CN114373900B - Positive electrode active material, electrochemical device, and electronic device - Google Patents
Positive electrode active material, electrochemical device, and electronic device Download PDFInfo
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Abstract
The application provides a positive electrode active material, an electrochemical device and an electronic device, so as to improve the cycle performance and interface stability of the active material under high working voltage. The present application provides an active material comprising: a core, the core comprising: a first compound; a shell layer present on at least a partial region of a surface of the core, wherein the shell layer comprises: with P6 3 A second compound having an mc crystal phase structure. The active material provided by the application has a core and a shell, wherein the shell comprises a core with P6 3 The second compound with the mc crystal phase structure has stable interface, can well protect the nucleus, prevent the collapse of the crystal structure of the first compound and the failure of the interface, inhibit the side reaction of the interface, and improve the cycle stability and the interface stability of the anode active material under high working voltage (for example, the working voltage is more than 4.6V).
Description
Technical Field
The present application relates to the field of electrochemical technology, and in particular, to a positive electrode active material, an electrochemical device, and an electronic device.
Background
Electrochemical devices, such as lithium ion batteries, are widely used in the fields of portable electronic products, electric traffic, national defense aviation, energy storage and the like because of their advantages of high energy density, good cycle performance, environmental protection, safety, no memory effect and the like. With the development of society, there is an increasing demand for electrochemical devices, in particular, for energy density of electrochemical devices, and increasing the operating voltage is one of the important means for increasing the energy density of electrochemical devices.
In the prior art, the cathode active material of the electrochemical device has a crystal structure collapsed and an interface failed under a high operating voltage (for example, 4.6V or more), resulting in rapid capacity decay and easy irreversible phase change, i.e., the existing cathode active material has poor cycle performance and interface stability under a high operating voltage. Therefore, improving the cycle performance of an electrochemical device at high operating voltages is a technical problem to be solved in the prior art.
Disclosure of Invention
In view of the above-described drawbacks of the prior art, it is an object of the present application to provide at least a positive electrode active material, an electrochemical device, and an electronic device to improve the cycle performance and interface stability of the positive electrode active material at high operating voltages.
The present application provides a positive electrode active material including:
a core, the core comprising: a first compound;
a shell layer present on at least a partial area of the surface of the core,
wherein, the shell layer includes: with P6 3 A second compound having an mc crystal phase structure.
In the above positive electrode active material, the first compound has an R-3m crystal phase structure.
In the above positive electrode active material, the first compound is a lithium cobalt composite oxide, and/or,
the second compound is lithium cobalt composite oxide.
In the above positive electrode active material, at least one of the conditions (a) to (d) is satisfied:
(a) The Dv50 of the positive electrode active material particles is 10 μm to 30 μm;
(b) The particle diameter of the core is 8-25 μm;
(c) The mass ratio of the core to the shell is 99:1 to 80:20;
(d) The first compound includes: li of R-3m crystal phase structure 1±b Co 1-a A a O 2 Wherein 0.ltoreq.b<0.1，0≤a<0.15, A is selected from at least one of Al, mg, ti, mn, fe, ni, zn, cu or Nb.
In the above positive electrode active material, the second compound includes: li (Li) x Na z Co 1-y M y O 2 Wherein, 0.6<x<0.95，0≤y<0.15，0≤z<0.02, M is selected from at least one of Al, mg, ti, mn, fe, ni, zn, cu or Nb.
In the positive electrode active material, an X-ray diffraction spectrum test is carried out on the positive electrode active material to obtain a first diffraction spectrum, and the number of peaks within the range of 15-22 degrees of the first diffraction spectrum is P1, wherein P1 is more than or equal to 2.
In the positive electrode active material, the positive electrode active material subjected to heat treatment for 5 hours at 200-400 ℃ is subjected to an X-ray diffraction spectrum test to obtain a second diffraction spectrum, and the number of peaks in the range of 15-22 DEG of the second diffraction spectrum is P2, wherein P2 is less than P1.
The present application also provides an electrochemical device comprising:
a positive electrode;
a negative electrode;
a separator disposed between the positive electrode and the negative electrode;
wherein the positive electrode comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector, and the positive electrode active material layer comprises any one of the positive electrode active materials.
In the above electrochemical device, the electrochemical device was cycled at a voltage of 4.8V for 20 cycles at a discharge gram capacity of not less than 200mAh/g, and the crack growth rate of the positive electrode active material particles at the 20 th cycle was not more than 5% as compared with the 1 st cycle.
The present application also provides an electronic device comprising an electrochemical device according to any one of the above.
The positive electrode active material provided by the application is provided with a core and a shell, wherein the shell comprises a positive electrode with P6 3 The second compound with the mc crystal phase structure has stable interface, can well protect the nucleus, prevent the collapse of the crystal structure of the first compound and the failure of the interface, inhibit the side reaction of the interface, and improve the cycle stability and the interface stability of the anode active material under high working voltage (for example, the working voltage is more than 4.6V).
Drawings
The above and other features, advantages, and aspects of embodiments of the present application will become more apparent by reference to the following detailed description when taken in conjunction with the accompanying drawings. The same or similar reference numbers will be used throughout the drawings to refer to the same or like elements. It should be understood that the figures are schematic and that elements and components are not necessarily drawn to scale.
Fig. 1 is a schematic view of a positive electrode active material according to an embodiment of the present application.
Fig. 2 is a schematic diagram of diffraction patterns before and after heat treatment of the positive electrode active material in examples 1 to 5 of the present application.
Fig. 3 is a raman test chart of the positive electrode active material in examples 1 to 5 after 20 cycles at 4.8V.
Fig. 4 is a raman test chart of the positive electrode active material in comparative example 3-1 after 20 cycles at 4.8V.
Detailed Description
Embodiments of the present application will be described in more detail below with reference to the accompanying drawings. While certain embodiments of the present application are shown in the drawings, it is to be understood that the present application may be embodied in various forms and should not be construed as limited to the embodiments set forth herein, but rather are provided to provide a more thorough and complete understanding of the present application. It should be understood that the drawings and examples of the present application are for illustrative purposes only and are not intended to limit the scope of the present application.
The following describes in detail the schemes provided in the embodiments of the present application with reference to the accompanying drawings.
Electrochemical devices, such as lithium ion batteries, are widely used in the fields of portable electronic products, electric traffic, national defense aviation, energy storage and the like because of their advantages of high energy density, good cycle performance, environmental protection, safety, no memory effect and the like. In order to increase the energy density and power density of an electrochemical device, a positive electrode active material of the electrochemical device is required to have a higher specific capacity and a higher operating voltage plateau.
The most currently used positive electrode active material is LiCoO 2 The positive electrode material has an R-3m crystalline phase structure, has a theoretical capacity of 273.8mAh/g, and has good circulation and safety performance. To obtain higher specific capacity, liCoO 2 Positive toward high working voltage>4.6Vvs. Li/Li+) direction. However, when LiCoO 2 The capacity of the battery can only reach 190mAh/g when the battery is charged to 4.5V. Researchers have attempted to remove more Li from the crystal structure + To obtain a higher specific capacity, but as the operating voltage further increases, li + The crystal structure will also undergo a series of irreversible phase changes (O3 to H1-3, H1-3 to O1) while largely taking off, R-3m phase LiCoO 2 After the working voltage reaches 4.6V, the collapse of the crystal structure and the failure of the interface can occur, the side reaction of the interface is very serious, the Co element is dissolved seriously, the capacity loss of the positive electrode active material is serious, and the cycle performance is deterioratedAnd a decrease in safety performance.
Some researchers have been made by doping (e.g., al, mg, ti) and cladding (e.g., al) 2 O3, mgO) and the like, which are used to improve the structural stability of the positive electrode active material by delaying the irreversible phase transition, the effect of these methods on improving the structural stability of the positive electrode active material after the operating voltage is higher than 4.6V is not obvious. If the doping amount is further increased, the theoretical capacity loss increases as the doping amount increases.
In order to at least partially solve the above-mentioned problems, in some embodiments of the present application, a positive electrode active material is proposed, referring to fig. 1, in some embodiments of the present application, the active material includes: a core 11 and a shell 12. The core 11 includes: a first compound; the shell layer 12 is present on at least a partial area of the surface of the core 11, wherein the shell layer 12 comprises: with P6 3 A second compound having an mc crystal phase structure.
In some embodiments of the present application, the positive electrode active material adopts a core-shell structure in which P6 is included in the shell layer 12 3 The second compound of mc crystalline phase structure is structurally stable, so that the shell layer 12 can well protect the core 11, prevent collapse of the crystal structure of the core 11 and interface failure and inhibit interface side reaction, thereby preventing capacity loss, and at the same time, since the shell layer 12 improves the structural stability of the active material as a whole, the active material has good cycle stability and interface stability at high operating voltage (e.g., operating voltage greater than 4.6V).
In some embodiments of the present application, the first compound has an R-3m crystalline phase structure. In some embodiments of the present application, the shell layer 12 coats the core 11 in situ. In some embodiments of the present application, the positive electrode active material is a positive electrode active material of an electrochemical device in which ions are intercalated and deintercalated in the first compound during charge and discharge of the electrochemical device, P6 3 The second compound of the mc crystal phase structure has ion vacancies, so that part of ions of the first compound which cannot return to the R-3m crystal phase structure can enter the vacancies of the second compound, thereby preventing energy loss due to the inability of the ions to return. In some of the present applicationIn an embodiment, the first compound is a lithium cobalt composite oxide and/or the second compound is a lithium cobalt composite oxide. When the first compound and the second compound are both lithium cobalt composite oxides, lithium vacancies exist in the second compound, and at this time, the second compound can accommodate lithium ions which cannot return to the first compound, so that irreversible capacity loss of the active material is prevented.
In some embodiments of the present application, the positive electrode active material satisfies at least one of the conditions (a) - (e):
(a) The Dv50 of the positive electrode active material particles is 10 μm to 30 μm; in some embodiments, when Dv50 of the positive electrode active material particles is too large, the specific surface area thereof is too small to be detrimental to the rate performance of the positive electrode active material, when Dv50 of the positive electrode active material is too small, the specific surface area is too large to accelerate consumption of the electrolyte and to be detrimental to the cycle stability of the positive electrode active material, and when Dv50 of the positive electrode active material is 10 μm to 30 μm, the rate performance and the cycle stability of the positive electrode active material can be ensured at the same time.
(b) The particle diameter of the core 11 is 8 μm to 25 μm; in some embodiments, the shell layer 12 is located on the surface of the core, and thus when the particle size of the core 11 is too small, the surface of the core 11 is not easily attached, possibly reducing the connection strength between the core 11 and the shell layer 12, and when the particle size of the core 11 is too large, the rate performance of the positive electrode active material as a whole is not good, and in some embodiments, when the particle size of the core 11 is 8 μm to 25 μm, the positive electrode active material has a good rate performance and sufficient connection strength between the core 11 and the shell layer 12.
In some embodiments of the present application, the active material core and shell are tested by SEM scanning electron microscopy, and the positive electrode active material is photographed by SEM, where the photographing magnification is not less than 5.0K, to obtain an image. And counting particles in a shooting range, wherein the counting number is 50-100, respectively recording the particle size of a shell layer in each positive electrode active material, and taking an average value as the particle size of the shell layer. The positive electrode active material particles were processed by an ion polisher (Japanese electron-IB-09010 CP) to obtain a cross section. And shooting the section by using SEM, wherein the shooting multiple is not lower than 5.0K, obtaining a high-resolution image, taking the particle size at the boundary of the nucleus, and taking the average value as the particle size of the nucleus.
(c) The mass ratio of the core 11 to the shell 12 is 99:1 to 80:20; in some embodiments, the total mass of the core and the core layer is 100 parts, the core comprises 80 to 99 parts, and the shell comprises 1 to 20 parts. In some embodiments, the core 11 serves to increase the capacity of the active material, and the shell 12 serves to protect the structural stability of the core 11, so that the overall specific capacity of the active material may be reduced when the content of the shell 12 is too high, and the structural stability of the active material crystalline phase may be insufficient when the content of the shell 12 is too low, and thus, in some embodiments, it is desirable to control the mass ratio between the core 11 and the shell 12, thereby avoiding the capacity reduction while stabilizing the structure of the active material crystalline phase. In some embodiments of the present application, the XRD diffractogram of the active material is obtained by XRD diffractometry of the positive electrode active material, and the mass ratio of the core to the shell is determined by analysis of the XRD diffractogram.
(d) The first compound includes: li containing R-3m crystal phase structure 1±b Co 1-a A a O 2 Wherein 0.ltoreq.b<0.1，0≤a<0.15, A is selected from at least one of Al, mg, ti, mn, fe, ni, zn, cu or Nb. In some embodiments, the first compound is a doped or undoped a element lithium cobaltate, which can improve structural stability at high operating voltages by incorporating an appropriate amount of a element.
In some embodiments of the present application, the second compound comprises: li (Li) x Na z Co 1-y M y O 2 Wherein, 0.6<x<0.95，0≤y<0.15，0≤z<0.03, M is selected from at least one of Al, mg, ti, mn, fe, ni, zn, cu or Nb. In some embodiments, P6 3 Li of mc crystal phase structure x Na z Co 1-y M y O 2 Because of the special oxygen structure, the interface is very stable, plays a good role in protecting the core, and has lithium vacancies which can contain lithium ions which cannot return to the core, so that capacity loss is avoided. Meanwhile, when the first compound includes Li of R-3m crystal phase structure 1±b Co 1-a A a O 2 When Li is due to the shell layer x Na z Co 1-y M y O 2 Li compared with the nucleus 1±b Co 1-a A a O 2 The lithium ion battery has lower lithium ion migration energy, so that the dynamics performance of the shell layer is better, and lithium ions are easier to insert and insert in and out of the shell layer, so that the insertion and the extraction of lithium ions of a core are reduced, the change of a core structure is reduced, the stability of the core structure is improved, the positive electrode active material is beneficial to exerting the theoretical capacity of the positive electrode active material, the charge cut-off voltage of the active material can reach 4.8V, and the lithium ion battery has excellent cycle performance and high-temperature storage performance.
In some embodiments of the present application, an X-ray diffraction spectrum test is performed on the positive electrode active material to obtain a first diffraction spectrum, wherein the number of peaks in the range of 15 DEG to 22 DEG of the first diffraction spectrum is P1, and P1 is more than or equal to 2. In some embodiments of the present application, P6 3 The number of diffraction peaks of the second compound with the mc crystal phase structure in the range of 15-22 DEG of the first diffraction spectrum is one, so that when the number P1 of the peaks in the range of 15-22 DEG of the first diffraction spectrum is more than or equal to 2, the positive electrode active material comprises two crystal phase structures.
In some embodiments of the present application, an X-ray diffraction spectrum test is performed on an active material subjected to heat treatment at 200 ℃ -400 ℃ for 5 hours to obtain a second diffraction spectrum, wherein the number of peaks in the range of 15 ° -22 ° of the second diffraction spectrum is P2, and P2 < P1. In some embodiments of the present application, the positive electrode active material after heat treatment has a reduced number of peaks in the range of 15 ° -22 ° of the diffraction spectrum, indicating that the positive electrode active material undergoes a phase change, and the positive electrode active material comprises two crystal phase structures.
In one embodiment of the present application, the positive electrode active material is of a core-shell structure, and the first compound includes Li of R-3m crystalline phase structure 1±b Co 1-a A a O 2 The second compound comprises Li x Na z Co 1-y M y O 2 . R-3m phase LiCoO in the prior art 2 Collapse of crystal structure and interface failure occur during charging when the voltage is higher than 4.6V, where the interface side reactions are severe, resulting in severe capacity loss, and in some embodiments of the present application, during delithiation/intercalationWherein the shell layer has P6 3 Li of mc crystal phase structure x Na z Co 1-y M y O 2 Li having R-3m crystal phase structure can be accommodated due to the existence of lithium vacancy in the self crystal structure 1± b Co 1-a A a O 2 Lithium ions which cannot return to the crystal, and Li in the shell layer when the operating voltage is greater than 4.6V x Na z Co 1-y M y O 2 Because of the special oxygen structure, the interface is very stable, has good protection effect on the surface of the core, and in addition, li in the shell layer x Na z Co 1- y M y O 2 Compared with Li in the nucleus 1±b Co 1-a A a O 2 The active material with the core-shell structure in the embodiment of the application has better interface stability and cycle stability, has lower lithium ion migration energy and good shell dynamics, and can not generate spinel phase change when the core is discharged under the working voltage of not lower than 4.6V, wherein the charge cut-off voltage is as high as 4.8V.
In some embodiments of the present application, a positive electrode is also presented, the positive electrode including a current collector and a positive electrode active material layer disposed on the current collector, the positive electrode active material layer including any of the active materials described above. In some embodiments of the present application, the positive electrode active material layer has a compacted density of 4.1g/cm 3 -4.5g/cm 3 。
In some embodiments, the positive electrode active material layer further includes a silicon-based material conductive agent and/or binder. In some embodiments, the binder may include at least one of carboxymethyl cellulose (CMC), polyacrylic acid, polyvinylpyrrolidone, polyaniline, polyimide, polyamideimide, polysiloxane, polybutene styrene rubber, epoxy resin, polyester resin, polyurethane resin, or polyfluorene. In some embodiments, the mass percentage of the binder in the positive electrode active material layer is 0.5% -10% based on the total weight of the positive electrode active material layer. In some embodiments, the thickness of the positive electrode active material layer is 50 μm to 200 μm. In some embodiments, the conductive agent may include at least one of conductive carbon black, ketjen black, acetylene black, carbon nanotubes, VGCF (Vapor Grown Carbon Fiber, vapor grown carbon fibers), or graphene.
Also presented in some embodiments of the present application is an electrochemical device comprising: the positive electrode, the negative electrode and the isolating film, and the isolating film is arranged between the positive electrode and the negative electrode; the positive electrode is the positive electrode described above.
In some embodiments of the present application, the electrochemical device is cycled for 20 cycles at a voltage of 4.8V at a discharge gram capacity of not less than 200mAh/g, and the crack growth rate of the positive electrode active material particles at the 20 th cycle is not more than 5% compared to the 1 st cycle. For example, when the discharge gram capacity of the electrochemical device is not lower than 200mAh/g, the positive electrode is cut into 10 positive electrode plates with the same size, the corresponding negative electrode is made of lithium foil, the button cell is assembled, 1 circle of the button cell is circulated under the voltage of 4.8V, 5 button cells are randomly selected, the cracking number of positive electrode active material particles is counted, and the average value is taken and recorded as the cracking number of the 1 st circle of positive electrode active material particles; and (5) continuously circulating the remaining 5 button cells to the 20 th turn, and after disassembling, performing a scanning electron microscope to count the number of cracking particles of the 20 th turn. The crack growth rate of the positive electrode active material particles was calculated at the 20 th turn compared with the 1 st turn. This shows that the shell layer of the positive electrode active material in the electrochemical device in the examples of the present application provides excellent protection for the core, thereby enabling the core to have sufficient structural stability at high operating voltages.
In some embodiments of the present application, the electrochemical device does not undergo a phase transition of the core when the gram discharge capacity is not less than 80% of the initial gram discharge capacity, in some embodiments the upper charge cutoff voltage is not less than 4.6V, in some embodiments the upper charge cutoff voltage is 4.8V. In some embodiments, li is included in the core of the active material 1±b Co 1-a A a O 2 At this point the nuclei do not undergo spinel phase transformation. In the prior art, lithium cobaltate serving as a positive electrode active material can generate spinel phase transition when the upper limit cutoff voltage is not lower than 4.6V, while the electrochemical device in the embodiment of the application does not generate spinel phase transition when the upper limit cutoff voltage is not lower than 4.6V, and the P6 in the core-shell structure and the shell layer can be seen 3 The second compound of mc crystalline phase structure greatly improves the stability of the core.
In some embodiments of the present application, the positive electrode active material layer may be coated on only a partial region of the current collector. The positive electrode active material layer may include a positive electrode active material, a conductive agent, and a binder. The current collector of the positive electrode may be an Al foil, and other positive electrode current collectors commonly used in the art may be used. The conductive agent of the positive electrode may include at least one of conductive carbon black, lamellar graphite, graphene, or carbon nanotubes. The binder in the positive electrode may include at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, a styrene-acrylate copolymer, a styrene-butadiene copolymer, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinyl acetate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, or polyhexafluoropropylene.
In some embodiments of the present application, the barrier film comprises at least one of polyethylene, polypropylene, polyvinylidene fluoride, polyethylene terephthalate, polyimide, or aramid. For example, the polyethylene includes at least one selected from high density polyethylene, low density polyethylene, or ultra high molecular weight polyethylene. In particular polyethylene and polypropylene, which have a good effect on preventing short circuits and can improve the stability of the battery through a shutdown effect. In some embodiments, the thickness of the release film is in the range of about 5 μm to 500 μm.
In some embodiments, the release film surface may further include a porous layer disposed on at least one surface of the release film, the porous layer including inorganic particles selected from aluminum oxide (Al 2 O 3 ) Silicon oxide (SiO) 2 ) Magnesium oxide (MgO), titanium oxide (TiO) 2 ) Hafnium oxide (HfO) 2 ) Tin oxide (SnO) 2 ) Cerium oxide (CeO) 2 ) Nickel oxide (NiO), zinc oxide (ZnO), calcium oxide (CaO), zirconium oxide (ZrO) 2 ) Yttria (Y) 2 O 3 ) At least one of silicon carbide (SiC), boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. In some embodiments, the pores of the barrier film have a pore size of between about 0.01Diameters in the range of μm-1 μm. The binder is at least one selected from polyvinylidene fluoride, copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene or polyhexafluoropropylene. The porous layer on the surface of the isolating membrane can improve the heat resistance, oxidation resistance and electrolyte infiltration performance of the isolating membrane, and enhance the adhesion between the isolating membrane and the pole piece.
In some embodiments of the present application, the electrochemical device is a wound or stacked type.
In some embodiments of the present application, the electrochemical device includes a lithium ion battery, but the present application is not limited thereto. In some embodiments, the electrochemical device may further include an electrolyte. In some embodiments, the electrolyte includes, but is not limited to, at least two of dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), diethyl carbonate (DEC), ethylene Carbonate (EC), propylene Carbonate (PC), propyl Propionate (PP). In addition, the electrolyte may additionally include at least one of Vinylene Carbonate (VC), fluoroethylene carbonate (FEC), or a dinitrile compound as an electrolyte additive. In some embodiments, the electrolyte further comprises a lithium salt.
The present application also proposes an electronic device comprising an electrochemical device according to any one of the above. In some embodiments, the electronic device may include, but is not limited to, a notebook computer, a pen-input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copier, a portable printer, a headset, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini-compact disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable audio recorder, a radio, a backup power source, a motor, an automobile, a motorcycle, a power assisted bicycle, a lighting fixture, a toy, a game machine, a clock, an electric tool, a flashlight, a camera, a household large-sized battery, a lithium ion capacitor, and the like.
The application also provides a preparation method of the positive electrode active material, which is used for preparing the active material, wherein the active material can be the positive electrode active material, and the preparation method comprises the following steps:
step (1) adopts a liquid phase precipitation and sintering method to synthesize the A element doped (Co) 1-a A a ) 3 O 4 Dissolving soluble cobalt salt (such as cobalt chloride, cobalt acetate, cobalt sulfate, cobalt nitrate and the like) and A salt (such as sulfate and the like) into solvent (such as deionized water) according to a molar ratio of Co: A= (1-a): a, adding precipitant (such as sodium carbonate) and complexing agent (such as ammonia water), and regulating pH (such as regulating pH value to 5-9) to precipitate; the precipitate was then sintered and ground to obtain (Co 1-a A a ) 3 O 4 And (3) powder. Finally, the (Co 1-a A a ) 3 O 4 Powder and Li 2 CO 3 Sintering according to stoichiometric ratio (such as 1+ -b: 1) to obtain Li with R-3m structure 1±b Co 1-a A a O 2 Active material (0.ltoreq.b)<0.1，0≤a<0.15)。
Step (2) adopts deionized water to carry out LiCo treatment 1-a A a O 2 Dispersing, mixing soluble cobalt salt (such as cobalt chloride, cobalt acetate, cobalt sulfate, cobalt nitrate and the like) and M salt (such as sulfate and the like) according to the proportion Co: m= (1-y): y dissolving in solvent (e.g. deionized water), adding precipitant (e.g. sodium carbonate) and complexing agent (e.g. ammonia water), adjusting pH (e.g. pH 5-9), precipitating, adding LiCo, adding sodium carbonate, and adding sodium carbonate to the solution 1-a A a O 2 Surface uniformity of precipitation of a layer (Co 1-a A a ) 3 O 4 Obtained (Co) 1-a A a ) 3 O 4 @LiCo 1- a A a O 2 Precipitating the product; by solid phase synthesis, the catalyst (Co 1-a A a ) 3 O 4 @LiCo 1-a A a O 2 Precipitation of the product with Na 2 CO 3 Mixing according to the proportion of Na:Co=0.7:1 to 0.74:1 for sintering to obtain Na z Co 1-y M y O 2 @Li 1±b Co 1-a A a O 2 。
Step (3) adopts Na z Co 1-y M y O 2 @Li 1±b Co 1-a A a O 2 As a precursor, the precursor and lithium-containing molten salt (such as lithium nitrate, lithium chloride, lithium hydroxide and the like) are mixed according to a proportion of preferably Na: li=1:5, the mixture is uniformly mixed, the mixture reacts for 2 to 8 hours in an air atmosphere at the temperature of 200 to 400 ℃, the reactant is washed by deionized water for multiple times, the molten salt is cleaned, and the powder is dried to obtain Li x Na z Co 1-y M y O 2 @Li 1±b Co 1-a A a O 2 (0≤b<0.1，0≤a<0.1 Active material. Washing reactants for many times by deionized water, cleaning the fused salt, and drying the powder to obtain the active material with a shell structure.
In some embodiments, in steps (1) and (2), the soluble cobalt salt is at least one of cobalt chloride, cobalt acetate, cobalt sulfate, or cobalt nitrate; in some embodiments, the soluble M, A salt is a sulfate salt; in some embodiments, the pH is 5-9 to achieve optimal complexation. In some embodiments, in step (1), the material sintering temperature ranges from 900 to 1100 °, the time ranges from 36 to 56 hours, and the calcination atmosphere is an air atmosphere; in some embodiments, in step (2), the material sintering temperature ranges from 700 ° to 900 °, the time ranges from 36h to 56h, and the calcination atmosphere is a pure oxygen atmosphere; in some embodiments, in step (3), the molten salt is at least one of lithium nitrate, lithium chloride, or lithium hydroxide, and the reaction temperature ranges; 200-400 deg. for 2-8 hr, and air as reaction atmosphere.
In order to better illustrate the beneficial effects of the active materials set forth in the embodiments of the present application, the following embodiments will be described with reference to examples, in which the active materials are used as positive electrode active materials of lithium ion batteries, and the differences between the embodiments are only that the positive electrode active materials are different, and in the following embodiments, performance tests are performed on the use of different positive electrode active materials, and the positive electrode active materials in the embodiments are all tested according to the following methods:
particle diameter test: randomly selecting one position of the positive electrode active material powder, shooting by using SEM (Zeiss Sigma 02-33), and obtaining an image, wherein the shooting multiple is not less than 5.0K. And counting particles in a shooting range, wherein the counting number is 50-100, respectively recording the particle size of a shell layer in each positive electrode active material, and taking the median value as the particle size of the shell layer. The active material particles were processed by an ion polisher (Japanese electron-IB-09010 CP) to obtain a cross section. And shooting the section by using SEM, wherein the shooting multiple is not less than 5.0K, obtaining a high-resolution image, measuring the particle size at the boundary of the nucleus, and taking the median value as the particle size of the nucleus.
The mass ratio detection method comprises the following steps: XRD testing is carried out on the positive electrode active material powder through XRD testing, diffraction peaks in an XRD diffraction pattern are collected, testing equipment is Bruker, D8 ADVANCE, germany, testing parameters are that the diffraction angle is 10-90 degrees, the step length is 0.02 degree/second, and the intensity of the strongest diffraction peak is not lower than 1 ten thousand, and the unit counts are provided. And refining the diffraction pattern by adopting GSAS, and determining the mass ratio of the P63mc crystal structure to the R-3m crystal structure according to the refined result.
The number of diffraction peaks before and after heat treatment is measured by the following steps: XRD diffraction patterns of the positive electrode active material before and after 300-degree sintering are obtained by adopting an XRD test (Bruker D8 ADVANCE), the diffraction patterns obtained by the test are compared, and the number P1 of diffraction peaks in the range of 15-20 degrees before heat treatment and the number P2 of diffraction peaks in the range of 15-20 degrees after heat treatment are determined.
Cycle performance and first cycle discharge capacity test: the lithium ion battery assembled by the positive electrode active material is charged and discharged for the first time in the environment of 25 ℃, and is charged with constant current and constant voltage under the charging current of 0.5C (namely, the current value of theoretical capacity is completely discharged in 2 hours) until the upper limit voltage is 4.8V; then, constant current discharge was performed at a discharge current of 0.5C until the final voltage was 3V. The discharge capacity of the first cycle was recorded as the first-cycle discharge capacity at a voltage of 4.8V, and the above charge-discharge process was repeated for 100 cycles.
High voltage cycle performance 100 cycles capacity retention = capacity after 100 cycles/initial capacity x 100%
The characterization method of the crystal structure before and after circulation comprises the following steps: lithium ion battery assembled with positive electrode active material is completely discharged at 0.5C (namely, the current value of theoretical capacity is completely discharged in 2 h) in 25 ℃ environmentAnd (3) constant-current and constant-voltage charging is carried out under the charging current, after the charging current is circulated for 20 circles under the high voltage of 4.8V, the positive pole piece is taken down, and Raman testing is carried out. The Raman device adopts HORIBA, labRAM HR Evol, and the incident wavelength is selected to be 532nm for testing. Co-O bond characteristic peak in lithium cobalt oxide is 480cm -1 And 600cm -1 Near wave number. Spinel phase transition corresponding Co 3 O 4 Co-O bond of (c).
The cracking growth rate test method comprises the following steps: when the discharge gram capacity is not lower than 200mAh/g, cutting the positive electrode into 10 positive electrode plates with the same size, wherein the corresponding negative electrode is lithium foil, assembling into a button cell, circulating for 1 circle under the voltage of 4.8V, randomly selecting 5 button cells, counting the cracking number of positive electrode active material particles, taking an average value, and recording as the cracking number of the positive electrode active material particles of the 1 st circle; and (5) continuously circulating the remaining 5 button cells to the 20 th turn, and after disassembling, performing a scanning electron microscope to count the number of cracking particles of the 20 th turn. Statistics of the number of cracked particles the active material was processed by an ion polisher (Japanese electron-IB-09010 CP) to obtain a cross section. And shooting the section by using SEM, wherein the shooting multiple is not less than 5.0K, and obtaining an image. And counting all different active material particles meeting the conditions in the shooting range, recording the total number of cracked positive electrode active material particles, counting the total number of cracks in the positive electrode active material particles in the positive electrode plate before and after charge and discharge cycles, and calculating to obtain the cracking growth rate, wherein the counting number is 50-100. Crack growth rate= (total number of cracks after 20 th cycle-total number of cracks after first cycle/total number of cracks after first cycle) ×100%.
The charge-discharge cycle process comprises the following steps: in an environment of 25 ℃, carrying out first charge and discharge circulation on a lithium ion battery prepared by adopting a positive electrode active material, and carrying out constant-current and constant-voltage charge under a charge current of 0.5C (namely, a current value of theoretical capacity is completely discharged in 2 h) until the upper limit voltage is 4.8V; then, constant-current discharge is carried out under the discharge current of 0.5C until the final voltage is 3V, and the discharge capacity of the first cycle is recorded, wherein the discharge capacity is not lower than 180mAh/g; finally, the charge and discharge cycles were continued for 20 times.
Examples 1-1 to 1-12, and comparative examples 1-1 to 1-8, in which the active materials have a core-shell structure, but the active materials in the comparative examples have no core-shell structure, only a core or only a shell layer, are described below, and the active materials in the above examples and comparative examples have the compositions shown in table 1.
TABLE 1
As can be seen from comparison of examples 1-1 to examples 1-12, and comparative examples 1-1 to comparative examples 1-4, the high voltage cycle performance 100 cycles capacity retention rate of examples 1-1 to examples 1-12 is significantly higher than that of comparative examples 1-1 to comparative examples 1-4 because when the positive electrode active material has a core-shell structure and the core layer has P6 3 When the mc crystalline phase structure is adopted, the stability of the crystal structure of the positive electrode active material at a voltage of 4.8V can be improved, so that the cycle performance of the lithium ion battery at a voltage of 4.8V can be improved, while comparative examples 1-1 to 1-4 have only the positive electrode active material same as the core without a shell layer, so that the crystal structure of the positive electrode active materials of comparative examples 1-1 to 1-4 is unstable at a voltage of 4.8V, irreversible phase transition is easily generated, and thus the cycle performance of comparative examples 1-1 to 1-4 is poor.
As can be seen from comparison of examples 1-1 to 1-12, and comparative examples 1-5 to 1-8, the initial ring discharge capacity was significantly higher than that of comparative examples 1-5 to 1-8 at a voltage of 4.8V for examples 1-1 to 1-12, because Li as a core when the positive electrode active material had a core-shell structure 1±b Co 1-a A a O 2 The capacity of the positive electrode active material can be ensured, the core layer can improve the stability of the crystal decoupling strand of the positive electrode active material, the core and the shell layer are mutually matched, so that the discharge capacity and the cycle performance of the lithium ion battery under the voltage of 4.8V are improved, and comparative examples 1-5 to 1-8 only have the components with the core layerThe same positive electrode active materials had no core, and thus the positive electrode active materials of comparative examples 1 to 5 to 1 to 8 were inferior in the first-turn discharge capacity at 4.8V.
Further, it can be noted that example 1-2 is the same as the core of comparative example 1-2, but the first-turn discharge capacity at a voltage of 4.8V of example 1-2 is still higher than comparative example 1-2, and it can be seen that the shell layer can cooperate with the core, not only improving the cycle performance at a high voltage, but also improving the first-turn discharge capacity.
In some examples of the present application, reference is made to FIG. 2, wherein the diffraction patterns of the active materials of examples 1-5 before and after heat treatment are schematically shown in FIG. 2, from which it can be seen that the active material has P6 before heat treatment (left half of FIG. 2) 3 mc crystalline phase structure and R-3m crystalline phase structure, after heat treatment (right half of FIG. 2), have only R-3m crystalline phase structure because of P6 3 The mc crystal phase structure is a metastable phase, the phase changes to a stable phase structure R-3m at high temperature, and the stability of the active material in high temperature circulation is ensured by the transition from the metastable phase to the stable phase, so as to meet the requirement of high temperature circulation under high voltage.
Examples 1-1, examples 1-6, examples 1-7, and comparative examples 2-1 to 2-7, in which the active materials each had a core-shell structure, the composition of the active materials, the particle size of the active materials, the core particle size, or the mass ratio of the core to the shell layer were different, and the compositions and test results of the active materials are shown in table 2, are described below.
TABLE 2
Note that: the total mass of the core and the shell is 100 parts, and the mass ratio of the core to the shell in the table is the part of the core in 100 parts.
The test results of examples 2-1 to 2-3, and comparative examples 2-1 to 2-7, revealed that the high voltage cycle performance 100 cycles of examples 2-1 to 2-3 was higher in capacity retention than those of comparative examples 2-1 to 2-7, because the core particle diameters of examples 2-1 to 2-3 were in the range of 8 μm to 25 μm, whereas the core particle diameters of comparative examples 2-1 to 2-7 were larger than 25 μm, whereby it was seen that the cycle performance at high voltage was likely to be impaired when the core particle diameters were larger than 25 μm, and thus the core particle diameters were defined to be in the range of 8 μm to 25 μm in some examples.
The test results of examples 2-1 to 2-3, and comparative examples 2-3 to 2-7, revealed that the first-turn discharge capacity at 4.8V voltage of examples 2-1 to 2-3 was higher than that of comparative examples 2-3 to 2-7, probably because the core content of examples 2-1 to 2-3 was smaller, resulting in a decrease in the capacity of the positive electrode active material, and thus in a decrease in the first-turn discharge capacity at 4.8V voltage, and thus the mass ratio of the core to the shell layer was defined to be 99:1 to 80:20 in some examples.
Examples 1-1, examples 1-6, examples 1-7, and comparative examples 3-1 to 3-7, in which the active materials have a core-shell structure, the active materials in the comparative examples have no core-shell structure, and only the components of the core or shell layer, the active material compositions and test results in the above examples and comparative examples are shown in table 3, are described below.
TABLE 3 Table 3
Referring to Table 3, it can be seen from the test results of comparative examples 3-1 to 3 that Li when not having a shell layer 1±b Co 1-a A a O 2 Spinel phase transition occurs after heat treatment and has a high rate of grain cracking growth, indicating nuclear Li 1±b Co 1-a A a O 2 Is poor in heat stability.
As can be seen from the test results of comparative examples 3-4 to 3-7, li x Na z Co 1-y M y O 2 No spinel phase change occurs before and after heat treatment and the grain cracking growth rate is extremely small, indicating Li x Na z Co 1-y M y O 2 Has better structural stability.
As can be seen from the results shown in examples 1-1, examples 1-6, and examples 1-7, the diffraction peaks decreased before and after the heat treatment of the positive electrode active material because of Li x Na z Co 1-y M y O 2 From P6 3 The mc crystal phase structure is converted to the R-3m crystal phase structure, but the positive electrode active material does not generate spinel phase change, which indicates that the core does not generate phase change, and the particle cracking growth rate of the active material is very small, so that the shell layer P6 is known 3 Li of mc crystal phase structure x Na z Co 1-y M y O 2 Li for improving nuclear R-3m crystal phase structure 1±b Co 1-a A a O 2 Structural stability at high temperatures. Since spinel phase transition does not occur, irreversible capacity loss does not occur in the positive electrode active material, which indicates that the positive electrode active material in the embodiments of the present application has superior high temperature storage performance.
Referring to fig. 3 and 4, fig. 3 is a raman test chart of examples 1 to 5 after 20 cycles at 4.8V, and fig. 4 is a raman test chart of comparative example 3 to 1 after 20 cycles at 4.8V. As can be seen from fig. 3 and 4, when the positive electrode active material has only a core without a shell layer, co corresponding to spinel phase transition is shown in raman test patterns corresponding to the core after cycling at a high operating voltage of 4.8V 3 O 4 This indicates that spinel phase transition occurs in the cathode active material without the shell layer, whereas Co is not shown in the Raman test patterns of the cathode active materials in examples 1 to 5 3 O 4 The peaks corresponding to Co-O bonds of (C) indicate that the positive electrode active material in examples of the present application does not undergo spinel phase transition, and thus it can be seen that the shell layer provides excellent protection for the core, as in examples 1 to 5 of the present applicationThe structural stability of the positive electrode active material is far superior to that of comparative example 3-1, and capacity loss caused by irreversible phase change is avoided because spinel phase change does not occur, which indicates that the positive electrode active material in the examples of the present application has good cycle performance at high operating voltage.
The foregoing description is only of the preferred embodiments of the present application and is presented as a description of the principles of the technology being utilized. It will be appreciated by persons skilled in the art that the scope of the disclosure referred to in this application is not limited to the specific combinations of features described above, but it is intended to cover other embodiments in which any combination of features described above or equivalents thereof is possible without departing from the spirit of the disclosure. Such as the above-described features and technical features having similar functions (but not limited to) disclosed in the present application are replaced with each other.
Moreover, although operations are depicted in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order. In certain circumstances, multitasking and parallel processing may be advantageous. Likewise, while several specific implementation details are included in the above discussion, these should not be construed as limiting the scope of the present application. Certain features that are described in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable subcombination.
Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are example forms of implementing the claims.
Claims (8)
1. A positive electrode active material, characterized by comprising:
a core, the core comprising: a first compound;
a shell layer present on at least a partial area of the surface of the core,
wherein the shell layer comprises: with P6 3 A second compound having an mc crystal phase structure;
the first compound has an R-3m crystalline phase structure;
the first compound is lithium cobalt composite oxide,
the second compound is lithium cobalt composite oxide.
2. The positive electrode active material according to claim 1, wherein at least one of the conditions (a) to (d) is satisfied:
(a) The Dv50 of the positive electrode active material particles is 10 μm to 30 μm;
(b) The particle size of the core is 8-25 μm;
(c) The mass ratio of the core to the shell is 99:1 to 80:20;
(d) The first compound includes: li containing R-3m crystal phase structure 1±b Co 1-a A a O 2 Wherein 0.ltoreq.b<0.1，0≤a<0.15, A is selected from at least one of Al, mg, ti, mn, fe, ni, zn, cu or Nb.
3. The positive electrode active material according to claim 1, wherein the second compound comprises: li (Li) x Na z Co 1- y M y O 2 Wherein, 0.6<x<0.95，0≤y<0.15，0≤z<0.03, M is selected from at least one of Al, mg, ti, mn, fe, ni, zn, cu or Nb.
4. The positive electrode active material according to claim 1, wherein the positive electrode active material is subjected to an X-ray diffraction spectrum test to obtain a first diffraction spectrum, and the number of peaks in the range of 15 ° -22 ° of the first diffraction spectrum is P1, and P1 is not less than 2.
5. The positive electrode active material according to claim 4, wherein the positive electrode active material subjected to heat treatment at 200 ℃ to 400 ℃ for 5 hours is subjected to an X-ray diffraction spectrum test to obtain a second diffraction spectrum, and the number of peaks in the range of 15 ° to 22 ° of the second diffraction spectrum is P2, P2 < P1.
6. An electrochemical device, comprising:
a positive electrode;
a negative electrode;
a separator disposed between the positive electrode and the negative electrode;
wherein the positive electrode includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the positive electrode active material layer including the positive electrode active material according to any one of claims 1 to 5.
7. The electrochemical device according to claim 6, wherein,
the electrochemical device circulates 20 circles under a voltage of 4.8V when the discharge gram capacity is not lower than 200mAh/g, and the cracking growth rate of the positive electrode active material particles at the 20 th circle is not higher than 5% compared with the first circle.
8. An electronic device comprising the electrochemical device of any one of claims 6-7.
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