A secondary battery

Abstract

This application discloses a secondary battery, belonging to the field of battery technology. The secondary battery uses lithium nickel cobalt manganese oxide as the positive electrode active material, and simultaneously controls the Li / Ni mixing ratio of the material, the viscosity change rate of the electrolyte at different temperatures, and the ionic conductivity of the electrolyte. This results in the positive electrode active material having ideal structural stability during charging and discharging, while having a low degree of side reaction with the electrolyte and good kinetic performance. The secondary battery has both excellent fast charging performance and cycle stability.

Description

Technical Field

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

[0002] With the increasing popularity of electric vehicles, users' expectations have shifted from simply "being able to drive" to "being easy to use," with long charging times being one of the most pressing concerns. Although battery range has improved significantly, issues such as low-temperature degradation and rapid power consumption at high speeds can reduce actual range, leaving users frequently facing uncertainty about battery capacity. Further improving battery fast-charging performance to achieve "charge and go" can help reduce users' anxiety about unexpected situations, such as temporary long-distance travel or when charging stations are scarce.

[0003] Ternary materials (such as NCM and NCA) have higher electronic conductivity, which can effectively reduce battery internal resistance and polarization during fast charging. Furthermore, the layered structure of ternary materials reduces the migration resistance of lithium ions, facilitating rapid insertion and extraction of lithium ions between the lattice layers. Therefore, using ternary materials as the cathode material can improve the battery's fast-charging performance. However, the intense electrochemical and thermodynamic shocks during fast charging accelerate the structural degradation of ternary materials, interfacial side reactions, and lithium loss, leading to a decrease in battery cycle life. Summary of the Invention

[0004] The purpose of this application is to overcome the shortcomings of the existing technology and provide a secondary battery. This secondary battery uses lithium nickel cobalt manganese oxide as the positive electrode active material, and simultaneously controls the Li / Ni mixing ratio of the material, the viscosity change rate of the electrolyte at different temperatures, and the ionic conductivity of the electrolyte. This results in the positive electrode active material having ideal structural stability during charging and discharging, while having a low degree of side reaction with the electrolyte and good kinetic performance. The secondary battery has both excellent fast charging performance and cycle stability.

[0005] To achieve the above objectives, in a first aspect of this application, this application provides a secondary battery, including a positive electrode sheet and an electrolyte, wherein the positive electrode sheet includes a positive electrode material layer, the positive electrode material layer includes a positive electrode material, the positive electrode material includes a positive electrode active material, and the positive electrode active material includes lithium nickel cobalt manganese oxide; The secondary battery satisfies: a×b×c=0.014~2.3; Where a is the Li / Ni mixing ratio of the positive electrode active material, b is the viscosity change rate of the electrolyte and b = 100% × |b1-b2| / b1, b1 cp is the viscosity of the electrolyte at 25℃, b2 cp is the viscosity of the electrolyte at 45℃, and cmS / cm is the ionic conductivity of the electrolyte at 25℃.

[0006] The beneficial effects of this application are as follows: This application provides a secondary battery that uses lithium nickel cobalt manganese oxide as the positive electrode active material. Simultaneously, the Li / Ni mixing ratio of the material, the viscosity change rate of the electrolyte at different temperatures, and the ionic conductivity of the electrolyte are controlled to ensure that the positive electrode active material has ideal structural stability during charging and discharging, while having low side reaction with the electrolyte and good kinetic performance. The secondary battery has both excellent fast charging performance and cycle stability. 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, characterized in that it comprises a positive electrode sheet and an electrolyte, wherein the positive electrode sheet comprises a positive electrode material layer, the positive electrode material layer comprises a positive electrode material, the positive electrode material comprises a positive electrode active material, and the positive electrode active material comprises lithium nickel cobalt manganese oxide; The secondary battery satisfies: a×b×c=0.014~2.3; Where a is the Li / Ni mixing ratio of the positive electrode active material, b is the viscosity change rate of the electrolyte and b = 100% × |b1-b2| / b1, b1 cp is the viscosity of the electrolyte at 25℃, b2 cp is the viscosity of the electrolyte at 45℃, and cmS / cm is the ionic conductivity of the electrolyte at 25℃.

[0011] To improve the fast-charging performance of lithium nickel cobalt manganese oxide (LCO) secondary batteries, this application uses ternary LCO material as the positive electrode active material. By reducing the Li / Ni mixing ratio (a) of the positive electrode material, the irreversible transition from an ordered layered structure to an electrochemically inert rock salt phase can be avoided, reducing the migration resistance of lithium ions and thus improving the fast-charging performance of the secondary battery. However, due to the similar ionic radii and small charge difference between divalent nickel ions and lithium ions, they are prone to positional exchange during high-temperature synthesis, making Li / Ni mixing unavoidable. Therefore, increasing the ionic conductivity (c) of the electrolyte in the secondary battery can further reduce the internal resistance and accelerate the lithium ion migration rate, thereby reducing polarization losses during charging and improving the battery's fast-charging performance. While the fast charging performance of the battery is improved, an excessively high c can exacerbate side reactions at the electrolyte-electrode interface, accelerate the consumption of active lithium and material degradation, thereby reducing the battery's cycle life. Furthermore, the secondary battery described in this application can increase the stability of the solid electrolyte interface (SEI) film by reducing the viscosity change rate b of the electrolyte at different temperatures (25°C for conventional battery evaluation and 45°C for accelerated battery evaluation), while ensuring the wettability of the electrolyte to the electrode material. This reduces repeated rupture and regeneration of the SEI film, decreases the consumption of active lithium and the decomposition of the electrolyte, improves the battery's cycle life, and reduces irreversible capacity loss. It also enhances the stability of the interface, reduces the continuous consumption of active lithium and electrolyte, and improves the battery's cycle life.

[0012] By controlling the above three factors and coordinating their relationship a×b×c within the range of 0.014 to 2.3, the lithium nickel cobalt manganese oxide in the secondary battery can achieve good kinetic performance while ensuring structural stability. At the same time, the interface stability between the electrode and the electrolyte is high, ultimately enabling the secondary battery to balance fast charging performance and cycle stability.

[0013] In some implementations, a×b×c = is a range of one or any two of the following: 0.014, 0.016, 0.02, 0.03, 0.05, 0.08, 0.1, 0.2, 0.5, 0.8, 0.9, 0.97, 1, 1.5, 2, 2.2.

[0014] More preferably, a×b×c=0.08~0.97.

[0015] When the secondary battery further meets the above preferred range, the lithium nickel cobalt manganese oxide in the secondary battery has better structural stability during fast charging, and the interface stability between the electrode and the electrolyte is excellent, thus achieving better fast charging performance and cycle life.

[0016] In some implementations, a = 1~15%.

[0017] In some implementations, a is a range of one or any two of 1%, 1.5%, 2%, 3%, 5%, 8%, 10%, 12%, and 15%.

[0018] More preferably, a = 3~10%.

[0019] Li / Ni mixing in cathode materials refers to the arrangement of nickel ions (Ni) in the crystal structure of the cathode material. 2+ ) and lithium ions (Li + This refers to a situation where nickel ions occupy lithium layer positions that should be occupied by lithium ions, while lithium ions enter the transition metal layer. When the Li / Ni mixing ratio of the cathode material increases, nickel ions physically block the migration path of lithium ions, increasing the energy barrier for lithium ion insertion / extraction, making lithium ion migration more difficult, and reducing the battery's fast-charging performance. Furthermore, severe Li / Ni mixing can promote the transformation of the layered structure to a rock-salt phase, disrupting the stability of the crystal framework, making it prone to microcracks during charging and discharging, exposing more active surfaces, promoting electrolyte decomposition and transition metal dissolution, and leading to a decrease in battery cycle life. However, in the secondary battery described in this application, after synchronously controlling a, b, and c, when a is further preferably within the above range, the secondary battery can achieve better stability and kinetic performance of the cathode active material, and better fast-charging performance and cycle stability.

[0020] It should be noted that the value of 'a' described in this application can be controlled by, but is not limited to, the molar ratio of lithium to transition metal elements, especially nickel, during the synthesis of the positive electrode active material. Within a certain range, the larger the molar ratio of lithium to transition metal elements, the lower the overall value of 'a'. Alternatively, the value of 'a' can be controlled by introducing doping elements and adjusting their content. Within a certain range, the higher the content of doping elements, the lower the overall value of 'a'.

[0021] It should be noted that the Li / Ni mixing ratio 'a' of the positive electrode active material described in this application can be confirmed by, but is not limited to, the following methods: the secondary battery is discharged to a voltage of 2.5V at a rate of 0.33, then disassembled, the obtained positive electrode is immersed in DMC (dimethyl carbonate) for 30 minutes, then removed and dried, and the electrode is directly placed in an XRD (X-ray powder diffractometer) tester and scanned at a scanning rate of 3° / min, with a scanning range of 10~80°. The characteristic peaks in the obtained test spectrum are refined and fitted using GSAS software to obtain the Li / Ni mixing ratio.

[0022] In some implementations, b = 20%~70%.

[0023] In some implementations, b is a range of one or any two of the following: 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, and 70%.

[0024] More preferably, b = 25~60%.

[0025] As mentioned above, in a secondary battery, the internal temperature rises during the charging and discharging process (due to internal chemical reactions and energy consumption). The viscosity change rate of the electrolyte at different temperatures affects lithium-ion transport and the interfacial stability of the electrodes. By controlling the viscosity change rate of the electrolyte after temperature rise, as the temperature increases, the thermal motion of molecules in the electrolyte intensifies, the intermolecular interaction force weakens, leading to a decrease in internal friction and a decrease in electrolyte viscosity. This improves the wettability of the electrodes, thereby promoting the rapid diffusion of lithium ions and improving the fast-charging performance of the battery. However, if the viscosity change rate of the electrolyte is too large, the fluidity and ion conductivity of the electrolyte will fluctuate drastically, causing the current to concentrate in certain areas, forming "hot spots," exacerbating polarization, and even inducing lithium dendrite precipitation. Furthermore, the capillary permeation rate of the electrolyte in the electrode pores increases sharply, generating instantaneous interfacial pressure pulses. These pressure impacts damage the originally dense and stable SEI film structure, causing SEI film rupture, exacerbating the side reactions at the electrolyte-electrode interface, consuming active lithium, and leading to a decrease in the cycle life of the battery. Therefore, it is necessary to coordinate and regulate a and c simultaneously. When the secondary battery meets the three synergistic constraints and the viscosity change rate of the electrolyte is preferably within the above range, not only can the lithium ion transport rate be guaranteed to be high, but also the probability of lithium metal deposition during lithium deintercalation and deintercalation at the negative electrode can be reduced, thus improving both fast charging performance and cycle stability.

[0026] It should be noted that the viscosity change rate b of the electrolyte of the secondary battery described in this application can be controlled by adjusting the composition ratio of the solvent in the electrolyte, such as the mass ratio of carboxylic acid ester solvent and carbonate solvent. Within a certain range, the smaller the ratio, the higher b tends to be.

[0027] It should be noted that the viscosity change rate b of the electrolyte in the secondary battery described in this application can be confirmed by, but is not limited to, the following methods: Discharge the secondary battery at a rate of 0.33C to a voltage of 2.5V, disassemble the secondary battery, seal it in an aluminum-plastic bag, squeeze the secondary battery with a tablet press to squeeze out the remaining electrolyte, puncture the electrolyte with a syringe to remove it, place it in a container in a constant temperature bath and control the sample temperature at 25°C, remove the electrolyte and test it with a Cambridge viscometer, and after the displayed value stabilizes, read the reading, which is the viscosity b1 of the electrolyte at 25°C. Then raise the temperature and control the sample temperature to 45°C and keep it constant, and test it again with a viscometer. After the displayed value stabilizes, read the reading, which is the viscosity b2 of the electrolyte at 45°C. Finally, b can be confirmed by calculation.

[0028] In some implementations, b1 = 2~10cp; In some implementations, b1 is a range of one or any two of 2cp, 3cp, 4cp, 5cp, 6cp, 7cp, 8cp, 9cp, and 10cp.

[0029] In some implementations, b2 = 1~5cp.

[0030] In some implementations, b2 is a range of one or any two of 1cp, 2cp, 3cp, 4cp, and 5cp.

[0031] In some implementations, c = 5~25 mS / cm.

[0032] In some implementations, c is a range of one or any two of the following: 5mS / cm, 6mS / cm, 8mS / cm, 10mS / cm, 12mS / cm, 15mS / cm, 18mS / cm, 20mS / cm, 22mS / cm, and 25mS / cm.

[0033] More preferably, c = 8~20mS / cm.

[0034] As mentioned above, controlling the ionic conductivity of the electrolyte in the secondary battery to meet the aforementioned range can reduce the battery's internal resistance, accelerate lithium-ion migration, thereby reducing polarization losses during charging and improving the battery's fast-charging performance. Simultaneously, it promotes the formation of a uniform and dense SEI film during the first charge-discharge cycle, effectively preventing subsequent electrolyte decomposition, reducing active lithium consumption, and improving the battery's cycle life. Once the secondary battery meets the defined range of a×b×c, further optimizing the electrolyte's ionic conductivity within the aforementioned range can simultaneously improve both the fast-charging performance and cycle stability of the secondary battery.

[0035] It should be noted that the c described in this application can be controlled by adjusting, but is not limited to, the concentration of lithium salt in the electrolyte, or the composition and content of lithium salt in the electrolyte, such as the mass percentage of LiFSi in the lithium salt.

[0036] It should be noted that the ionic conductivity c of the electrolyte in the secondary battery described in this application can be confirmed by, but is not limited to, the following methods: Discharge the secondary battery to 2.5V at a rate of 0.33C, disassemble the secondary battery, seal it in an aluminum-plastic bag, squeeze the secondary battery with a tablet press to squeeze out the remaining electrolyte, puncture the electrolyte with a syringe to remove it, place it in a container in a constant temperature bath and control the sample temperature at 25℃, remove the conductivity meter test electrode, calibrate the electrode constant (K) using a standard KCl solution (12.85 mS / cm@25℃), place the sample in an Ar glove box, immerse the electrode in the electrolyte, apply a small amplitude AC signal (10 mV, frequency 1 MHz~0.1 Hz), and record the Nyquist plot; fit the intersection of the semicircle in the high-frequency region with the real axis to obtain Rb; calculate the single ionic conductivity of the electrolyte according to the formula σ=K / Rb, measure 3 times, take the average value, and the relative standard deviation should be <5%, which is the c.

[0037] In some embodiments, the cathode material contains transition metal elements, and the molar content of nickel in the transition metal elements is 0.5 to 0.95.

[0038] In some embodiments, the cathode material contains a transition metal element, wherein the molar content of nickel in the transition metal element is a range of one or both of 0.5, 0.6, 0.65, 0.7, 0.8, 0.9, and 0.95.

[0039] During charging, a large number of lithium ions are released from the cathode material, creating numerous vacancies in the lithium layer that provide landing sites for nickel ions to migrate. However, if the nickel content in the cathode material is too high, it can increase the local lattice stress during deep delithiation of the lithium nickel cobalt manganese oxide material, further promoting the diffusion of nickel into the lithium layer, exacerbating Li / Ni mixing, and increasing the resistance to lithium ion transport and migration. This is detrimental to improving the fast-charging performance of the secondary battery. Furthermore, this stress can cause cracks on the material surface, intensifying side reactions with the electrolyte and affecting the cycle performance of the secondary battery. When the molar content of nickel in the cathode material is preferably within the above-mentioned range, the transport resistance of lithium ions in the secondary battery is kept at a low level, less lattice stress is generated in the active material, and the fast-charging and cycle performance of the secondary battery are both better.

[0040] It should be noted that the molar content of nickel among transition metal elements described in this application can be confirmed by, but is not limited to, the following methods: The secondary battery was discharged to 2.5V at a rate of 0.33C. The battery was then disassembled to obtain the positive electrode. The positive electrode was immersed in DMC (dimethyl carbonate) at room temperature for 60 minutes, removed, and air-dried at room temperature with humidity ≤15%. The positive electrode material was scraped off the surface of the current collector. A certain amount of positive electrode material powder was accurately weighed and dispersed in 20mL of water. Then, 10mL of nitric acid was added, and the mixture was heated until dissolved. After the positive or negative electrode material powder dissolved, the material was diluted with water to 100mL to obtain the test solution. The test solution was subjected to ICP testing. Before the test, a standard solution must be 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 (generally 0, 1 mg / 100mL, 2 mg / 100mL, 3 mg / 100mL), and the element detection wavelength was selected. The experimental conditions were set as follows: Based on the characteristics of the sample and the element to be detected, the appropriate ICP instrument operating conditions were set: gas flow rate 0.5 L / min, power 1150 W, and the element detection wavelength was selected, depending on the element to be tested (Ni wavelength 231.60 nm, Co: 228.616 nm, Mn: 257.61 nm). The mass content of Ni, Co, and Mn in the sample can be read by the self-analysis function of the ICP testing software. The molar content is then obtained by dividing by their respective molar mass. Then, the molar content of nickel in the transition metal elements is calculated as: nNi / (nNi+nCo+nMn).

[0041] In some embodiments, the cathode material contains a dopant element, which includes at least one selected from Mg, Cr, Na, Al, Mn, Zr, Nb, Co, Ni, Ti, and V.

[0042] More preferably, the mass content of the dopant element in the cathode material is 100ppm to 10000ppm.

[0043] It should be noted that the mass content of the dopant elements described in this application can be confirmed by, but is not limited to, the following methods: After discharging the secondary batteries to 2.5V at a rate of 0.33C, multiple secondary batteries were disassembled. The resulting positive electrode sheets were soaked in DMC for 2 hours. 200g of positive electrode material powder was obtained by scraping off the positive electrode material layer from multiple sheets and placed in a polypropylene sample container. A magnetic rod and 300mL of ultrapure water were added. The sample container was sealed and placed on a ball mill at 80rpm for 60min. The magnetic rod was then retained, and the remaining sample was cleaned. Ultrapure water was added to the container to wash the magnetic rod. The magnetic rod was then transferred to a 250mL beaker and ultrapure water was added. The magnetic rod was soaked in water and ultrasonically cleaned. After removing the water, it was soaked in 60 mL of 10% hydrochloric acid solution for 30 min. The soaking solution was then transferred to a centrifuge tube and recorded as -100. The remaining magnetic rod was soaked in a beaker containing 10 mL of aqua regia and 50 mL of ultrapure water. The mixture was heated to boiling at 230°C for 30 min, then allowed to cool naturally to room temperature. The magnetic rod was rinsed three times with a small amount of ultrapure water. The rinsed solution was transferred to a 50 mL volumetric flask and diluted to volume, recording the solution as -50. The Thermo ICP analyzer was then turned on. Fisher Scientific's iCAP PRO series (RF power 1150W, atomizer and auxiliary gas flow rate set to 0.5L / min, rinsing time 30s), selects the elemental testing wavelength, depending on the element being tested (e.g., Mg wavelength 285.2nm, Cr wavelength 267.72nm, Al wavelength 308.21nm, Mn wavelength 257.61nm, Zr wavelength 336.12nm, Nb wavelength 309.4nm, Co wavelength 240.7nm, Ni wavelength 232nm, Ti wavelength 337.28nm, V wavelength 292.4nm). The ICP testing software's self-service analysis function can read the content of the corresponding dopant element in the sample. In some embodiments, the surface of the positive electrode active material is further provided with a coating layer, which includes at least one of alumina, zirconium oxide, titanium dioxide, zinc oxide, magnesium oxide, aluminum fluoride, lithium fluoride, polypyrrole, tungsten oxide, and boron oxide.

[0044] By further coating the surface of the positive electrode active material, the stability of the positive electrode active material during the cycle charge and discharge process can be effectively improved. At the same time, it can also prevent it from directly contacting the electrolyte, reduce side reactions, prevent the precipitation of lattice oxygen and lattice collapse in the material, maintain the order of lithium ion insertion and extraction channels, and thus improve the fast charging performance and cycle performance of the secondary battery.

[0045] Furthermore, the average thickness of the coating layer is 10~100nm.

[0046] It should be noted that the average thickness of the coating layer can be confirmed by, but is not limited to, the following methods: The secondary battery was discharged to 2.5V at a rate of 0.33C, disassembled, and the obtained positive electrode sheet was soaked in DMC for 2 hours and dried. The positive electrode material layer was then scraped off as a sample. The powder sample was uniformly dispersed in ethanol and tested by TEM (transmission electron microscopy). The magnification was adjusted to 300KX to identify the positive electrode active material particles. Five different positions were selected, and the magnification was further increased to 500KX to measure the thickness of the coating layer on the outer surface of the core. The number of particles was counted as 200, and the average value was calculated to obtain the average thickness of the coating layer.

[0047] In some embodiments, the particle size D of the positive electrode material v50 The range is 2~16μm.

[0048] In some embodiments, the particle size D of the positive electrode material v50 The range is one or any two of the following: 2μm, 3μm, 5μm, 8μm, 10μm, 12μm, 15μm, and 16μm.

[0049] When the particle size of the cathode material is preferably within the above range, it can not only shorten the diffusion path of lithium ions during lithium insertion and extraction, but also appropriately reduce the specific surface area of ​​the overall material, so that the contact area between the cathode material and the electrolyte is smaller, and the direct contact side reactions are reduced, which is more conducive to improving the fast charging performance and cycle performance of the secondary battery at the same time.

[0050] In some embodiments, the particle size D of the positive electrode material v10 The value is 1~12μm, D v90 The value is 4~20μm.

[0051] In some embodiments, the particle size D of the positive electrode material v10 Particle size D v50 Particle size D v90 Confirmation can be made through, but is not limited to, the following methods: The secondary battery was discharged to 2.5V at 0.33C, and the positive electrode was obtained by disassembly. It was then immersed in dimethyl carbonate (DMC) solution at room temperature for 2 hours, removed, and dried in a vacuum environment. The positive electrode material powder on the surface of the electrode was scraped off with a ceramic knife. The particle size distribution was measured using a laser particle size analyzer (Mastersizer 3000) according to the laser diffraction method (specific steps refer to GB / T19077-2016). The particle size corresponding to the cumulative particle size distribution percentage reaching 10%, 50%, and 90% is D. v10 D v50 D v90 .

[0052] In some embodiments, the lithium nickel cobalt manganese oxide has the chemical formula LiNi. b Coc Mn d O2, where 0.4≤b≤0.99, 0.05≤c≤0.3, and 0.05≤d≤0.3.

[0053] In some embodiments, the lithium nickel cobalt manganese oxide has a mass percentage content of 92% to 99% in the cathode material, such as 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or any range formed by any two of the above values.

[0054] In some embodiments, the positive electrode material further includes a conductive agent and a binder.

[0055] 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.).

[0056] Specifically, the adhesive is selected from polytetrafluoroethylene or polyvinylidene fluoride.

[0057] In some embodiments, the mass percentage of binder in the cathode material is 0.5% to 4.0%, such as 0.5%, 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.

[0058] 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.

[0059] In some embodiments, the mass percentage of the conductive agent in the positive electrode material is 0.5% to 4%, such as 0.5%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.5%, 3.0%, 4.0%, or any range formed by any two of the above values.

[0060] In some embodiments, the positive electrode sheet can be prepared by, but is not limited to, the following methods: 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.

[0061] In some embodiments, the solvent includes N-methylpyrrolidone (NMP).

[0062] In some embodiments, the areal density of the positive electrode sheet is 200~550 g / m³. 2 The compacted density is 3.3~3.8 g / cm³. 3 .

[0063] When the compaction density of the positive electrode sheet is preferably within the above range, it can make the material particles more closely arranged, significantly reduce the diffusion resistance of ions inside the electrode, build a shorter and more direct transport path for lithium ions, improve the fast charging performance of the secondary battery, and avoid the problem of excessive accumulation of electrolyte inside the positive electrode sheet, which would lead to an increase in side reactions and improve the cycle life of the battery.

[0064] It should be noted that the areal density and compacted density of the positive electrode sheet described in this application can be confirmed by, but not limited to, the following methods: The positive electrode sheet is obtained by disassembling a secondary battery in its empty state; the positive electrode sheet is immersed in DMC at room temperature (25°C) for 60 minutes, removed, and dried; the pretreated positive electrode sheet is punched into circular sheets of a fixed area using a punching machine, the area of ​​which is denoted as S0; three circular sheets are taken as parallel samples, and the mass of each of the three circular sheets is weighed using an electronic balance, the average value is taken and denoted as M1; the density is measured using a micrometer. The average thickness of the active material layer in the three discs (i.e., the total thickness minus the current collector) is recorded as H. Finally, an appropriate amount of deionized water is dropped onto each of the three discs, and the coating on the discs is gently wiped off with lint-free paper to expose the copper foil. The discs are left to stand at room temperature (or dried) for 10 minutes. After the copper foil is dry, the mass of the three copper foils is weighed and the average mass is recorded as M0. The compaction density of the positive electrode is calculated according to the following formulas: A = (M1-M0) / (H*S0), and the areal density is B = (M1-M0) / S0.

[0065] In some embodiments, the lithium nickel cobalt manganese oxide can be a commercially available product, or it can be prepared by, but is not limited to, the following methods: 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, a flux, and an oxide or salt containing doped elements. The mixture is then calcined once at 400-600°C in air, followed by a second calcination at 1000-1200°C. The temperature is then adjusted to 900-1000°C and held for treatment. After cooling and crushing, the lithium nickel cobalt manganese oxide particles are obtained.

[0066] 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. More preferably, the lithium source is molten salt; In some embodiments, the precipitant includes at least one of sodium hydroxide, sodium carbonate, oxalic acid, and ammonia.

[0067] In some embodiments, the solvent includes water.

[0068] In some embodiments, the precipitation reaction takes 0.5 to 24 hours.

[0069] In some embodiments, the flux comprises a compound containing at least one of the elements Sr, Na, and K. Further, the compound may be, but is not limited to, oxides and / or carbonates containing the aforementioned elements.

[0070] More preferably, the amount of flux added is 0.08 to 0.42% of the total mass of the mixed precursors during a single calcination.

[0071] In some embodiments, the heating rate during the first calcination is 1~15℃ / min, and the time is 2~4h.

[0072] In some embodiments, the heating rate during the secondary calcination is 1~15℃ / min, and the time is 3~10min.

[0073] In some embodiments, the heat preservation treatment lasts for 8 to 12 hours.

[0074] In some embodiments, the crushing is carried out using an air jet mill, with an induced draft frequency of 10-50 Hz. In some embodiments, the particle size D of the resulting lithium nickel cobalt manganese oxide is... v50 The particle size is 2~16μm, and the span value is 1~1.7, where the span value is (the particle size D of lithium nickel cobalt manganese oxide). v90 - Particle size D of lithium nickel cobalt manganese oxide v10 Particle size D of lithium nickel cobalt manganese oxide v50 The particle size D of the lithium nickel cobalt manganese oxide v50 D v10 and D v90 Confirmed using a laser particle size analyzer.

[0075] In some embodiments, the electrolyte comprises a solvent and a lithium salt; More preferably, the lithium salt includes at least one of LiPF6 and LiFSi.

[0076] More preferably, the concentration of the lithium salt in the electrolyte is 0.8~1.5 mol / L.

[0077] When the preferred high-dissociation lithium salt is selected, and the lithium salt concentration is preferably within the above range, the transference number of lithium ions can be increased, concentration polarization can be reduced, the fast-charging performance of the battery can be improved, and a uniform and dense SEI film can be formed on the negative electrode surface, reducing the consumption of active lithium and the oxidative decomposition of the electrolyte, thereby improving the cycle life of the battery.

[0078] 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.

[0079] 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 methyl ethyl carbonate (EMC); carboxylic acid ester solvents include, but are not limited to, at least one of ethyl acetate (EA), 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.

[0080] 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.

[0081] In some embodiments, the solvent includes at least one of carboxylic acid ester solvents and carbonate solvents.

[0082] Furthermore, the solvent includes carboxylic acid ester solvents and carbonate solvents, wherein the mass ratio of the carboxylic acid ester solvents to the carbonate solvents is 0.002 to 0.5.

[0083] In some embodiments, the solvent includes carboxylic acid ester solvents and carbonate solvents, wherein the mass ratio of the carboxylic acid ester solvent to the carbonate solvent is one or any two of the following: 0.002, 0.005, 0.008, 0.01, 0.05, 0.08, 0.1, 0.2, 0.25, 0.3, 0.4, 0.5.

[0084] Carboxylic acid ester solvents have low viscosity and therefore high ionic conductivity. When used as solvents in electrolytes, they can reduce the overall electrolyte impedance and increase the lithium-ion transport rate, thereby improving the fast-charging performance of secondary batteries. Carbonate solvents, on the other hand, help form a more stable solid electrolyte interface film on the surface of the negative electrode, suppressing side reactions in the electrolyte and the large loss of lithium ions, thus improving the cycle stability of secondary batteries. When the two are combined and the ratio is preferably within the above range, they can synergistically improve the lithium-ion transport efficiency and cycle stability of secondary batteries, resulting in better fast-charging and cycle performance.

[0085] It should be noted that the mass ratio of carboxylic acid ester solvents to carbonate solvents in the electrolyte can be confirmed by, but is not limited to, the following methods: The secondary battery was discharged to 2.5V at a rate of 0.33C. The battery was then disassembled and the electrolyte collected in a glove box (H2O ≤ 0.1ppm, O2 ≤ 0.1ppm). There are three methods for collecting the electrolyte: After removing the battery cover, ① if there is free electrolyte, collect it into a 5mL sample tube using a pipette and seal it with adhesive tape to prevent leakage. ② if there is no free electrolyte, a hydraulic press (Beijing Heng'ao Technology Co., Ltd.'s FY-30 hydraulic press) can be used to continuously pressurize until free electrolyte appears. Collect the electrolyte into a sample tube and seal it. ③ Add an appropriate amount of dichloromethane extractant to the battery and record the dichloromethane content. After adding dichloromethane, put the battery into an aluminum-plastic bag and seal it with a heat sealer. Transfer it to an ultrasonic oscillator and oscillate for 12 hours to allow the electrolyte in the electrode to mix thoroughly with the dichloromethane. Then, use a pipette to draw the mixture of dichloromethane and electrolyte into a 5mL sample tube and seal the sample tube with sealing glue. The collected electrolyte samples were injected into an Agilent Intuvo 9000 gas chromatograph-mass spectrometer using a microsyringe for testing, yielding GC-MS spectra. Electrolyte additives were dissolved in EMC solvent to prepare solutions of different concentrations, which were then injected into the Agilent Intuvo 9000 gas chromatograph-mass spectrometer to obtain standard GC-MS spectra. The GC-MS spectra of the electrolyte to be tested were compared with the standard GC-MS spectra to confirm the presence of corresponding carboxylic esters and carbonates. The mass content of each carboxylic ester and carbonate in the electrolyte to be tested was then determined based on the peak area, and finally, the mass ratio of carboxylic esters to carbonates was calculated.

[0086] In some embodiments, the carboxylic acid ester solvent includes at least one of ethyl acetate, ethyl propionate, propyl acetate, methyl propionate, and propyl propionate.

[0087] In some embodiments, the carbonate solvent includes at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0088] Furthermore, the carboxylic acid ester solvent includes ethyl acetate, and the carbonate solvent includes at least one of ethylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0089] Furthermore, the carbonate solvent includes EC, EMC and DEC, and the mass ratio of EC, EMC and DEC is (1~6):(1~6):(1~3).

[0090] In some embodiments, the electrolyte also includes additives.

[0091] More preferably, the additive includes at least one of fluoroethylene carbonate, vinylene carbonate, methane disulfonate, tris(trimethylsilane)borate, propylene sulfite, vinyl ethylene carbonate, vinyl sulfate, lithium dioxaborate, lithium difluorophosphate, lithium difluorooxaborate, lithium bis(fluorosulfonyl)imide, lithium bis(oxaborate), lithium tetrafluoroborate, lithium bis(trifluoromethanesulfonyl)imide, tris(trimethylsilane) phosphate, tris(trimethylsilane) phosphite, tris(trimethylsilane) phosphate, trimethyl phosphate, and triphenyl phosphate.

[0092] More preferably, the additive has a mass percentage content of 1-15% in the electrolyte.

[0093] In some embodiments, the additive is present in the electrolyte at a mass percentage of one or any two of the following: 1%, 2%, 3%, 4%, 5%, 6%, 8%, 10%, 12%, and 15%.

[0094] When the aforementioned preferred additives are further introduced into the electrolyte, the thermal stability of the electrolyte can be effectively improved, thereby inhibiting the decomposition of the solvent under different environments, and thus keeping the viscosity change rate of the electrolyte within a moderate range at different temperatures, thereby improving the fast charging performance and cycle stability of the secondary battery.

[0095] In some embodiments, the secondary battery further includes a negative electrode and a separator.

[0096] In some embodiments, the negative electrode sheet includes a negative electrode material layer with a porosity of 20-60%.

[0097] In some embodiments, the porosity of the negative electrode material layer is a range of one or both of 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, and 60%.

[0098] When the secondary battery is synergistically regulated for the positive electrode and electrolyte, and the porosity of the negative electrode material layer is further optimized within the above-mentioned range, the lithium ion insertion / extraction sites can be effectively guaranteed, the lithium ion diffusion path can be shortened, and appropriate wetting with the electrolyte can be guaranteed, avoiding excessive interfacial side reactions caused by excessive electrolyte wetting, thereby further improving the fast charging performance and cycle performance of the secondary battery.

[0099] It should be noted that the porosity of the negative electrode material layer described in this application can be confirmed by, but is not limited to, the following methods: The secondary battery was discharged to 2.5V at a rate of 0.33C. The negative electrode was disassembled and soaked in dimethyl carbonate (DMC) solution for 4 hours, then dried. The electrode was then cut into 12mm diameter discs using an electrode punching machine. The thicknesses of the electrode and current collector were measured using a thickness gauge, denoted as h1 and h2 respectively. The mass was weighed using a balance with an accuracy of 0.00001g and recorded as m1. According to the formula v=πr... 2 Calculate the volume v of the cut electrode sheet (h1-h2); Immerse the electrode sheet in a sealed container with a certain volume of hexadecane for 1 hour (the volume of hexadecane in the sealed solution is not required, but the amount must be sufficient to completely submerge the electrode sheet); after 1 hour, remove the electrode sheet with tweezers and place it on filter paper to absorb dry until constant weight (generally, 1 hour is sufficient to absorb dry to constant weight). Weigh it using a balance and record the mass as m2. Calculate the porosity using the formula X / v, where X = (m2-m1) / ρ, and ρ is the density of hexadecane, 0.7734 g / cm³. 3 .

[0100] In some embodiments, the negative electrode material layer includes a negative electrode material containing manganese. After the secondary battery is cycled 300 times at a rate of 0.33C in a voltage range of 2.5 to 4.25V, the mass percentage of manganese in the negative electrode material layer is 0.1 to 100 ppm.

[0101] In some embodiments, after the secondary battery is cycled 300 times at a rate of 0.33C in a voltage range of 2.5 to 4.25V, the mass percentage of manganese in the negative electrode material layer is one or any two of the following values: 0.1ppm, 0.5ppm, 1ppm, 5ppm, 10ppm, 20ppm, 50ppm, 80ppm, 90ppm, and 100ppm.

[0102] After cycling, the manganese in the negative electrode material layer of the secondary battery mainly originates from the positive electrode. During cycling, it dissolves and enters the electrolyte along with lithium ions, transferring to the negative electrode and depositing there. By controlling the mass percentage of manganese in the negative electrode material layer after cycling, the reduction degree of manganese ions on the negative electrode can be reduced, thus decreasing its catalytic activity for electrolyte oxidation reactions, reducing the frequency of SEI film rupture / regeneration, thereby reducing interfacial impedance and increasing the diffusion rate of lithium ions. This can further improve the fast-charging performance and cycle performance of the secondary battery.

[0103] It should be noted that, after the secondary battery described in this application is cycled 300 times at a rate of 0.33C within a voltage range of 2.5 to 4.25V, the mass percentage content of manganese in the negative electrode material layer can be confirmed in the following way: The secondary battery was charged at a constant current rate of 0.33C to a voltage of 4.25V, then charged at a constant voltage rate to a cutoff current of 0.05C, and then discharged at a constant current rate of 0.33C to a voltage of 2.5V. This constituted one cycle, and the cycle was repeated 300 times. The secondary battery was then disassembled, and the negative electrode sheet was soaked in DMC (dimethyl carbonate) at room temperature for 60 minutes. After removal, 200g of negative electrode material powder was scraped off from the electrode sheet and placed in a polypropylene sample container. A magnetic rod and 300mL of ultrapure water were added, the sample container was sealed, and the container was placed on a ball mill and ball-milled at 80rpm for 60 minutes. The magnetic rod was then retained, and the remaining sample was cleaned up. Add ultrapure water to the tank to wash the magnetic rod, then transfer the magnetic rod to a 250mL beaker, add ultrapure water to soak the magnetic rod and ultrasonically clean it, then remove the water and add 60mL of 10% hydrochloric acid solution to soak for 30min. Then place the soaking solution into a centrifuge tube and record the solution as -100. Add 10mL of aqua regia and 50mL of ultrapure water to the remaining beaker of the magnetic rod and soak it. Heat at 230℃ to boiling and continue for 30min. Let it cool naturally to room temperature, rinse the magnetic rod 3 times with a small amount of ultrapure water, transfer the rinsed solution to a 50mL volumetric flask, make up to volume and record the solution as -50. Turn on the Thermo Fisher Scientific iCAP PRO series ICP analyzer (RF power 1150W, nebulizer flow and auxiliary gas flow set to 0.5L / min, rinsing time 30s), select the elemental test wavelength (Mn wavelength: 257.61nm), and the mass percentage of the corresponding Mn element in the sample can be read by the self-analysis function of the ICP test software.

[0104] In some embodiments, the negative electrode material includes a negative electrode active material, which includes at least one of graphite and silicon carbide materials.

[0105] Furthermore, the negative electrode active material includes graphite, and the particle size D of the negative electrode material is... v50 The value is 4~20μm.

[0106] When the negative electrode active material is selected as highly conductive graphite, and the particle size of the negative electrode material is preferably within the above range, it can shorten the diffusion path of lithium ions, improve the ion transport rate, reduce the specific surface area of ​​the overall material, and reduce the side reactions between the material and the electrolyte, thereby improving the fast charging performance and cycle life of the secondary battery.

[0107] In some embodiments, the particle size D of the negative electrode material v10 The particle size is 1.5~10μm, and the particle size D is... v90 The value is 8~35μm.

[0108] In some embodiments, the particle size D of the negative electrode material v10 Particle size Dv50 Particle size D v90 Confirmation can be made through, but is not limited to, the following methods: The secondary battery was discharged to 2.5V at 0.33C, and the negative electrode sheet was obtained by disassembly. It was then immersed in dimethyl carbonate (DMC) solution at room temperature for 2 hours, removed, and dried in a vacuum environment. The negative electrode material powder on the surface of the electrode sheet was scraped off with a ceramic knife. The particle size distribution was measured using a laser particle size analyzer (Mastersizer 3000) according to the laser diffraction method for particle size distribution (specific steps refer to GB / T19077-2016). The particle size corresponding to the cumulative particle size distribution percentage reaching 10%, 50%, and 90% is D. v10 D v50 D v90 .

[0109] In some embodiments, the negative electrode material further includes a conductive agent, a thickener, and a binder.

[0110] More preferably, the conductive agent includes at least one of carbon black, acetylene black, artificial graphite, natural graphite, carbon nanotubes, and graphene; the thickener includes at least one of sodium carboxymethyl cellulose, polyvinylpyrrolidone, polyethylene glycol, and sodium alginate; and the binder includes at least one of polyvinylidene fluoride, styrene-butadiene rubber, polyacrylic acid, polyimide, and polyurethane.

[0111] More preferably, the mass ratio of the negative electrode active material, conductive agent, thickener, and binder is (96~98):(0.5~1):(0.5~1):(2~3).

[0112] In some embodiments, the separator is disposed between the positive and negative electrode plates to separate them and prevent short circuits caused by contact. The separator can be at least one of glass fiber, nonwoven fabric, polyethylene (PE), polypropylene (PP), and polyvinylidene fluoride. The separator surface can also be coated with an inorganic coating and / or an organic coating. The inorganic coating material includes at least one of alumina, silicon oxide, titanium oxide, magnesium oxide, zirconium oxide, and boehmite. The organic coating includes at least one of aramid coating and polyvinylidene fluoride (PVDF) coating.

[0113] More preferably, the air permeability of the diaphragm is 100~400s / 100mL.

[0114] In some embodiments, the permeability of the diaphragm is a range of one or both of the following: 100s / 100mL, 150s / 100mL, 200s / 100mL, 250s / 100mL, 300s / 100mL, 350s / 100mL, and 400s / 100mL.

[0115] It should be noted that the air permeability of the diaphragm described in this application can be confirmed in the following way: The secondary battery was discharged at 0.33C to the lower limit voltage of 2.5V. Then, the empty battery was disassembled, the separator was removed, and soaked in DMC solution for 2 hours. After drying, the separator permeability was measured using a separator permeability meter. In a low-humidity, normal-pressure environment, with the instrument applying a pressure of 1.21 kPa, the permeability of 100 mL of air was measured to be 6.45 cm². 2 The time required for the sample (the cut diaphragm) to be processed is the air permeability of the diaphragm.

[0116] In the technical solution of this application, based on the regulation of the positive electrode and the electrolyte, the lithium ion transport efficiency inside the battery and the stability of the positive electrode active material layer during the lithium intercalation and deintercalation process can be effectively balanced. The interface stability between the electrode and the electrolyte is high. When the permeability of the separator of the secondary battery is further optimized within the above range, it can ensure that the electrolyte is fully wetted and the lithium ions migrate efficiently, thereby improving the fast charging performance of the battery. At the same time, it can avoid excessive side reactions between the electrolyte and the positive and negative electrode interfaces, thereby improving the cycle life of the battery.

[0117] 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 positive electrode active material: Nickel sulfate, cobalt sulfate and manganese sulfate were mixed in water at a molar ratio of nickel, cobalt and manganese of 60:20:20. Sodium hydroxide was added as a precipitant to adjust the pH to 12.1 and a precipitation reaction was carried out at 110℃. After standing, filtration, washing and drying were performed. The mixture was then mixed with LiOH-LiNO3 molten salt at a molar ratio of lithium atoms to the total atoms of nickel, cobalt and manganese in the precursor of 1.06:1. Strontium oxide flux was added at 0.1 wt% of the total mass of the molten salt and precursor. Zirconia dopant was added and ball-milled. The mixture was calcined for 3 hours at a rate of 5℃ / min to 500℃ in air atmosphere, followed by a second calcination at 5℃ / min to 1050℃ for 5 minutes. The temperature was then adjusted to 980℃ and held for 10 hours. After cooling, the mixture was crushed by air milling at a frequency of 20Hz to obtain the lithium nickel cobalt manganese oxide (LiNi). 0.6 Co 0.2 Mn 0.2 O2; The lithium nickel cobalt manganese oxide has a particle size Dv50 of 4.3 μm and a Span value of 1.42; (2) Preparation of the positive electrode: LiNi nickel cobalt manganese oxide LiNi 0.6 Co 0.2 Mn 0.2O2, conductive agent acetylene black, and binder polyvinylidene fluoride are dispersed in N-methylpyrrolidone at a mass ratio of 98:1:1. The mixture is then vacuum-stirred to prepare a slurry, which is subsequently coated onto 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 250 g / m³. 2 The compacted density is 3.51 g / cm³. 3 ; (3) Preparation of negative electrode sheet: The particle size D v50 Natural graphite (12μm negative electrode active material), acetylene black (conductive agent), sodium carboxymethyl cellulose (binder), and styrene-butadiene rubber (binder) were dispersed in water at a mass ratio of 96.4:1:1.2:1.4. The mixture was then vacuum-stirred to prepare a slurry, which was subsequently coated onto both sides of a copper current collector foil. After drying, cold pressing, and slitting, the negative electrode sheet was obtained. The compacted density of the negative electrode sheet was 1.55 g / cm³. 3 The surface density is 185 g / m³ 2 ; (4) Selection of diaphragm: The diaphragm is commercially available. Its base membrane is a PE membrane with a thickness of 9μm. One side of the base membrane is provided with an alumina coating with a thickness of 3μm and a PVDF coating with a thickness of 2μm. The other side of the base membrane is provided with a PVDF coating with a thickness of 2μm.

[0118] (5) Preparation of electrolyte: After mixing carbonate EC, EMC and DEC in a mass ratio of 1:1:1, add 17.7% of carboxylic acid ester EA based on the total mass of carbonate to obtain a mixed organic solvent. Add lithium salt LiPF6 and dissolve it in the mixed organic solvent to prepare an electrolyte with a molar concentration of 0.92 mol / L.

[0119] (6) The positive electrode, separator and negative electrode are stacked, wound and assembled into a cell in sequence. The cell is placed in the outer packaging shell, dried and injected with electrolyte. After vacuum sealing, standing for 24 hours, formation and volume adjustment, the secondary battery is obtained. The formation process is as follows: After the battery is placed at 25°C for 24 hours, it is placed in a glass clamp with a clamping pressure of 0.5 MPa. 1) Let stand at 45℃ for 10 minutes; 2) Charge at a rate of 0.05C, with a cutoff voltage of 3.4V; 3) Let stand for 10 minutes; 4) Charge at a rate of 0.05C, with a cutoff voltage of 3.75V; 5) End.

[0120] The parameters of the secondary battery were statistically analyzed, and the results are shown in Tables 1 and 2.

[0121] Examples 2-19 A secondary battery differs from Example 1 only in that the preparation process parameters of the positive electrode active material and the characteristic parameters of the obtained positive electrode material are different, as shown in Tables 1-2.

[0122] Comparative Examples 1-4 A secondary battery differs from Example 1 only in that the preparation process parameters of the positive electrode active material and the characteristic parameters of the obtained positive electrode material are different, as shown in Tables 1-2.

[0123] Table 1 Table 2 Example of effect The cathode materials and secondary batteries obtained in each embodiment and comparative example were tested as follows: (1) Fast charging performance test: Copper wires were inserted into the secondary batteries obtained in each embodiment and comparative example during assembly. After the secondary batteries were assembled, they were left to stand for 24 hours, then charged at 0.02C to 4.1V, then charged at a constant current of 0.33C to 4.25V, and then charged at a constant voltage to 0.05C. After standing for 10 minutes, they were discharged at a constant current of 0.33C to 2.5V to complete the formation. Then they were discharged at a constant current of 0.33C to 4.25V, then charged at a constant voltage to 0.05C. After standing for 10 minutes, they were discharged at a constant current of 0.33C to 2.5V. This was considered one cycle of capacity determination. After two cycles of capacity determination, First, lithium plating was performed on the copper wire at a rate of 0.02C for 4 hours. Then, the battery was charged at a rate of 0.33C to 10% of its capacity for the second cycle of constant-capacity charging. Immediately afterwards, constant-current charging was performed at a rate of 4C until the negative parameter potential reached 0 or the terminal voltage reached 4.25V. Then, a step-down charging was performed at a rate of 0.2C, 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 to 4.25V. The test temperature was 25℃, and the equipment 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. (2) Cyclic performance test: The secondary batteries obtained in each embodiment and comparative example were charged at a constant current of 0.33C to 4.25V at room temperature, then charged at a constant voltage to 0.05C to cut off charging, and then discharged at a constant current of 0.33C to 2.5V. The above steps were repeated for a total of 3 charge-discharge cycles. The discharge capacity Q1 of the third charge-discharge cycle was obtained as the fixed capacity. Then the battery was charged at a constant current of 0.33C to 4.25V, then charged at a constant voltage to 0.05C to cut off charging, and then discharged at a constant current of 0.33C to 2.5V. This was one cycle, and the cycle was repeated 300 times. The discharge capacity Q2 of the battery in the 300th cycle was recorded. The capacity retention rate = Q2 / Q1*100% The test results are shown in Table 3.

[0124] Table 3 As can be seen from Table 3: (1) In the secondary battery described in this application, on the one hand, reducing the Li / Ni mixing ratio a of the positive electrode material can effectively suppress the irreversible rock salt phase transformation of the ternary material in the positive electrode material and improve the fast charging performance of the secondary battery. On the other hand, considering the inevitability of lithium-nickel mixing, further synergistic optimization of the viscosity change rate and ionic conductivity of the electrolyte at different temperatures can further improve the lithium ion migration rate in the battery and reduce the polarization loss of active lithium during charging, so that the fast charging performance of the secondary battery can reach an excellent level. As shown in Examples 1 to 19, the fast charging time can be controlled within 20 minutes, and the fastest can reach about 8 minutes. At the same time, this synergistic regulation can also ensure the wettability of the electrolyte in the battery to the electrode material, and the stability of the interface film formed at the electrode interface is better. During the cycle, the degree of rupture / regeneration of the interface film is small, the side reaction is less, and the cycle stability of the secondary battery is also better. The capacity retention rate of each embodiment can reach more than 85% after cycle testing. In contrast, the various ratios cannot coordinate the balance between the mixing rate 'a', the electrolyte viscosity change rate, and the ionic conductivity, making it difficult to achieve both fast charging and cycle performance of the secondary battery.

[0125] (2) At the same time, when a, b and c are synergistically regulated, when a×b×c is further optimized and adjusted to the range of 0.08~0.97, the ternary material in the secondary battery undergoes fewer irreversible phase transitions during cycling, the lithium ion insertion / extraction migration resistance is smaller, and the stability between the electrode and the electrolyte is better. The overall performance of the secondary battery is better, the fast charging time can be further improved to within 15 minutes, and the battery's cycle capacity retention rate can reach more than 89%.

[0126] (3) In addition, when the Li / Ni mixing ratio a of the cathode material increases, nickel ions will physically block the migration path of lithium ions, increase the energy barrier for lithium ion insertion and extraction, and make it more difficult for lithium ions to migrate, thus reducing the fast charging performance of the battery. Moreover, severe Li / Ni mixing will also promote the transformation of the layered structure to the rock salt phase, destroy the stability of the crystal skeleton, and easily generate microcracks during charging and discharging, exposing more active surfaces, promoting electrolyte decomposition and transition metal dissolution, thus reducing the cycle life of the battery. When the Li / Ni mixing ratio decreases, the lithium ion insertion and extraction rate increases, but the structural stability of the cathode active material decreases, and cracks are easily generated due to volume stress, which also affects the cycle life of the secondary battery. In the secondary battery described in this application, after synchronously controlling a, b and c, when a is further preferably in the range of 3~10%, the secondary battery can achieve better cathode active material stability and kinetic performance, and better fast charging performance and cycle stability. Based on this, the viscosity change rate b of the electrolyte at different temperatures also exhibits a similar phenomenon. When it is large, the fluidity and ion conductivity of the electrolyte will fluctuate drastically, causing the current to concentrate in certain areas, forming "hot spots," exacerbating polarization, and even inducing lithium dendrite precipitation, leading to a decrease in the cycle life of the battery. However, if it is too small, it is impossible to simultaneously improve the lithium-ion transport rate. It is necessary to simultaneously control the ion conductivity c of the electrolyte. However, if c is too large, it will exacerbate the side reactions between the electrode and the electrolyte. Therefore, both need to be controlled in a coordinated manner. Based on this, when b is further preferably 25~60%, or c is preferably 8~20 mS / cm, the secondary battery can achieve a better balance between fast charging performance and cycle performance.

Claims

1. A secondary battery, characterized in that, It includes a positive electrode sheet and an electrolyte, wherein the positive electrode sheet includes a positive electrode material layer, the positive electrode material layer includes a positive electrode material, the positive electrode material includes a positive electrode active material, and the positive electrode active material includes lithium nickel cobalt manganese oxide; The secondary battery satisfies: a×b×c=0.014~2.3; Where a is the Li / Ni mixing ratio of the positive electrode material, b is the viscosity change rate of the electrolyte and b = 100% × |b1-b2| / b1, b1 cp is the viscosity of the electrolyte at 25℃, b2 cp is the viscosity of the electrolyte at 45℃, and c mS / cm is the ionic conductivity of the electrolyte at 25℃.

2. The secondary battery as described in claim 1, characterized in that, The value of a×b×c is 0.08~0.

97.

3. The secondary battery as described in claim 1, characterized in that, a = 1~15%, and / or b = 20~70%, and / or c = 5~25mS / cm.

4. The secondary battery as described in claim 1, characterized in that, The molar content of nickel in the transition metal elements in the cathode material is 0.5~0.

95.

5. The secondary battery as described in claim 1, characterized in that, The cathode material contains doping elements, including at least one of Mg, Cr, Na, Al, Mn, Zr, Nb, Co, Ni, Ti, and V.

6. The secondary battery as described in claim 5, characterized in that, The mass content of the dopant element in the cathode material is 100ppm to 10000ppm.

7. The secondary battery as described in claim 1, characterized in that, The surface of the positive electrode active material is further provided with a coating layer, which includes at least one of aluminum oxide, zirconium oxide, titanium dioxide, zinc oxide, magnesium oxide, aluminum fluoride, lithium fluoride, polypyrrole, tungsten oxide, and boron oxide.

8. The secondary battery as described in claim 7, characterized in that, The average thickness of the coating layer is 10~100nm.

9. The secondary battery as described in claim 1, characterized in that, The particle size D of the positive electrode material v50 The range is 2~16μm.

10. The secondary battery as described in claim 1, characterized in that, The compaction density of the positive electrode sheet is 3.3~3.8 g / cm³. 3 .

11. The secondary battery as described in claim 1, characterized in that, The electrolyte comprises a solvent and a lithium salt; the lithium salt comprises at least one of LiPF6 and LiFSi; the concentration of the lithium salt in the electrolyte is 0.8~1.5 mol / L.

12. The secondary battery as described in claim 11, characterized in that, The solvent includes at least one of carboxylic acid ester solvents and carbonate solvents.

13. The secondary battery as described in claim 12, characterized in that, The solvent includes carboxylic acid ester solvents and carbonate solvents, and the mass ratio of the carboxylic acid ester solvents to the carbonate solvents is 0.002~0.

5.

14. The secondary battery as described in claim 12, characterized in that, The carboxylic acid ester solvent includes at least one of ethyl acetate, ethyl propionate, propyl acetate, methyl propionate, and propyl propionate.

15. The secondary battery as described in claim 12, characterized in that, The carbonate solvents include at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

16. The secondary battery as described in claim 12, characterized in that, The carboxylic acid ester solvent includes ethyl acetate, and the carbonate solvent includes at least one of ethylene carbonate, dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

17. The secondary battery as described in claim 1, characterized in that, The electrolyte further includes additives; the additives include at least one of the following: fluoroethylene carbonate, vinylene carbonate, methane disulfonate, tris(trimethylsilane)borate, propylene sulfite, vinyl ethylene carbonate, vinyl sulfate, lithium dioxalate borate, lithium difluorophosphate, lithium difluorooxalate borate, lithium bis(fluorosulfonyl)imide, lithium bis(oxalate borate), lithium tetrafluoroborate, lithium bis(trifluoromethanesulfonyl)imide, tris(trimethylsilane) phosphate, tris(trimethylsilane) phosphite, tris(trimethylsilane) phosphate, trimethyl phosphate, and triphenyl phosphate, wherein the mass percentage of the additives in the electrolyte is 1% to 15%.

18. The secondary battery as described in claim 1, characterized in that, The secondary battery also includes a negative electrode sheet, which includes a negative electrode material layer with a porosity of 20-60%.

19. The secondary battery as described in claim 18, characterized in that, The negative electrode material layer includes a negative electrode material containing manganese. After the secondary battery is cycled 300 times at a rate of 0.33C in a voltage range of 2.5 to 4.25V, the mass content of manganese in the negative electrode material layer is 0.1 to 100 ppm.

20. The secondary battery as described in claim 19, characterized in that, The negative electrode material includes a negative electrode active material, which includes graphite, and the particle size D of the negative electrode material is... v50 The value is 4~20μm.

21. The secondary battery as described in claim 1, characterized in that, The secondary battery includes a separator, the air permeability of which is 100~400s / 100mL.

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