Secondary battery and electric device

Abstract

The invention relates to a secondary battery and an electric device, and belongs to the technical field of batteries. The secondary battery comprises a positive electrode plate, the positive electrode plate comprises a positive electrode current collector and a positive electrode material located on at least one surface of the positive electrode current collector, the positive electrode material comprises a positive electrode active material, and the positive electrode active material comprises lithium nickel manganese oxide particles. By controlling the charging constant current ratio b at 45 DEG C and 1C multiplying power and the sum c of the dissolution rates of nickel ions and manganese ions after the positive electrode material is stored in a hydrofluoric acid solution for 72 hours at 70 DEG C to meet the condition that b * c is more than or equal to 6 and less than or equal to 45, the balance of the positive electrode interface stability and the dynamic performance of the battery can be realized, and the battery is ensured to have excellent cycle performance and fast charge performance.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a secondary battery and an electrical device. Background Art

[0002] In the new energy sector, the application of lithium-ion power batteries is becoming increasingly widespread. The market has put forward higher requirements for their energy density and output power, which has prompted researchers to shift their research focus to the development of high-voltage cathode materials. Spinel-type lithium nickel manganese oxide (LNMO) materials stand out due to their unique crystal structure. This structure can provide three-dimensional lithium ion transmission channels, giving the material good ion conductivity and enabling it to have a 4.7V (vs Li / Li + ) high voltage platform, with a theoretical specific capacity of up to 147 mAh / g, showing great application potential in the field of high voltage positive electrode materials.

[0003] However, in high-voltage applications, LNMO materials present the following challenges: The positive electrode metal ions readily dissolve and deposit on the negative electrode surface, reducing the structural stability of the positive electrode and affecting the negative electrode, resulting in poor integrity of the negative electrode SEI film and consumption of active lithium, which severely impacts the battery's cycle performance and shortens its lifespan. Lithium nickel manganese oxide also has relatively poor electronic conductivity, which severely limits its fast-charging capabilities, making it difficult to charge the battery quickly.

[0004] Therefore, it is urgent to develop a technology that can balance the positive electrode interface stability and kinetic performance of LNMO batteries so that LNMO batteries can take into account both cycle performance and fast charging performance. Summary of the Invention

[0005] The purpose of the present application is to overcome the deficiencies of the above-mentioned prior art and provide a secondary battery and an electrical device, wherein the secondary battery has both excellent cycle performance and fast charging performance.

[0006] To achieve the above objectives, in a first aspect, the present application provides a secondary battery, comprising a positive electrode sheet, the positive electrode sheet comprising a positive electrode current collector and a positive electrode material located on at least one surface of the positive electrode current collector, the positive electrode material comprising a positive electrode active material, the positive electrode active material comprising lithium nickel manganese oxide particles; The secondary battery satisfies: 6≤b×c≤45; Wherein, b is the constant current ratio of charge at 45℃ and 1C rate, and the unit is %; c is the sum of the dissolution rates of nickel ions and manganese ions of the positive electrode material after being stored in a hydrofluoric acid solution at 70°C for 72 hours, in %.

[0007] In a second aspect, the present application provides an electrical device comprising the secondary battery.

[0008] Compared with the prior art, the beneficial effect of the present application is as follows: by controlling the charging constant current ratio (b) at 45°C and 1C rate and the sum of the nickel ion and manganese ion dissolution rates (c) of the positive electrode material after being stored in a hydrofluoric acid solution at 70°C for 72 hours to meet a specific relationship, the present application can achieve a balance between the positive electrode interface stability and kinetic performance of the LNMO battery, thereby ensuring that the LNMO battery has both excellent cycle performance and fast charging performance. DETAILED DESCRIPTION

[0009] To make the purpose, technical solutions, and advantages of the embodiments of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are part of the embodiments of this application, not all of them. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0010] In this application, the technical features described in an open manner include closed technical solutions composed of the listed features, and also include open technical solutions containing the listed features.

[0011] In this application, when referring to numerical ranges, unless otherwise specified, the numerical ranges are considered continuous and include the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, every integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges subsumed therein.

[0012] In the present application, there is no particular limitation on the specific dispersion and stirring treatment methods.

[0013] The reagents and instruments used in this application without manufacturer indication are all conventional products that can be purchased commercially.

[0014] In this application, “first time”, “second time” and other similar expressions are not used to limit the number of operations, processes or events. Their only purpose is to distinguish different operations, processes or events.

[0015] secondary batteries The present application provides a secondary battery, comprising a positive electrode sheet, the positive electrode sheet comprising a positive electrode current collector and a positive electrode material located on at least one surface of the positive electrode current collector, the positive electrode material comprising a positive electrode active material, the positive electrode active material comprising lithium nickel manganese oxide particles; The secondary battery satisfies: 6≤b×c≤45; Wherein, b is the constant current ratio of charge at 45℃ and 1C rate, and the unit is %; c is the sum of the dissolution rates of nickel ions and manganese ions of the positive electrode material after being stored in a hydrofluoric acid solution at 70°C for 72 hours, in %.

[0016] The charge constant current ratio (b) at 45°C and a 1C rate is calculated by charging and discharging the battery at 45°C at a 1C rate for two cycles and calculating the ratio of the charge capacity during the second constant current period to the total charge capacity. The 45°C test condition both prevents secondary battery performance degradation and accelerates its kinetics, making it easier to observe differences in the charge constant current ratios at a 1C rate for different secondary batteries. A higher value of the charge constant current ratio (b) at 45°C and a 1C rate indicates smoother lithium-ion intercalation and deintercalation, leading to better short-term fast-charging performance. However, this also results in greater stress on the electrode material during lithium-ion intercalation and deintercalation, resulting in a gradual decrease in the structural stability of the electrode material and reduced battery cycling performance. The value of the charge constant current ratio (b) at 45°C and a 1C rate can be adjusted by adjusting the electrolyte conductivity, the size of the lithium nickel manganese oxide particles, the structure of the lithium nickel manganese oxide particles, or the thickness of the surface coating (if present) on the lithium nickel manganese oxide particles.

[0017] This application does not limit the detection method of the charging constant current ratio (b) at 45°C and 1C rate. Those skilled in the art can detect the charging constant current ratio (b) at 45°C and 1C rate according to conventional technical means. For example, the charging constant current ratio (b) at 45°C and 1C rate can be detected by the following method: At 45°C, the empty battery is cycled for two cycles at a charge and discharge rate of 1C. The ratio of the charging capacity in the constant current section of the second cycle to the total charging capacity is calculated. The result is the charge constant current ratio (b) at 45°C and a rate of 1C. The voltage range is 3.5~4.8V. When charging, first charge at a constant current of 1C to the upper limit voltage of 4.8V, and then charge at a constant voltage until the current is less than or equal to 0.05C; when discharging, discharge at 1C to 3.5V.

[0018] The sum of the nickel and manganese ion dissolution rates (c) of the cathode material after storage in a hydrofluoric acid solution at 70°C for 72 hours is calculated by storing the cathode material in a hydrofluoric acid solution with a mass fraction of 30% HF at 70°C at a material-liquid ratio of 1g:5mL for 72 hours and then calculating the ratio of the nickel and manganese dissolved to the total mass (i.e., the total mass of nickel and manganese). At 70°C, the difference in the sum of the nickel and manganese ion dissolution rates of different cathode materials can be clearly observed within a short period of time. A lower value for the sum of the nickel and manganese ion dissolution rates (c) after storage in a hydrofluoric acid solution at 70°C for 72 hours indicates greater stability of the cathode interface. This value also reflects, to a certain extent, the degree of passivation of the cathode material. By controlling the nickel and manganese ion dissolution rate and adjusting the passivation level, the oxidizability of the cathode active material can be reduced, thereby protecting the cathode electrode, reducing manganese dissolution, and improving the diffusion of electrons and ions, thereby enhancing the battery's fast charging capability. The value of the sum of the dissolution rates of nickel ions and manganese ions (c) of the positive electrode material after being stored in a hydrofluoric acid solution at 70°C for 72 hours can be adjusted by adjusting the diameter of the lithium nickel manganese oxide particles, the thickness of the coating layer of the lithium nickel manganese oxide particles (if a coating layer is provided), etc.

[0019] Regarding the method for detecting the sum of the dissolution rates (c) of nickel ions and manganese ions of the positive electrode material after being stored in a hydrofluoric acid solution at 70°C for 72 hours, this application does not limit this. Those skilled in the art can detect the sum of the dissolution rates (c) of nickel ions and manganese ions of the positive electrode material after being stored in a hydrofluoric acid solution at 70°C for 72 hours using conventional technical means. For example, the sum of the dissolution rates (c) of nickel ions and manganese ions of the positive electrode material after being stored in a hydrofluoric acid solution at 70°C for 72 hours can be detected using the following method: Disassemble the empty battery to obtain the positive electrode sheet, soak the positive electrode sheet in DMC (dimethyl carbonate) at room temperature (25°C, the same below) for 60 minutes to remove the residual electrolyte and by-products on the surface of the electrode sheet, take it out, dry it at room temperature with a humidity of ≤15%, scrape the positive electrode material on the surface of the current collector and digest it to obtain a sample solution, and then use ICP (inductively coupled plasma) to determine the content of Ni and Mn in the sample solution. Based on this, the mass percentage of Ni and Mn in the positive electrode material is calculated. Three parallel samples are measured according to this method and the average mass percentage of Ni and Mn in the positive electrode material is calculated. The empty battery was disassembled according to the above method to obtain the positive electrode material. The obtained positive electrode material was added to an HF solution with a HF mass percentage content of 30% at a material-liquid ratio of 1 g:5 mL. The solution was stored at a constant temperature of 70°C for 72 hours and then centrifuged. The supernatant was taken and the Ni and Mn contents were determined by the ICP method. The dissolved mass percentages of Ni and Mn in the positive electrode material were calculated based on the above results. Three parallel samples were measured according to this method and the average M2 of the dissolved mass percentages of Ni and Mn in the positive electrode material was calculated. The sum of the dissolution rates of nickel ions and manganese ions (c) of the positive electrode material after storage in a hydrofluoric acid solution at 70°C for 72 hours was calculated according to the formula c=M2 / M1×100%.

[0020] The digestion method is as follows: the cathode material is dispersed in 20 mL of water, 10 mL of nitric acid (HNO3 mass percentage of 66%) is added, and the dispersion is heated until the cathode material is completely dissolved, and the volume is adjusted to 100 mL with water to obtain a test solution, and the test solution is subjected to ICP testing; The ICP instrument can be the ICAP7400 from Thermo Fisher Scientific, USA, with the radio frequency power set to 1150 W and the carrier gas flow rate to 0.5 L / min.

[0021] The charging constant current ratio (b) at 45°C and 1C rate and the sum of the nickel ion and manganese ion dissolution rates of the positive electrode material after being stored in a hydrofluoric acid solution at 70°C for 72 hours (c) affect the fast charging capability and cycle performance of lithium-ion secondary batteries to varying degrees. Moreover, the dissolution of nickel ions and manganese ions will cause the stability of the positive electrode structure to deteriorate, the positive electrode structure to collapse, and the internal impedance of the battery to increase, affecting the charging constant current ratio. At the same time, the improvement of the latter (i.e., the sum of the nickel ion and manganese ion dissolution rates (c) of the positive electrode material after being stored in a hydrofluoric acid solution at 70°C for 72 hours) will affect the fast charging performance to a certain extent. In order to improve the transition ion dissolution rate, a coating layer can be set, the particle diameter can be changed, etc., which will affect the transmission of lithium ions and affect the former (i.e., the charging constant current ratio (b) at 45°C and 1C rate). b and c have a mutual influence on each other, and it is difficult to achieve a battery with both excellent fast charging capability and cycle performance by controlling a single variable. This application controls the charging constant current ratio (b) at 45°C and 1C rate and the sum of the nickel ion and manganese ion dissolution rates (c) of the positive electrode material after being stored in a hydrofluoric acid solution at 70°C for 72 hours to meet the above-mentioned specific relationship, thereby achieving a balance between the positive electrode interface stability and kinetic performance of the LNMO battery, ensuring that the LNMO battery has both excellent cycle performance and fast charging performance.

[0022] In the present application, 6≤b×c≤45. The value of b×c can be selected from 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, or an interval formed by any two of the above values.

[0023] In one preferred embodiment, the secondary battery satisfies: 13≤b×c≤28. Controlling the value of b×c within this specific range helps to improve the balance between positive electrode interface stability and kinetic performance, thereby optimizing the coordination between the battery's fast charging capability and cycle performance.

[0024] In some embodiments, the range of b is 60%≤b≤93%, for example, b is 60%, 62%, 65%, 67%, 70%, 72%, 75%, 78%, 80%, 83%, 85%, 88%, 90%, 93%, or an interval formed by any two of the above values. In some embodiments, the range of b is 60%≤b≤90%. In a preferred embodiment, the range of b is 75%≤b≤85%.

[0025] When the charging constant current ratio (b) is in the range of 60%~90% at 45°C and 1C rate, especially in the range of 75%~85%, lithium ion deintercalation is smoother, and the stress on the electrode material during the lithium ion deintercalation process is lower. The electrode material has good stability, which is conducive to optimizing the coordinated relationship between the battery's fast charging capability and cycle performance.

[0026] In some embodiments, the range of c is 0.07%≤c≤0.65%, for example, c is 0.07%, 0.08%, 0.09%, 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 0.45%, 0.50%, 0.55%, 0.60%, 0.65%, or an interval formed by any two of the above values. In some embodiments, the range of c is 0.08%≤c≤0.60%. In a preferred embodiment, the range of c is 0.15%≤c≤0.35%.

[0027] When the sum of the dissolution rates of nickel ions and manganese ions (c) of the positive electrode material after storage in a hydrofluoric acid solution at 70°C for 72 hours is in the range of 0.08%~0.60%, especially in the range of 0.15%~0.35%, not only is the stability of the positive electrode interface better, but the degree of "passivation" of the positive electrode surface is also low, and the diffusion of electrons and ions is good, which is conducive to optimizing the coordinated relationship between the battery's fast charging capability and cycle performance.

[0028] In some embodiments, the diameter of the lithium nickel manganese oxide particles ranges from 3 to 8 μm. This application does not limit the method for detecting the diameter of the lithium nickel manganese oxide particles. Those skilled in the art can detect the diameter of the lithium nickel manganese oxide particles using conventional technical means. For example, the diameter of the lithium nickel manganese oxide particles can be detected using the following method: The sample was placed in a sample tube, anhydrous ethanol was added, and ultrasonication was performed at 25°C for 40 min to obtain a suspension; Use a pipette to drop the suspension onto a copper grid and place it in a vacuum drying oven to dry completely; The dried sample was placed on a test bench and transferred to a cavity. The particle diameter was measured using TEM. 100 particles were selected and the average particle diameter was calculated to obtain the diameter of the lithium nickel manganese oxide particles.

[0029] The lithium nickel manganese oxide particles are spinel-type, and the chemical formula is LiNi x Mn 2-x O4, wherein x>0, such as 0.1, 0.2, 0.3, 0.4, 0.5 or the range formed by any two of the above values. The lithium nickel manganese oxide particles may contain no doping elements or may contain doping elements. The present application does not limit the type of doping element in the lithium nickel manganese oxide, for example, it may be selected from at least one of Al, P, Mg, Zr, etc. The surface of the lithium nickel manganese oxide particles may not be provided with a coating layer, or may be provided with a coating layer.

[0030] In some embodiments, the lithium nickel manganese oxide particles are further provided with a coating layer, which may be disposed on part or all of the surface of the lithium nickel manganese oxide particles. The coating layer may be made of, but is not limited to, at least one of aluminum oxide, tungsten oxide, and zirconium oxide. This coating of the lithium nickel manganese oxide particles can suppress side reactions between the positive electrode and the electrolyte, thereby improving the battery's cycling performance.

[0031] In some embodiments, the ratio of the thickness of the coating layer to the diameter of the lithium nickel manganese oxide particles is in the range of 0.5 to 4.5 nm / μm, for example, the ratio is 0.5 nm / μm, 0.8 nm / μm, 1 nm / μm, 1.5 nm / μm, 2 nm / μm, 2.5 nm / μm, 3 nm / μm, 3.5 nm / μm, 4 nm / μm, 4.5 nm / μm, or an interval formed by any two of the foregoing values. In one embodiment, the ratio of the thickness of the coating layer to the diameter of the lithium nickel manganese oxide particles is in the range of 0.5 to 4.5 nm / μm.

[0032] When the ratio of the thickness of the coating layer to the diameter of the lithium nickel manganese oxide particles is controlled within the range of 0.5 to 4.5 nm / μm, the lithium nickel manganese oxide particles can have both high strength and dynamic properties, while the coating layer can better suppress the side reaction between the positive electrode sheet and the electrolyte and has moderate resistance to ion and electron transmission.

[0033] In some embodiments, the coating layer has a thickness ranging from 2 to 35 nm, for example, 2 nm, 4 nm, 6 nm, 8 nm, 10 nm, 12 nm, 14 nm, 16 nm, 18 nm, 20 nm, 22 nm, 24 nm, 26 nm, 28 nm, 30 nm, 32 nm, 35 nm, or a range formed by any two of the foregoing values. In one embodiment, the coating layer has a thickness ranging from 2 to 20 nm.

[0034] The present application does not limit the method for detecting the thickness of the coating layer and the ratio of the thickness of the coating layer to the diameter of the lithium nickel manganese oxide particles. Those skilled in the art can detect the thickness of the coating layer and the ratio of the thickness of the coating layer to the diameter of the lithium nickel manganese oxide particles using conventional technical means. For example, the thickness of the coating layer and the diameter of the lithium nickel manganese oxide particles can be detected using the following method: The sample was placed in a sample tube, anhydrous ethanol was added, and ultrasonication was performed at 25°C for 40 min to obtain a suspension; Use a pipette to drop the suspension onto a copper grid and place it in a vacuum drying oven to dry completely; The dried sample was placed on the test bench and transferred to the cavity. The thickness of the coating layer and the diameter of the core were measured by TEM. 50 particles were selected and the average thickness of the particle coating layer was calculated to obtain the thickness of the coating layer. At the same time, the average diameter of the core of these particles was calculated to obtain the diameter of the lithium nickel manganese oxide particles.

[0035] The present application does not limit the preparation method of the lithium nickel manganese oxide particles. Those skilled in the art can prepare the lithium nickel manganese oxide particles according to conventional technical means. For example, the lithium nickel manganese oxide particles can be prepared by a method comprising the following steps: The lithium source, nickel source and manganese source are mixed and dispersed, calcined and cooled to obtain lithium nickel manganese oxide particles. The calcination conditions can be selected as follows: calcination temperature of 600-900°C and calcination time of 6-48 hours.

[0036] Wherein, the lithium source includes but is not limited to at least one of lithium hydroxide, lithium carbonate, lithium oxalate, and lithium acetate; and / or Nickel sources include but are not limited to at least one of nickel hydroxide, nickel carbonate, and nickel nitrate; and / or The manganese source includes, but is not limited to, at least one of manganese dioxide, manganese hydroxide, manganese oxide, and manganese sulfate.

[0037] The lithium nickel manganese oxide particles obtained by the above preparation method can also be coated as needed. For example, the coating material can be coated on the surface of the lithium nickel manganese oxide particles using atomic layer deposition (ALD) technology, wherein the coating material can be selected from at least one of aluminum oxide, tungsten oxide, zirconium oxide, etc. The number of ALD deposition cycles can be selected from 20 to 350 cycles, and the thickness of one ALD deposition cycle can be selected from 0.1 to 0.3 nm.

[0038] When preparing lithium nickel manganese oxide particles, a certain amount of doping element source (if any) can be mixed and dispersed with the lithium source, nickel source and manganese source as needed to prepare lithium nickel manganese oxide particles. The doping element source can be at least one of Si, Mg, P, Co, Al, Cr, Nb, etc. to obtain lithium nickel manganese oxide particles containing a certain amount of doping elements.

[0039] In some embodiments, the compaction density of the positive electrode sheet is in the range of 2.5 to 3.5 g / cm 3 , for example, the compacted density is 2.5g / cm 3 , 2.6g / cm 3 , 2.7g / cm 3 , 2.8g / cm 3 , 2.9g / cm 3 , 3.0g / cm 3 , 3.1g / cm 3 、3.2g / cm 3 , 3.3g / cm 3 、3.4g / cm 3 , 3.5g / cm 3 Or the interval range formed by any two of the above values ​​is not only conducive to electrolyte infiltration, but also good contact between the positive electrode material particles, which helps to improve the lithium ion transmission efficiency, thereby improving the battery's fast charging performance.

[0040] This application does not limit the method for detecting the compaction density of the positive electrode sheet. Those skilled in the art can detect the compaction density of the positive electrode sheet using conventional technical means. For example, the compaction density of the positive electrode sheet can be detected using the following method: Disassemble the empty battery to obtain the positive electrode sheet. Soak the positive electrode sheet in DMC (dimethyl carbonate) at room temperature (25°C, the same below) for 60 minutes to remove the residual electrolyte and by-products on the surface of the electrode sheet. Take it out and dry it at room temperature with a humidity of ≤15%; Use a punching machine to punch the pretreated positive electrode sheet into discs of fixed area, and the area is recorded as S0. In order to ensure the accuracy of the test, select a disc from the middle and flat position of the electrode sheet, take three discs as parallel samples, and then use an electronic balance to weigh the mass of the three discs respectively, take the average value and record it as M1; use a micrometer to test the thickness of the three discs respectively, take the average value and record it as H, finally add an appropriate amount of deionized water to each of the three discs, gently wipe off the coating on the disc with dust-free paper to expose the copper foil, let it stand at room temperature (or dry) for 10 minutes, and after the copper foil is dry, weigh the mass of the three copper foils respectively, take the average value and record it as M0, and calculate the coating compaction density A according to the following formula: A=(M1-M0) / (H×S0).

[0041] In some embodiments, the mass percentage of the positive electrode active material in the positive electrode material is 93% to 98%, such as 93%, 94%, 95%, 96%, 97%, 98% or an interval formed by any two of the above values.

[0042] In addition to the above-mentioned positive electrode active material, the positive electrode material further includes a conductive agent and a binder.

[0043] The conductive agent in the positive electrode material is used to provide electrical conductivity. Any conductive agent can be used without particular limitation, as long as it has suitable electronic conductivity and does not significantly cause adverse chemical changes in the battery. Exemplary conductive agents in the positive electrode material include, but are not limited to, at least one of carbon nanotubes (CNTs), carbon black, graphite, carbon fibers, activated carbon, mesoporous carbon, and fullerenes, wherein carbon fibers include carbon nanofibers; and carbon blacks include SP (Super P), acetylene black, and Ketjen black.

[0044] In some embodiments, the mass percentage of the conductive agent in the positive electrode material is 0.05% to 4%, such as 0.05%, 1%, 2%, 3%, 4% or an interval formed by any two of the above values.

[0045] The binder in the positive electrode material is used to improve the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Any binder can be used without particular limitation, as long as it has suitable adhesion and does not significantly cause adverse chemical changes in the battery. Exemplary, the binder in the positive electrode material layer includes but is not limited to fluorinated polyolefin binders, and fluorinated polyolefin binders include but are not limited to polyvinylidene fluoride (PVDF), vinylidene fluoride copolymers or their modified (for example, carboxylic acid, acrylic acid, acrylonitrile, etc.) derivatives.

[0046] In some embodiments, the mass percentage of the binder in the positive electrode material is 0.5% to 3%, such as 0.5%, 1%, 2%, 3% or an interval formed by any two of the above values.

[0047] The positive electrode material can be located on one side of the positive electrode current collector or on both sides of the positive electrode current collector.

[0048] This application places no particular restrictions on the positive electrode current collector, as long as it has conductivity and does not cause adverse chemical changes in the battery, and for example, aluminum, nickel, titanium, stainless steel, fired carbon; or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. can be used.

[0049] The positive electrode sheet of this application can be prepared according to conventional methods in the art. For example, the positive electrode active material, conductive agent, and binder are dispersed in a solvent to obtain a positive electrode slurry, and then the positive electrode slurry is coated on at least one side of the positive electrode current collector, and after processes such as drying, cold pressing, and slitting, a positive electrode sheet is obtained. Among them, the solvents used to prepare the positive electrode slurry include but are not limited to at least one of N-methylpyrrolidone (NMP) and deionized water.

[0050] In some of these embodiments, the secondary battery further includes a negative electrode sheet.

[0051] The negative electrode sheet of this application includes a negative electrode current collector and a negative electrode material located on at least one surface of the negative electrode current collector, and the negative electrode material contains a negative electrode active material.

[0052] This application places no particular restrictions on the negative electrode active material. Exemplarily, the negative electrode active material includes but is not limited to natural graphite, artificial graphite, hard carbon, soft carbon, mesocarbon microbeads (MCMB), silicon, SiO x (0 < x < 2, such as x = 1), silicon carbide, Li4Ti5O 12 and at least one of them.

[0053] In some of these embodiments, the mass percentage content of the negative electrode active material in the negative electrode material is 94% - 98%, such as 94%, 95%, 96.4%, 97%, 98% or the range formed by any two of the above values.

[0054] The negative electrode material may further contain a conductive agent and / or a binder.

[0055] The conductive agent in the negative electrode material is used to provide conductivity, and any conductive agent can be used without particular restrictions, as long as it has appropriate electron conductivity and does not significantly cause adverse chemical changes in the battery. Exemplarily, the conductive agent in the negative electrode material includes but is not limited to at least one of carbon nanotubes, carbon black, graphite, carbon fiber, activated carbon, mesoporous carbon, fullerenes, etc. Among them, carbon fiber such as carbon nanofiber, etc.; carbon black such as SP, acetylene black, Ketjen black, etc.

[0056] In some embodiments, the mass percentage of the conductive agent in the negative electrode material is 0.5% to 2.5%, such as 0.5%, 1%, 1.5%, 2%, 2.5% or an interval formed by any two of the above values.

[0057] The binder in the negative electrode material is used to improve the adhesion between the negative electrode active material particles and the adhesion between the negative electrode active material and the negative electrode current collector. Any binder can be used without particular limitation, as long as it has suitable adhesion and does not significantly cause adverse chemical changes in the battery. Exemplary binders in the negative electrode material include, but are not limited to, at least one of styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), polyacrylic acid (PAA), polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl butyral, and a water-based acrylic resin.

[0058] In some embodiments, the mass percentage of the binder in the negative electrode material is 1% to 4%, such as 1%, 1.4%, 2%, 3%, 4% or an interval formed by any two of the above values.

[0059] The negative electrode material may further include a thickener as needed. The thickener in the negative electrode material is used to prevent agglomeration and sedimentation of components such as the negative electrode active material and the conductive agent in the slurry, thereby maintaining good dispersion of the components in the slurry. Any thickener can be used without particular limitation, as long as it has suitable thickening properties and does not significantly cause adverse chemical changes in the battery. Exemplary thickeners in the negative electrode material include, but are not limited to, at least one of carboxymethyl cellulose (CMC) and styrene-butadiene rubber.

[0060] In some embodiments, the mass percentage of the thickener in the negative electrode material is 0.5% to 1.5%, such as 0.5%, 0.8%, 1%, 1.2%, 1.5% or an interval formed by any two of the above values.

[0061] The negative electrode material may be located on one side of the negative electrode current collector or on both sides of the negative electrode current collector.

[0062] The present application has no particular limitation on the negative electrode current collector, as long as it has conductivity and does not cause adverse chemical changes in the battery, and can use, for example: copper, stainless steel, aluminum, nickel, titanium, fired carbon, copper or stainless steel surface-treated with at least one of carbon, nickel, titanium, silver, etc., or aluminum-cadmium alloy.

[0063] In some embodiments, the secondary battery further includes an electrolyte.

[0064] The electrolyte of the present application can be selected from various electrolytes suitable for batteries in the art.

[0065] In some embodiments, the ionic conductivity of the electrolyte is in the range of 2 to 6 mS / cm, for example, the ionic conductivity is 2 mS / cm, 2.2 mS / cm, 2.4 mS / cm, 2.6 mS / cm, 2.8 mS / cm, 3 mS / cm, 3.2 mS / cm, 3.4 mS / cm, 3.6 mS / cm, 3.8 mS / cm, 4 mS / cm, 4.2 mS / cm, 4.4 mS / cm, 4.6 mS / cm, 4.8 mS / cm, 5 mS / cm, 5.2 mS / cm, 5.4 mS / cm, 5.6 mS / cm, 5.8 mS / cm, 6 mS / cm, or an interval formed by any two of the above values. In one embodiment, the ionic conductivity of the electrolyte is in the range of 3 to 5 mS / cm. Controlling the ionic conductivity of the electrolyte to a range of 2 to 6 mS / cm, especially 3 to 5 mS / cm, can not only enhance the lithium ion transport capacity of the electrolyte itself, but also make the viscosity of the electrolyte appropriate, thereby reducing the viscosity's hindrance to lithium ion diffusion. At the same time, it can also reduce the oxidation resistance of the electrolyte and reduce its side reactions with the positive electrode interface, thereby improving the fast charging capability and cycle performance of the battery.

[0066] This application does not limit the method for detecting the ionic conductivity of the electrolyte. Those skilled in the art can detect the ionic conductivity of the electrolyte using conventional techniques. For example, the ionic conductivity of the electrolyte can be tested according to the national standard GB / T 11007-2008.

[0067] The electrolyte solution includes an electrolyte and a solvent. The electrolyte may generally include a lithium salt.

[0068] Exemplarily, the lithium salt includes, but is not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobis(oxalatophosphate) (LiDFOP), and lithium tetrafluorooxalatophosphate (LiTFOP). The concentration of the electrolyte in the electrolyte can be selected from 0.5 to 2 mol / L, such as 0.5 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L, or an interval formed by any two of the above values.

[0069] Exemplary, the solvent includes but is not limited to at least one of ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), propylene carbonate (PC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE). The mass percentage of the solvent in the electrolyte can be selected to be 70% to 98%, such as 70%, 75%, 80%, 85%, 90%, 95%, 98%, or the interval range formed by any two of the above values.

[0070] In some embodiments, the electrolyte includes a solvent, and the solvent comprises an ester solvent. The addition of an ester solvent can improve oxidation resistance while maintaining high ionic conductivity. In a preferred embodiment, the ester solvent is selected from at least one of ethylene carbonate (EC), dimethyl carbonate (DMC), and fluoroethylene carbonate (FEC).

[0071] Additives may be added to the electrolyte as needed. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve high-temperature battery performance, additives that improve overcharge performance, and additives that improve low-temperature battery performance.

[0072] In some embodiments, the secondary battery further includes a separator.

[0073] The separator is located between the positive and negative electrodes, separating them and preventing short circuits. The separator can be made of any of a variety of battery-compatible materials. Exemplarily, the separator includes, but is not limited to, at least one of polypropylene and polyethylene.

[0074] Electrical devices The present application further provides an electrical device comprising the secondary battery, wherein the secondary battery serves as a power supply for the electrical device.

[0075] The term "electrical device" refers to any device that can utilize electrical energy and convert it into one or more other forms of energy, such as mechanical energy, thermal energy, or light energy, and includes, for example, electric motors, electric heat generators, and electric light sources. Specifically, these devices include, but are not limited to, mobile devices, electric vehicles, electric trains, ships, satellites, and energy storage systems. Mobile devices include mobile phones, laptops, drones, robot vacuums, and electronic cigarettes. Electric vehicles include pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, and electric trucks.

[0076] The present application is further described below with reference to specific examples. It should be noted that, unless otherwise specified, the sintering is carried out in an air atmosphere. Example 1 This embodiment provides a secondary battery, and the specific preparation method is as follows: (1) Preparation of positive electrode Li2CO3, NiCO3 and MnO2 are mixed according to the chemical formula LiNi 0.5 Mn 1.5 The stoichiometric ratio of Li, Ni, and Mn in O4 was added to a ball mill and mixed and dispersed to obtain a mixture. The mixture was added to a muffle furnace and heated to 850°C (i.e., calcination temperature) for 12 hours. It was cooled to room temperature to obtain a bare lithium nickel manganese oxide sample. An aluminum oxide coating layer was then deposited on the surface of the bare lithium nickel manganese oxide sample using an ALD instrument. The deposition was repeated 100 times, with a deposition thickness of 0.1 nm per turn, to obtain an aluminum oxide-coated LiNi 0.5 Mn 1.5 O4; The obtained alumina-coated LiNi 0.5 Mn 1.5 O4 was used as the positive electrode active material, and the positive electrode active material, the conductive agent CNTs, and the binder PVDF were mixed in a mass ratio of 97:1:2, and the solvent NMP was added, and the mixture was stirred and dispersed under the action of a vacuum mixer to obtain a positive electrode slurry; The positive electrode slurry was coated on both surfaces of the positive electrode current collector aluminum foil, dried at room temperature and then transferred to an oven for further drying. After that, the compacted density was 2.8 g / cm3 by cold pressing and slitting. 3 The positive electrode.

[0077] (2) Preparation of isolation membrane A polyethylene (PE) separator with a thickness of 15 μm is used.

[0078] (3) Preparation of negative electrode sheet The negative electrode active material graphite, the conductive agent acetylene black, the thickener CMC, and the binder SBR were mixed in a mass ratio of 96.4:1:1.2:1.4, deionized water was added as a solvent, and the mixture was stirred and dispersed in a vacuum mixer to obtain a negative electrode slurry; The negative electrode slurry is coated on both surfaces of the negative electrode current collector copper foil, dried at room temperature, and then transferred to an oven for further drying. The negative electrode sheet is then obtained by cold pressing and slitting.

[0079] (4) Preparation of electrolyte Ethylene carbonate (EC), dimethyl carbonate (DMC) and fluoroethylene carbonate (FEC) are mixed in a volume ratio of 5:3:2 to obtain a mixed organic solvent, and then dry lithium salt LiPF6 is dissolved in the above mixed organic solvent to prepare an electrolyte with a LiPF6 concentration of 1 mol / L.

[0080] (5) Preparation of secondary batteries The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator placed between the positive and negative electrode sheets to serve as an isolation. A copper wire is then inserted as a reference electrode, and the cells are wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried and then injected with electrolyte, vacuum-sealed and allowed to stand for 24 hours, and then subjected to formation to obtain a secondary battery. The specific formation steps are as follows: the battery is first charged to 4.5V at a constant current rate of 0.05C using a LAND system, then charged to 4.8V at a constant current and constant voltage rate of 0.33C, and then discharged to 3.5V at a constant current rate of 0.33C. Then, the above charge and discharge procedure is cycled for another cycle (i.e., a total of 2 cycles).

[0081] Examples 2 to 22 and Comparative Examples 1 to 2 These examples and comparative examples all provide a secondary battery, and the preparation method is similar to that of Example 1, except that: (a) In step (1), the calcination temperature, the type and number of ALD deposition materials, and the compaction density of the positive electrode sheet are shown in Table 1; (b) In step (4), the volume ratios of EC, DMC, and FEC are shown in Table 1.

[0082] Table 1 The following method was used to detect the coated LiNi in each embodiment and comparative example. 0.5 Mn 1.5 The diameter of the lithium nickel manganese oxide particles (i.e., the core diameter) and the coating thickness of O4 (i.e., the positive electrode active material), the compaction density of the positive electrode sheet, the ionic conductivity of the electrolyte, the charging constant current ratio of the secondary battery at 45°C and 1C rate (b), and the sum of the nickel ion and manganese ion dissolution rates of the positive electrode material after storage in a hydrofluoric acid solution at 70°C for 72 hours (c), and the cycle performance and fast charging performance of the secondary battery were tested. The specific test methods are as follows: (1) Coated LiNi 0.5 Mn 1.5Detection of lithium nickel manganese oxide particle diameter and coating thickness of O4 The sample was placed in a sample tube, anhydrous ethanol was added, and ultrasonication was performed at 25°C for 40 min to obtain a suspension; Use a pipette to drop the suspension onto a copper grid and place it in a vacuum drying oven to dry completely; The dried sample was placed on the test bench and transferred to the cavity. The thickness of the coating layer and the diameter of the core were measured by TEM. 50 particles were selected and the average thickness of the particle coating layer was calculated to obtain the thickness of the coating layer. At the same time, the average diameter of the core of these particles was calculated to obtain the diameter of the lithium nickel manganese oxide particles.

[0083] (2) Compaction density test of positive electrode Disassemble the empty battery to obtain the positive electrode sheet. Soak the positive electrode sheet in DMC (dimethyl carbonate) at room temperature (25°C) for 60 minutes to remove the residual electrolyte and by-products on the surface of the electrode sheet. Take it out and dry it at room temperature with a humidity of ≤15%; Use a punching machine to punch the pretreated positive electrode sheet into discs of fixed area, and the area is recorded as S0. In order to ensure the accuracy of the test, select a disc from the middle and flat position of the electrode sheet, take three discs as parallel samples, and then use an electronic balance to weigh the mass of the three discs respectively, take the average value and record it as M1; use a micrometer to test the thickness of the three discs respectively, take the average value and record it as H, finally add an appropriate amount of deionized water to each of the three discs, gently wipe off the coating on the disc with dust-free paper to expose the copper foil, let it stand at room temperature (or dry) for 10 minutes, and after the copper foil is dry, weigh the mass of the three copper foils respectively, take the average value and record it as M0, and calculate the coating compaction density A according to the following formula: A=(M1-M0) / (H×S0).

[0084] (3) Detection of ionic conductivity of electrolyte Tested in accordance with national standard GB / T 11007-2008.

[0085] (4) Charging constant current ratio (b) detection at 45°C and 1C rate At 45°C, the battery is cycled for two cycles at a charge and discharge rate of 1C in an empty state. The ratio of the charging capacity in the constant current section in the second cycle to the total charging capacity is calculated. The result is the charging constant current ratio (b) at 45°C and a rate of 1C. The voltage range is 3.5~4.8V. When charging, first charge at a constant current of 1C to the upper limit voltage of 4.8V, and then charge at a constant voltage until the current is less than or equal to 0.05C; when discharging, discharge at 1C to 3.5V.

[0086] (5) Detection of the sum of the dissolution rates of nickel ions and manganese ions (c) of the cathode material after storage in hydrofluoric acid solution at 70°C for 72 hours Disassemble the empty battery to obtain the positive electrode sheet, soak the positive electrode sheet in DMC (dimethyl carbonate) at room temperature (25°C, the same below) for 60 minutes to remove the residual electrolyte and by-products on the surface of the electrode sheet, take it out, dry it at room temperature with a humidity of ≤15%, scrape the positive electrode material on the surface of the current collector and digest it to obtain a sample solution, and then use ICP (inductively coupled plasma) to determine the content of Ni and Mn in the sample solution. Based on this, the mass percentage of Ni and Mn in the positive electrode material is calculated. Three parallel samples are measured according to this method and the average mass percentage of Ni and Mn in the positive electrode material is calculated. The empty battery was disassembled according to the above method to obtain the positive electrode material. The obtained positive electrode material was added to an HF solution with a HF mass percentage content of 30% at a material-liquid ratio of 1 g:5 mL. The solution was stored at a constant temperature of 70°C for 72 hours and then centrifuged. The supernatant was taken and the Ni and Mn contents were determined by the ICP method. The dissolved mass percentages of Ni and Mn in the positive electrode material were calculated based on the above results. Three parallel samples were measured according to this method and the average M2 of the dissolved mass percentages of Ni and Mn in the positive electrode material was calculated. The sum of the nickel ion and manganese ion dissolution rates (c) of the cathode material after storage in a hydrofluoric acid solution at 70°C for 72 hours was calculated according to the formula c=M2 / M1×100%. The digestion method is as follows: the positive electrode material is dispersed in 20 mL of water, and then 10 mL of nitric acid (HNO3 mass percentage of 66%) is added, and the dispersion is heated until the positive electrode material is completely dissolved, and the volume is adjusted to 100 mL with water to obtain a test solution, and the test solution is subjected to ICP testing; The ICP instrument can be the ICAP7400 from Thermo Fisher Scientific, USA, with the radio frequency power set to 1150 W and the carrier gas flow rate to 0.5 L / min.

[0087] (6) 45℃ cycle test The LAND system was used to test the cycling performance of secondary batteries. The batteries were cycled at 45°C at a 1C / 1C charge / discharge rate for 100 cycles. One cycle consisted of charging at a constant current of 1C to 4.8V with a cutoff current of less than or equal to 0.05C, followed by discharging at 1C to 3.5V. After the cycling was complete, the cycling data was processed to determine the capacity retention (capacity retention after n cycles = discharge capacity at the nth cycle / discharge capacity at the first cycle × 100%). This was then used to determine the capacity retention after 100 cycles.

[0088] (7) 10%-80% SOC fast charging time test at room temperature (i.e. 25°C) At 25°C, the LAND system is used to cycle the battery for two cycles at a constant capacity of 3.5V to 4.8V at a rate of 0.33C. The constant capacity of the last cycle is set to C1. First, it is charged to 10% SOC at a constant current rate of 0.33C1, and then charged to 4.80V at a constant current rate of 4C1, 3C1, 2.5C1, 2C1, 1.2C1, and 0.8C1. After each constant current charge at a different rate, constant voltage charging is required until the cutoff current is ≤0.05C or the negative reference electrode potential is lower than 0 mV. Finally, it is charged to 4.80V at a constant current and constant voltage rate of 0.33C1, with a cutoff current of 0.05C. After the test, the time used to charge the battery from 10% to 80% SOC is calculated, which is the fast charging time. (Starting from 4C1, the charging capacity at each rate is accumulated. If the cumulative capacity is ≥80% SOC, the time consumed in these processes is accumulated).

[0089] The above test results are shown in Table 2.

[0090] Table 2 As can be seen from Table 2, the batteries prepared in each embodiment of the present application have a capacity retention rate of ≥82% after 100 cycles at 45°C, and a fast charge time of ≤33.2 min at room temperature from 10% to 80% SOC. It can be seen that the batteries of the present application have both excellent cycle performance and fast charge performance.

[0091] It can be seen from the comparison of Examples 1, 4 to 5 with Examples 2 to 3 and 12, and from the comparison of Examples 6 to 7, 10 to 11, and 13 to 15 with Examples 8 to 9 that when the charging constant current ratio (b) at 45°C and 1C rate and the sum of the nickel ion and manganese ion dissolution rates of the positive electrode material after storage in a hydrofluoric acid solution at 70°C for 72 hours (c) meet the preferred ranges described in this application, the balance between the cycle performance and the fast charging performance is better.

[0092] From the comparison between Examples 1, 4-5 and Examples 8-9, and between Examples 2-3 and 12 and Examples 6-7, 10-11, and 13-15, it can be seen that when the secondary battery satisfies 13≤b×c≤28, the balance between the cycle performance and the fast charging performance is better.

[0093] According to Comparative Examples 1 and 2, even though the charging constant current ratio (b) at 45°C and 1C rate and the sum of the dissolution rates of nickel ions and manganese ions (c) of the positive electrode material after storage in a hydrofluoric acid solution at 70°C for 72 hours are respectively in the appropriate range, when the value of b×c exceeds the range of 6 to 45, the cycle performance and fast charging performance of the battery are relatively poor.

[0094] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this article and are not intended to limit the scope of protection of this article. Although this application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of this article may be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of this article.

Claims

1. A secondary battery, characterized in that: The positive electrode sheet includes a positive electrode current collector and a positive electrode material located on at least one surface of the positive electrode current collector, wherein the positive electrode material includes a positive electrode active material, and the positive electrode active material includes lithium nickel manganese oxide particles; The secondary battery satisfies: 6≤b×c≤45; Wherein, b is the constant current ratio of charge at 45℃ and 1C rate, and the unit is %; c is the sum of the dissolution rates of nickel ions and manganese ions of the positive electrode material after being stored in a hydrofluoric acid solution at 70°C for 72 hours, in %.

2. The secondary battery according to claim 1, wherein The secondary battery satisfies: 13≤b×c≤28.

3. The secondary battery according to claim 1 or 2, wherein: The range of b is 60%≤b≤90%.

4. The secondary battery according to claim 1 or 2, wherein: The range of c is 0.08%≤c≤0.60%.

5. The secondary battery according to claim 1 or 2, wherein: The surface of the lithium nickel manganese oxide particles is further provided with a coating layer, and the material of the coating layer is selected from at least one of aluminum oxide, tungsten oxide, and zirconium oxide.

6. The secondary battery according to claim 5, wherein The ratio of the thickness of the coating layer to the diameter of the lithium nickel manganese oxide particles is in the range of 0.5 to 4.5 nm / μm; and / or The thickness of the coating layer ranges from 2 to 20 nm.

7. The secondary battery according to claim 1 or 2, wherein: The diameter of the lithium nickel manganese oxide particles ranges from 3 to 8 μm.

8. The secondary battery according to claim 1 or 2, wherein: The compaction density of the positive electrode sheet is in the range of 2.5-3.5 g / cm 3 .

9. The secondary battery according to claim 1 or 2, wherein: The secondary battery further includes an electrolyte, and the ion conductivity of the electrolyte is in the range of 3-5 mS / cm.

10. The secondary battery according to claim 1 or 2, wherein The secondary battery further includes an electrolyte solution containing a solvent, and the solvent contains an ester solvent.

11. The secondary battery according to claim 10, wherein The ester solvent is selected from at least one of ethylene carbonate, dimethyl carbonate and fluoroethylene carbonate.

12. An electrical device, characterized in that: The invention comprises the secondary battery according to any one of claims 1 to 11.

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