Positive pole piece, secondary battery and electronic device

Abstract

The invention provides a positive electrode plate, a secondary battery and an electronic device, the positive electrode plate comprises a positive electrode material layer, the positive electrode material layer comprises a positive electrode active material and an additive, and the additive comprises at least one of barium titanate, magnesium metatitanate and strontium titanate; and the dielectric constant E1 of the positive pole piece is 5-30. The positive electrode material layer of the positive electrode plate comprises the additive, so that the low-temperature discharge performance of the secondary battery can be improved, and meanwhile, the secondary battery has relatively high rate capability.

Description

Technical Field

[0001] The present application relates to the field of electrochemical technologies, and particularly to a positive electrode sheet, a secondary battery, and an electronic device. Background Art

[0002] Secondary batteries, such as lithium-ion batteries, have advantages such as high specific energy, high working voltage, low self-discharge rate, small size, and light weight, and are widely used in fields such as consumer electronics. With the development of technology, the requirements for the performance of secondary batteries are increasing day by day, and the voltage system of secondary batteries gradually shows an upward trend. At the same time, in order to pursue higher energy density and better charge-discharge performance, the thickness of the positive electrode sheet of the secondary battery increases, and the low-temperature discharge performance of the secondary battery faces great challenges.

[0003] Therefore, how to improve the low-temperature discharge performance of secondary batteries has become an urgent problem to be solved by those skilled in the art. Summary of the Invention

[0004] The purpose of the present application is to provide a positive electrode sheet, a secondary battery, and an electronic device to reduce the resistance of lithium ions to desolvation at low temperatures, improve the low-temperature discharge performance of the secondary battery, and at the same time, the secondary battery also has high rate performance.

[0005] It should be noted that in the summary of the invention of the present application, lithium-ion batteries are used as examples of secondary batteries to explain the present application, but the secondary batteries of the present application are not limited to lithium-ion batteries. The specific technical solutions are as follows:

[0006] The first aspect of the present application provides a positive electrode sheet, which includes a positive electrode material layer. The positive electrode material layer includes a positive electrode active material and an additive. The additive includes at least one of barium titanate, magnesium metatitanate, and strontium titanate; the dielectric constant E1 of the positive electrode sheet is 5 to 30. The positive electrode material layer includes the above types of additives, and the dielectric constant E1 of the positive electrode sheet can be adjusted within the above range. The positive electrode sheet has a high dielectric constant, which changes the polarity and electric field distribution of the electrolyte near the positive electrode sheet. It can not only weaken the interaction between lithium ions and solvent molecules, but also be beneficial to the migration of lithium ions, which reduces the resistance of lithium ions to desolvation at low temperatures. Therefore, it is beneficial to improve the low-temperature discharge performance of the secondary battery. In addition, the above additives exist on the surface of the positive electrode active material of the positive electrode sheet and have little influence on the electronic conductive network constructed by the positive electrode active material, so the rate performance of the secondary battery is basically not affected.

[0007] In some embodiments of the present application, based on the mass of the positive electrode material layer, the mass percentage content of the additive is m%, where 0.01 ≤ m ≤ 0.1. By regulating the mass percentage content of the additive within the above range, it is beneficial to reduce the resistance of lithium ions to desolvation at low temperatures. Moreover, since the content of the additive is relatively low and it exists on the surface of the positive electrode active material, it has little impact on the electronic conductive network formed by the positive electrode active material, thereby improving the low-temperature discharge performance of the secondary battery. At the same time, the secondary battery has high rate performance.

[0008] In some embodiments of the present application, 0.03 ≤ m ≤ 0.07. By regulating the mass percentage content of the additive within the above range, the low-temperature discharge performance of the secondary battery can be further improved. At the same time, the secondary battery has high rate performance.

[0009] In some embodiments of the present application, the thickness of the positive electrode material layer is H μm, where 10 ≤ H ≤ 50 and 0.001 ≤ m / H ≤ 0.005. By regulating the thickness H of the positive electrode active material layer within the above range and adjusting H and m to satisfy the above relationship, not only can lithium ions have a suitable transmission path during charge and discharge, but also the dielectric constant E1 of the positive electrode sheet can be within the range of the present application, jointly realizing the rapid and stable transmission of lithium ions, thereby improving the low-temperature discharge performance of the secondary battery. In addition, the additive can exist on the surface of the positive electrode active material particles and has little impact on the electronic conductive network formed by the positive electrode active material, and the secondary battery also has high rate performance.

[0010] In some embodiments of the present application, the additive further includes a doping element; the doping element includes at least one of niobium, zinc, tin, lanthanum, yttrium, barium, and magnesium. The additive containing the above doping elements has a high dielectric constant and can improve the low-temperature discharge performance of the secondary battery.

[0011] In some embodiments of the present application, the average particle size D of the additive is from 0.1 μm to 3 μm. Generally, the particles of the positive electrode active material are larger than the average particle size of the above additive. By regulating D within the above range, the agglomeration between the additive particles can be weakened, and the additive can be uniformly dispersed in the positive electrode material layer and exist on the surface of the positive electrode active material, which helps to reduce the resistance of lithium ions to desolvation at low temperatures and reduce the impact on the electronic conductive network, thereby improving the low-temperature discharge performance of the secondary battery. At the same time, the secondary battery has high rate performance.

[0012] In some embodiments of the present application, the dielectric constant E2 of the additive is from 10 to 300. By regulating the dielectric constant E2 of the additive within the above range, the dielectric constant E1 of the positive electrode sheet can be regulated within the range of 5 to 30, which helps to reduce the resistance of lithium ions to desolvation at low temperatures and improve the low-temperature discharge performance of the secondary battery.

[0013] In some embodiments of the present application, the film resistance of the positive electrode sheet is R, where 0.1 Ω ≤ R ≤ 1.0 Ω. When R is within the above range and R is relatively small, it helps lithium ions to be quickly inserted and extracted from the inside of the positive active material, thereby improving the low-temperature discharge performance of the secondary battery. At the same time, the secondary battery has high rate performance.

[0014] In some embodiments of the present application, the dielectric constant E1 of the positive electrode sheet is from 7 to 20. By adjusting the dielectric constant E1 of the positive electrode sheet within the above range, the low-temperature discharge performance of the secondary battery can be further improved. At the same time, the secondary battery has high rate performance.

[0015] The second aspect of the present application provides a secondary battery, which includes the positive electrode sheet in any of the foregoing embodiments. Therefore, the secondary battery provided by the present application has good low-temperature discharge performance and rate performance.

[0016] The third aspect of the present application provides an electronic device, which includes the secondary battery in any of the foregoing embodiments. Thus, the electronic device provided by the present application has good performance in use.

[0017] Advantages of the present application:

[0018] The present application provides a positive electrode sheet, a secondary battery, and an electronic device. The positive electrode sheet includes a positive electrode material layer, the positive electrode material layer includes a positive active material and an additive, and the additive includes at least one of barium titanate, magnesium metatitanate, and strontium titanate; the dielectric constant E1 of the positive electrode sheet is from 5 to 30. On the one hand, by adding an additive with a relatively high dielectric constant to the positive electrode material layer, the positive electrode sheet has a suitable dielectric constant, which helps to reduce the resistance of lithium ions to desolvate at low temperatures during discharge and improve the low-temperature discharge ability of the secondary battery. On the other hand, the above additive exists on the surface of the positive active material and has little influence on the electronic conductive network constructed by the positive active material, so that the secondary battery has high rate performance.

[0019] Of course, it is not necessary for any product or method implementing the present application to achieve all the above-mentioned advantages simultaneously. Detailed implementation manners

[0020] The technical solutions in the present application will be clearly and completely described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. All other embodiments obtained by those skilled in the art based on the present application belong to the scope of protection of the present application.

[0021] It should be noted that in the following content, a lithium-ion battery is taken as an example of a secondary battery to explain the present application. However, the secondary battery of the present application is not limited to lithium-ion batteries.

[0022] The first aspect of the present application provides a positive electrode sheet, which includes a positive electrode material layer. The positive electrode material layer includes a positive electrode active material and an additive. The additive includes at least one of barium titanate, magnesium metatitanate, and strontium titanate. The dielectric constant E1 of the positive electrode sheet is 5 to 30. In some embodiments of the present application, the dielectric constant E1 of the positive electrode sheet is 7 to 20. For example, the dielectric constant E1 of the positive electrode sheet can be 5, 6, 7, 12, 15, 20, 23, 25, 26, 30, or a range composed of any two of these values.

[0023] In a low-temperature environment, the ionic conductivity of the electrolyte decreases, and the migration rate of lithium ions in the electrolyte decreases. Moreover, at low temperatures, the viscosity of the electrolyte increases, and the interaction between lithium ions and solvent molecules in the electrolyte is enhanced, significantly reducing the diffusion rate of lithium ions in the electrolyte. This results in difficulty for lithium ions to transform from a solvated state to a free ion state at low temperatures, increasing the resistance to desolvation of lithium ions. In the present application, the above-mentioned low temperature refers to a temperature less than or equal to 0 °C.

[0024] By adding the above-mentioned types of additives to the positive electrode sheet, the present application can regulate the dielectric constant E1 of the positive electrode sheet within the above range. The positive electrode sheet has a high dielectric constant, which changes the polarity and electric field distribution of the electrolyte near the positive electrode sheet, changes the distance and arrangement of solvent molecules, can weaken the interaction between lithium ions and solvent molecules, reduce the energy barrier in the desolvation process of lithium ions, and thus reduce the resistance to desolvation of lithium ions at low temperatures. Moreover, the positive electrode sheet with the dielectric constant E1 within the above range can improve the migration environment of lithium ions in the electrolyte near the positive electrode sheet, facilitate the migration of lithium ions, and indirectly reduce the resistance to desolvation of lithium ions at low temperatures. This is beneficial to improving the low-temperature discharge performance of secondary batteries.

[0025] The inventors found that when the dielectric constant E1 of the positive electrode sheet is too large, for example, greater than 30, the electric field polarization effect on lithium ions will be enhanced, resulting in a decrease in the migration rate of lithium ions and a reduction in the charge and discharge performance of lithium-ion batteries. When the dielectric constant E1 of the positive electrode sheet is too small, for example, less than 5, it is not conducive to the dissociation of lithium salts, reducing the number of freely movable lithium ions in the solution, resulting in a decrease in the conductivity of secondary batteries and affecting the charge and discharge performance and rate performance of secondary batteries.

[0026] The inventors also found that the above-mentioned additive in the form of powder particles exists on the surface of the cathode active material particles of the cathode electrode sheet, has little influence on the electron conductive network formed by the cathode active material, and the sheet resistance of the cathode electrode sheet is low, which is beneficial to the transmission of lithium ions and electrons, and can enable the electrochemical reaction during the charge and discharge process of the secondary battery to proceed smoothly, thus not affecting the rate performance of the secondary battery.

[0027] Therefore, the cathode electrode sheet provided by this application has a relatively high dielectric constant, which helps to reduce the resistance of lithium ions to desolvate at low temperatures during the discharge process, thereby improving the low-temperature discharge performance of the secondary battery. At the same time, the above-mentioned additive exists on the surface of the cathode active material particles and has little influence on the electron conductive network formed by the cathode active material, and the secondary battery also has relatively high rate performance.

[0028] In some embodiments of this application, based on the mass of the cathode material layer, the mass percentage content of the additive is m%, and 0.01 ≤ m ≤ 0.1. In some embodiments of this application, 0.03 ≤ m ≤ 0.07. For example, m can be 0.01, 0.03, 0.05, 0.07, 0.08, 0.1 or a range composed of any two of these values. By adjusting the mass percentage content m% of the additive within the above range, the cathode electrode sheet can have an appropriate dielectric constant, which is beneficial to reducing the resistance of lithium ions to desolvate at low temperatures, and the content of the additive is relatively low. It exists on the surface of the cathode active material and has little influence on the electron conductive network formed by the cathode active material. Thereby, the low-temperature discharge performance of the secondary battery can be improved, and at the same time, the secondary battery has relatively high rate performance.

[0029] In some embodiments of the present application, the thickness of the positive electrode material layer is H μm, where 10 ≤ H ≤ 50 and 0.001 ≤ m / H ≤ 0.005. For example, H can be 10 μm, 15 μm, 20 μm, 26 μm, 32 μm, 36 μm, 42 μm, 45 μm, 50 μm or a range composed of any two of these values; m / H can be 0.001, 0.002, 0.003, 0.004, 0.005 or a range composed of any two of these values. By controlling the thickness H of the positive electrode active material layer within the above range and adjusting H and m to satisfy the above relationship, when H is within the above range, the secondary battery has a high energy density, and during the charge and discharge process, lithium ions have a suitable transmission path, enabling a faster charge and discharge speed; when H and m satisfy the above relationship, as H increases, m also increases correspondingly, which can keep the dielectric constant of the positive electrode plate within the range of the present application, helping to reduce the resistance of lithium ions to desolvate at low temperatures and jointly achieving the fast and stable transmission of lithium ions, thereby improving the low-temperature discharge performance of the secondary battery. In addition, when H and m satisfy the above relationship, the thickness of the positive electrode material layer matches the mass percentage content of the additive, and the additive can exist on the surface of the positive electrode active material particles, having little impact on the electron conductive network constructed by the positive electrode active material, and the secondary battery also has high rate performance.

[0030] In some embodiments of the present application, the additive further includes a doping element; the doping element includes at least one of niobium, zinc, tin, lanthanum, yttrium, barium, and magnesium. Doping the above elements into barium titanate, magnesium metatitanate, and strontium titanate can change the dielectric constant of the additive, thereby changing the dielectric constant of the positive electrode plate and achieving the purpose of improving the low-temperature discharge performance of the secondary battery.

[0031] The above additives and the additives containing doping elements can be obtained by purchase. Based on the mass of the above additives, the mass percentage content of the doping element is 1% to 5%. At the same time, by combining the test of the content of the doping element with an inductively coupled plasma atomic emission spectrometer, the additive with the required content of the doping element can be selected.

[0032] In some embodiments of the present application, the average particle size D of the additive is from 0.1 μm to 3 μm. For example, D can be 0.1 μm, 0.5 μm, 1 μm, 1.3 μm, 1.7 μm, 2 μm, 2.3 μm, 2.6 μm, 3 μm or a range composed of any two of these values. Generally, the particles of the positive electrode active material are larger than the average particle size of the above additive. By controlling D within the above range, the aggregation between the additive particles can be weakened, and the additive can be uniformly dispersed in the positive electrode material layer and present on the surface of the positive electrode active material, which helps to reduce the electric field distortion and excessive local electric field intensity near the positive electrode sheet, improve the overall dielectric constant and the uniformity and stability of the dielectric constant of the positive electrode sheet, reduce the resistance of lithium ions to desolvate at low temperature, and reduce the influence on the electronic conduction network, thereby improving the low-temperature discharge performance of the secondary battery. At the same time, the secondary battery has high rate performance.

[0033] In the present application, the above additives with different average particle sizes can be obtained by purchase, and their average particle sizes can be measured by combining with a scanning electron microscope, and the additives with the required particle sizes can be selected.

[0034] In some embodiments of the present application, the dielectric constant E2 of the additive is from 10 to 300. For example, the dielectric constant E2 of the additive can be 10, 40, 70, 100, 135, 170, 200, 220, 250, 280, 300 or a range composed of any two of these values. By controlling the dielectric constant E2 of the additive within the above range, the dielectric constant E1 of the positive electrode sheet can be controlled within the range of 5 to 30, which helps to reduce the resistance of lithium ions to desolvate at low temperature, thereby improving the low-temperature discharge performance of the secondary battery.

[0035] In some embodiments of the present application, the sheet resistance of the positive electrode sheet is R, and 0.1 Ω ≤ R ≤ 1.0 Ω. For example, R can be 0.1 Ω, 0.2 Ω, 0.4 Ω, 0.5 Ω, 0.6 Ω, 0.8 Ω, 1.0 Ω or a range composed of any two of these values. When the sheet resistance R of the positive electrode sheet is within the above range and R is small, it helps lithium ions to quickly intercalate and deintercalate from the inside of the positive electrode active material, improves the charge and discharge performance of the secondary battery, enables the secondary battery to charge and discharge quickly, thereby improving the low-temperature discharge performance of the secondary battery, and the secondary battery has high rate performance. At the same time, it can also reduce the internal resistance of the secondary battery and extend the cycle life of the secondary battery.

[0036] In this application, the positive electrode sheet further includes a positive electrode current collector, and the positive electrode material layer is disposed on at least one surface of the positive electrode current collector. The above-mentioned "disposed on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be disposed on one surface of the positive electrode current collector along its own thickness direction, or can be disposed on two surfaces of the positive electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of the surface of the positive electrode current collector, or can be a partial area of the surface of the positive electrode current collector. There is no special limitation in this application, as long as the purpose of this application can be achieved.

[0037] This application has no special limitation on the positive electrode current collector, as long as the purpose of this application can be achieved. For example, it can include aluminum foil, aluminum alloy foil or composite current collector (such as aluminum-carbon composite current collector), etc.

[0038] This application has no special limitation on the positive electrode active material, as long as the purpose of this application can be achieved. For example, the positive electrode active material can include but is not limited to lithium nickel cobalt manganese oxide (such as NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminate, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide (LiCoO 2 ), lithium manganese oxide, lithium manganese iron phosphate or lithium titanate, etc.

[0039] The positive electrode material layer can also include a conductive agent and a binder. This application has no special limitation on the types of the conductive agent and the binder, as long as the purpose of this application can be achieved. For example, the conductive agent can include but is not limited to at least one of conductive carbon black (SuperP), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metal materials or conductive polymers. The conductive carbon black can include but is not limited to at least one of acetylene black or Ketjen black. The above carbon nanotubes can include but are not limited to single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above carbon fibers can include but are not limited to vapor-grown carbon fibers (VGCF) and / or nanofibers. The above metal materials can include but are not limited to metal powders and / or metal fibers. Specifically, the metal can include but is not limited to at least one of copper, nickel, aluminum or silver. The above conductive polymers can include but are not limited to at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene or polypyrrole. The binder can include but is not limited to at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamideimide, styrene-butadiene rubber or polyvinylidene fluoride. This application has no special limitation on the mass ratio of the positive electrode active material, conductive agent and binder in the positive electrode material layer. Those skilled in the art can select according to actual needs as long as the purpose of this application can be achieved.

[0040] The present application has no particular limitation on the thickness of the positive current collector, as long as the object of the present application can be achieved. For example, the thickness of the positive current collector is 5 μm to 20 μm.

[0041] The second aspect of the present application provides a secondary battery, which includes the positive electrode sheet in any of the foregoing embodiments. Therefore, the secondary battery provided by the present application has good low-temperature discharge performance and rate performance.

[0042] In the present application, the secondary battery further includes a negative electrode sheet, and the negative electrode sheet includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector. The above-mentioned "the negative electrode material layer is disposed on at least one surface of the negative current collector" means that the negative electrode material layer can be disposed on one surface of the negative current collector along its own thickness direction, or can be disposed on two surfaces of the negative current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of the surface of the negative current collector, or a partial area of the surface of the negative current collector. The present application has no particular limitation, as long as the object of the present application can be achieved.

[0043] The present application has no particular limitation on the negative current collector, as long as the object of the present application can be achieved. For example, it can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam or composite current collector. Exemplarily, the composite current collector can be a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc.

[0044] The negative electrode material layer includes a negative electrode active material. The present application has no particular limitation on the negative electrode active material, as long as the object of the present application can be achieved. For example, the negative electrode active material can include but is not limited to at least one of natural graphite, artificial graphite, mesocarbon microbeads, hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO 2 , lithiated TiO with a spinel structure 2 -Li 4 Ti 5 O 12 or Li-Al alloy.

[0045] The negative electrode material layer may further include a dispersant. The present application has no particular limitation on the type of the dispersant, as long as the object of the present application can be achieved. For example, the dispersant can include but is not limited to at least one of sodium carboxymethyl cellulose, lithium carboxymethyl cellulose or carboxymethyl cellulose.

[0046] The negative electrode material layer may further include a conductive agent and a binder. The present application does not particularly limit the types of the conductive agent and the binder, as long as the object of the present application can be achieved. For example, it may be at least one of the above-mentioned conductive agent and the above-mentioned binder. The present application does not particularly limit the mass ratio of the negative electrode active material, the conductive agent, the binder, and the dispersant in the negative electrode material layer, and those skilled in the art can select according to actual needs as long as the object of the present application can be achieved.

[0047] The present application does not particularly limit the thickness of the negative electrode material layer, as long as the object of the present application can be achieved. For example, the thickness of the negative electrode material layer is 30 μm to 120 μm. The present application does not particularly limit the thickness of the negative electrode current collector, as long as the object of the present application can be achieved. For example, the thickness of the negative electrode current collector is 4 μm to 15 μm.

[0048] Optionally, the negative electrode sheet may further include a conductive layer, and the conductive layer is located between the negative electrode current collector and the negative electrode material layer. The present application does not particularly limit the composition of the conductive layer, and it may be a commonly used conductive layer in the art. For example, the conductive layer includes a conductive agent and a binder. The present application does not particularly limit the conductive agent and the binder in the conductive layer. For example, it may be at least one of the above-mentioned conductive agent and the above-mentioned binder.

[0049] In the present application, the secondary battery further includes an electrolyte, and the electrolyte includes a lithium salt and a non-aqueous solvent.

[0050] The present application does not particularly limit the lithium salt, as long as the object of the present application can be achieved. For example, the lithium salt may include, but is not limited to, LiPF 6 , LiBF 4 , LiAsF 6 , LiClO 4 , LiB(C 6 H 5 ) 4 , LiCH 3 SO 3 , LiCF 3 SO 3 , LiN(SO 2 CF 3 ) 2 , LiC(SO 2 CF 3 ) 3 , Li 2 SiF 6 , lithium bis(oxalato)borate (LiBOB) or lithium difluoroborate, or at least one of them. The present application does not particularly limit the content of the lithium salt in the electrolyte, as long as the object of the present application can be achieved.

[0051] The non-aqueous solvent in the present application is not particularly limited as long as the object of the present application can be achieved. For example, the non-aqueous solvent may include, but is not limited to, at least one of carbonate compounds, carboxylate compounds, ether compounds, or other organic solvents.

[0052] The above carbonate compounds may include, but are not limited to, at least one of linear carbonate compounds, cyclic carbonate compounds, or fluorinated carbonate compounds. The above linear carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). The above cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or vinylene ethylene carbonate (VEC). The fluorinated carbonate compounds may include, but are not limited to, at least one of fluorinated ethylene carbonate (FEC), 1,2-difluoroethylene 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, or trifluoromethyl ethylene carbonate. The above carboxylate compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolide, valerolactone, or caprolactone. The above ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The above other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. The content of the non-aqueous solvent in the electrolyte in the present application is not particularly limited as long as the object of the present application can be achieved.

[0053] In the present application, the secondary battery further includes a separator. The separator in the present application is not particularly limited as long as the object of the present application can be achieved. For example, the material of the separator may include, but is not limited to, at least one of polyolefins (PO) mainly composed of polyethylene (PE) and polypropylene (PP), polyesters (for example, polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of the separator may include at least one of woven membranes, non-woven membranes, microporous membranes, composite membranes, rolled membranes, or spun membranes.

[0054] In some embodiments of the present application, the separator may include a substrate layer and a surface treatment layer. The substrate layer may be a non-woven fabric, a film or a composite film having a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate or polyimide. Optionally, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite film may be used.

[0055] Optionally, a surface treatment layer is provided on at least one surface of the substrate layer, and the surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic substance.

[0056] In some embodiments of the present application, the inorganic layer includes inorganic particles and a binder. The present application places no particular limitation on the inorganic particles. For example, the inorganic particles may include at least one of alumina, silica, magnesia, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide or barium sulfate. The present application places no particular limitation on the binder. For example, the binder may be at least one of the above binders. In some embodiments of the present application, the polymer layer includes a polymer, and the material of the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyethylene ether or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene).

[0057] In the present application, the thickness of the separator is not particularly limited as long as the object of the present application can be achieved. For example, the thickness of the separator may be 3 μm to 30 μm.

[0058] The secondary battery further includes a housing for accommodating the positive electrode sheet, the separator, the negative electrode sheet and the electrolyte, as well as other components known in the field of secondary batteries. The present application places no limitation on the above other components. The present application places no particular limitation on the housing, and it may be a housing well-known in the art as long as the object of the present application can be achieved. For example, the housing may be a hard shell housing or a flexible housing. The material of the hard shell housing may be metal. The present application places no limitation on the type of metal, and a metal hard shell housing known in the art may be used as long as the object of the present application can be achieved. The flexible housing may be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0059] The preparation process of the secondary battery of the present application is well-known to those skilled in the art, and there is no particular limitation in the present application. For example, the preparation process of the secondary battery may include, but is not limited to, the following steps: stacking the positive electrode sheet, the separator, and the negative electrode sheet in sequence, and winding, folding, etc. as needed to obtain a wound electrode assembly, placing the electrode assembly into a casing, injecting electrolyte into the casing and sealing it to obtain a secondary battery. Alternatively, stack the positive electrode sheet, the separator, and the negative electrode sheet in sequence, and then fix the four corners of the entire laminated structure with tape to obtain a laminated electrode assembly, place the electrode assembly into the casing, inject electrolyte into the casing and seal it to obtain a secondary battery. In addition, an overcurrent protection element, a guide plate, etc. can also be placed in the casing as needed to prevent the pressure inside the secondary battery from rising and overcharging / discharging.

[0060] The third aspect of the present application provides an electronic device, which includes the secondary battery in any of the foregoing embodiments. Thus, the electronic device provided by the present application has good performance in use.

[0061] The present application does not particularly limit the type of the electronic device, and it can be any electronic device known in the prior art. In some embodiments of the present application, the electronic device may include, but is not limited to, 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 earphone, a video recorder, a liquid crystal TV, a portable cleaner, a portable CD player, a minidisc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power source, a motor, an automobile, a motorcycle, a moped, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flash, a camera, a large household battery, and a lithium-ion capacitor, etc.

[0062] Examples

[0063] Hereinafter, examples and comparative examples are given to more specifically illustrate the embodiments of the present application. Various tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0064] Testing method and equipment:

[0065] Sampling of the positive electrode sheet:

[0066] Under the condition of 25°C, the lithium-ion battery is discharged at a constant current of 0.2C until the discharge cut-off voltage of 3.0V, the lithium-ion battery is disassembled, and then the residual electrolyte on the surface of the positive electrode sheet is wiped off with a dust-free paper to obtain the positive electrode sheet.

[0067] The following test methods use the positive electrode sheet obtained by the above method unless otherwise specified.

[0068] Positive electrode sheet dielectric constant E1 test:

[0069] At 25 °C, use the parallel plate capacitor method with a TH2839 parallel plate capacitor device to test the dielectric constant E1 of the positive electrode sheet. Among them, two mutually parallel aluminum metal conductors are used as the two parallel electrode plates.

[0070] Positive electrode material layer thickness H test:

[0071] 1) Cut the positive electrode sheet under plasma to obtain its cross-section;

[0072] 2) Observe the cross-section of the positive electrode sheet obtained in 1) under a scanning electron microscope (SEM), and measure the thickness H of the single-sided positive electrode material layer. The adjacent test points are spaced 2 mm to 3 mm apart, and 15 different points are tested. Record the average value of all test points as the thickness H of the positive electrode material layer.

[0073] Additive average particle size D test:

[0074] Observe the surface of the positive electrode material layer of the positive electrode sheet through SEM at a magnification of 10,000 times. Randomly select 50 additive particles, measure the diameters of the 50 particles respectively, and calculate the average value to obtain the average particle size D of the additive. Among them, the particles with high contrast and particle size much smaller than the positive electrode active material in the SEM image are additive particles.

[0075] Additive dielectric constant E2 test:

[0076] 1) Test at 25 °C using a TH2839 parallel plate capacitor device.

[0077] 2) Clean the parallel plate capacitor electrode plates to ensure that the surface is flat, smooth, and free of foreign objects.

[0078] 3) Measure and record the capacitance value C of the empty capacitor 0 .

[0079] 4) Uniformly fill the additive powder sample between the two electrode plates, and try to ensure that the filling density is uniform and consistent, avoiding voids or uneven accumulation.

[0080] 5) Measure the capacitance value again to obtain the capacitance value C 1 .

[0081] According to the formula, dielectric constant = C 1 / C 0 Calculate the dielectric constant E2 of the additive.

[0082] For step 4), when there are two or more types of additives, first mix the powder of different types of additives evenly according to the ratios in the examples or comparative examples, and then evenly fill it between the two electrodes.

[0083] Positive electrode sheet film resistance R test:

[0084] Tested with the Yuaneng Technology BER2500 film resistance tester, with a pressure holding time of 10 s, a pressure of 0.4 t, and a test area of 153.9 mm 2 , after testing 12 values, output a test report, and obtain the film resistance R of the positive electrode sheet by taking the average of the 12 film resistances.

[0085] Low-temperature (-20 °C) discharge performance test:

[0086] Let the lithium-ion battery stand for 5 min at a test temperature of 25 °C, then charge the lithium-ion battery at a constant current of 0.7C to 4.53V, and then charge it at a constant voltage of 4.53V until the cut-off current of 0.05C; stand for 5 min, and then discharge it at a constant current of 0.2C to 3.0V at a test temperature of 25 °C, and record the 0.2C discharge capacity at 25 °C; then stand for 5 min, repeat the above charging process, after full charge, stand for 60 min at -20 °C, and then discharge it at a constant current of 0.2C to 3.0V, and record the 0.2C discharge capacity at -20 °C. Low-temperature discharge performance = 0.2C discharge capacity at -20 °C / 0.2C discharge capacity at 25 °C × 100%.

[0087] Rate performance test:

[0088] Let the lithium-ion battery stand for 5 min at a test temperature of 25 °C, then charge the lithium-ion battery at a constant current of 0.7C to 4.53V, and then charge it at a constant voltage of 4.53V until the cut-off current of 0.05C; stand for 5 min, and then discharge it at a constant current of 0.2C to 3.0V at a test temperature of 25 °C, and record the 0.2C discharge capacity; then stand for 5 min, repeat the above charging process, after full charge, discharge it at a constant current of 2C at a test temperature of 25 °C, and record the 2C discharge capacity. Rate performance = 2C discharge capacity / 0.2C discharge capacity × 100%.

[0089] Example 1

[0090] <Preparation of positive electrode sheet>

[0091] Put the positive electrode active material LiCoO 2, the additive strontium titanate, the conductive agent Super P, and the binder polyvinylidene fluoride are mixed in a mass ratio of 97:0.01:1.5:1.49. N-methylpyrrolidone (NMP) is added as a solvent to formulate a slurry with a solid content of 75 wt%, and the positive electrode slurry is obtained after vacuum stirring evenly. The positive electrode slurry is evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 10 μm, and the coating weight of the positive electrode material layer is 70 mg / 1540 mm 2 . After drying at 120 °C and then cold pressing, a positive electrode plate with a single-sided coated positive electrode material layer with a thickness H of the positive electrode material layer of 10 μm is obtained. After cutting and welding the tab, a positive electrode plate with a specification of 74 mm × 867 mm is obtained for use. Among them, the average particle size D of the additive is 1.0 μm.

[0092] <Preparation of negative electrode plate>

[0093] The negative electrode active material artificial graphite, the binder styrene-butadiene rubber, the dispersant carboxymethyl cellulose, and the conductive agent acetylene black are mixed in a mass ratio of 97.4:1.4:0.6:0.6. Deionized water is added as a solvent to formulate a slurry with a solid content of 45 wt%, and the negative electrode slurry is obtained after vacuum stirring evenly. The negative electrode slurry is evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 6 μm, and after drying at 120 °C, a negative electrode plate with a single-sided coated negative electrode material layer is obtained, and the coating weight of the negative electrode material layer is 50 mg / 1540 mm 2 . After drying at 120 °C and then cold pressing, and then cutting and welding the tab, a negative electrode plate with a specification of 78 mm × 875 mm is obtained for use. Among them, the thickness of the single-sided negative electrode material layer is 8 μm.

[0094] <Preparation of electrolyte>

[0095] In an environment with a water content of less than 10 ppm, dimethyl carbonate, diethyl carbonate, and ethylene carbonate are mixed in a mass ratio of 1:1:1 to obtain an organic solvent, and then an electrolyte salt LiPF 6 is added to the organic solvent and mixed evenly to obtain an electrolyte. Among them, based on the mass of the electrolyte, the mass percentage content of the electrolyte salt is 12.5%, and the rest is the organic solvent.

[0096] <Preparation of separator>

[0097] A porous polyethylene film with a thickness of 7 μm (provided by Celgard) is used as the separator.

[0098] <Preparation of lithium-ion battery>

[0099] Stack the prepared positive electrode sheet, separator, and negative electrode sheet in sequence, with the separator in the middle of the positive electrode sheet and the negative electrode sheet to play an isolation role, and wind to obtain an electrode assembly. Place the electrode assembly in an aluminum-plastic film packaging bag, remove moisture at 80 °C, inject the electrolyte prepared above, and obtain a lithium-ion battery through processes such as vacuum packaging, standing, forming, degassing, and trimming.

[0100] Examples 2 to 19

[0101] Except for adjusting the relevant parameters according to Table 1, the rest are the same as in Example 1. Among them, when the mass percentage content of the additive in the positive electrode material layer changes, the mass percentage content of the positive electrode active material LiCoO 2 changes accordingly, and the mass percentage contents of the conductive agent Super and the binder polyvinylidene fluoride remain unchanged.

[0102] In Example 17, the mass ratio of the additives strontium titanate and barium titanate is 1:1; in Example 18, based on the mass of the additive strontium titanate, the mass percentage content of the doped element barium is 3%; in Example 19, based on the mass of the additive strontium titanate, the mass percentage content of the doped element magnesium is 3%.

[0103] Comparative Examples 1 to 4

[0104] Except for adjusting the relevant parameters according to Table 1, the rest are the same as in Example 1. Among them, when the mass percentage content of the additive in the positive electrode material layer changes, the mass percentage content of the positive electrode active material LiCoO 2 changes accordingly, and the mass percentage contents of the conductive agent Super and the binder polyvinylidene fluoride remain unchanged.

[0105] The preparation parameters and electrical performance parameters of each example and comparative example are shown in Table 1.

[0106]

[0107] Note: " / " in Table 1 indicates no relevant parameters.

[0108] It can be seen from Examples 1 to 19 and Comparative Examples 1 to 4 that the positive electrode material layer of the examples includes additives within the scope of this application, and the mass percentage content m% of the additives is within the scope of this application. Comparative Example 1 does not include additives, and the mass percentage content m% of the additives in Comparative Examples 2 and 3 is not within the scope of this application. The additives in Comparative Example 4 are not within the scope of this application. Compared with the comparative examples, the obtained positive electrode sheet of the examples has a suitable dielectric constant Ε1, and the dielectric constant Ε1 of the positive electrode sheet is within the scope of this application. The obtained lithium-ion battery has both good low-temperature discharge performance and high rate performance.

[0109] It can be seen from Examples 1 to 6, Comparative Example 2 and Comparative Example 3 that increasing the additive content can increase the dielectric constant E1 of the positive electrode sheet, thereby reducing the resistance of lithium ions to desolvate at low temperatures and improving the low-temperature discharge performance of lithium-ion batteries. However, if the additive content is continuously increased, the dielectric constant E1 of the positive electrode sheet will be too large, which will instead reduce the lithium-ion migration rate and affect the low-temperature discharge performance of lithium-ion batteries. The rate performance of lithium-ion batteries also decreases with the increase of the additive content.

[0110] The thickness H and m / H of the positive electrode material layer will affect the low-temperature discharge performance and rate performance of lithium-ion batteries. It can be seen from Example 3 and Examples 7 to 9 that when the thickness H and m / H of the positive electrode material layer are within the scope of this application, the lithium-ion battery has good low-temperature discharge performance and rate performance at the same time. When the thickness H of the positive electrode material layer is less than 10 μm, the lithium-ion battery exhibits good low-temperature discharge performance and rate performance, but due to the low thickness of the positive electrode material layer, its energy density is also low.

[0111] The average particle size D of the additive will affect the low-temperature discharge performance and rate performance of lithium-ion batteries. It can be seen from Example 3 and Examples 10 to 13 that when the average particle size D of the additive is within the scope of this application, the lithium-ion battery has good low-temperature discharge performance and rate performance at the same time.

[0112] When the type of additive changes, its dielectric constant E2 also changes accordingly. It can be seen from Example 3 and Examples 14 to 19 that when the type of additive is within the scope of this application, the dielectric constant E2 of the additive is within the scope of this application, the dielectric constant E1 of the obtained positive electrode sheet is also within the scope of this application, and the obtained lithium-ion battery has good low-temperature discharge performance and high rate performance at the same time.

[0113] The sheet resistance R of the positive electrode sheet changes with the mass percentage content m%, type of additive, average particle size D of the additive, thickness H of the positive electrode material layer and m / H. It can be seen from Examples 1 to 19 that when the above parameters are within the scope of this application, the sheet resistance R of the obtained positive electrode sheet is also within the scope of this application, and the obtained lithium-ion battery has good low-temperature discharge performance and high rate performance at the same time.

[0114] The above are only the preferred embodiments of this application and are not intended to limit this application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of this application shall be included within the scope of protection of this application.

Claims

1. A positive electrode sheet, comprising a positive electrode material layer, wherein the positive electrode material layer comprises a positive electrode active material and an additive, wherein the additive comprises at least one of barium titanate, magnesium metatitanate, and strontium titanate; The dielectric constant E1 of the positive electrode plate is 5 to 30.

2. The positive electrode sheet according to claim 1, wherein: Based on the mass of the positive electrode material layer, the mass percentage of the additive is m%, and 0.01≤m≤0.

1.

3. The positive electrode sheet according to claim 2, wherein: 0.03≤m≤0.07。 4. The positive electrode sheet according to claim 2, wherein: The thickness of the positive electrode material layer is H μm, 10≤H≤50, 0.001≤m / H≤0.

005.

5. The positive electrode sheet according to claim 1, wherein: The additives also include doping elements; The doping element includes at least one of niobium, zinc, tin, lanthanum, yttrium, barium and magnesium.

6. The positive electrode sheet according to any one of claims 1 to 5, wherein: The average particle size D of the additive is 0.1 μm to 3 μm.

7. The positive electrode sheet according to any one of claims 1 to 5, wherein: The dielectric constant E2 of the additive is 10 to 300.

8. The positive electrode sheet according to any one of claims 1 to 5, wherein: The membrane resistance of the positive electrode plate is R, 0.1Ω≤R≤1.0Ω.

9. The positive electrode sheet according to any one of claims 1 to 5, wherein: The dielectric constant E1 of the positive electrode plate is 7 to 20. 10 . A secondary battery comprising the positive electrode sheet according to claim 1 . 11 . An electronic device comprising the secondary battery according to claim 10 .

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