%.

[0050] In some embodiments, the lithium cobalt oxide particles further contain an R element, and the R element includes at least one of Al, Mg, Ti, Ca, La, Y, Zr, Fe, Mn, Ni, Nb, Mo, W, Ta, Zn, Cr, Sn, V, Ce, Sr, Ge, Ga, Pb, Na, K, Ba, or Cu.

[0051] In some embodiments, based on the molar amount of metal elements other than Li in the lithium cobalt oxide particles, the molar percentage of the R element in the lithium cobalt oxide particles is 0.1% to 10%.

[0052] In some embodiments, the particle size Dv50 of the lithium cobalt oxide particles satisfies the following: 6 μm ≤ Dv50 ≤ 30 μm. For example, the particle size Dv50 of the lithium cobalt oxide particles is 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 22 μm, 24 μm, 26 μm, 28 μm, 30 μm, or a range consisting of any two of the foregoing values.

[0053] Preparation of positive electrode active materials

[0054] The present application further provides a method for preparing the above-mentioned positive electrode active material, which comprises at least the following steps: (1) mixing a cobalt source and a lithium source to obtain a first mixture; (2) mixing the first mixture with a S-containing coating additive to obtain a second mixture; (3) sintering the second mixture at 110-120°C for 2-3 hours to obtain an intermediate product; (4) sintering the intermediate product at 1000-1100°C for 5-6 hours; thus, the positive electrode active material is obtained.

[0055] In some embodiments, during the sintering process of step (4), the S element in the S-containing coating additive first robs Li, resulting in the formation of lithium-deficient spinel (i.e., the formation of the first region of the coating layer). At the same time, S is also oxidized to form SO4 2- (i.e., the formation of the second region of the coating layer), SO4 2- It is a polyanion conductor that helps ion transport. In addition, a small amount of S element will dope the third region of the surface layered structure, increasing the Li-O layer spacing and improving the crystallinity of the material.

[0056] In some embodiments, the S-containing coating additive includes elemental S.

[0057] In some embodiments, in step (2), the amount of the S-containing coating additive added is 1000 ppm to 10000 ppm based on the mass of LiCoO2 converted based on the molar amount of Co in the cobalt source. After melting, an appropriate amount of the coating additive first forms a liquid phase coating on the substrate, which makes the coating more uniform. The appropriate amount of the coating additive is conducive to the formation of the first region and the second region. At the same time, it can also achieve doping of the third region, thereby improving the cycle performance and rate performance of the electrochemical device.

[0058] Among them, in step (3), 110°C to 120°C is conducive to promoting the melting of the S-containing coating additive (such as S elemental) to form a liquid phase coating, and the coating effect is good.

[0059] In the present application, finished lithium cobalt oxide can also be directly used to replace the first mixture, mixed with the S-containing coating additive, and then the mixture is sintered at 700-800° C. for 6-7 hours to obtain the positive electrode active material.

[0060] The present application does not impose any particular restrictions on the positive electrode current collector, as long as the objectives of the present application can be achieved. For example, it may include aluminum foil, aluminum alloy foil, or a composite current collector (e.g., a composite current collector with a metal layer disposed on the surface of a polymer layer). The present application does not impose any particular restrictions on the thickness of the positive electrode current collector, as long as the objectives of the present application can be achieved. For example, the thickness of the positive electrode current collector is 5μm to 12μm. The present application does not impose any particular restrictions on the thickness of the positive electrode active layer, as long as the objectives of the present application can be achieved. For example, the thickness of the positive electrode active layer is 30μm to 120μm. The positive electrode active layer may also include a conductive agent and a binder. The present application does not impose any particular restrictions on the types of conductive agents and binders, as long as the objectives of the present application can be achieved. For example, the conductive agent may include, but is not limited to, at least one of conductive carbon black, carbon nanotubes (CNTs), carbon fibers, Ketjen black, graphene, a metal material, or a conductive polymer. The above-mentioned metal materials may include, but are not limited to, metal powder and / or metal fibers. Specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The conductive polymer may include, but is not limited to, at least one of a polyphenylene derivative, polyaniline, polythiophene, polyacetylene, or polypyrrole. The binder may include, but is not limited to, at least one of polyacrylic acid, a polyacrylate, an acrylate polymer, polyvinyl alcohol, polyvinylidene fluoride, polytetrafluoroethylene, or a vinylidene fluoride-hexafluoropropylene copolymer. The present application does not particularly limit the mass ratio of the positive electrode active material, conductive agent, and binder in the positive electrode active layer. Those skilled in the art may select the binder according to actual needs, as long as the purpose of the present application can be achieved.

[0061] other

[0062] The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on the surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material. The present application does not particularly limit the thickness of the negative electrode active layer, as long as it can achieve the objectives of the present application. For example, the thickness of the negative electrode active layer is 30μm to 120μm. In some embodiments, the negative electrode active material may include at least one of a carbon material or a silicon-based material. In some embodiments, the carbon material includes, but is not limited to, at least one of natural graphite, artificial graphite, mesophase microcarbon beads, hard carbon, or soft carbon. In some embodiments, the silicon-based material includes, but is not limited to, at least one of silicon, a silicon-oxygen composite material, or a silicon-carbon composite material. The present application does not particularly limit the thickness of the negative electrode current collector, as long as it can achieve the objectives of the present application. For example, it may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a composite current collector (e.g., a composite current collector with a metal layer disposed on the surface of a polymer layer). The present application does not particularly limit the thickness of the negative electrode current collector, as long as it can achieve the objectives of the present application. For example, the thickness of the negative electrode current collector is 5μm to 12μm. The negative electrode active layer may also include a binder and a thickener. This application does not specifically limit the types of binder and thickener, as long as they can achieve the objectives of this application. For example, the binder may include, but is not limited to, at least one of polyvinyl alcohol, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, styrene-butadiene rubber, or acrylated styrene-butadiene rubber; the thickener may include, but is not limited to, at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose. The negative electrode active layer may also include a conductive agent. This application does not specifically limit the type of conductive agent, as long as it can achieve the objectives of this application. For example, the conductive agent may include, but is not limited to, at least one of conductive carbon black, carbon nanotubes (CNTs), carbon fibers, Ketjen black, graphene, a metallic material, or a conductive polymer. This application does not specifically limit the mass ratio of the negative electrode active material, conductive agent, binder, and thickener in the negative electrode active layer. Those skilled in the art may select the ratio based on actual needs, as long as it can achieve the objectives of this application. Optionally, the negative electrode plate may also include a conductive layer, which is located between the negative electrode current collector and the negative electrode active layer. The present application does not particularly limit the composition of the conductive layer, and it may be any conductive layer commonly used in the art. For example, the conductive layer may include a conductive agent and a binder. The present application does not particularly limit the conductive agent and binder in the conductive layer, and for example, it may be at least one of the conductive agent and binder used in the negative electrode active layer described above.

[0063] The isolation membrane may include a substrate layer and a surface treatment layer. The substrate layer may be a non-woven fabric, a film, or a composite film having a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a surface treatment layer is provided on at least one surface of the substrate layer, and the surface treatment layer may be a bonding layer or a heat-resistant layer. For example, the bonding layer contains a binder, and the binder material includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or vinylidene fluoride-hexafluoropropylene copolymer. The heat-resistant layer includes inorganic particles and a binder, and the inorganic particles are not particularly limited, for example, they may include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. The binder is not particularly limited, for example, it may be at least one of the binders in the above-mentioned bonding layer.

[0064] The electrolyte includes an organic solvent and a lithium salt; the organic solvent includes a carbonate solvent, a carboxylate solvent, or a combination thereof; wherein the carbonate solvent includes at least one of diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), ethylmethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), or butylene carbonate (BC); and the carboxylate solvent includes at least one of ethyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl butyrate, ethyl difluoroacetate, difluoroethyl acetate, ethyl trifluoroacetate, trifluoroethyl acetate, or methyl trifluoropropionate. The lithium salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium bis(oxalatoborate) (LiB(C2O4)2, LiBOB), lithium difluorooxalatoborate (LiBF2(C2O4), LiDFOB), lithium tetrafluoroborate (LiBF4), lithium bis(fluorosulfonyl)imide (LiFSI) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0065] The electrolyte may further include an electrolyte additive, which may include but is not limited to at least one of fluoroethylene carbonate (FEC), vinyl ethylene carbonate (VC) or 1,3-propane sultone (PS).

[0066] The electrochemical device of the present application may include any device that generates an electrochemical reaction, and specific embodiments thereof include all types of primary batteries or secondary batteries. In particular, the electrochemical device is a lithium secondary battery, including a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

[0067] electronic devices

[0068] The electronic devices described in this application include any of the above-mentioned electrochemical devices. The electronic devices described in this application include, but are not limited to, mobile phones, laptops, tablet computers, game consoles, drones, electric cars, electric bicycles, power tools, and Bluetooth earphones.

[0069] Hereinafter, embodiments of the present application will be described in more detail with reference to examples and comparative examples.

[0070] Example 1

[0071] (1) Preparation of lithium-ion batteries

[0072] (1) Preparation of positive electrode active materials

[0073] 3.33 kg of precursor Co3O4 was placed in a mixing tank, and 1.623 kg of Li2CO3 and sublimed S were added, wherein the amount of sublimed S added was 6000 ppm based on the amount of the added precursor Co3O4 converted to the mass of LiCoO2 containing an equal molar amount of Co. The mixture was ball milled for 6 hours to mix evenly. The mixed powder was sintered in a tube furnace at a low temperature of 115°C for 2 hours, and then sintered at a high temperature of 1050°C for 6 hours, and then naturally cooled to room temperature. The obtained solid was crushed and sieved to obtain a lithium cobalt oxide positive electrode active material, and its S element content is shown in Table 3.

[0074] (2) Preparation of positive electrode sheet

[0075] The positive electrode active material, the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) are mixed uniformly in an N-methylpyrrolidone solvent system at a mass ratio of 97:1.5:1.5, coated on the surface of the Al foil and dried, and then cold pressed to obtain a positive electrode sheet.

[0076] (4) Preparation of isolation membrane

[0077] A 9 μm thick polyethylene (PE) porous membrane was selected as the separator.

[0078] (5) Preparation of electrolyte

[0079] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a mass ratio of 1:1:1 to obtain an organic solvent. LiPF6 is dissolved in the organic solvent, and fluoroethylene carbonate (FEC) and 1,3-propane sultone (PS) are added to obtain an electrolyte. Based on the total mass of the electrolyte, the mass percentage of LiPF6 is 12.5%, the mass percentage of FEC is 2%, and the mass percentage of PS is 1%.

[0080] (6) Preparation of lithium-ion button batteries

[0081] The positive electrode sheet was punched into a disc with a diameter of 1.4 cm, and placed into a battery case in sequence with the separator and the negative lithium sheet. The electrolyte was injected and the battery was packaged on a packaging machine to obtain a lithium-ion button battery.

[0082] Examples 2 to 15

[0083] Examples 2-10 differ from Example 1 in that the amount of sublimated sulfur added was adjusted. Examples 11-15 differ in that finished lithium cobalt oxide was directly ball-milled with sublimated sulfur in the amounts shown in Table 3, followed by sintering at 750°C for 6 hours. See Table 3 for details.

[0084] Comparative Examples 1-2

[0085] Comparative Example 1 differs from Example 1 in that the surface coating of the lithium cobalt oxide particles' substrate is absent. Specifically, no sublimed sulfur was added during the preparation of the positive electrode active material, and no low-temperature sintering step was performed. Comparative Example 2 differs from Example 1 in that the coating on the surface of the lithium cobalt oxide particles' substrate only contains the second region. In Comparative Example 2, the finished lithium cobalt oxide was mixed with Li2SO4 salt (with 6000 ppm of sulfur added based on the mass of the lithium cobalt oxide) and sintered at 350°C for 5 hours. See Table 3 for details.

[0086] (2) Test method

[0087] (1) Lithium ion diffusion coefficient test: potentiostatic intermittent titration (PITT)

[0088] A lithium-ion button cell was tested with a voltage range of 3.95-4.60V. The upper limit voltage of the test was consistent with the upper limit voltage of charge and discharge, and the voltage step during the test was 25mV. Before testing PITT, the test cell was first charged to 3.95V at a constant voltage. When the current was less than 5μA, the battery was considered to have basically reached equilibrium. During the test, a 25mV step potential was applied to the battery to obtain a chronoamperometric curve of current versus time (It curve). When the current decayed to 5μA, the step experiment was terminated. The battery was left to stand for a period of time. When the electrode basically reached equilibrium, the next potential PITT test was performed. The solid-phase diffusion coefficient of lithium ions was obtained by obtaining the It curve under the constant voltage through the above experiment, calculating the ln(I)-t curve, and performing a linear fit on the exponential part. The slope obtained was d(ln(I)) / d(t), and then the lithium ion diffusion coefficient was calculated using the following formula: D = -d(ln(I)) / d(t)×(r 2 / π 2 ), where r is the volume average particle size Dv50 of the positive electrode active material.

[0089] (2) High temperature cycle test

[0090] Lithium-ion button cells were tested for cycling performance at 45°C. The battery was charged at a constant current of 1C to 4.25V, then at a constant current of 0.5C to 4.53V. The battery was then charged at a constant voltage of 4.53V until the current reached 0.05C, reaching the full charge voltage plateau of 4.53V. The battery was then discharged at a constant current rate of 0.2C until the voltage reached 3.0V. Each cycle was repeated with a 5-minute interval until the capacity retention rate fell below 70%.

[0091] Capacity retention rate at the 50th cycle = (discharge capacity corresponding to the 50th cycle / discharge capacity at the first cycle) × 100%.

[0092] (3) Rate test

[0093] The lithium-ion button battery was placed at 25°C for 30 minutes, then charged to 4.53V at a rate of 0.2C, charged at a constant voltage of 4.53V to a cutoff current of 0.05C, and placed for 10 minutes. Finally, it was discharged to 3V at different rates. The capacity of the discharge rate of 0.1C, 0.2C, 0.5C, 1C, and 2C was tested respectively, and the capacity retention rate was calculated as discharge capacity at different rates / discharge capacity at 0.1C rate × 100%.

[0094] (4) S content in the positive electrode active material D

[0095] The positive electrode active material sample was added to aqua regia for digestion, and the contents of elements such as Li, Co and S in the positive electrode active material were obtained by inductively coupled plasma optical emission spectrometry (ICP-OES).

[0096] (5) Crystal structure, coating thickness, and S content D′ in the third region

[0097] The positive electrode sheet was transferred to the chamber of a scanning electron microscope (FEI Vion Plasma FIB) equipped with a focused ion beam and processed to obtain a sample suitable for analysis using a transmission scanning electron microscope (STEM, FEI Titan3 G2 60-300). The sample surface was protected with Pt and processed using a Ga ion beam, with a thickness of no more than 100 nm. The sample was then cleaned using a low-voltage mode to remove any residual surface residue from the sample processing. The sample was observed under the STEM, and the crystal structures of the positive electrode active material matrix, the first region, the second region, and the third region were determined by electron diffraction analysis. Simultaneously, electron micrographs of the sample were taken, and the thicknesses of the first region and the second region were measured. The thickness of the coating layer was the greater of the thicknesses of the first and second regions.

[0098] Use X-ray energy dispersive spectroscopy (EDS) at an appropriate magnification to collect data to determine the element types and contents in the cathode active material matrix, the first region, and the third region. Collect data from at least three different locations and take the average of the results. The molar percentage D′ of the S element in the third region can be calculated.

[0099] (6) Volume average particle size Dv50 test

[0100] The volume average particle size Dv50 of the positive electrode active material was measured using a laser particle size analyzer.

[0101] (7) X-ray photoelectron spectroscopy test

[0102] The X-ray photoelectron spectra of the positive electrode active materials were obtained using an X-ray photoelectron spectrometer.

[0103] (8) XRD test

[0104] The cathode active material powder was placed on the sample stage of an XRD test instrument (model Bruker, D8) and an X-ray diffraction pattern was obtained using a scanning rate of 2° / min and a scanning angle range of 10° to 90°. The positions and intensities of the characteristic peaks corresponding to the (003) and (104) crystal planes were read to calculate I A / I B value.

[0105] The microscopic internal stress is calculated according to the formula: ε = β / (4 × tanθ), where ε represents the microscopic internal stress, β is the half-peak width of the characteristic peak corresponding to the (003) crystal plane, and θ is the diffraction angle of the characteristic peak corresponding to the (003) crystal plane.

[0106] Among them, the cycle performance comparison diagram of the lithium-ion batteries prepared in Example 1 and Comparative Example 1 is shown in Figure 1. It can be seen from Figure 1 that the co-coating layer containing both the first region and the second region can significantly improve the cycle performance of the lithium-ion battery. The rate performance comparison diagram of the lithium-ion batteries prepared in Example 1 and Comparative Example 1 is shown in Figure 2. It can be seen from Figure 2 that the co-coating layer containing both the first region and the second region is also beneficial to the improvement of the rate performance. Figure 3 is an SEM image of the morphology of the positive electrode active material particles after coating in Example 1, and Figure 4 is an SEM image of the morphology of the uncoated positive electrode active material particles of Comparative Example 1. It can be seen from Figures 3 and 4 that the particle morphology does not change significantly before and after coating. The XRD test spectrum of the lithium cobalt oxide particles prepared in Example 1 and Comparative Example 1 is shown in Figure 5. It can be seen from Figure 5 that no impurity phase is generated after coating, and the crystallinity is better. FIG6 is an XPS test spectrum of the lithium cobalt oxide particles prepared in Example 1 and Comparative Example 1 of the present application. As can be seen from FIG6 , the coated lithium cobalt oxide particles have a characteristic peak in the range of 167 eV to 170 eV.

[0107] Table 1 shows the XRD test results of the positive electrode active materials of Example 1 and Comparative Example 1, and Table 2 shows the ICP test results of the positive electrode active materials of Example 1 and Comparative Example 1.

[0108] Table 1

[0109] It can be seen from Table 1 that the Li-O interlayer spacing increases and there is S element doping in the third region, which is manifested as a significant increase in the C axis.

[0110] Table 2

[0111] It can be seen from Table 2 that Example 1 contains an appropriate amount of S element, while the S element in Comparative Example 1 exists in a small amount as an impurity element.

[0112] The preparation parameters and test results of Examples 1 to 15 and Comparative Examples 1 to 2 are shown in Table 3.

[0113] Table 3

[0114] Compared with Example 1, it can be seen that the co-coating layer on the substrate surface can significantly improve the cycle performance and rate performance of the lithium-ion battery. Compared with Example 1 and Comparative Example 2, it can be seen that the single SO4 2- While a polyanion conductor coating can also improve the rate capability of lithium-ion batteries, its effect on improving cycling performance at high voltages is far less pronounced than a co-coating layer formed from two materials. This may be because the spinel-structured first region has greater structural stability at high voltages, and because it forms an in-situ symbiotic relationship with the matrix, it shares a good eutectic lattice structure with the matrix, further enhancing the material's interfacial stability.

[0115] Comparing Examples 1-15 with Comparative Example 1, it can be seen that due to the doping of S element in the third region of the layered structure, the C axis increases, indicating that the Li-O interlayer spacing on the surface of the lithium cobalt oxide particles increases, which is beneficial to promote the deintercalation of Li ions, thereby reducing the interfacial impedance of the lithium cobalt oxide particles and improving the rate performance of the lithium ion battery; at the same time, I A / I B The increase indicates that the degree of cation mixing is reduced, which makes the crystallinity of the material better, thereby improving its structural stability at high voltage; and the doping of S element in the third region can improve the interface compatibility between the second region and the third region, thereby further improving the stability of the lithium cobalt oxide particle interface, and thus improving the cycle performance of the lithium-ion battery at high voltage.

[0116] Compared to Example 1, Examples 2-10 show that the amount of sublimated S added in Examples 2-10 varies, resulting in changes in the S content in the positive electrode active material and the S content in the third region. This changes the C-axis of the positive electrode active material, and the lithium ion diffusion coefficient and microstress also change accordingly. The addition of an appropriate amount of sublimated S, such as in Example 1, is more conducive to improving the cycle performance and rate performance of the electrochemical device. Compared to Example 1, Examples 11-15 show that directly obtaining a finished lithium cobalt oxide product and reacting it with an appropriate amount of sublimated S can also achieve the formation of a co-coating layer.

[0117] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A positive electrode active material, characterized in that: The positive electrode active material includes lithium cobalt oxide particles; The lithium cobalt oxide particles include a substrate and a coating layer located on the surface of the substrate; The coating layer includes a first region and a second region; The first region has a spinel structure; The second region contains the S element.

2. The positive electrode active material according to claim 1, characterized in that At least one of the following conditions is met: (1) In the X-ray photoelectron spectrum of the positive electrode active material, there is a characteristic peak in the range of 167 eV to 170 eV; (2) The second region has a monoclinic crystal structure.

3. The positive electrode active material according to claim 1, characterized in that The coating layer further comprises a third region having a layered structure, wherein the third region contains an S element; Based on the molar amount of the metal elements except the Li element in the third region, the molar percentage of the S element in the third region is 0.01% to 1.0%.

4. The positive electrode active material according to claim 1, characterized in that In the XRD spectrum of the positive electrode active material, the peak intensity of the (003) crystal plane of the lithium cobalt oxide particle is I A The peak intensity of the crystal surface of the lithium cobalt oxide particle (104) is I B , satisfying: 1.75≤I A / I B ≤1.

95.

5. The positive electrode active material according to claim 4, characterized in that Satisfy: 1.8≤I A / I B ≤1.

95.

6. The positive electrode active material according to claim 1, characterized in that At least one of the following conditions is met: (1) The C axis of the lithium cobalt oxide particle is (2) The Li ion diffusion coefficient of the positive electrode active material is greater than or equal to 2×10 -13 cm 2 s -1 ; (3) The microscopic internal stress of the lithium cobalt oxide particles is 3×10 -3 ~4×10 -1 .

7. The positive electrode active material according to claim 1, characterized in that At least one of the following conditions is met: (1) The thickness of the coating layer is 5 nm to 65 nm; (2) a eutectic lattice structure exists between the substrate and the first region; (3) Based on the mass of the positive electrode active material, the mass percentage of the S element in the positive electrode active material is 0.05% to 1%.

8. The positive electrode active material according to claim 1, characterized in that At least one of the following conditions is met: (1) The thickness of the first region is 5 to 65 nm; (2) The thickness of the second region is 5 to 65 nm; (3) Based on the molar amount of the metal elements other than the Li element in the lithium cobalt oxide particles, the molar percentage of the Co element in the lithium cobalt oxide particles is 90% to 100%; (4) The lithium cobalt oxide particles further contain R elements; the R elements include at least one of Al, Mg, Ti, Ca, La, Y, Zr, Fe, Mn, Ni, Nb, Mo, W, Ta, Zn, Cr, Sn, V, Ce, Sr, Ge, Ga, Pb, Na, K, Ba or Cu; based on the molar amount of the metal elements other than the Li element in the lithium cobalt oxide particles, the molar percentage of the R element in the lithium cobalt oxide particles is 0.1% to 10%.

9. An electrochemical device, characterized in that: The invention comprises a positive electrode sheet, wherein the positive electrode sheet comprises a positive electrode active layer, and the positive electrode active layer comprises the positive electrode active material according to any one of claims 1 to 8.

10. An electronic device, characterized in that: An electrochemical device comprising the electrochemical device of claim 9.

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