A positive electrode sheet, a secondary battery

Abstract

The application discloses a positive pole piece and a secondary battery, and belongs to the technical field of batteries. The positive pole piece is synergistically controlled in terms of the exothermic characteristic parameter of a ternary material in the pole piece and the tortuosity of the pole piece, so that the safety and stability of the whole positive pole piece can be effectively improved, the efficiency of the ternary material in the pole piece in the lithium ion transmission process can be improved, and good kinetic performance is ensured.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a positive electrode sheet and a secondary battery. Background Technology

[0002] Ternary cathode materials in lithium-ion secondary batteries possess high energy density, but improving safety, especially safety and stability under special conditions, remains a key area for improvement. However, some safety enhancements for ternary cathode materials can reduce lithium-ion transport efficiency, decrease discharge capacity, and degrade kinetic performance, failing to achieve the desired overall product performance. Summary of the Invention

[0003] The purpose of this application is to overcome the shortcomings of the existing technology and provide a positive electrode sheet. By synergistically controlling the exothermic characteristic parameters of the ternary material in the positive electrode sheet and the tortuosity of the electrode sheet, the overall safety and stability of the positive electrode sheet can be effectively improved, while the efficiency of the ternary material in the electrode sheet in the lithium-ion transport process is improved, ensuring better kinetic performance.

[0004] To achieve the above objectives, in a first aspect of this application, this application provides a positive electrode sheet, the positive electrode sheet comprising a positive electrode material layer, the positive electrode material layer comprising a positive electrode active material, the positive electrode active material comprising lithium nickel cobalt manganese oxide particles;

[0005] The positive electrode plate satisfies: a / b = 0.5~8;

[0006] The a=S / T, where SJ / g is the total peak area of ​​the exothermic characteristic peak of the positive electrode sheet in the range of 190~290℃ during DSC (differential scanning calorimetry) testing, and T℃ is the temperature corresponding to the highest peak value of the exothermic characteristic peak.

[0007] b represents the tortuosity of the positive electrode sheet.

[0008] The beneficial effects of this application are as follows:

[0009] This application provides a positive electrode sheet. By synergistically controlling the exothermic characteristic parameters of the ternary material in the positive electrode sheet and the tortuosity of the electrode sheet, the overall safety and stability of the positive electrode sheet can be effectively improved, while the efficiency of the ternary material in the electrode sheet in the lithium-ion transport process is improved, ensuring better kinetic performance. Attached Figure Description

[0010] Figure 1 This is a schematic diagram of the exothermic characteristic peak of the positive electrode sheet described in Embodiment 5 of this application during DSC testing. Detailed Implementation

[0011] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0012] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0013] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0014] The present application is further illustrated below with specific embodiments:

[0015] A positive electrode sheet, the positive electrode sheet comprising a positive electrode material layer, the positive electrode material layer comprising a positive electrode active material, the positive electrode active material comprising lithium nickel cobalt manganese oxide particles;

[0016] The positive electrode plate satisfies: a / b = 0.5~8;

[0017] The value of a = S / T is given by SJ / g, where SJ / g is the total peak area of ​​the exothermic characteristic peaks of the positive electrode sheet in the range of 190~290℃ during DSC testing, and T℃ is the temperature corresponding to the highest peak value of the exothermic characteristic peaks.

[0018] b represents the tortuosity of the positive electrode sheet.

[0019] When lithium nickel cobalt manganese oxide ternary materials are used as cathode materials, they have high energy density, but there is still considerable room for improvement in terms of safety. In particular, after being prepared into secondary batteries, the critical thermal runaway temperature of secondary batteries is relatively low when performing ARC (adiabatic thermal instrument) thermal runaway tests, which limits their use in some special working environments. Therefore, in this application, after using lithium nickel cobalt manganese oxide particles as the positive electrode active material, the overall exothermic range characteristics of the positive electrode are controlled, that is, the ratio of the total peak area of ​​the exothermic characteristic peaks to the temperature corresponding to the highest peak of the characteristic peaks. This improves the thermal stability of the positive electrode after it is made into a secondary battery, and the thermal runaway critical temperature can be significantly improved. On the other hand, when controlling the thermodynamic stability of the positive electrode, its kinetic performance during the charge and discharge process also needs to be considered. Therefore, the tortuosity of the positive electrode needs to be controlled in a coordinated manner, so that the lithium nickel cobalt manganese oxide particles can achieve a better electrochemical environment distribution on the positive electrode, with high electrochemical reaction uniformity and lithium-ion transport efficiency maintained at a high level. When applied to secondary batteries, it can achieve excellent effects in both safety and kinetic performance.

[0020] In some implementations, a / b = 0.5~8.

[0021] More preferably, the a / b value is a range of one or any two of the following: 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8.

[0022] More preferably, a / b = 3~6.

[0023] The exothermic characteristic parameter S / T of the positive electrode sheet represents the interaction between the positive electrode material and the electrolyte when applied to a secondary battery. During this process, as the internal temperature of the battery increases, the electrolyte decomposes, and the lithium nickel cobalt manganese oxide particles undergo phase transition decomposition, increasing the internal heat generation of the battery. By controlling the ratio of the exothermic peak area to the exothermic peak temperature, the internal heat generation of the battery when the positive electrode sheet is applied to the battery can be effectively regulated, resulting in a higher critical thermal runaway temperature and ensuring sufficient thermal stability and safety. At the same time, the control of thermal stability needs to take into account the kinetic performance of the lithium nickel cobalt manganese oxide particles in the positive electrode sheet during charge and discharge. Therefore, the tortuosity of the electrode sheet needs to be controlled simultaneously. When the a / b ratio of the positive electrode sheet is preferably within the above range, not only is the overall thermal stability of the positive electrode sheet higher, but also the electrolyte wetting degree of the electrode sheet is higher after it is assembled into a secondary battery, and the structural stability is higher. The stress dispersion uniformity during charge and discharge is high, which can achieve rapid lithium-ion intercalation and deintercalation while maintaining good electrochemical stability, better kinetic performance, and lower DCR.

[0024] In some implementations, a = 1.5~12.

[0025] More preferably, a is a range of one or any two of the following: 1.5, 2, 2.4, 2.45, 2.5, 3, 3.5, 3.75, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 7.8, 8, 9, 10, 11, 12.

[0026] More preferably, a = 3~9.

[0027] In this application, a = S / T, where S represents the area of ​​the exothermic peak, reflecting the intensity of the exothermic reaction, T represents the temperature corresponding to the highest peak of the exothermic peak, and S / T represents the exothermic intensity ratio of the positive electrode. When the characteristic peak area is small and the peak temperature is large, the thermal stability of the positive electrode is good. However, at this time, the passivation degree of the positive electrode surface will also increase, resulting in a decrease in the lithium-ion transport rate and a decline in kinetic performance. When the ratio of the characteristic peak area to the peak temperature is preferably within the above range, the positive electrode can achieve better thermodynamic stability. At this time, the interaction reaction between the positive electrode material layer interface and the electrolyte is reduced, the internal heat generation is reduced, and the thermal stability of the secondary battery is improved, thus the safety is better. At the same time, the positive electrode can also achieve good kinetic performance and low DCR during charging and discharging.

[0028] In some embodiments, S = 400~2600 J / g.

[0029] More preferably, S is a value within the range of one or any two of the following: 400 J / g, 700 J / g, 800 J / g, 900 J / g, 1000 J / g, 1200 J / g, 1400 J / g, 1500 J / g, 1800 J / g, 2000 J / g, 2200 J / g, 2400 J / g, and 2600 J / g.

[0030] More preferably, S = 900~2200 J / g.

[0031] When the total peak area of ​​the exothermic characteristic peak between 190 and 290°C during DSC testing of the positive electrode is preferably within the above range, it can not only ensure that the internal heat generation is low and the thermal stability is good when it is applied to secondary batteries, but also ensure that the positive electrode will not be over-passivated, and the lithium-ion transport kinetics performance is at a high level, resulting in better overall performance.

[0032] In some implementations, T = 190~290℃.

[0033] More preferably, T is a range of one or any two of the following: 190℃, 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, 260℃, 270℃, 280℃, and 290℃.

[0034] More preferably, the temperature T is 220~260℃.

[0035] Similar to S, when the peak temperature of the exothermic characteristic of the positive electrode during DSC testing is preferably within the above range, it can not only ensure its thermal stability when applied to secondary batteries, but also take into account better kinetic performance, high lithium-ion transport efficiency, and lower DCR.

[0036] It should be noted that, in the technical solution of this application, the 'a' can be tested by, but is not limited to, the following method: the secondary battery is discharged to 0% SOC at 0.33C, then charged to 4.25V at 0.33C, with a cutoff current less than 0.05C. The secondary battery is disassembled in a glove box, and the obtained positive electrode sheet is soaked in DMC for 2 hours and then dried. The positive electrode material layer on the current collector is scraped and placed into the test chamber of a DSC (Differential Scanning Calorimeter) testing instrument (model DSC214), and then electrolyte is added. The electrolyte dosage was matched with the powder addition, with 1 mg of powder corresponding to 1 μL of electrolyte. EC (ethylene carbonate), EMC (ethyl methyl carbonate), and DEC (diethyl carbonate) were mixed in a mass ratio of 1:1:1 as a solvent, and then lithium hexafluorophosphate was added to prepare a 1 mol / L solution. A nitrogen atmosphere was used as the protective atmosphere, and the temperature was increased from 25 °C to 400 °C at a rate of 5 K / min. Test spectra were obtained, and the S and T values ​​were analyzed and integrated to obtain the S and T values. a was then calculated.

[0037] In some implementations, b = 1 to 5.

[0038] More preferably, b is a range of one or any two of the following: 1, 1.2, 1.5, 1.8, 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, 4.8, 5.

[0039] More preferably, b = 1.5~3.5.

[0040] The tortuosity of the positive electrode sheet affects its wettability in the secondary battery, as well as the electrochemical environment of lithium nickel cobalt manganese oxide particles during charging and discharging, and the efficiency of the lithium-ion transport network. When the tortuosity of the positive electrode sheet is preferably within the above range, it can not only effectively avoid the negative impact of anisotropy of lithium nickel cobalt manganese oxide particles, resulting in better structural stability, but also ensure that the charge and discharge activities of the particles on the surface and bottom layers are both high, resulting in higher lithium-ion transport efficiency, lower initial DCR, and better kinetic performance.

[0041] In some embodiments, the lithium nickel cobalt manganese oxide particles further include a shell, the shell comprising at least one of Al2O3, ZrO2, LiAlO2, Li3PO4, LATP (lithium aluminum titanium phosphate), LLZO (lithium lanthanum zirconium oxide), and LLTO (lithium lanthanum titanate).

[0042] In this application, relatively inert metal oxides such as alumina and solid electrolytes such as LATP are used to coat lithium nickel cobalt manganese oxide particles, which can effectively improve the thermal and chemical stability of lithium nickel cobalt manganese oxide. Furthermore, after simultaneously controlling the exothermic characteristics of the particle material and the tortuosity characteristics of the electrode itself, the secondary battery corresponding to the electrode can achieve a good balance between thermal stability and kinetic performance.

[0043] More preferably, the average thickness of the outer shell is 5~500 nm.

[0044] It should be noted that, in this application, the average thickness of the outer shell can be confirmed by, but is not limited to, the following methods: the positive electrode sheet is soaked in DMC for 2 hours and then dried; the positive electrode material layer is scraped to obtain powder; the resulting powder is sintered at 450°C for 6 hours in air; the cross-section of the sample to be tested is polished using an argon ion beam; four samples to be tested are grouped together; the polished samples are prepared by EPMA (electron probe X-ray micro-area analysis) using anhydrous ethanol as the dispersion liquid; the prepared samples are placed on the sample stage and the imaging is started; the imaging position is selected at least four locations on the edge of the outer shell of the lithium nickel cobalt manganese oxide particles; the coating thickness is calculated according to the scale; and the final result is the average value of 16 sets of data.

[0045] In some embodiments, the particle size Dv50 of the positive electrode active material is 1.5~6μm.

[0046] More preferably, the particle size Dv50 of the positive electrode active material is one or any two of the following: 1.5μm, 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, 5.5μm, and 6μm.

[0047] In some embodiments, the SPAN value of the positive electrode active material is 1 to 1.5; the SPAN value is equal to (particle size Dv90 of the positive electrode active material and particle size Dv10 of the positive electrode active material) / particle size Dv50 of the positive electrode active material.

[0048] It should be noted that the particle size Dv50 and SPAN value of the positive electrode active material in the positive electrode sheet described in this application can be confirmed by, but is not limited to, the following methods:

[0049] The positive electrode sheet was soaked in DMC for 2 hours and then dried. The positive electrode material layer was scraped to remove powder. The resulting powder was sintered at 450°C for 6 hours in air atmosphere. Subsequently, the particle size distribution was measured using a laser particle size distribution measuring instrument (Mastersizer 3000) according to the particle size distribution laser diffraction method (specific steps refer to GB / T19077-2016). The particle size that accumulated to 50% of the volume distribution was defined as Dv50; the particle size that accumulated to 10% of the volume distribution was defined as Dv10; and the particle size that accumulated to 90% of the volume distribution was defined as Dv90.

[0050] In some embodiments, the specific surface area of ​​the positive electrode active material is 0.4~1.2 m². 2 / g.

[0051] More preferably, the specific surface area of ​​the positive electrode active material is 0.4 m². 2 / g, 0.6 m 2 / g, 0.8 m 2 / g, 1m 2 / g、1.1 m 2 / g, 1.2 m 2 The range of one or both of the values ​​in / g.

[0052] It should be noted that the specific surface area of ​​the positive electrode active material in the positive electrode sheet described in this application can be confirmed by, but is not limited to, the following methods:

[0053] The positive electrode sheet was soaked in DMC for 2 hours and then dried. The positive electrode material layer was scraped to obtain powder, and the resulting powder was sintered at 450°C for 6 hours in air atmosphere. Then, the BET specific surface area was tested by nitrogen adsorption-desorption method.

[0054] In some embodiments, the porosity of the positive electrode sheet is 13-35%.

[0055] More preferably, the porosity of the positive electrode sheet is a value within the range of one or any two of 13%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, 32%, and 35%.

[0056] It should be noted that the porosity of the positive electrode sheet described in this application can be confirmed using, but is not limited to, the following methods:

[0057] The positive electrode sheet was cut into circular pieces with a diameter of D = 12 mm. Simultaneously, a thickness gauge was used to measure the thickness of the electrode sheet and the current collector, recording the results as h1 and h2 respectively. This was then calculated using V1 = πR. 2 Calculate the volume V1 of the positive electrode active material layer (h1-h2), then weigh the electrode and record the mass as m1. Next, immerse the electrode completely in a sealed container of hexadecane for 1 hour (the volume of hexadecane in the sealed container is not critical, but the amount must be sufficient to completely submerge the electrode). Remove the electrode and dry it with filter paper until a constant weight is achieved (generally after 1 hour). Weigh the electrode and record the weight as m2. Calculate the porosity of the positive electrode using the formula porosity% = (m2-m1 / ρ) / V1 × 100%, where ρ is the density of hexadecane, 0.7734 g / cm³. 3 .

[0058] In some embodiments, the compaction density of the positive electrode sheet is 3.25~3.65 g / cm³. 3 .

[0059] It should be noted that the compaction density of the positive electrode sheet described in this application can be confirmed by, but is not limited to, the following methods:

[0060] The pretreated positive electrode sheet is punched into circular pieces of a fixed area using a punching machine. The area is denoted as S0, and the unit is mm. 2 To ensure testing accuracy, select a flat location in the middle of the electrode sheet and take three or more circular pieces as parallel samples. Weigh each of the three circular pieces using an electronic balance, and record the mass of each circular piece as M1 (g). Measure the thickness of the active material layer (after removing the current collector) in each of the three circular pieces using a micrometer, and record it as H (mm). Take the average value. Finally, add an appropriate amount of deionized water to each of the three circular pieces, gently wipe off the coating with lint-free paper to expose the copper foil, and let it stand (dry) at room temperature for 10 minutes. After the copper foil is dry, weigh each of the three copper foil pieces and record it as M0. Take the average value and calculate the coating compaction density A using the following formula: A = (M1 - M0) / (H * S0).

[0061] In some embodiments, the lithium nickel cobalt manganese oxide has the structural formula LiNi. a Co b Mn c O2, where a is greater than 0 and less than 1; b is greater than 0 and less than 1; c is greater than 0 and less than 1; a+b+c=1.

[0062] In some embodiments, the lithium nickel cobalt manganese oxide particles further contain a doping element, including at least one selected from Al, Mg, and Ti. In some embodiments, the lithium nickel cobalt manganese oxide particles can be commercially available products or obtained through a self-prepared method. Specifically, the lithium nickel cobalt manganese oxide particles can be obtained using the following preparation method:

[0063] Nickel, cobalt, and manganese sources are mixed in a solvent, a precipitant is added to precipitate the reaction, and the mixture is allowed to stand. After filtration, washing, and drying, the resulting mixed precursor is mixed with a lithium source and calcined once. The resulting material is then crushed, sieved, and mixed with a coating material, calcined a second time, and sieved again to obtain the lithium nickel cobalt manganese oxide particles.

[0064] In some embodiments, the nickel source includes at least one of nickel sulfate, nickel nitrate, nickel chloride, nickel oxalate, and nickel acetate;

[0065] In some embodiments, the cobalt source used includes at least one of cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt oxalate, and cobalt acetate;

[0066] In some embodiments, the manganese source includes at least one of manganese sulfate, manganese nitrate, manganese chloride, manganese oxalate, and manganese acetate;

[0067] In some embodiments, the lithium source includes at least one of lithium hydroxide, lithium carbonate, lithium nitrate, lithium oxalate, lithium dihydrogen phosphate, lithium citrate, and lithium acetate.

[0068] In some embodiments, the precipitant includes at least one of sodium hydroxide, sodium carbonate, oxalic acid, and ammonia.

[0069] In some embodiments, the solvent includes water.

[0070] In some embodiments, the precipitation reaction takes 0.5 to 1.5 hours.

[0071] In some embodiments, the heating rate during the first calcination is 1~10℃ / min, the calcination temperature is 800~920℃, and the time is 10~15h.

[0072] In some embodiments, the coating material includes at least one of Al2O3, ZrO2, LiAlO2, Li3PO4, LATP (lithium aluminum titanium phosphate), LLZO (lithium lanthanum zirconium oxide), and LLTO (lithium lanthanum titanate).

[0073] In some embodiments, the heating rate during the secondary calcination is 1~10℃ / min, the calcination temperature is 450~470℃, and the time is 4~5h.

[0074] In some embodiments, the mixing can be achieved by ball milling, with a milling time of 1.5h to 2.5h and a rotation speed of 400 to 500 r / min.

[0075] In some embodiments, the mass percentage of manganese ions in the surface layer of the lithium nickel cobalt manganese oxide particles is greater than the mass percentage of manganese ions in the inner layer.

[0076] By setting a gradient of manganese ion concentrations, lithium nickel cobalt manganese oxide particles can exhibit better thermal stability and safety performance when used as positive electrode active materials.

[0077] In some embodiments, the material obtained during the secondary calcination is also mixed with manganese dioxide.

[0078] In some embodiments, the positive electrode material layer in the positive electrode sheet includes a positive electrode material, a binder, and a conductive agent. The positive electrode material includes lithium nickel cobalt manganese oxide particles, and the mass percentage of the positive electrode material in the positive electrode material layer is 92-99%.

[0079] In some embodiments, the binder is used to improve the adhesion between 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 adhesive properties and does not significantly cause adverse chemical changes in the battery. For example, the binder includes fluorinated polyolefin binders, including but not limited to polyvinylidene fluoride (PVDF), PVDF copolymers, or their modified derivatives (e.g., modified with carboxylic acids, acrylic acid, acrylonitrile, etc.).

[0080] Specifically, the adhesive is selected from polytetrafluoroethylene or polyvinylidene fluoride.

[0081] In some embodiments, the mass percentage of the binder in the positive electrode material layer is 1% to 4.0%, such as 1%, 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.7%, 3.0%, 3.2%, 3.5%, 3.8%, 4.0%, or any range formed by any two of the above values.

[0082] In some embodiments, the conductive agent is used to provide 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 examples of conductive agents in the positive electrode active material layer include, but are not limited to, at least one of carbon nanotubes, carbon black, graphite, carbon fibers, activated carbon, mesoporous carbon, and fullerenes, wherein carbon fibers are, for example, carbon nanofibers; and carbon black is, for example, SP (Super P), acetylene black, Ketjen black, etc.

[0083] In some embodiments, the mass percentage of the conductive agent in the positive electrode material layer is 1.0% to 4.0%, such as 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.5%, 3%, 3.5%, 4%, or any range formed by any two of the above values.

[0084] In some embodiments, the positive electrode sheet can be prepared by, but is not limited to, the following methods:

[0085] The positive electrode active material, conductive agent, and binder are mixed in a solvent and then stirred to prepare a slurry. The slurry is coated onto a current collector in one or two layers, dried, rolled, and cut to obtain the positive electrode sheet.

[0086] In some embodiments, the solvent includes N-methylpyrrolidone (NMP).

[0087] In some embodiments, the tortuosity of the positive electrode sheet of this application can be controlled, but is not limited to, during the preparation of the positive electrode sheet by selecting the particle size of the positive electrode material, the compaction density of the positive electrode sheet (i.e., controlling the pressure of the positive electrode sheet rolling), and setting concave holes by drilling the electrode sheet (controlling the drilling depth, etc.). Those skilled in the art can also use other means to control the tortuosity, which are not specifically limited here.

[0088] In some embodiments, the positive electrode material layer of the positive electrode sheet is further provided with a plurality of recesses, the ratio of the depth of the recesses to the thickness of the positive electrode material layer being (0.2~0.96):1, the pore diameter of the recesses being 2~40μm, and the pore density of the recesses being 6~500pt / m³. 2 .

[0089] Furthermore, the recessed hole is located on the side of the active material layer away from the current collector.

[0090] Another object of this application is to provide a secondary battery, including the positive electrode sheet described in this application.

[0091] In some embodiments, the lithium-ion battery further includes a negative electrode and an electrolyte.

[0092] In some embodiments, the electrolyte includes additives, solvents, and lithium salts.

[0093] In some embodiments, the solvent includes at least one of carbonate solvents, carboxylic acid ester solvents, ether solvents, sulfone solvents, nitrile solvents, and phosphate ester solvents.

[0094] Exemplary examples include, but are not limited to, at least one of propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); carboxylic acid ester solvents include, but are not limited to, at least one of ethyl acetate, methyl formate, and 1,4-butyrolactone; ether solvents include, at least one of dimethyltetrahydrofuran, tetrahydrofuran, and 1,2-dimethoxyethane; sulfone solvents include, at least one of methyl sulfone and dimethyl sulfoxide; nitrile solvents include, at least one of propionitrile, butyronitrile, 1-(2-cyanoethyl)pyrrole, and 1,3,6-hexanetrionitrile; and phosphate ester solvents include, at least one of trimethyl triphosphate and triethyl phosphate.

[0095] More preferably, the carbonate solvent has a mass percentage of 70-100% based on the total solvent mass in the electrolyte.

[0096] When the tortuosity and thermal stability parameters of the positive electrode meet the specified requirements, further adaptation using the above-mentioned preferred electrolyte solvent formulation can greatly improve the lithium-ion transport rate in the secondary battery, thereby improving the kinetic performance of the secondary battery.

[0097] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium dioxalate borate, lithium difluorooxalate borate, lithium trifluoromethanesulfonate, lithium difluoromethanesulfonylimide, lithium ditrifluoromethanesulfonylimide, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0098] In some embodiments, the additives include, but are not limited to, vinylene carbonate.

[0099] More preferably, the additive includes at least one of 2,2,2-trifluoroethyl methyl carbonate, pentafluoroethoxycyclotriphosphazene, and vinyl sulfate.

[0100] When using a lithium nickel cobalt manganese oxide system for the positive electrode, selecting an electrolyte containing the aforementioned additives can effectively balance the safety performance and fast charging performance of the secondary battery.

[0101] In some embodiments, the negative electrode sheet includes a negative electrode material layer, the negative electrode material layer including a negative electrode material, the negative electrode material including at least one of natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, elemental silicon, silicon suboxide, silicon-carbon composite material, and lithium titanate.

[0102] In some embodiments, the negative electrode material comprises graphite, and the positive electrode sheet satisfies: b = 1.8~5.

[0103] More preferably, the graphite includes at least one of artificial graphite and natural graphite.

[0104] When lithium nickel cobalt manganese oxide particles are selected as the active material for the positive electrode, and the negative electrode material contains graphite, the overall kinetic performance is superior to other systems due to the good strength and conductivity of graphite itself. Therefore, the tortuosity of the positive electrode can be preferably within the aforementioned range, resulting in higher lithium-ion conduction efficiency.

[0105] In some embodiments, the negative electrode material includes a silicon-based material, and the positive electrode sheet satisfies: b=1~4.5.

[0106] More preferably, the silicon-based material includes at least one of elemental silicon, silicon suboxide, silicon carbide, and silicon-carbon composite materials.

[0107] Silicon-based materials have poorer kinetic performance compared to carbon-based materials. In order to achieve better lithium-ion conduction efficiency, when the negative electrode material contains silicon-based materials, it is further preferred that the tortuosity of the positive electrode sheet is within the above-mentioned range. The high kinetic performance of the positive electrode sheet can improve the overall lithium-ion conduction efficiency of the secondary battery.

[0108] The negative electrode material layer may also contain a conductive agent and / or a binder.

[0109] The conductive agent in the negative electrode active material layer is used to provide 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. For example, the conductive agent includes, but is not limited to, at least one of carbon nanotubes, carbon black, graphite, carbon fibers, activated carbon, mesoporous carbon, and fullerenes, wherein carbon fibers are, for example, carbon nanofibers; and carbon black is, for example, SP, acetylene black, Ketjen black, etc.

[0110] In some embodiments, the mass percentage of the conductive agent in the negative electrode material layer is 0.4% to 2%, such as 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, or any range formed by any two of the above values.

[0111] The binder in the negative electrode active layer is used to improve the adhesion between 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 adhesive properties and does not significantly cause adverse chemical changes in the battery. For example, the binder includes, but is not limited to, at least one of carboxymethyl cellulose (CMC), styrene-butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl butyral, and aqueous acrylic resin.

[0112] In some embodiments, the mass percentage of the binder in the negative electrode material layer is 1.0% to 4.5%, such as 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.7%, 3.0%, 3.2%, 3.5%, 3.8%, 4.0%, 4.2%, 4.5%, or any range formed by two of the above values. In some embodiments, the negative electrode material layer in the negative electrode sheet includes a negative electrode material, a binder, and a conductive agent, and the mass percentage of the negative electrode material in the positive electrode material layer is 70% to 99%.

[0113] In some embodiments, the secondary battery further includes a separator;

[0114] More preferably, the diaphragm includes at least one of a polypropylene diaphragm and a polyethylene diaphragm.

[0115] More preferably, the diaphragm has a porosity of 40-60%.

[0116] More preferably, the diaphragm is provided with an aramid coating.

[0117] By optimizing the porosity settings or applying the aramid coating, the interfacial reactivity can be effectively reduced, thereby improving the overall transmission efficiency and safety performance of the secondary battery during lithium-ion transport.

[0118] The present invention is further illustrated below with specific embodiments, which should not be construed as limiting the scope of protection claimed by the present invention:

[0119] Example 1

[0120] A positive electrode sheet and a secondary battery, the preparation method of which includes the following steps:

[0121] (1) Preparation of lithium nickel cobalt manganese oxide particles: Nickel sulfate solution, cobalt sulfate solution and manganese sulfate solution were mixed according to the stoichiometric ratio, and then sodium hydroxide precipitant was added. The pH was adjusted to be within the range of 12.1 and the temperature was 55℃ for 1h. After filtration, washing and drying, the obtained precursor and lithium carbonate were mixed and ball-milled according to the molar ratio of lithium atoms to the total atoms of nickel, cobalt and manganese in the precursor of 1.05:1. The mixture was heated to 800℃ at 5℃ / min under oxygen atmosphere and calcined for 10h. The obtained material was crushed and sieved and then mixed with the coating material. The mixture was calcined to 450℃ at 5℃ / min under oxygen atmosphere for 4h and sieved to obtain lithium nickel cobalt manganese oxide particles.

[0122] (2) Preparation of the positive electrode sheet: Lithium nickel cobalt manganese oxide particles were used as the positive electrode material. Subsequently, the positive electrode material, conductive agent acetylene black, conductive agent carbon nanotubes, and binder polyvinylidene fluoride were dispersed in N-methylpyrrolidone at a mass ratio of 98:0.5:0.5:1. The mixture was vacuum stirred to prepare a slurry, which was then coated onto aluminum foil as a current collector. After coating, drying, cold pressing, slitting, and rolling, concave holes were formed by mechanical punching to obtain the positive electrode sheet. The areal density of the positive electrode sheet was 249.5 g / m³. 2 The compacted density is 3.35 g / cm³. 3 ;

[0123] (2) Preparation of negative electrode sheet: The negative electrode material artificial graphite, conductive agent acetylene black, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber are dispersed in water at a mass ratio of 96.4:1:1.2:1.4. The mixture is then vacuum stirred to prepare a slurry, which is then coated onto the current collector copper foil. After coating, drying, cold pressing, slitting, and rolling, the negative electrode sheet is obtained. The areal density of the negative electrode sheet is 172 g / m³. 2 The compacted density is 1.66 g / cm³. 3 ;

[0124] (3) Preparation of electrolyte: EC, EMC and DEC are mixed in a volume ratio of 1:1:1 as solvent. Then, based on the total mass of the electrolyte, lithium hexafluorophosphate and additive 2,2,2-trifluoroethyl methyl carbonate are added to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1 mol / L and a 2,2,2-trifluoroethyl methyl carbonate mass content of 1.8 wt%.

[0125] (4) The positive electrode, commercially available PP separator and negative electrode are stacked in sequence to form a battery cell. The battery cell is placed in the outer packaging shell, dried and injected with electrolyte. After vacuum sealing, standing, formation and volume adjustment, the lithium-ion secondary battery is obtained.

[0126] Examples 2-31, Comparative Examples 1-6

[0127] A positive electrode sheet and the lithium-ion battery prepared therefrom differ from Example 1 only in the preparation process of the positive electrode sheet.

[0128] The lithium nickel cobalt manganese oxide particles described in Examples 13, 21, 26, 29, Comparative Examples 4 and 6 were prepared by calcining once, and the resulting material was crushed and sieved before being mixed with a coating material, which included 2000 ppm manganese dioxide and other coating materials. The mixture was then calcined again at 450°C for 4 hours under an oxygen atmosphere at a rate of 5°C / min, and sieved to obtain lithium nickel cobalt manganese oxide particles.

[0129] The parameters and test results of each positive electrode during preparation are shown in Tables 1 and 2. The test methods for each product parameter are as described above. Taking 'a' as an example, in Example 5, 'a' of the positive electrode is obtained by performing DSC testing on the positive electrode beforehand. Figure 1 The characteristic peak spectrum was then fitted to obtain S and T, and finally a was calculated. " / " indicates that the product of the example / comparative example did not undergo this process during preparation. For example, the positive electrode sheet in Example 2 was not perforated, therefore there are no corresponding concave holes on the surface. The pore diameter, depth, and pore density of the concave holes in Table 1 are indicated by " / ".

[0130] Table 1

[0131]

[0132] Table 2

[0133]

[0134] Example of effect

[0135] The lithium-ion batteries obtained in each embodiment and comparative example were tested as follows:

[0136] (1) Security testing:

[0137] (I) The secondary batteries obtained in each embodiment and comparative example were placed at 45°C and left to stand for 24 hours. Then, they were fixed with a glass clamp under a pressure of 0.4~0.6MPa and left to stand for 10 minutes. They were pre-charged at 0.02C to the cutoff voltage of 3.2V and left to stand for 10 minutes. Then, they were charged at 0.05C to the cutoff voltage of 3.4V and left to stand for 10 minutes. Finally, they were charged at 0.08C to the cutoff voltage of 3.75V to complete the formation. The formed secondary batteries were charged at a constant current rate of 0.33C to the upper limit voltage and then switched to constant voltage charging until the current I ≤ 0.05C. The cells were wrapped with heating wire and thermocouples were attached. Then, they were placed in an insulated reaction chamber and the ambient temperature was raised to the initial test temperature at a constant rate. Heating was stopped and the chamber was put into an insulated mode. The cell temperature was kept at equilibrium with the ambient temperature and the cell temperature was continuously monitored. When the rate of heat generation of the battery cell exceeds the set threshold (0.02℃ / min), it is determined to be the self-heating initiation temperature (T1); when the rate of heat generation of the battery cell exceeds the set threshold (1℃ / min), it is determined to be the thermal runaway initiation point (T2), and the temperature of T2 is recorded.

[0138] (2) DCR test:

[0139] The secondary battery was charged at 25°C with a constant current and constant voltage of 0.33C to 4.25V, with a cutoff current of 0.05C. After resting for 10 minutes, it was discharged at a constant current of 0.33C to 2.5V. This cycle was repeated twice. After resting for 10 minutes, it was charged at a constant current and constant voltage of 0.33C to 4.25V. After discharging to 50% of the discharge capacity of the second cycle, it was rested for 2 hours. The initial voltage was recorded as V1. The battery was discharged at a 1C rate for 18 seconds with a sampling interval of 0.1 seconds. After the discharge ended, the voltage was recorded as V2, and the discharge current for 18 seconds was recorded as I1. The DCR discharge (I1, 18s) = |V2-V1| / I1×1000.

[0140] The test results are shown in Table 3.

[0141] Table 3

[0142]

[0143] As can be seen from Table 3:

[0144] (1) The secondary battery described in this application achieves a / b ratio within the range of 0.5 to 8 by synergistically regulating the exothermic characteristic parameter a of the ternary material in the positive electrode and the tortuosity b of the electrode. This not only effectively improves the safety and stability of the electrode, but also enhances the efficiency of the ternary material in lithium-ion transport, resulting in a lower DCR. The thermal runaway initiation temperature of the secondary battery is as high as 130°C or more, and the DCR can be controlled within 32mΩ, demonstrating excellent overall performance. In contrast, the positive electrode of the secondary batteries described in the comparative examples does not simultaneously take into account the thermodynamic stability and kinetic performance of the positive electrode, and therefore cannot simultaneously achieve the safety and DCR test levels of the products in each embodiment.

[0145] (2) When the exothermic characteristic parameter a and the tortuosity b of the positive electrode are adjusted simultaneously, not only can the internal heat generation of the battery be controlled and the thermal stability and safety of the battery be adjusted, but also the wettability of the electrode and the electrolyte after contact can be high, the stress can be highly dispersed and uniform during charging and discharging, the electrochemical stability during lithium ion insertion and extraction is excellent, and the kinetic performance is better. According to the test results of each embodiment, when the ratio a / b is further optimized in the range of 3 to 6, the electrochemical performance of the secondary battery is obviously better.

[0146] (3) On the other hand, when the exothermic characteristic parameter a of the electrode is further preferably in the range of 3 to 9, the interaction reaction between the interface of the positive electrode of the secondary battery and the electrolyte is further reduced, and the internal heat generation is lower; when the tortuosity b of the electrode is further preferably in the range of 1.5 to 3.5, the ternary material not only avoids the negative impact caused by anisotropy, but also further improves its structural stability in the positive electrode, and the charge and discharge activity of the surface and bottom particles reaches a higher level, with a lower DCR value. Therefore, when a and / or b of the secondary battery are further optimized, the electrochemical performance of the secondary battery will also be effectively improved.

[0147] (4) According to the comparison of various embodiments, after optimization, the initial temperature of thermal runaway of the secondary battery can reach more than 160°C, and the DCR value can be further controlled at around 20mΩ.

Claims

1. A positive electrode sheet, characterized by, The positive electrode sheet comprises a positive electrode material layer, the positive electrode material layer comprises a positive electrode active material, and the positive electrode active material comprises lithium nickel cobalt manganese oxide particles; The positive electrode sheet satisfies a / b=0.5-8; The a=S / T, wherein S J / g is the total peak area of the exothermic characteristic peak of the positive electrode sheet at 190-290 DEG C when the DSC test is performed, and T DEG C is the temperature corresponding to the highest peak value of the test exothermic characteristic peak; the a=1.5-12; The b is the tortuosity of the positive electrode sheet; the b=1-5.

2. The positive electrode sheet according to claim 1, wherein The a / b=3-6.

3. The positive electrode sheet according to claim 1, wherein The S=400-2600 J / g, and / or, the T=190-290 DEG C.

4. The positive electrode sheet according to claim 1, wherein The lithium nickel cobalt manganese oxide particles further comprise a shell, and the shell comprises at least one of Al2O3, ZrO2, LiAlO2, Li3PO4, lithium aluminum titanium phosphate, lithium lanthanum zirconium oxide, and lanthanum lithium titanate.

5. The positive electrode sheet according to claim 1, wherein The particle size Dv50 of the positive electrode active material is 1.5-6 mu m.

6. The positive electrode sheet according to claim 1, wherein The SPAN value of the positive electrode active material=1-1.

5.

7. The positive electrode sheet according to claim 1, wherein The specific surface area of the positive electrode active material is 0.4 to 1.2 m 2 / g.

8. The positive electrode plate of claim 1, wherein the positive electrode plate has a thickness of 100 to 300 μm. The porosity of the positive electrode sheet is 13-35%.

9. The positive electrode plate of claim 1, wherein the positive electrode plate has a thickness of 100 to 300 microns. The compacted density of the positive electrode plate is 3.25-3.65 g / cm 3 .

10. The positive electrode plate of claim 1, wherein the positive electrode plate has a thickness of 100 to 300 micrometers. The nickel cobalt manganese lithium has a structural formula of LiNi a Co b Mn c O2, wherein a is greater than 0 and less than 1; b is greater than 0 and less than 1; c is greater than 0 and less than 1; a+b+c=1.

11. The positive electrode plate of claim 1, wherein the positive electrode plate has a thickness of 100 to 300 micrometers. The positive electrode material layer is provided with a plurality of recessed holes, and the ratio of the depth of the recessed holes to the thickness of the positive electrode material layer is (0.2-0.96):1, and the pore size of the recessed holes is 2-40 mu m.

12. A secondary battery characterized by comprising: The secondary battery comprises the positive electrode sheet according to any one of claims 1-11.

13. The secondary battery as described in claim 12, characterized in that, The secondary battery further comprises a negative electrode sheet, the negative electrode sheet comprises a negative electrode material layer, the negative electrode material layer comprises a negative electrode active material, and the negative electrode active material comprises at least one of graphite and silicon-based material.

14. The secondary battery as described in claim 13, characterized in that, The negative electrode active material comprises graphite, and the positive electrode sheet in the secondary battery satisfies b=1.8-5.

15. The secondary battery as described in claim 13, characterized in that, The negative electrode active material comprises silicon-based material, and the positive electrode sheet in the secondary battery satisfies b=1-4.

5.

16. The secondary battery of claim 12, wherein the cathode comprises a cathode active material, a cathode binder, and a cathode conductive agent. The secondary battery further comprises an electrolyte, and the electrolyte comprises a solvent, the solvent comprises at least one of carbonate solvent, carboxylic acid ester solvent, ether solvent, and nitrile solvent.

17. The secondary battery of claim 16, wherein the cathode comprises a cathode active material, a cathode binder, and a cathode conductive agent. The carbonate solvent comprises at least one of dimethyl carbonate, diethyl carbonate, ethylene carbonate, and methyl ethyl carbonate, and / or the carboxylic acid ester solvent comprises at least one of ethyl acetate, ethyl propionate, and methyl acetate.

18. The secondary battery of claim 16, wherein the cathode comprises a cathode active material, a cathode binder, and a cathode conductive agent. The mass percentage content of the carbonate solvent in the total solvent in the electrolyte is 70-100%.

19. The secondary battery of claim 16, wherein the cathode comprises a cathode active material, a cathode binder, and a cathode conductive agent. The electrolyte further comprises an additive, and the additive comprises at least one of 2,2,2-trifluoroethyl methyl carbonate, pentafluoroethoxy cyclotriphosphazene, and ethylene sulfate.

20. The secondary battery of claim 12, wherein the cathode comprises a cathode active material, a cathode binder, and a cathode conductive agent. The secondary battery further comprises a separator, the porosity of the separator is 40-60%, and / or the separator is provided with an aramid coating.

Images