Secondary battery

Abstract

The invention discloses a secondary battery, which belongs to the technical field of batteries, and can effectively improve the compaction density of a positive pole piece, guarantee better dynamic performance and improve the fast charging performance of the secondary battery by cooperatively regulating and controlling the heat release characteristic temperature parameter of the secondary battery and the nickel content and the particle size of a ternary material in the positive pole piece. And meanwhile, the internal stability of the secondary battery during lithium deintercalation can be guaranteed, and the service life of the battery is prolonged.

Description

Technical Field

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

[0002] In lithium-ion rechargeable batteries, ternary cathode materials exhibit superior fast-charging performance compared to lithium iron phosphate (LFP) cathode materials. This is because, unlike LFP cathode materials with olivine structures that only provide one-dimensional channels, the layered structure of ternary cathode materials offers two-dimensional channels for lithium-ion transport. This channel difference results in a significantly higher lithium-ion diffusion coefficient for ternary cathode materials. However, the layered structure of ternary cathode materials undergoes greater volume changes during lithium-ion insertion and extraction, making them prone to particle cracking and structural degradation over long-term cycling. Consequently, the cycle life of ternary cathode materials is generally lower. Summary of the Invention

[0003] The purpose of this application is to overcome the shortcomings of the existing technology and provide a secondary battery. By synergistically controlling the exothermic characteristic temperature parameters of the secondary battery, the nickel content and particle size of the ternary material in the positive electrode sheet, it is possible to effectively improve the compaction density of the positive electrode sheet, ensure better dynamic performance, improve the fast charging performance of the secondary battery, and at the same time ensure the internal stability of the secondary battery during lithium intercalation and deintercalation, thus extending the battery life.

[0004] To achieve the above objectives, in a first aspect of this application, this application provides a secondary battery, including a positive electrode sheet, the positive electrode sheet including a positive electrode material layer, the positive electrode material layer including a positive electrode active material, and the positive electrode active material including lithium nickel cobalt manganese oxide; The positive electrode plate satisfies: (a×b) / c=0.07~13.3; The formula a = T2 / (T2-T1) is used, where T2℃ is the self-generated heat temperature of the secondary battery during ARC testing (adiabatic accelerated calorimetry), and T1℃ is the thermal runaway temperature; b is the percentage of nickel in the positive electrode active material relative to the total mass of nickel, cobalt, and manganese; and cμm is the particle size D of the positive electrode active material. v50 .

[0005] The beneficial effects of this application are as follows: This application provides a secondary battery that, by synergistically controlling the exothermic characteristic temperature parameters of the secondary battery, the nickel content and particle size of the ternary material in the positive electrode sheet, can effectively improve the compaction density of the positive electrode sheet, ensure good kinetic performance, enhance the fast charging performance of the secondary battery, and at the same time ensure the internal stability of the secondary battery during lithium intercalation and deintercalation, thus extending the battery life. Attached Figure Description

[0006] Figure 1 This is an ARC test result diagram obtained when parameter a of the secondary battery described in Embodiment 9 of this application is tested. Detailed Implementation

[0007] 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.

[0008] 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.

[0009] 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.

[0010] The present application is further illustrated below with specific embodiments: A secondary battery includes a positive electrode sheet, the positive electrode sheet including a positive electrode material layer, the positive electrode material layer including a positive electrode active material, the positive electrode active material including lithium nickel cobalt manganese oxide; The positive electrode plate satisfies: (a×b) / c=0.07~13.3; a = T2 / (T2-T1), where T2℃ is the self-generated heat temperature of the secondary battery during ARC testing, and T1℃ is the thermal runaway temperature; b is the percentage of the mass of the positive electrode active material relative to the total mass of nickel, cobalt, and manganese; and cμm is the particle size D of the positive electrode active material. v50 .

[0011] While lithium nickel cobalt manganese oxide (LCA), a ternary material, can theoretically provide sufficient active sites and reduce the energy barrier during lithium-ion insertion / extraction when used as the positive electrode active material in secondary batteries by increasing the nickel content, thus improving fast-charging performance, the thermal decomposition temperature of the material decreases as the nickel content increases. This leads to increased lattice oxygen release at high temperatures, triggering a chain-like exothermic reaction. Furthermore, the highly active tetravalent nickel ions oxidize the electrolyte, causing irreversible decomposition and consuming active lithium, thereby reducing the cycle life of the secondary battery and making it difficult to achieve the overall performance of the secondary battery. Therefore, in the technical solution of this application, the parameter relationship 'a' between the self-generated heat temperature and the thermal runaway temperature of the secondary battery during ARC testing is adjusted to ensure that the probability of overheating or even thermal runaway is reduced. Simultaneously, the nickel content and particle size of the positive electrode active material are controlled: As mentioned above, excessively high nickel content in the positive electrode active material leads to a decrease in the cycle life of the secondary battery, but if the content is too low, lithium ions are hindered during insertion / extraction, resulting in a decrease in the fast-charging performance of the secondary battery. On the other hand, when the particle size of the positive electrode active material increases, lithium ions need to migrate through a longer solid-phase diffusion path to reach the particle surface, leading to a decrease in ion migration rate and a decrease in the fast-charging performance of the secondary battery. However, if the particle size is too small, the overall specific surface area of ​​the material increases, which significantly increases the contact area between the material and the electrolyte, thereby exacerbating interfacial side reactions, accelerating irreversible capacity loss during cycling, and ultimately leading to a decrease in the cycle life of the secondary battery. When the secondary battery simultaneously regulates the exothermic characteristic parameter a, the nickel content b of the positive electrode active material, and the particle size c, and controls their relationship within the above range, the secondary battery can balance cycle life and fast charging performance.

[0012] In some implementations, (a×b) / c = one or any two of the following values: 0.07, 0.08, 0.1, 0.15, 0.2, 0.21, 0.25, 0.5, 0.8, 1, 1.2, 1.4, 1.5, 1.8, 2, 2.5, 3, 3.8, 4, 5, 8, 10, 11, 12, 12.5, 13, 13.3.

[0013] More preferably, (a×b) / c = 0.26~7.2.

[0014] In order to balance the cycle life and fast charging performance of the secondary battery, the secondary battery described in this application synchronously controls the macroscopic battery heat dissipation characteristic parameter a and the microscopic nickel content and particle size c of the positive electrode active material. When the relationship between the three is further optimized within the above range, the fast charging performance and cycle life of the secondary battery can be further improved.

[0015] In some implementations, a = 1.6~30.

[0016] Preferably, a is a range of one or any two of the following: 1, 1.5, 1.6, 1.7, 1.75, 1.8, 2, 2.2, 2.5, 3, 3.5, 4, 4.5, 4.6, 5, 8, 10, 12, 15, 16, 18, 20, 22, 24, 25, 26, 30.

[0017] More preferably, a = 1.9~26.

[0018] The exothermic relationship parameter 'a' of a secondary battery has a crucial impact on its cycle life. When the self-generated heat temperature T1 and thermal runaway temperature T2 of a secondary battery change, the exothermic characteristics of the secondary battery will also change. If parameter 'a' is too small, the thermal runaway temperature of the battery is low and much higher than the self-generated heat temperature. There is a slow accumulation of side reactions inside the battery (such as continuous decomposition of the electrolyte and repeated rupture and repair of the SEI film), which leads to an increase in lithium-ion transport impedance during fast charging and a decrease in the battery's fast charging performance. However, if parameter a is too large, the thermal runaway temperature is close to the self-generated heat temperature, the transition time from self-generated heat to thermal runaway is very short, the kinetics of exothermic reactions inside the battery (such as membrane melting, electrolyte decomposition, etc.) are accelerated, the consumption of active lithium is large, and the cycle life of the secondary battery is short. Therefore, it is necessary to jointly regulate the nickel content and particle size of the positive electrode active material in the battery. When the regulation relationship of the three is combined with the limitations of this application, and the parameter a is further preferably within the above range, the secondary battery has higher exothermic stability, longer cycle life, and can also maintain a high level of fast charging performance under high rate conditions.

[0019] In some embodiments, T1 = 82~125℃; Preferably, T1 is a range of one or any two of 82℃, 85℃, 88℃, 90℃, 95℃, 100℃, 105℃, 110℃, 115℃, 120℃, and 125℃, and more preferably 95~125℃.

[0020] In some embodiments, T2 = 130~220℃; Preferably, T2 is a range of one or any two of 130℃, 135℃, 140℃, 145℃, 150℃, 155℃, 160℃, 165℃, 170℃, 175℃, 180℃, 190℃, 200℃, 210℃, 215℃, and 220℃, and more preferably 160~220℃.

[0021] It should be noted that T1 and T2 in parameter a described in this application can be confirmed, but are not limited to, in the following ways: The secondary battery was placed in an ARC tester (ARC-130, detection sensitivity 0.03℃ / min, minimum threshold temperature 0.02℃ / min, maximum search temperature 250℃, maximum search pressure 4 bar, maximum tracking temperature 500℃, maximum tracking pressure 4 bar). A long calibration was performed with an initial temperature of 40℃. After calibration, the battery was heated with a resistance wire, and the temperature change rate was tracked intermittently. If the battery temperature change rate did not reach the detection sensitivity, a short calibration was performed. After the short calibration, the temperature was increased again to complete the cycle. If the secondary battery temperature change rate reached the detection sensitivity, the heating wire was stopped, and the instrument switched to continuous tracking of the battery temperature until the secondary battery thermally ran away and exploded. When the heating rate of the lithium-ion battery reached 0.02℃ / min, the temperature of the lithium-ion battery at this time was the self-heating temperature, denoted by T1, in ℃; when the heating rate of the lithium-ion battery reached 1℃ / min, the temperature of the lithium-ion battery at this time was the thermal runaway temperature, denoted by T2, in ℃. The self-heating temperature T1 and the thermal runaway temperature T2 of the secondary battery were recorded. Figure 1 As shown.

[0022] It should be noted that in this application, parameter 'a' can be controlled by changing the content of doping elements in the positive electrode active material and the aging temperature of the battery. A higher doping element content results in a more stable crystal structure of the positive electrode active material, making it less prone to side reactions with the electrolyte. A smaller 'a' corresponds to a higher aging temperature and more intense internal side reactions within the battery. Those skilled in the art can set parameter 'a' based on this common knowledge, but are not limited to it. Furthermore, other methods known in the art can be used to modify the value of 'a'.

[0023] In some implementations, b = 80~98%.

[0024] Preferably, b is a range of one or any two of the following: 80%, 82%, 84%, 85%, 88%, 90%, 92%, 95%, 96%, and 98%.

[0025] More preferably, b = 85~92%.

[0026] Increasing the nickel content in the positive electrode active material can lower the migration barrier of lithium ions between layered structural sites and optimize the diffusion path of lithium ions. However, excessively high nickel content can lead to increased cation mixing in the positive electrode active material. Furthermore, lithium nickel cobalt manganese oxide materials with high nickel content are more prone to irreversible phase transitions from the H2 phase to the H3 phase during electrochemical cycling, resulting in greater volume anisotropy. This causes a series of problems such as material structure degradation, microcrack formation, side reactions, and gas generation, ultimately leading to a reduction in battery cycle life. When the nickel content of the positive electrode active material in the secondary battery is preferably within the above-mentioned range, the kinetic performance of the battery can be improved while ensuring the thermal stability within the battery, ultimately achieving both a longer cycle life and higher fast-charging performance.

[0027] It should be noted that the nickel content b of the positive electrode active material in the actual scheme of this application can be confirmed, but is not limited to, by the following methods: The secondary battery is pre-discharged at 0.33C, then the positive electrode sheet is disassembled, soaked in dimethyl carbonate (DMC) for 1 hour, dried, the positive electrode material layer on the positive electrode sheet is scraped off, 1g of material powder is accurately weighed, dispersed in 20mL of water, and then 10mL of nitric acid is added. After mixing evenly, the mixture is heated until the powder dissolves. The resulting material is then diluted with water to 100mL to obtain the test solution. The test solution is subjected to ICP testing (Thermo Fisher iCAPPRO ICP-OES). Before the test, a standard solution is prepared. The linear correlation coefficient of the standard concentration must be above 0.999 to be used as a normal standard. The 1000 mg / L standard solution was diluted with deionized water to different concentrations (0, 1 mg / 100 mL, 2 mg / 100 mL, 3 mg / 100 mL), and the element detection wavelength was selected. The experimental conditions were set as follows: Based on the characteristics of the sample and the elements to be detected, appropriate ICP instrument operating conditions were set: gas flow rate 0.5 L / min, power 1150 W, and element detection wavelengths were selected (Ni: 231.6 nm, Co: 228.6 nm, Mn: 257.6 nm). The contents of nickel, cobalt, and manganese in the sample were read by the self-analysis function of the ICP testing software. Then, the percentage of nickel mass in the total mass of nickel, cobalt, and manganese was calculated.

[0028] In some embodiments, the particle size D of the positive electrode active material v50 cμm = 2~20μm.

[0029] Preferably, cμm is a range of one or any two of the following: 2μm, 2.5μm, 3μm, 4μm, 5μm, 6μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm, and 20μm.

[0030] More preferably, the cμm = 3~7μm.

[0031] The particle size of the positive electrode active material is directly related to the lithium-ion transport efficiency. As the particle size gradually increases, the path for lithium-ion insertion / extraction transport also lengthens, resulting in lower kinetic performance and consequently reducing the fast-charging performance of the secondary battery. On the other hand, smaller particle size of the positive electrode active material increases the specific surface area, which increases the probability of side reactions after contact with the electrolyte, leading to increased heat generation and affecting the cycle stability of the secondary battery. When the secondary battery achieves synergistic regulation of a, b, and c, and the particle size c of the positive electrode active material is further optimized within the aforementioned range, the secondary battery can achieve a better balance between lithium-ion transport efficiency and electrode interface stability, resulting in superior fast-charging and cycle performance.

[0032] It should be noted that the particle size D of the positive electrode active material described in this application... v50 Confirmation can be made through, but is not limited to, the following methods: The secondary battery is discharged at 0.33C, then the positive electrode is disassembled, soaked in DMC (dimethyl carbonate) for 1 hour, dried, and then the positive electrode material layer on the electrode is scraped off. The scraped powder is calcined in a muffle furnace at 450°C in air for 6 hours. The remaining lithium nickel cobalt manganese oxide particles after calcination are then dispersed in an aqueous solution containing 3% sodium hexametaphosphate dispersant. After continuous stirring with a glass rod, the dispersion sample is poured into a test sample cell, and the particle size distribution is measured using a laser particle size analyzer (Mastersizer 3000) according to GB / T190777-2016. v50 .

[0033] It should be noted that, in this application, the particle size D of the positive electrode active material is... v50 The particle size of the positive electrode active material can be adjusted by parameters during the preparation process, such as the temperature and time during sintering, but it is not limited to this. The particle size can also be adjusted by other means, such as the amount of material added. At the same time, the positive electrode active material can also be a commercially available product, and further particle size screening can be used for adjustment. There are no specific limitations on this.

[0034] In some embodiments, the porosity of the positive electrode material layer is 20-45%.

[0035] Furthermore, the porosity of the positive electrode material layer is a value within the range of one or any two of 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, 42%, and 45%.

[0036] The porosity of the positive electrode material layer refers to the percentage of void volume in the total volume of the positive electrode sheet. Porosity can affect the cycle life of the battery. Excessive porosity may lead to a loose electrode structure, weakening the mechanical strength of the positive electrode material. During charging and discharging, the bonding force between active material particles is insufficient, making them prone to detachment. In addition, excessive porosity can also increase side reactions between the electrode and the electrolyte, leading to a decrease in the battery's cycle life. Conversely, excessively low porosity will make electrolyte wetting difficult, affecting lithium-ion transport and thus impacting the battery's fast-charging performance. By regulating and optimizing the porosity of the positive electrode material within the aforementioned range, it is possible to help the positive electrode material layer accommodate more electrolyte, allowing for smoother lithium-ion transport during charging and discharging, further improving the fast-charging performance of the secondary battery, and simultaneously maintaining a high level of battery cycle life.

[0037] It should be noted that the porosity of the positive electrode material layer described in this application can be obtained by, but is not limited to, the mass difference method. The positive electrode is cut into circular sheets with a radius r of 0.95 cm using an electrode blanking machine, and the thicknesses L and L0 of the electrode and current collector are measured using a thickness gauge. The volume of the negative electrode active material on the cut electrode is calculated as V = π × r. 2 ×(L-L0); Weigh the cut electrode sheet using a balance with an accuracy of 0.00001g and record the mass as m1. Soak the electrode sheet in hexadecane for 1 hour (completely immersed). Remove the electrode sheet with tweezers and blot it dry with filter paper until a constant weight is achieved. Weigh the electrode sheet again and record the mass as m2. Substitute the experimental data into the formula to calculate: ε=(m2-m1) / (V×ρ0)×100%, where ρ0 is the density of hexadecane, 0.7734g / cm³. 3 The porosity ε of the cathode material layer was obtained.

[0038] In some embodiments, the lithium nickel cobalt manganese oxide has the structural formula LiNi. m Co n Mn o O2, where m is greater than 0 and less than 1; n is greater than 0 and less than 1; o is greater than 0 and less than 1; m+n+o=1.

[0039] In some embodiments, the positive electrode active material further contains doping elements, including at least one selected from Zr, Ti, Si, Mo, La, Ce, Te, Nb, Ta, Al, W, Sr, V, Y, Mg, Co, F, Cl, and S.

[0040] In some embodiments, the mass content of the dopant element in the cathode material layer is 100~10000ppm.

[0041] After doping, ternary cathode active materials can suppress lithium-nickel mixing in the material, improve the stability of the overall crystal structure, reduce the probability of irreversible phase transitions and microcrack formation during charging and discharging, and thus improve the cycle stability of the battery. At the same time, under the above-mentioned preferred doping content range, the lithium-ion transport efficiency will not be affected, and the fast charging performance is excellent.

[0042] It should be noted that, in the technical solution of this application, the content of the doping element can be confirmed by, but is not limited to, using the same test method as described in b above.

[0043] More preferably, the doping element includes Al, and the mass content of Al in the cathode material layer is 200~800ppm.

[0044] Because of its small ionic radius, Al is a better choice for doping ternary cathode active materials. It can better adapt to and improve the layered structure of the material, reduce the risk of high-temperature oxygen release and lattice distortion, thereby improving the overall thermal stability of the cathode active material, ensuring long-term lithium ion insertion and extraction, and resulting in better cycle stability of the secondary battery.

[0045] In some embodiments, a coating layer is provided on the surface of the positive electrode active material.

[0046] In some embodiments, the coating layer includes at least one of cobalt oxide, cobalt hydroxide, aluminum oxide, aluminum hydroxide, phosphate, solid electrolyte, titanium oxide, tungsten oxide, tungstic acid, boric acid, and boron nitride.

[0047] More preferably, the average thickness of the coating layer is 2~10 nm.

[0048] By setting a coating layer on the surface of the positive electrode active material, a continuous conductive film can be formed on the surface of the active material particles, further improving the conductivity of the material. At the same time, it can also physically block the active material particles from direct contact with the electrolyte, reducing the dissolution of transition metals and corrosion by the byproduct hydrofluoric acid under high voltage, thereby improving the overall stability of the material.

[0049] It should be noted that the average thickness of the coating layer on the surface of the positive electrode active material described in this application can be confirmed by, but is not limited to, the following test methods: the secondary battery is discharged at 0.33C, then the positive electrode sheet is disassembled, the positive electrode sheet is scraped to remove powder, the resulting powder is dispersed in ethanol and ultrasonically treated for 10 minutes, then samples are transferred to a copper grid for transmission electron microscopy, dried, and the thickness of the particle coating layer is observed and measured under a 500kx discharge magnification of the transmission electron microscope. The number of samples measured is 20, and the average thickness of the coating layer of the samples is calculated, which is the average thickness of the coating layer.

[0050] In some embodiments, the positive electrode active material can be a commercially available product or can be obtained by a self-made method. Specifically, the positive electrode active material can be obtained by the following preparation method: Nickel, cobalt, and manganese sources are mixed in a solvent, a precipitant is added to precipitate the reaction, the mixture is allowed to stand, filtered, washed, and dried. The resulting mixed precursor is then mixed with a lithium source and a substance containing doped elements, ball-milled, calcined, and sieved to obtain the lithium nickel cobalt manganese oxide particles.

[0051] In some embodiments, the nickel source includes at least one of nickel sulfate, nickel nitrate, nickel chloride, nickel oxalate, and nickel acetate; In some embodiments, the cobalt source used includes at least one of cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt oxalate, and cobalt acetate; In some embodiments, the manganese source includes at least one of manganese sulfate, manganese nitrate, manganese chloride, manganese oxalate, and manganese acetate; 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. In some embodiments, the precipitant includes at least one of sodium hydroxide, sodium carbonate, oxalic acid, and ammonia.

[0052] In some embodiments, the solvent includes water.

[0053] In some embodiments, the precipitation reaction takes 0.5 to 1.5 hours.

[0054] In some embodiments, the heating rate during calcination is 1~10℃ / min, the calcination temperature is 750~900℃, and the time is 10~15h.

[0055] In some embodiments, the doped substance includes an oxide of the dopant element or a salt thereof.

[0056] Specifically, the substance includes, but is not limited to, at least one of aluminum oxide, titanium dioxide, zirconium oxide, and aluminum chloride.

[0057] 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.

[0058] In some embodiments, the compaction density of the positive electrode sheet is 3~4 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: After the secondary battery is discharged to zero, the positive electrode is disassembled and cut 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).

[0060] In some embodiments, the positive electrode material layer in the positive electrode sheet includes a positive electrode active material, a binder, a conductive agent, and a dispersant. The positive electrode active material includes lithium nickel cobalt manganese oxide, and the mass percentage of the positive electrode active material in the positive electrode material layer is 94-98%.

[0061] 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 adhesion 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.).

[0062] Specifically, the adhesive is selected from polytetrafluoroethylene or polyvinylidene fluoride.

[0063] In some embodiments, the mass percentage of the binder in the positive electrode material layer is 1% to 4%, such as 1%, 1.5%, 2%, 3%, 4%, or any range formed by any two of the above values.

[0064] 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.

[0065] In some embodiments, the mass percentage of the conductive agent in the positive electrode material layer is 1% to 2%, such as 1%, 1.1%, 1.2%, 1.4%, 1.5%, 1.8%, 2%, or any range formed by any two of the above values.

[0066] In some embodiments, the secondary battery includes a negative electrode sheet, the negative electrode sheet includes a negative electrode material layer, and the negative electrode material layer includes a negative electrode active material.

[0067] In some embodiments, the compaction density of the negative electrode sheet is 1.2~1.8 g / cm³. 3 .

[0068] The test method for the compaction density of the negative electrode sheet described in this application is the same as that for the positive electrode sheet, and will not be repeated here.

[0069] In some embodiments, the negative electrode active material includes at least one of graphite materials and silicon-based materials.

[0070] Preferably, the negative electrode active material comprises graphite material, and b = 91~98%.

[0071] In secondary batteries, when graphite is selected as the negative electrode active material, it can not only achieve good ion / electron conduction efficiency, but also has excellent structural stability and can withstand lithium-ion impact at high rates. In this case, the nickel content of the positive electrode active material is preferably in the above-mentioned higher range, which can further improve the fast charging performance of the secondary battery.

[0072] More preferably, the graphite material includes at least one of artificial graphite and natural graphite.

[0073] In some embodiments, the negative electrode active material comprises a silicon-based material, and b = 80~90%.

[0074] In rechargeable batteries, when silicon-based materials are chosen as the negative electrode active material, they exhibit lower desolvation energy and faster lithium-ion transport rates, effectively resisting lithium dendrite formation during fast charging and improving the battery's fast-charging performance. However, silicon particles experience volume expansion / contraction during lithium-ion insertion / extraction, leading to repeated rupture of the SEI film and lower electrode interface stability. Further optimization of the nickel content in the positive electrode active material within the aforementioned range can effectively balance overall battery stability, preventing further degradation of the negative electrode's structural stability due to ion impacts, thereby ensuring superior battery stability and fast-charging performance.

[0075] More preferably, the silicon-based material includes at least one of silicon-carbon composite material, elemental silicon, and silicon suboxide.

[0076] In some embodiments, the mass percentage of silicon in the negative electrode material layer is 1-30%.

[0077] When silicon is introduced into the negative electrode active material and the content is preferably within the above range, not only can its fast charging performance be effectively improved, but also the stability problem of the battery electrode caused by the volume change of silicon can be avoided. It can also achieve better cycle stability and long service life.

[0078] It should be noted that, in the technical solution of this application, the mass percentage of silicon in the negative electrode material layer can be confirmed by, but is not limited to, the following methods: The secondary battery is discharged, then the negative electrode sheet is disassembled, cleaned in DMC solvent for 2 hours, dried at 60°C, and then scraped off. The powder is weighed and its mass recorded, placed in a nickel crucible pre-filled with potassium hydroxide, and a small amount of potassium hydroxide is added to cover the sample surface. Two drops of ethanol are added, and the mixture is heated on an electric furnace until the potassium hydroxide melts and dehydrates. Then, it is transferred to a muffle furnace at 1100°C and kept at the melting temperature for 8 hours. After cooling, the crucible is placed in a 300 mL plastic beaker, and hot water is added for extraction. After the reaction, the crucible is removed, and hydrochloric acid is added to the beaker for acidification. Hydrogen peroxide is added dropwise. After cooling, other impurities are filtered out, and the filtered solution is transferred to a 100 mL volumetric flask, diluted to volume, and shaken well. After standing, the solution is transferred to another 100 mL volumetric flask, diluted to volume, shaken well, and allowed to stand until clear before analysis. Simultaneously, a blank solution was prepared. The blank solution refers to a solution without any sample added. Then, following the steps above, the solution was obtained. The operating conditions of the ICP instrument were set (Thermo Fisher iCAPPRO ICP-OES, gas flow rate 0.5 L / min, power 1150 W; Si element measurement wavelength 288.158 nm). The Si content of the element was determined by ICP.

[0079] In some embodiments, the negative electrode active material comprises a silicon-carbon composite material, wherein the particle size D of the silicon-carbon composite material is... v50 The size is 6~15μm.

[0080] When silicon-carbon composite material is selected as the negative electrode active material of the secondary battery described in this application, the particle size of the material will also affect the electrochemical performance of the secondary battery. If the particles are too large, similar to the positive electrode active material, the lithium ion transport efficiency will also be worse, thereby reducing the fast charging performance of the secondary battery. If the particles are too small, they will also lead to an increased tendency for side reactions with the electrolyte, thereby affecting the cycle stability of the battery. Therefore, when the particle size range is preferably within the above range, the fast charging performance and cycle performance of the battery can be balanced, resulting in excellent overall performance.

[0081] In some embodiments, the negative electrode material layer may also contain a conductive agent and / or a binder.

[0082] 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.

[0083] 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.

[0084] 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 adhesion 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.

[0085] 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%.

[0086] In some embodiments, the secondary battery further includes an electrolyte having a viscosity of 1.5~5 mPa·s at 25°C.

[0087] After simultaneously regulating the exothermic characteristics of the secondary battery and the positive electrode active material, the viscosity of the electrolyte further affects its wettability on the electrode and the stability of the SEI film formed on the surface of the negative electrode material. When the electrolyte is preferably within the above-mentioned range, the electrolyte wets the electrode well, the lithium-ion transport rate is faster, the secondary battery can exhibit more ideal fast-charging performance, and the degree of side reactions is lower, resulting in better cycle stability. Those skilled in the art can also use other parameters for related regulation based on the electrochemical behavior characteristics of the secondary battery, and are not limited to this.

[0088] It should be noted that the viscosity of the electrolyte described in this application can be confirmed by, but is not limited to, the following methods: disassemble the secondary battery in an empty state, seal it with an aluminum-plastic bag, squeeze the secondary battery with a hydraulic press (Beijing Heng'ao Technology Co., Ltd.'s FY-30 hydraulic press) to squeeze out the remaining electrolyte, puncture the hole with a needle to remove the electrolyte, place it in a beaker and control the sample temperature at 25°C, take out the electrolyte and test it with a Cambridge viscometer, and read the reading after the display value stabilizes, which is the viscosity of the electrolyte.

[0089] In some embodiments, the electrolyte includes a solvent, which includes at least one of carbonate solvents, carboxylic acid ester solvents, ether solvents, sulfone solvents, nitrile solvents, and phosphate ester solvents.

[0090] 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 (EA), methyl acetate (MA), 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.

[0091] More preferably, the solvent may also include, but is not limited to, at least one of carbonate solvent fluorinated derivatives, carboxylic acid ester solvent fluorinated derivatives, ether solvent fluorinated derivatives, sulfone solvent fluorinated derivatives, nitrile solvent fluorinated derivatives, and phosphate ester solvent fluorinated derivatives.

[0092] More preferably, the solvent in the electrolyte includes at least one of DMC, EMC, and EA, and the mass content of the solvent in the electrolyte is 10-70%.

[0093] When using the preferred low-viscosity solvents to formulate the electrolyte, in addition to effectively improving the lithium-ion transport efficiency, it can also effectively control the probability of decomposition and gas generation of the solvent components in the electrolyte to a low level, avoid reducing the cycle performance of the battery due to excessive heat generation, and ensure high cycle stability.

[0094] In some embodiments, the electrolyte also includes additives.

[0095] More preferably, the additive includes at least one of vinylene carbonate, fluoroethylene carbonate, vinyl sulfate, 1,3-propanesulfonate lactone, and methanedisulfonate.

[0096] More preferably, the additive has a mass percentage content of 0.5-5% in the electrolyte.

[0097] After selecting the solvent, the introduction of the above-mentioned additives can make the SEI film on the surface of the material layer more dense, further protect the material from contact with the electrolyte, improve the interface stability, and also improve the thermal stability of the electrolyte and the migration rate of lithium ions, thus optimizing the fast charging performance and cycle life of the secondary battery.

[0098] More preferably, the concentration of lithium salt in the electrolyte is 0.6~1.8 mol / L.

[0099] More preferably, the concentration of lithium salt in the electrolyte is one or any two of the following: 0.6 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.5 mol / L, and 1.8 mol / L.

[0100] 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.

[0101] In some embodiments, the secondary battery further includes a separator; More preferably, the diaphragm includes at least one of a polypropylene diaphragm and a polyethylene diaphragm.

[0102] More preferably, the air permeability of the diaphragm is 172~244s / 100mL.

[0103] By optimizing the permeability settings described above, the interfacial reactivity of the separator in the battery can be effectively reduced, thereby improving the overall transmission efficiency of the secondary battery during the lithium-ion transport process.

[0104] 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: Example 1 A secondary battery, the preparation method comprising the following steps: (1) Preparation of lithium nickel cobalt manganese oxide: Nickel sulfate solution, cobalt sulfate solution and manganese sulfate solution were mixed in stoichiometric ratio, and then sodium hydroxide precipitant was added. The pH was adjusted to be within the range of 11.0 ~ 11.8. The precursor reaction temperature was controlled at 45℃ for 11h. 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. At the same time, aluminum oxide containing doped elements was added. Based on the mass of lithium nickel cobalt manganese oxide, the mass content of Al element in aluminum oxide was 800 ppm. Then, the temperature was raised to 800℃ at 5℃ / min under oxygen atmosphere and calcined for 11h. After crushing and sieving, lithium nickel cobalt manganese oxide was obtained and used as positive electrode active material. (2) Preparation of the positive electrode sheet: The positive electrode active material, conductive agent acetylene black, and binder polyvinylidene fluoride are dispersed in N-methylpyrrolidone at a mass ratio of 92:4:4. The mixture is then vacuum stirred to prepare a slurry, which is subsequently coated on both sides of the current collector aluminum foil. After drying, cold pressing, and slitting, the positive electrode sheet is obtained. The areal density of the positive electrode sheet is 330 g / m³. 2 The compacted density is 3.55 g / cm³. 3 The porosity of the positive electrode material layer is 20.3%. (2) Preparation of negative electrode sheet: Graphite, a negative electrode active material, and silicon-carbon composite material (silicon content 45wt%) were mixed at a ratio of 9:1 (w:w), acetylene black (conductive agent), CMC (thickening agent), and SBR (binder) at a mass ratio of 96.4:1:1.2:1.4. Deionized water was added as solvent, and the mixture was stirred under vacuum until homogeneous to obtain a negative electrode slurry. The negative electrode slurry was uniformly coated on both surfaces of the copper foil current collector, dried at room temperature, and then transferred to an oven for further drying. The negative electrode sheet was then cold-pressed at different compaction densities and slit to obtain the negative electrode sheet. The areal density of the negative electrode sheet was 160 g / m³. 2 The compacted density is 1.45 g / cm³. 3 ; (3) Preparation of electrolyte: EC / EMC were mixed at a volume ratio of 3:7 as solvent, and then lithium hexafluorophosphate was added based on the total mass of the electrolyte to prepare an electrolyte with a lithium hexafluorophosphate concentration of 1 mol / L. (4) Stack the positive electrode, commercially available PP separator, and negative electrode in sequence to assemble the cell. Place the cell in the outer packaging shell, dry it, inject electrolyte, let it stand for 24 hours, charge it to 2.8V with a current of 0.02C, let it stand at 45℃ for 720 minutes, charge it to 3.4V with a current of 0.05C, let it stand for 10 minutes, and charge it to 3.92V with a current of 0.08C to obtain the lithium-ion secondary battery.

[0105] Examples 2-25, Comparative Examples 1-4 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.

[0106] Table 1 Table 2 Example of effect The secondary batteries obtained in each embodiment and comparative example were tested as follows: (1) Cyclic performance test: At 25°C, the secondary battery is pre-charged at a constant current of 1C to 4.35V, then charged at a constant voltage to the cutoff current of 0.05C, left to stand for 20 minutes, and then discharged at a constant current of 1C to 2.75V. This constitutes one cycle, and the discharge capacity A0 of the secondary battery at this time is recorded. The above steps are repeated until the discharge capacity of the secondary battery is ≤80%A0, and the number of cycles is recorded.

[0107] (2) Fast charging time test: During secondary battery assembly, a copper wire is added to the negative electrode side. After assembly, the battery is left to stand for 24 hours, then charged at 0.02C to 4.1V, followed by constant current charging at 0.33C to 4.25V, and then constant voltage charging to 0.05C. After standing for 10 minutes, it is discharged at 0.33C to 2.5V to complete formation. Subsequently, it is discharged and charged at 0.33C to 4.25V, then charged at constant voltage to 0.05C. After standing for 10 minutes, it is discharged at 0.33C to 2.5V. This constitutes one cycle of capacity determination. After two cycles of capacity determination, the copper wire is first lithium-plated at 0.02C for 4 hours, and then charged at 0.33C to 10% of the capacity from the second cycle of capacity determination. Immediately afterwards, constant current charging was performed at a 4C rate until the negative parameter potential reached 0 or the terminal voltage reached 4.25V. Then, decreasing-order charging was performed at a 0.2C gradient, with the cutoff condition remaining unchanged, until the charging rate dropped to 0.33C or the capacity reached 80%. Subsequently, constant current and constant voltage charging was performed at 0.33C until 4.25V was reached. The test temperature was 25℃, and the device signal acquisition frequency was 100ms. After the test, the charging time t required for the secondary battery to go from 10% to 80% SOC was calculated. The test results are shown in Table 3.

[0108] Table 3 As can be seen from Table 3: (1) The secondary battery described in this application is based on the exothermic relationship parameter a between the self-generated heat temperature and the thermal runaway temperature of the secondary battery, as well as the proportion of nickel in the total transition elements b and its particle size c in the positive electrode active material. This not only ensures the exothermic stability of the battery during charging and discharging, but also takes into account the migration rate of lithium ions during the insertion and extraction process and the interface stability after the electrode and electrolyte come into contact. In electrochemical testing, it not only has good fast charging performance with a fast charging time of no more than 20 minutes, but also excellent cycle stability. At a 1C rate, the number of high-capacity stable cycles can reach more than 1600 cycles, and the overall performance is good.

[0109] (2) After the positive electrode of the secondary battery meets the limitation of (a×b) / c=0.07~13.3 of this application, the product of the embodiment can take into account both fast charging performance and cycle performance compared with the comparative product that does not meet the limitation. When the coordination range of the above three parameters is further preferably in the range of 0.26~7.2, the fast charging performance and cycle life of the secondary battery can be further optimized.

[0110] (3) At the same time, as mentioned above, when the exothermic relationship parameter a of the secondary battery changes, the accumulation of side reactions and the kinetic performance inside the battery will be different, which will affect the lithium-ion transport rate during fast charging and the irreversible consumption of lithium-ions during cycling. Therefore, it is necessary to coordinate and regulate parameters b and c. When the parameter a is further optimized in the range of 1.6 to 4.6, the secondary battery will not only have a longer cycle life, but also better fast charging performance. Similarly, different nickel contents in the positive electrode active material result in varying degrees of lithium-nickel mixing and phase transition reversibility. The nickel content also affects the diffusion efficiency of lithium ions within the positive electrode active material structure. After synergistic regulation of these three parameters, further optimization of the nickel content (b) within the range of 85-92% allows the secondary battery to achieve both longer cycle life and superior fast-charging performance. The particle size of the positive electrode active material also influences the length of the lithium-ion insertion / extraction path, thus affecting the transport rate. Furthermore, changes in particle size alter the overall specific surface area of ​​the material, leading to variations in the degree of contact with the electrolyte after wetting. When the particle size D of the material changes... v50 When the size cμm is preferably between 3 and 7μm, the overall performance of the secondary battery is better.

Claims

1. A secondary battery, characterized in that, It includes a positive electrode sheet, the positive electrode sheet includes a positive electrode material layer, the positive electrode material layer includes a positive electrode active material, and the positive electrode active material includes lithium nickel cobalt manganese oxide; The positive electrode plate satisfies: (a×b) / c=0.07~13.3; a = T2 / (T2-T1), where T2℃ is the self-generated heat temperature of the secondary battery during ARC testing, and T1℃ is the thermal runaway temperature; b is the percentage of nickel in the positive electrode active material relative to the total mass of nickel, cobalt, and manganese; and cμm is the particle size D of the positive electrode active material. v50 .

2. The secondary battery as described in claim 1, characterized in that, The value of (a×b) / c is 0.26~7.

2.

3. The secondary battery as described in claim 1, characterized in that, a = 1.6~30, and / or b = 80~98%, and / or c = 2~20.

4. The secondary battery as described in claim 3, characterized in that, a = 1.9~26, and / or b = 85~92%, and / or c = 3~7.

5. The secondary battery as described in claim 1, characterized in that, The porosity of the positive electrode material layer is 20-45%.

6. The secondary battery as described in claim 1, characterized in that, The cathode material layer also contains doping elements, including at least one of Zr, Ti, Si, Mo, La, Ce, Te, Nb, Ta, Al, W, Sr, V, Y, Mg, Co, F, Cl, and S.

7. The secondary battery as described in claim 6, characterized in that, The mass content of the doping element in the cathode material layer is 100~10000ppm.

8. The secondary battery as described in claim 7, characterized in that, The doping element includes Al, and the mass content of Al in the cathode material layer is 200~800ppm.

9. The secondary battery as described in claim 1, characterized in that, The surface of the positive electrode active material is provided with a coating layer, and the average thickness of the coating layer is 2~10nm.

10. The secondary battery as described in claim 1, characterized in that, The secondary battery includes a negative electrode sheet, which includes a negative electrode material layer, and the compaction density of the negative electrode sheet is 1.2~1.8 g / cm³. 3 .

11. The secondary battery as described in claim 10, characterized in that, The negative electrode material layer includes a negative electrode active material; the negative electrode active material includes at least one of graphite material and silicon-based material.

12. The secondary battery as described in claim 11, characterized in that, The negative electrode active material includes graphite material, and b = 91~98%.

13. The secondary battery as described in claim 12, characterized in that, The particle size D of the graphite material v50 The value is 10~30μm.

14. The secondary battery as described in claim 11, characterized in that, The negative electrode active material includes silicon-based materials, and b = 80~90%.

15. The secondary battery as described in claim 14, characterized in that, The mass percentage of silicon in the negative electrode material layer is 1-30%.

16. The secondary battery as described in claim 11, characterized in that, The negative electrode active material includes a silicon-based material, which in turn includes a silicon-carbon composite material, wherein the particle size D of the silicon-carbon composite material is... v50 The value is 6~15μm.

17. The secondary battery as described in claim 1, characterized in that, The secondary battery also includes an electrolyte, the viscosity of which is 1.5~5 mPa·s at 25°C.

18. The secondary battery as described in claim 17, characterized in that, The electrolyte includes additives, which include at least one of vinylene carbonate, fluoroethylene carbonate, vinyl sulfate, 1,3-propanesulfonate lactone, and methanedisulfonate.

19. The secondary battery as described in claim 18, characterized in that, The electrolyte contains additives at a mass percentage of 0.5% to 5%.

20. The secondary battery as described in claim 1, characterized in that, The secondary battery also includes a separator; the air permeability of the separator is 172~244s / 100mL.

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