A positive electrode sheet and its application

Abstract

The present invention discloses a positive electrode plate and its application, comprising a positive electrode current collector, a first positive electrode active material layer and a second positive electrode active material layer; the first positive electrode active material layer is arranged on the positive electrode current collector, the first positive electrode active material layer is composed of a first positive electrode active material, and the first positive electrode active material contains polycrystalline particles and single crystal particles; the second positive electrode active material layer is arranged on the first positive electrode active material layer, the second positive electrode active material layer is composed of a second positive electrode active material, the second positive electrode active material is a single crystal particle, and the second positive electrode active material is selected from a polyanionic lithium-containing silicate compound and / or a cationic disordered rock salt phase structure lithium-containing positive electrode material.

Description

Technical Field

[0001] The present invention relates to the field of electrochemical technology, and in particular to a positive electrode sheet and applications thereof. Background Art

[0002] Lithium-ion batteries, with their high energy density, long cycle life, and lack of memory effect, are widely used in wearable devices, smartphones, drones, electric vehicles, and large-scale energy storage systems. They have become the most promising new green chemical power source. As these applications continue to expand, higher requirements are being placed on the comprehensive performance of lithium-ion batteries.

[0003] The purpose of this application is to provide a positive electrode plate containing a high-nickel positive electrode material and its application in electrochemical devices and electronic devices, so as to reduce the gas production during high-temperature storage of the electrochemical device and improve the high-temperature cycle life of the electrochemical device.

[0004] The key reasons for the poor gas production and high-temperature cycling performance of high-nickel materials during high-temperature storage are the cracks at grain boundaries and within grains caused by lattice contraction and expansion during the electrode cold pressing process and the charge-discharge process, as well as electrolyte penetration and accelerated interfacial side reactions. The previous case optimized the high-nickel material by doping and coating it to improve the bulk and surface structural stability. The compressive strength of the particles was also optimized through particle micromorphology (single crystallization), particle size adjustment, and internal grain orientation. Furthermore, the electrode design was further optimized by mixing large and small particles to improve particle breakage.

[0005] The micro-morphology of the particles is adjusted to change the high-nickel polycrystalline secondary particles into single-crystal morphology particles. Due to the rapid increase of the primary grains of single-crystal materials, the dynamic performance deteriorates, and the first efficiency and discharge capacity will be greatly reduced.

[0006] By mixing large and small particles, an appropriate proportion of single crystal particles are mixed into the large polycrystalline particles to reduce particle breakage and reduce the loss of discharge capacity. However, during the cold pressing process of the electrode, the polycrystalline particles located on the surface of the electrode will also be crushed in large quantities, accelerating the interface side reactions, affecting high-temperature storage and high-temperature cycle retention, and the improvement effect is limited. Summary of the Invention

[0007] In response to the technical problem that during the cold pressing process of existing electrode sheets, polycrystalline particles located on the electrode sheet surface are also crushed in large quantities, accelerating interfacial side reactions, affecting high-temperature storage and high-temperature cycle retention, etc., the present application discloses a positive electrode sheet and its application, with the purpose of providing a positive electrode sheet containing high-nickel positive electrode material, an electrochemical device, and an electronic device to reduce gas production during high-temperature storage of the electrochemical device and improve the cycle life of the electrochemical device.

[0008] A positive electrode plate proposed in the present invention includes a positive electrode current collector, a first positive electrode active material layer and a second positive electrode active material layer; the first positive electrode active material layer is arranged on the positive electrode current collector, and the first positive electrode active material layer contains a first positive electrode active material; the first positive electrode active material contains polycrystalline morphology particles and single crystal morphology particles; the second positive electrode active material layer is arranged on the first positive electrode active material layer, and the second positive electrode active material layer contains a second positive electrode active material, the second positive electrode active material is a single crystal morphology particle, and the second positive electrode active material is selected from a polyanionic lithium-containing silicate compound and / or a cationic disordered rock salt phase structure lithium-containing positive electrode material.

[0009] Furthermore, the ICP test shows that the total molar amount of metal elements other than Li in the first positive electrode active material is nM1, the molar amount of Ni element is nNi1, the molar amount of Co element is nCo1, the molar amount of Mn element is nMn1, the molar amount of Al element is nAl1, the molar amount of R element is nR1, 0.75≤nNi1 / nM1<1, 0≤nCo1 / nM1≤0.15, 0≤nMn1 / nM1≤0.1, 0≤nAl1 / nM1≤0.005, 0≤nR1 / nM1≤0.05, and the R element is at least one of B, P, Mg, Ti, Zr, Nb, Y, Cr, V, Ge, Mo, Fe, Cu, Zn, Ga, Ag, W, In, Sn, Pb, Sb, La, Ce, Ca, and Sr.

[0010] Furthermore, the mass ratio of polycrystalline particles to single crystal particles in the first positive electrode active material is in the range of 90:10 to 50:50.

[0011] Furthermore, the polyanionic lithium silicate compound has a general formula of Li2MSiO4, wherein M is at least one of Ni, Fe, Mn, Co, and V, and the surface of the polyanionic lithium silicate compound is coated with amorphous carbon.

[0012] Furthermore, the cationic disordered rock salt phase structure lithium-containing positive electrode material has the general formula Li 1+x TM 1-x O2 and / or Li2TMO3, wherein 0.2≤x≤1, wherein TM is selected from one or more composites of transition metals Co, Fe, Mn, Mo, Nb, Ni, V, and Ti.

[0013] Furthermore, the positive electrode sheet compression range is 3.15g / cm 3 ≤ρ≤3.7g / cm 3 .

[0014] Furthermore, the particle size of the second positive electrode active material satisfies the following relationship: 3.5 μm <Dv50 ≤7.0μm.

[0015] Furthermore, the particle size of the single crystal morphology material in the first positive electrode active material satisfies the following relationship: 3.5 μm ≤ D v50 ≤7.0μm; the particle size of the polycrystalline morphology material in the first positive electrode active material satisfies the following relationship: 8.0μm≤D v50 ≤15.0μm.

[0016] The present application also discloses an application of a positive electrode sheet in an electrochemical device.

[0017] The present application also discloses an application of the electrochemical device in an electronic device.

[0018] Beneficial effects:

[0019] The positive electrode sheet and its application provided by the present invention have the following advantages compared with the prior art:

[0020] 1. The purpose of the present application is to provide a positive electrode sheet containing a high-nickel positive electrode material, comprising a positive electrode current collector, a first positive electrode active material layer and a second positive electrode active material layer; the first positive electrode active material layer is disposed on the positive electrode current collector and comprises a first positive electrode active material; the first positive electrode active material comprises polycrystalline particles and single crystal particles; the first positive electrode active material is also a layered lithium-containing metal oxide; the second positive electrode active material layer is disposed on the first positive electrode active material layer and comprises a second positive electrode active material; the second positive electrode active material is Single crystal morphology particles; wherein the positive electrode active material in the second positive electrode active material layer is set to a single crystal morphology for the purpose of alleviating the generation of grain boundaries and intracrystalline cracks on the surface of the electrode during cold pressing and charge and discharge, thereby causing deterioration of high-temperature storage gas production and high-temperature cycle retention rate; the positive electrode active material in the second positive electrode active material layer is selected from at least one of a polyanionic lithium-containing silicate compound or a cationic disordered rock salt structure lithium-containing positive electrode material, the main purpose of which is to compensate for the reversible lithium consumed by the SEI film formation of the anode, increase the reversible capacity of the cathode, and thereby increase the energy density of the electrochemical device;

[0021] 2. The positive electrode active material in the first positive electrode active material layer contains polycrystalline particles and single-product morphology particles. The purpose is to alleviate the breakage of polycrystalline particles during the cold pressing process, which in turn leads to deterioration of high-temperature storage gas production and high-temperature cycle retention rate. It can also reduce the elongation of the electrode during the cold pressing process and improve the problem of broken belts during the cold pressing process. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a schematic diagram of the positive electrode structure of this application. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solutions, and advantages of the present application more clearly understood, the present application is further described in detail below with reference to the accompanying drawings and examples. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field fall within the scope of protection of the present application. It should be noted that in the specific implementation of the present application, lithium-ion batteries are used as an example of electrochemical devices to explain the present application, but the electrochemical devices of the present application are not limited to lithium-ion batteries.

[0024] A positive electrode sheet, comprising a positive electrode current collector, a first positive electrode active material layer, and a second positive electrode active material layer; the first positive electrode active material layer is disposed on the top and bottom surfaces of the positive electrode current collector and comprises a first positive electrode active material: the first positive electrode active material is a layered lithium-containing metal oxide; the first positive electrode active material comprises polycrystalline particles and single crystal particles;

[0025] The second positive electrode active material layer is arranged on the first positive electrode active material layer and includes a second positive electrode active material; the second positive electrode active material is a single crystal particle; the purpose of setting the positive electrode active material in the second positive electrode active material layer to have a single crystal morphology is mainly to alleviate the generation of grain boundaries and intracrystalline cracks on the surface of the electrode during cold pressing and charge and discharge, reduce the direct interface side reaction between the active material and the electrolyte, and thereby alleviate the gas production during high-temperature storage and the deterioration of the high-temperature cycle retention rate; and the positive electrode active material in the second positive electrode active material layer is selected from polyanionic lithium-containing silicate compounds and / or cationic disordered rock salt phase structure lithium-containing positive electrode materials, the main purpose of which is to make up for the reversible lithium consumed by the SEI film formation of the anode, improve the reversible charge and discharge capacity of the positive electrode active material, and the excess active lithium stored in the anode can also be gradually released during the cycle to improve the cycle life.

[0026] The positive electrode active material in the first positive electrode active material layer comprises polycrystalline particles and single crystal particles, the purpose of which is to alleviate the breakage of polycrystalline particles during the cold pressing process, which in turn leads to deterioration of high-temperature storage gas production and high-temperature cycle retention rate, and can also reduce the elongation of the electrode during the cold pressing process, thereby improving the problem of broken bands during the cold pressing process; if polycrystalline particles are used alone, the elongation of the electrode during the cold pressing process will be greatly increased, causing the electrode to break, reducing the processable limit compaction, and increasing the probability of particle breakage, deteriorating high-temperature storage gas production and cycle retention rate; if a single particle is used alone, the reversible specific capacity of the single crystal is lower than that of the polycrystalline material, which will greatly reduce the capacity of the electrochemical device; the mass ratio of polycrystalline particles to single crystal particles in the first positive electrode active material is in the range of 90:10 to 50:50;

[0027] The total molar amount of the metal elements other than Li in the first positive electrode active material is measured by ICP test, the molar amount of the Ni element is nNi1, the molar ratio of the Co element is nCo1, the molar amount of the Mn element is nMn1, the molar amount of the Al element is nAl1, and the molar amount of the R element is nR1, wherein 0.75≤nNi1 / nM1<1, 0≤nCo1 / nM1≤0.15, 0≤nMn1 / nM1≤0.1, 0≤nAl1 / nM1≤0.005, and 0≤nR1 / nM1≤0.05; the R element includes at least one of B, P, Mg, Ti, Zr, Nb, Y, Cr, V, Ge, Mo, Fe, Cu, Zn, Ga, Ag, W, In, Sn, Pb, Sb.La, Ce, Ca or Sr; preferably, the first positive electrode active material is selected from LiNi 0.75 Co 0.12 Mn 0.13 O2, LiNi 0.75 Co 0.12 Mn 0.125 Zr 0.004 Al 0.001 O2, LiNi 0.82 Co 0.12 Mn 0.06 O2, LiNi 0.82 Co 0.12 Mn 0.055 Zr 0.004 Al 0.001 O2,LiNi 0.9 Co 0.05 Mn 0.045 Zn 0.004 Al 0.001 O2, LiNi 0.9 Co 0.05 Mn 0.042 Zn 0.004 Al 0.001 B 0.003 At least one of O2.

[0028] The polyanionic lithium-containing silicate compound has a general formula of Li2M2SiO4, wherein M is at least one of Ni, Fe, Mn, Co, and V. The surface of the polyanionic lithium-containing silicate compound is coated with amorphous carbon. The polyanionic lithium-containing silicate compound has a theoretical capacity of up to 333 mAh / g and a low lithium insertion potential. It can be used as a good positive electrode lithium supplement material to compensate for the reversible active lithium consumption caused by SEI film formation at the anode, thereby improving the reversible charge and discharge capacity of the material. The active lithium stored in the anode can also be gradually released in the later stages of the cycle to improve the cycle life.

[0029] The general formula of the lithium-containing positive electrode material with a cationic disordered rock salt phase structure is Li1+x TM 1-x O2 and / or Li2TMO3, where TM is selected from one or more combinations of transition metals Co, Fe, Mn, Mo, Nb, Ni, V, Ti; preferably, the cation-disordered rock salt phase-structured lithium-containing cathode material is selected from Li 1.25 Nb 0.25 Mn 0.5 O2, Li 1.23 Mo 0.467 Cr 0.3 O2, Li 1.3 Nb 0.3 Mn 0.4 O2, Li 1.23 Ni 0.155 Ru 0.615 O2, at least one of Li2MnO3; the first charge capacity of the cation-disordered rock salt phase-structured lithium-containing cathode material can be as high as over 350 mAh / g, and the lithium insertion / extraction potential is moderate, which can be used as a good cathode lithium supplement material to compensate for the reversible active lithium consumption caused by the formation of the anode SEI film, improve the charge-discharge reversible capacity of the material, and the active lithium stored in the anode can also be gradually released in the later stage of cycling to improve the cycle life;

[0030] In some embodiments of the present application, the thickness T0 of the positive electrode current collector satisfies: 8.0 μm < T0 < 20.0 μm; the selection of the current collector thickness is mainly related to the requirements of cold pressing processing, energy density requirements, and the internal resistance requirements of the battery cell. Reducing the current collector thickness increases the energy density, but the cold pressing processing is difficult and the internal resistance of the battery cell increases; preferably, the thickness T of the positive electrode current collector is selected to be 10 μm, which can meet the requirements of coating cold pressing processing and maximize the energy density;

[0031] In some embodiments of the present application, the compaction of the positive electrode sheet is 3.15 g / cm 3 ≤ρ≤3.7 g / cm 3 , if the compaction is too large, the high-temperature storage will also deteriorate sharply, and if the compaction is too low, the energy density will decrease significantly;

[0032] In some embodiments of the present application, the particle size of the second positive electrode active material satisfies the following relationship: 3.5 μm ≤ D v50 ≤ 7.0 μm. The processable thickness of the second positive electrode active material layer will increase correspondingly, which will reduce the energy density. If Dv50 is too large, the specific capacity of the material will also decrease, reducing the energy density; if the particle size is too small, the BET of the particles on the surface of the electrode sheet will increase correspondingly, which will also deteriorate the gas generation during high-temperature storage and the high-temperature cycle retention rate;

[0033] In some embodiments of the present application, the particle size of the single-crystal morphology material in the first positive electrode active material satisfies the following relationship 3.5 μm ≤ D V50≤7.0μm; the particle size of the polycrystalline morphology material in the first positive electrode active material satisfies the following relationship 8.0μm≤D V50 ≤15.0μm; the particle size requirements of polycrystalline and single crystal morphology particles can achieve the closest packing of particles and improve the compaction density.

[0034] A positive electrode plate is used in an electrochemical device. The positive electrode plate provided in the present application has good structural stability and low interfacial charge transfer impedance, so that the electrochemical device provided in the present application has good cycle performance and kinetic performance.

[0035] In the present application, the positive electrode material layer further includes a positive electrode binder, which is at least one of a fluorine-containing resin, a polypropylene resin, a fiber-type binder, a rubber-type binder, and a polyimide-type binder;

[0036] In the present application, the positive electrode material layer also includes a conductive agent, which is at least one of conductive carbon black, carbon nanotubes (CNTS), carbon fibers, acetylene black, graphite, Ketjen black, graphene metal materials, and conductive polymers. The carbon nanotubes are single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The carbon fibers are vapor-grown carbon fibers (VGCF) and / or nanocarbon fibers. The metal material is metal powder and / or metal fibers, and the metal is at least one of copper, nickel, aluminum, and silver. The conductive polymer is at least one of a polyphenylene derivative, polyaniline, polythiophene, polyacetylene, and polypyrrole.

[0037] Based on the mass of the positive electrode material layer, the mass percentage of the positive electrode active material is 90% to 98%, the mass percentage of the positive electrode binder is 0.5% to 5%, and the mass percentage of the conductive agent is 0.5% to 5%.

[0038] Preferably, the positive electrode plate further includes a conductive layer, which is located between the positive electrode current collector and the positive electrode material layer, and includes the above-mentioned conductive agent and positive electrode binder.

[0039] In the present application, the electrochemical device further includes a negative electrode plate. The negative electrode plate in the present application includes a negative electrode current collector and a negative electrode material layer, and the negative electrode material layer is provided on the negative electrode current collector.

[0040] In the present application, the negative electrode current collector is copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foam nickel, foam copper or a composite current collector, and the thickness of the negative electrode current collector is 4 μm to 12 μm.

[0041] In the present application, the negative electrode active material in the negative electrode material layer is a carbon material that reversibly intercalates and deintercalates lithium ions. The carbon material is crystalline carbon and / or amorphous carbon. The crystalline carbon is amorphous, flaky, platelet-shaped, spherical, or fibrous natural graphite or artificial graphite. The amorphous carbon is at least one of soft carbon, hard carbon, mesophase pitch carbide, or calcined coke.

[0042] Furthermore, the negative electrode active material is natural graphite, artificial stone, mesophase carbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon oxide (SiO x , x is 1 to 2) or at least one of a silicon-carbon composite. Wherein the mass ratio of silicon to carbon in the silicon-carbon composite is 1:10 to 10:1, D v50 3μm to 15μm.

[0043] In the present application, the negative electrode material layer also includes a conductive agent and a negative electrode binder, and the negative electrode binder is at least one of polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoride, polyethylene, polypropylene, polyacrylic acid, styrene-butadiene rubber, acrylic (ester) styrene-butadiene rubber, epoxy resin, and nylon.

[0044] Preferably, the negative electrode plate is further provided with a conductive layer, and the conductive layer is located between the negative electrode current collector and the negative electrode material layer.

[0045] In the present application, the electrochemical device also includes an isolation membrane, which is at least one of polyethylene (PE), polypropylene (PP), polytetrafluoroethylene-based polyolefin (PO) isolation membranes, polyester membranes (such as polyethylene terephthalate (PET) membranes), cellulose membranes, polyimide membranes (PI), polyamide membranes (PA), spandex, aramid membranes, woven membranes, non-woven membranes (non-woven fabrics), microporous membranes, composite membranes, diaphragm paper, rolled membranes, and spinning membranes, preferably polyethylene or polypropylene, which have a good effect on preventing short circuits and can improve the stability of electrochemical devices through the shutdown effect. The isolation membrane of the present application has a porous structure with a pore size of 0.01pm to 1μm. In the present application, the thickness of the isolation membrane is 5μm to 10μm.

[0046] The separator includes a substrate layer and a surface treatment layer. The substrate layer is a nonwoven fabric, film, or composite film having a porous structure. The material of the substrate layer includes, but is not limited to, at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Preferably, a polypropylene porous film, a polyethylene porous film, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite film is used. A surface treatment layer is provided on the surface of the substrate layer. The surface treatment layer is a polymer layer, an inorganic layer, or a layer formed by a mixed polymer and inorganic material.

[0047] The inorganic layer includes, but is not limited to, inorganic particles and an inorganic layer binder, wherein the inorganic particles include, but are not limited to, at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, chromium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The inorganic layer binder of the present application is at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, polyamide polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylic acid salt, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer includes a polymer, wherein the polymer is at least one of polyamide, polyacrylonitrile, an acrylate polymer, polyacrylic acid, polyacrylic acid salt, polyvinylpyrrolidone, polyvinyl ether, polyvinylidene fluoride, or poly(vinylidene fluoride hexafluoropropylene).

[0048] In the present application, the electrochemical device further includes an electrolyte, which is one or more of a gel electrolyte, a solid electrolyte, and an electrolyte, wherein the electrolyte includes a lithium salt and a non-aqueous solvent.

[0049] In the present application, the lithium salt is selected from at least one of LiPF6, LiBF4, LiAsF6, LiCIO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, LiSiF6, and lithium difluoroborate. Preferably, the lithium salt is LiPF6.

[0050] The non-aqueous solvent of the present application is at least one of a carbonate compound, a carboxylate compound, an ether compound, and other organic solvents. The carbonate compound is at least one of a chain carbonate compound, a cyclic carbonate compound, and a fluorinated carbonate compound. The chain carbonate compound is at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), and ethylmethyl carbonate (MEC). The cyclic carbonate is at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylethylene carbonate (VEC). The fluorinated carbonate compound is at least one of fluorinated Zn-alkylene carbonate (FEC), 1,2-difluoroacetic acid carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, and trifluoromethylethylene carbonate. The carboxylate compound is at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, r-butyrolactone, decanoic acid lactone, valerolactone, mevalonolactone, and caprolactone. The ether compound is at least one of dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran, and tetrahydrofuran. The other organic solvent is at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate, and a phosphate ester.

[0051] The electrochemical device of the present application is a primary battery, a secondary battery, a fuel cell, a solar cell, or a capacitor. Preferably, the electrochemical device is a lithium secondary battery, which is a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery. More preferably, the electrochemical device is a lithium ion battery.

[0052] The electrochemical device is prepared by stacking the positive electrode sheet, separator, and negative electrode sheet in sequence, and winding and folding them as needed to obtain a wound electrode assembly, placing the electrode assembly in a packaging bag, injecting an electrolyte into the packaging bag, and sealing the bag to obtain the electrochemical device; or alternatively, stacking the positive electrode sheet, separator, and negative electrode sheet in sequence, and then securing the four corners of the entire stack with tape to obtain a stacked electrode assembly, placing the electrode assembly in a packaging bag, injecting an electrolyte into the packaging bag, and sealing the bag to obtain the electrochemical device. In addition, overcurrent protection components, guide plates, etc. can also be placed in the packaging bag as needed to prevent pressure buildup and overcharging and discharging within the electrochemical device.

[0053] An application of a positive electrode sheet in an electronic device includes an electrochemical device containing the positive electrode sheet. The electrochemical device provided in the present application has good cycle performance and kinetic performance, so that the electronic device provided in the present application has a long service life and good safety performance.

[0054] The electronic device of the present application is a laptop computer, a pen-input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, an LCD TV, a portable cleaner, a portable CD player, a mini optical machine, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a power tool, a drone, a handheld vacuum cleaner, a bicycle, a lighting fixture, a toy, a game console, a clock, an electric tool, a flashlight, a camera, a large household battery or a lithium-ion capacitor.

[0055] The following examples and comparative examples are provided to further illustrate the embodiments of the present invention. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are by mass.

[0056] Example 1

[0057] An application of a positive electrode sheet in a lithium-ion battery comprises the following steps:

[0058] 1. Preparation of positive electrode sheet

[0059] The first positive electrode active material shown in Table 1, the conductive agent acetylene black and carbon nanotubes, and the binder polyvinylidene fluoride were mixed in a mass ratio of 97.0:1.0:0.8:1.2, wherein the first positive electrode active material contained polycrystalline particles and single crystal particles, the mass ratio of polycrystalline to single crystal materials was 75:25, and the average particle size of the polycrystalline material was D v50 The average particle size D of the single crystal material is 10 μm. v50 4μm, the chemical composition is LiNi 0.75Co 0.12 Mn 0.13 O2, add N-methylpyrrolidone, stir evenly under the action of a vacuum mixer to obtain a positive electrode slurry, wherein the solid content of the positive electrode slurry is 70%. The positive electrode slurry is evenly coated on one surface of the positive electrode current collector aluminum foil, and the aluminum foil is dried at 120°C for 0.5h to obtain a positive electrode sheet coated with the first positive electrode active material layer on one side, and the above steps are repeated on the other surface of the aluminum foil to obtain a positive electrode sheet coated with the first positive electrode active material layer on both sides. The second positive electrode active material shown in Table 1, the conductive agent acetylene black and carbon nanotubes, and the adhesive polyvinylidene fluoride are mixed in a mass ratio of 97:1.0:0.8:1.2, wherein the second positive electrode active material is Li2Fe 0.5 Mn 0.5 SiO4, the second positive electrode active material is coated with amorphous carbon, the amorphous carbon content in the material is 1.5wt% by mass, the first cycle charging capacity of the material is 320mAh / g, and the average particle size of the second positive electrode active material is D v50 The particle size of the positive electrode sheet is 4.5 μm, N-methylpyrrolidone is added, and the mixture is stirred evenly in a vacuum mixer to obtain a positive electrode slurry, wherein the solid content of the positive electrode slurry is 70%. The second positive electrode active material layer slurry is coated on the surface of the positive electrode sheet with the first positive electrode active material layer in the above step, and the above steps are repeated on the other side to obtain a composite positive electrode sheet coated with the first positive electrode active material layer and the second positive electrode active material layer on both sides. The composite positive electrode sheet is then cold pressed, wherein the density of the composite positive electrode sheet is controlled at 3.45 g / cm 3 , after cutting, slitting and tab welding, the positive electrode sheet is obtained;

[0060] 2. Preparation of negative electrode sheet

[0061] The negative electrode active material artificial graphite, binder SBR, and thickener sodium carboxymethyl cellulose (CMC) are mixed in a mass ratio of 97.4%:1.1%:1.4% and added to deionized water. The mixture is stirred evenly in a vacuum mixer to obtain a negative electrode slurry with a solid content of 67%. The negative electrode slurry is evenly coated on one surface of the negative electrode current collector copper foil (6um), and the copper foil is dried at 120°C to obtain a negative electrode sheet coated with a negative electrode material layer on one side. The above steps are repeated on the other surface of the copper foil to obtain a negative electrode sheet coated with a negative electrode material layer on both sides, which is then cold pressed. The negative electrode sheet is obtained after cutting, slitting, and tab welding.

[0062] 3. Electrolyte preparation

[0063] In a dry argon atmosphere glove box, propylene carbonate, ethylene carbonate, and diethyl carbonate were mixed in a mass ratio of 1:1:1 to obtain an organic mixed solvent, and then lithium salt LiFP6 was added to the organic solvent to dissolve and mix evenly to obtain an electrolyte, wherein the mass concentration of LiPF6 in the electrolyte was 12.5%.

[0064] 4. Preparation of isolation membrane

[0065] A polycrystalline polyethylene film with a thickness of 7 μm was used as the isolation membrane;

[0066] 5. Preparation of lithium-ion batteries

[0067] The positive electrode sheet, separator, and negative electrode sheet prepared above are stacked in order, and the separator is placed between the positive and negative electrode sheets to play an isolating role. The electrode assembly is wound and vacuum dried to remove moisture. The electrode assembly after moisture removal is placed in an aluminum-plastic film packaging bag, and the electrolyte is injected after drying. The lithium-ion battery is obtained through vacuum packaging, standing, formation, degassing and trimming processes. The lithium-ion battery is charged for the first time, and the upper limit of the charging voltage is set to 4.8V, so that the active Li+ in the polyanionic lithium-containing silicate compound or the cationic disordered rock salt structure lithium-containing positive electrode material can be fully released and stored in the anode to make up for the reversible active lithium consumption caused by the SEI film formation at the anode and the active Li consumption during the cycle, thereby improving the cycle performance.

[0068] Example 2

[0069] The only difference compared with Example 1 is the following aspects of the positive electrode sheet preparation steps: the chemical components of the first positive electrode active material containing polycrystalline and single crystal positive electrode materials are LiNi 0.75 Co 0.12 Mn 0.125 Zr 0.004 Al 0.001 O2.

[0070] Example 3

[0071] The only difference compared with Example 1 is the following aspects of the positive electrode sheet preparation steps: the chemical components of the first positive electrode active material containing polycrystalline and single crystal positive electrode materials are LiNi 0.82 Co 0.12 Mn 0.06 O2.

[0072] Example 4

[0073] The only difference compared with Example 1 is the following aspects of the positive electrode sheet preparation steps: the chemical components of the first positive electrode active material containing polycrystalline and single crystal positive electrode materials are LiNi 0.82 Co0.12 Mn 0.055 Zr 0.004 Al 0.001 O2.

[0074] Example 5

[0075] The only difference compared with Example 1 is the following aspects of the positive electrode sheet preparation steps: the chemical components of the first positive electrode active material containing polycrystalline and single crystal positive electrode materials are LiNi 0.9 Co 0.05 Mn 0.045 Zr 0.004 Al 0.001 O2.

[0076] Example 6

[0077] The only difference compared with Example 1 is the following aspects of the positive electrode sheet preparation steps: the chemical components of the first positive electrode active material containing polycrystalline and single crystal positive electrode materials are LiNi 0.9 Co 0.05 Mn 0.042 Zr 0.004 Al 0.001 B 0.00 3O2.

[0078] Example 7

[0079] The only difference compared with Example 6 is the following aspects of the positive electrode sheet preparation steps: the second positive electrode active material is Li2FeSiO4 / C, and the first cycle charging capacity of the material is 300mAh / g.

[0080] Example 8

[0081] The only difference compared with Example 6 is the following aspects of the positive electrode sheet preparation steps: the second positive electrode active material is Li2MnSiO4 / C, and the first cycle charging capacity of the material is 330mAh / g.

[0082] Example 9

[0083] The only difference compared with Example 6 is the following aspects of the positive electrode sheet preparation steps: the second positive electrode active material is Li 1.25 Nb 0.25 Mn 0.5 O2, the material's first cycle charging capacity is 356mAh / g.

[0084] Example 10

[0085] The only difference compared with Example 6 is the following aspects of the positive electrode sheet preparation steps: the second positive electrode active material is Li 1.23 Mo 0.467 Cr0.3 O2, the material's first cycle charging capacity is 378mAh / g.

[0086] Example 11

[0087] The only difference compared with Example 6 is the following aspects of the positive electrode sheet preparation steps: the second positive electrode active material is Li 1.3 Nb 0.3 Mn 0.4 O2, the material's first cycle charging capacity is 367mAh / g.

[0088] Example 12

[0089] The only difference compared with Example 6 is the following aspects of the positive electrode sheet preparation steps: the second positive electrode active material is Li 1.23 Ni 0.155 Ru 0.615 O2, the material's first cycle charging capacity is 334mAh / g.

[0090] Example 13

[0091] The only difference compared with Example 6 is the following aspects of the positive electrode sheet preparation steps: the second positive electrode active material is Li2MnO3, and the first cycle charging capacity of the material is 342mAh / g.

[0092] Example 14

[0093] The only difference compared with Example 6 is the following aspects of the positive electrode sheet preparation steps: the average particle size D of the second positive electrode active material is v 50 is 7.5μm.

[0094] Example 15

[0095] The only difference compared with Example 6 is the following aspects of the positive electrode sheet preparation steps: the average particle size D of the second positive electrode active material is v 50 is 3.5μm.

[0096] Example 16

[0097] The only difference compared with Example 6 is the following aspects of the positive electrode sheet preparation steps: the density of the composite positive electrode sheet is controlled at 3.7g / cm 3 .

[0098] Example 17

[0099] The only difference compared with Example 6 is the following aspects of the positive electrode sheet preparation steps: the first positive electrode active material contains polycrystalline and single crystal positive electrode materials, and the mass ratio of polycrystalline to single crystal materials is 50:50.

[0100] Example 18

[0101] The only difference compared with Example 6 is the following aspects of the positive electrode sheet preparation steps: the first positive electrode active material contains polycrystalline and single crystal positive electrode materials, and the mass ratio of polycrystalline to single crystal materials is 90:10.

[0102] Comparative Example 1

[0103] The only difference compared with Example 3 is the following aspect of the positive electrode sheet preparation steps: no second positive electrode active material layer is provided in the composite positive electrode sheet.

[0104] Comparative Example 2

[0105] The only difference compared with Example 3 is the following aspect of the positive electrode sheet preparation steps: the first positive electrode active material layer contains only polycrystalline material particles.

[0106] Comparative Example 3

[0107] The only difference compared with Example 3 is the following aspect of the positive electrode sheet preparation steps: the first positive electrode active material layer contains only single crystal material particles.

[0108] The technical effects achieved by the specific comparative examples and embodiment settings are shown in Table 1;

[0109] Compared with Examples 1 and 3 and Examples 4 and 5, increasing the Ni content in the first positive electrode active material increases the amount of lithium removed from the material, deteriorates the bulk structural stability of the material, and enhances the oxidizing property of the electrolyte. During high-temperature storage and high-temperature cycling, the interfacial side reactions are aggravated, the metal dissolution increases, and the high-temperature storage and cycle retention rates are correspondingly deteriorated.

[0110] Comparing Example 1 with Example 2, Example 3 with Example 4, and Example 5 with Example 6, by doping and coating the first positive electrode active material, the structural stability of the material during charge and discharge is enhanced, the oxygen activity of the bulk and surface of the material is reduced, and the dissolution of transition metals and interfacial side reactions are inhibited. The modified solution reduces gas generation during high-temperature storage and improves high-temperature cycle retention.

[0111] Compared with Examples 6 to 13, the higher the capacity of the second positive electrode active material, the higher the active lithium content stored in the anode after the first cycle charge capacity, in addition to compensating for the active lithium consumed by the SEI film formation at the anode. In the later stages of the cycle, as the active lithium is lost, the anode has more active lithium to replenish, thereby improving the high-temperature cycle retention rate of the battery cell. These materials have relatively good structural stability and relatively little impact on high-temperature storage.

[0112] Compared with Examples 6, 14, and 15, the smaller the particle size of the second positive electrode active material in the second positive electrode active material layer, the greater the BET and the increased interfacial side reactions between the electrolyte, which will deteriorate the high-temperature storage and high-temperature cycle retention rates; the larger the particle size of the second positive electrode active material, the lower the first-cycle lithium removal capacity, which will affect the high-temperature cycle life;

[0113] Compared with Example 16, when the positive electrode sheet is compacted, the degree of particle breakage in the first positive electrode active material layer increases accordingly, and the high-temperature storage and high-temperature cycle retention rates deteriorate.

[0114] Compared with Examples 6, 17, and 18, the mixing ratio of polycrystalline particles to single-crystal particles in the first positive electrode active material layer is reduced. That is, in the mixed design, polycrystalline particles are reduced and single crystal particles are increased. The degree of particle breakage during the cold pressing of the electrode is further alleviated, and the high-temperature storage gas production and high-temperature cycle retention rate are improved. In addition, the increase in the single crystal ratio will further reduce the reversible capacity of the electrode and reduce the energy density of the battery cell.

[0115] Comparative Example 1 Compared with Example 3, in the single-layer electrode setting, when the first positive electrode active material layer contains a mixed design of polycrystalline and single crystal, although the mixed design of polycrystalline and single crystal will alleviate the particle breakage, the polycrystalline particles on the surface of the electrode will basically be flattened under the effect of cold pressing, and obvious particle breakage and grain boundary cracks will appear. After electrolyte infiltration and penetration, the interface side reaction will still be aggravated. In Example 2, a second positive electrode active material layer is further set on the first positive electrode active material layer, wherein the positive electrode active material in the second positive electrode active material layer is set to a single crystal morphology. The purpose is to alleviate the grain boundary and intracrystalline cracks of the electrode surface particles during cold pressing and charge and discharge. The positive electrode active material in the second positive electrode active material layer is selected from a polyanionic lithium-containing silicate compound, the main purpose of which is to reduce the reversible lithium consumed by the SEI film formation at the anode, increase the reversible capacity of the cathode, and thus increase the energy density of the electrochemical device. In addition, the polyanionic lithium-containing silicate compound or the cationic disordered rock salt structure lithium-containing positive electrode material can be used as a good positive electrode lithium supplement material to make up for the reversible active lithium consumption caused by the SEI film formation at the anode, improve the charge and discharge reversible capacity of the material, and the active lithium stored in the anode can also be gradually released in the later stage of the cycle to increase the cycle life.

[0116] Comparing Comparative Example 1 with Comparative Example 2, in a single-layer electrode arrangement, when the first positive electrode active material layer includes a polycrystalline and single crystal mixed design, the cold pressing process of pure polycrystalline will cause severe particle breakage, exacerbating the interface side reaction. In comparison, Comparative Example 1 has better high-temperature storage gas production and cycle retention rate. Compared with Comparative Examples 1, 2, and 3, the single crystal high-temperature storage gas production and cycle retention rate are better than the polycrystalline and polycrystalline and single crystal mixed designs. However, the single crystal has low specific capacity and poor kinetic performance, which makes it unsuitable for more scenarios.

[0117] Lithium-ion battery testing methods

[0118] (1) High temperature storage gas production test

[0119] Four lithium-ion batteries from each group, prepared using the positive electrodes from the examples and comparative examples, were charged at a constant current rate of 0.5C to a voltage of 4.3V in an environment of approximately 25°C. The batteries were then charged at a constant voltage of 4.3V until the current dropped below 0.05C, bringing the batteries to a fully charged state of 4.3V. The thickness d0 of the fully charged batteries before storage was measured. The fully charged batteries were then placed in an oven at approximately 85°C for approximately 24 hours, and their post-storage thickness d1 was measured.

[0120] Thickness expansion ratio S = ((d1-d0) / d0×100%).

[0121] (2) Cyclic performance test

[0122] The lithium-ion battery was charged and discharged for the first time in an environment of 45°C. Constant current charging was performed at a charging current of 1.5C to a voltage of 4.30V, then constant voltage charging was performed to a current of 0.05C, and then constant current discharge was performed at a discharge current of 4C until the final voltage reached 2.8V. The discharge capacity Q0 of the first cycle was recorded, and then the above charge and discharge cycles were repeated 1000 times, and the discharge capacity Qe of the 1000th cycle was recorded. The cycle capacity retention rate W = Qe / Q0×100%.

[0123] Four lithium-ion batteries prepared in each of the examples and comparative examples were tested, and the average value was taken as the final result.

[0124] Table 1 Example and comparative example settings and technical effects

[0125]

[0126]

[0127] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A positive electrode plate, characterized in that: The positive electrode sheet includes a positive electrode current collector, a first positive electrode active material layer and a second positive electrode active material layer; the first positive electrode active material layer is disposed on the positive electrode current collector, the first positive electrode active material layer comprises a first positive electrode active material; the first positive electrode active material comprises polycrystalline particles and single crystal particles; the second positive electrode active material layer is disposed on the first positive electrode active material layer, the second positive electrode active material layer comprises a second positive electrode active material, the second positive electrode active material is a single crystal particle, and the second positive electrode active material is selected from a polyanionic lithium-containing silicate compound and / or a cationic disordered rock salt phase structure lithium-containing positive electrode material; The mass ratio of polycrystalline particles to single crystal particles in the first positive electrode active material is in the range of 90:10 to 50:50; The particle size of the second positive electrode active material satisfies the following relationship: 3.5 μm <D v50 ≤7.0 μm.

2. The positive electrode sheet according to claim 1, characterized in that: The ICP test shows that the total molar amount of metal elements other than Li in the first positive electrode active material is nM1, the molar amount of Ni element is nNi1, the molar amount of Co element is nCo1, the molar amount of Mn element is nMn1, the molar amount of Al element is nAl1, the molar amount of R element is nR1, 0.75≤nNi1 / nM1<1, 0≤nCo1 / nM1≤0.15, 0≤nMn1 / nM1≤0.1, 0≤nAl1 / nM1≤0.005, 0≤nR1 / nM1≤0.05, and the R element is at least one of B, P, Mg, Ti, Zr, Nb, Y, Cr, V, Ge, Mo, Fe, Cu, Zn, Ga, Ag, W, In, Sn, Pb, Sb, La, Ce, Ca, and Sr.

3. The positive electrode sheet according to claim 1, characterized in that: The polyanionic lithium-containing silicate compound has a general formula of Li2MSiO4, wherein M is at least one of Ni, Fe, Mn, Co, and V. The surface of the polyanionic lithium-containing silicate compound is coated with amorphous carbon.

4. The positive electrode sheet according to claim 1, characterized in that: The general formula of the lithium-containing positive electrode material with a cationic disordered rock salt phase structure is Li 1+x TM 1-x O2 and / or Li2TMO3, where 0.2≤ x ≤1, wherein TM is selected from one or more transition metals selected from Co, Fe, Mn, Mo, Nb, Ni, V, and Ti.

5. The positive electrode sheet according to claim 1, characterized in that: The positive electrode sheet compression range is 3.15 g / cm 3 ≤ρ≤3.7 g / cm 3 .

6. The positive electrode sheet according to claim 1, characterized in that: The particle size of the single crystal morphology material in the first positive electrode active material satisfies the following relationship: 3.5 μm≤D v50 ≤7.0 μm; the particle size of the polycrystalline morphology material in the first positive electrode active material satisfies the following relationship: 8.0 μm≤D v50 ≤15.0 μm.

7. An electrochemical device, characterized in that: The positive electrode sheet comprises the positive electrode sheet according to any one of claims 1 to 6.

8. An electronic device, characterized in that: An electrochemical device comprising the electrochemical device according to claim 7.

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