Lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium ion secondary battery. Comprising a positive plate and an electrolyte, the positive plate comprises a lithium metal transition oxide, and the lithium metal transition oxide comprises elements Ni, Co and Mn; on the basis of the total mole number of the elements Ni, Co and Mn, the content X of the element Ni is 90%-98%, and the content ratio R of the element Co to the element Mn is 1.2-5; the positive electrode active material further comprises at least one of elements Al, Mg, Ti, Zr, Y, La, Te, W, P and B; the positive electrode active material comprises a substrate and a coating layer positioned on the surface of the substrate; the electrolyte comprises a nitrile additive and a carbonic ester solvent, and the carbonic ester solvent comprises chain carbonic ester and cyclic carbonic ester; and the ratio of the content of the chain carbonate to the content of the cyclic carbonate is greater than 1. The battery disclosed by the invention has relatively good thermal stability, rate capability and cycling stability while ensuring that the capacity of the positive electrode active material is ultrahigh and the energy density is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a lithium ion secondary battery. BACKGROUND

[0002] With the rapid development of lithium ion battery technology, people have put forward higher requirements for the energy density of lithium ion batteries. High energy density lithium batteries are the development trend of consumer lithium ion batteries. As a core component of lithium ion batteries, the performance of the positive electrode material directly affects the battery endurance and safe operation. Among the many positive electrode materials, high-nickel positive active materials (the molar content of Ni in the positive active material is ≥0.8) have become the first choice for battery positive electrode materials due to their significant high specific capacity advantage (usually greater than 200 mAh / g). Compared with high-voltage lithium cobalt oxide (only 190 mAh / g) and lithium iron phosphate (less than 160 mAh / g), high-nickel positive active materials have become the first choice for battery positive electrode materials.

[0003] However, as the nickel content in high-nickel positive active materials continues to increase, the cation mixing phenomenon of transition metal ions in the layered structure becomes more serious, resulting in a significant decrease in the structural stability and safety of the positive active material. In particular, when the nickel content exceeds 90%, the thermal runaway temperature of the battery is lower, and other electrochemical properties such as cycle stability, rate performance, and charge-discharge power have all declined to varying degrees, greatly limiting their further application and development in the field of high energy density batteries. Therefore, it is very important to improve the overall electrochemical performance of high-nickel positive active material batteries. SUMMARY

[0004] The present application aims to overcome the above-mentioned problems in the prior art and provides a lithium ion secondary battery. The lithium ion secondary battery (hereinafter referred to as the battery) of the present application ensures that the capacity of the positive active material is super high, realizes the improvement of energy density, and improves the thermal stability, rate performance, and cycle stability performance of the battery.

[0005] The inventors of the present application found that the balance of rate capability and cycle stability can be achieved by regulating the molar content ratio of element Co and element Mn in the positive electrode active material. The reason is that: the introduction of element Co can inhibit the mixing of nickel ions and lithium ions in the positive electrode active material, maintain the stability of the layered structure of the material; and cobalt atoms can reduce lattice distortion through strong Co-O bonds, reduce the energy barrier of lithium ion migration in the charging and discharging process, thereby improving the rate capability of the battery. And element Mn usually exists in +4 valence in the positive electrode active material, which can stabilize the layered structure and inhibit the irreversible phase transition (such as the transition from layered to spinel or rock salt phase) in the charging and discharging process. When Co / Mn is too large (for example, >5), the high Co content actually reduces the Mn content, which shows that the Mn element content is too low, which will make the crystal stability of the material poor. When Co / Mn is too small (for example, <1.2), too much manganese element is easy to induce Jahn-Teller distortion, which destroys the structural stability. Therefore, by regulating the molar content ratio of Co / Mn elements in the positive electrode active material, the balance of the rate capability and cycle stability of the battery can be achieved.

[0006] Further, by introducing a coating layer containing element Zr to the positive electrode active material, the structural stability and chemical inertness of the coating layer can effectively improve the structural stability of the positive electrode active material; and the dense coating layer can effectively block the direct contact of the positive electrode active material with the electrolyte, inhibit the interface side reaction caused by HF corrosion and transition metal dissolution, and reduce the precipitation of lattice oxygen, thereby reducing the risk of thermal runaway of the positive electrode material, and making the battery have good thermal stability.

[0007] However, the charging and discharging process of the battery is an electrochemical process involving the positive electrode, the negative electrode and the electrolyte. In the cycle process of the battery, the charging and discharging reaction needs the positive electrode and the negative electrode to efficiently carry out the embedding and extraction of lithium ions at the same time, and to build a smooth ion transmission channel between the two through the electrolyte. Therefore, if only the positive electrode is improved, the effect is far from enough. The present application further improves the electrolyte, by introducing a nitrile additive and a carbonate solvent into the electrolyte, which can significantly enhance the rate performance and cycle stability of the battery. Among them, the nitrile additive can play a role in protecting the positive electrode. The nitrogen atom in the nitrile group has a pair of lone pair electrons, which can coordinate with transition metal ions in the positive electrode material, effectively inhibit the dissolution of transition metal ions, alleviate the cracking and structure collapse of the positive electrode material in the cycle process, and improve the structure stability of the positive electrode material. And the nitrile additive can also form a polymer film rich in LiF and nitrogen-containing compounds on the surface of the positive electrode, which can protect the positive electrode / electrolyte interface, reduce the oxidative decomposition of the electrolyte at high voltage, and thus improve the cycle stability of the battery. The carbonate solvent includes chain carbonate and cyclic carbonate, and the mass content ratio of the chain carbonate to the cyclic carbonate in the electrolyte is controlled in the present application, which can further improve the rate performance and cycle stability of the battery. Among them, the chain carbonate has a relatively simple molecular structure, small intermolecular force, low viscosity and high flowability, which is not only beneficial to heat dissipation, but also can improve the transmission efficiency of lithium ions and reduce the internal resistance of the battery, thereby improving the rate performance of the battery; while the cyclic carbonate plays an important role in forming a stable solid electrolyte interface (SEI) film and improving the stability of the electrolyte. By controlling the mass content of the chain carbonate to be higher than that of the cyclic carbonate in the electrolyte, the ionic conductivity and the electrode interface stability of the electrolyte can be effectively improved. In addition, the stable solvation layer formed by the chain structure on the electrode surface can inhibit the excessive reduction and decomposition of the cyclic carbonate, reduce the impedance increase of the SEI film, and thus prolong the cycle life of the battery.

[0008] Based on this, the inventors of the present application propose the following scheme:

[0009] A lithium ion secondary battery, comprising a positive electrode sheet and an electrolyte, the positive electrode sheet comprising a positive electrode active material, the positive electrode active material comprising a lithium metal transition oxide, the lithium metal transition oxide comprising elements Ni, Co and Mn; the content X of element Ni being 90%-98% based on the total moles of elements Ni, Co and Mn, and the ratio R of the contents of element Co and element Mn being 1.2-5; the positive electrode active material further comprising at least one of elements Al, Mg, Ti, Zr, Y, La, Te, W, P and B; the positive electrode active material comprising a substrate and a coating layer located on the surface of the substrate, the coating layer comprising element Zr, the mass content of element Zr in the positive electrode active material being 200ppm-2500ppm; the electrolyte comprising a nitrile additive and a carbonate solvent, the carbonate solvent comprising a chain carbonate and a cyclic carbonate, the content G of the nitrile additive being 0.3%-6% based on the total mass of the electrolyte, and the ratio of the content of the chain carbonate to the content of the cyclic carbonate being >1.

[0010] By means of the technical scheme, the present application has at least the following advantages compared with the prior art:

[0011] The battery of the present application has good thermal stability, rate performance and cycle stability while ensuring that the capacity of the positive electrode active material is super high and the energy density is improved.

[0012] The endpoints of the ranges and any values disclosed herein are not limited to the precise values recited as the exact dimensions are not critical to the present application. Any numeric range recited is intended to include all values from the lower value to the upper value. For values which are less than one, one part in 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1 parts per 100 are also specifically intended. These are only examples of what is specifically intended and all other ranges are also intended, which are essentially the same. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 A scanning electron microscope (SEM) image of a positive electrode active material in an example of the present application is shown.

[0014] Figure 2 A structural schematic diagram of a substrate in an example of the present application is shown.

[0015] Figure 3 A structural schematic diagram of a positive electrode sheet and a tab in an example of the present application is shown.

[0016] Figure 4 A structural schematic diagram of a negative electrode current collector in an example of the present application is shown. DETAILED DESCRIPTION

[0017] The specific embodiments of the present application are described in detail below. It should be understood that the specific embodiments described herein are merely illustrative and explanatory in nature and are not intended to limit the application.

[0018] The present application provides a lithium ion secondary battery, which comprises a positive electrode sheet and an electrolyte, the positive electrode sheet comprises a positive electrode active material, the positive electrode active material comprises a lithium metal transition oxide, the lithium metal transition oxide comprises elements Ni, Co and Mn; the content X of element Ni is 90%-98% (for example, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97% or 98%) based on the total moles of elements Ni, Co and Mn; the ratio R of the contents of element Co and element Mn is 1.2-5 (for example, 1.2, 2, 2.5, 3, 3.5, 4, 4.5 or 5); the positive electrode active material further comprises at least one of elements Al, Mg, Ti, Zr, Y, La, Te, W, P and B; the positive electrode active material comprises a matrix and a coating layer on the surface of the matrix, the coating layer comprises element Zr, and the mass content of element Zr in the positive electrode active material is 200-2500 ppm (for example, 200 ppm, 500 ppm, 1000 ppm, 1500 ppm, 2000 ppm or 2500 ppm); the electrolyte comprises a nitrile additive and a carbonate solvent, the carbonate solvent comprises a chain carbonate and a cyclic carbonate, and the content G of the nitrile additive in the electrolyte is 0.3%-6% (for example, 0.3%, 1%, 2%, 3%, 4%, 5% or 6%) based on the total mass of the electrolyte; the ratio of the content of the chain carbonate to the content of the cyclic carbonate is >1 (for example, 1.1, 2, 3, 4, 5, 6, 7, 8, 9 or 10).

[0019] In an example, R is 1.2-4.

[0020] In an example, the coating layer comprises ZrO2.

[0021] In an example, the mass content of element Zr is 1000-1500 ppm.

[0022] When the R value is large, the molar content of element Co is high, and the molar content of element Mn is low, by adding an appropriate amount of nitrile additive, transition metal ions can be complexed, the dissolution of Co can be reduced, the cracking and structure collapse of the positive electrode material during the cycle process can be alleviated, the structural stability of the positive electrode material can be improved, and the positive electrode material can be effectively protected. However, if the content of the nitrile additive is too high, a too thick polymer film can be formed on the surface of the positive electrode, the interface impedance can be increased, the conduction of Li + can be hindered, and the rate performance of the battery can be reduced.

[0023] In the present invention, the molar contents of the element Co and the element Mn can be obtained by testing by conventional methods in the art, such as discharging the battery to 0% SOC (for example, discharging the battery to 2.7 V), disassembling the positive electrode sheet, polishing the cross-section of the positive electrode sheet with an argon ion milling instrument, and then testing it using inductively coupled plasma (ICP).

[0024] In the present invention, the mass content of the element Zr in the positive electrode active material can be tested by conventional methods in the art, for example, discharging the battery to 0% SOC, disassembling and removing the positive electrode sheet, using an argon ion milling (CP) laser to cut the positive electrode sheet, observing the coating layer in an SEM or TEM device, and combining it with ICP testing to calculate the mass content of the element Zr.

[0025] In the present invention, the positive electrode active material includes single crystal particles and polycrystalline particles; the average particle size of the single crystal particles is 2μm-6μm (for example, 2μm, 3μm, 4μm, 5μm or 6μm), and the average particle size of the polycrystalline particles is 6μm-17μm (for example, 6μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm or 17μm).

[0026] In one example, the mass ratio of the single crystal particles to the polycrystalline particles is (0.2-4):1, for example, 0.2:1, 0.5:1, 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1 or 4:1.

[0027] In the present invention, the nitrile additive includes at least one of benzonitrile, succinonitrile, fluorobenzonitrile, adiponitrile, 1,3,6-hexanetrinitrile, glyceroltrinitrile, 1,4-dicyano-2-butene and ethylene glycol bis(propionitrile) ether.

[0028] In the present invention, the chain carbonate includes at least one of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) and methyl propyl carbonate (MPC); based on the total mass of the electrolyte, the content of the chain carbonate is 30%-65% (for example, 30%, 35%, 40%, 45%, 50%, 55%, 60% or 65%).

[0029] In the present invention, the cyclic carbonate includes at least one of propylene carbonate (PC), ethylene carbonate (EC) and fluoroethylene carbonate (FEC); based on the total mass of the electrolyte, the content of the cyclic carbonate is 15%-35% (for example, 15%, 20%, 25%, 30% or 35%).

[0030] Single crystal particles are complete crystals formed by the arrangement of microscopic particles such as atoms, ions or molecules in three-dimensional space according to a single and periodic rule. They are anisotropic and do not have defects such as grain boundaries. They can inhibit microcracks caused by volume changes during charging and discharging, reduce side reactions caused by electrolyte penetration, and thus improve battery cycle life. Polycrystalline particles usually have a smaller size and a higher specific surface area, which can increase the diffusion rate of lithium ions and thus improve the rate performance of the battery. Figure 1 The figure shows a scanning electron microscope (SEM) image of the positive electrode active material in an example of the present invention. As can be seen from the figure, the single crystal particles and polycrystalline particles of the positive electrode active material of the present invention are mixed. By regulating the ratio of the two, the stability of the single crystal structure and the advantages of the polycrystalline structure in optimizing the ion transmission dynamics can be fully utilized, effectively reducing the internal resistance growth of the battery during the cycle, improving the transmission path of lithium ions, further optimizing the charge and discharge performance of the battery, and extending the cycle life of the battery. Usually, under high voltage and high cycle requirements, the proportion of single crystal particles is higher than that of polycrystalline particles. This is because single crystal particles have a more uniform structure and do not have defects such as grain boundaries, which helps to reduce stress concentration during the electrochemical cycle and improve the cycle stability of the battery. Polycrystalline particles can provide more active sites and faster ion diffusion paths, thereby improving the rate performance of the battery. Therefore, under the demand for high-rate discharge, the proportion of polycrystalline particles should be controlled to be higher than that of single crystal particles.

[0031] In the present invention, the content of the nitrile additive, the content of the chain carbonate and the content of the cyclic carbonate can be measured by conventional methods in the art, such as by gas chromatography (GC) or gas chromatography-mass spectrometry (GCMS).

[0032] In the present invention, the average particle size of the single crystal particles and polycrystalline particles can be measured by conventional methods in the art. For example, the battery is discharged to 0% SOC, the positive electrode sheet is disassembled and removed, and the cross-section of the positive electrode sheet is polished using an argon ion milling apparatus. Then, at least 20 single crystal particles or polycrystalline particles are selected from a scanning electron microscope (SEM) image, and the particle size of each particle is measured and the average value is calculated. If the single crystal particle / polycrystalline particle is a regular circle in the image, the particle size of the particle is the diameter of the circle. If the single crystal particle / polycrystalline particle is not a "regular circle" in the image, the particle size of the particle is the diameter of an equivalent circle with the same area as the "regular circle".

[0033] In the present application, the particle size Dv10 of the positive electrode active material is 0.5 μm-5 μm (for example, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm or 5 μm), the Dv50 is 1 μm-15 μm (for example, 1 μm, 3 μm, 5 μm, 7 μm, 9 μm, 11 μm, 13 μm or 15 μm), and the Dv90 is 10 μm-40 μm (for example, 10 μm, 20 μm, 30 μm or 40 μm).

[0034] In the present application, the positive electrode sheet comprises a positive electrode current collector and a positive electrode active layer located on at least one side surface of the positive electrode current collector, and the positive electrode active layer further comprises a positive electrode conductive agent, wherein the positive electrode conductive agent comprises carbon nanotubes and conductive carbon black; the tube diameter of the carbon nanotubes is 1 nm-100 nm (for example, 1 nm, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm or 100 nm), and the ratio of the tube length of the carbon nanotubes to the tube diameter of the carbon nanotubes is 50-10000 (for example, 50, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000 or 10000).

[0035] In an example, the content of the positive electrode conductive agent in the positive electrode active layer is 0.05%-3% (for example, 0.05%, 1%, 1.5%, 2%, 2.5% or 3%).

[0036] The carbon nanotube is a one-dimensional fiber structure, which can form a linear conductive channel in the positive electrode material and construct a long-range conductive transmission path. The conductive carbon black is in a granular shape, which can form a point-to-point contact with the positive electrode active material and construct a short-range multi-point contact conductive network. The present application uses the carbon nanotube and the conductive carbon black to construct a long and short range combined conductive network in the positive electrode active layer, which can reduce the hindrance in the electronic transmission process, improve the transmission efficiency of the electrons, reduce the polarization phenomenon of the battery, thereby well compensating for the problem of low conductivity of the active material caused by the reduction of the cobalt content in the positive electrode active material, and further improving the charge and discharge efficiency and the rate performance of the battery. The diameter ratio of the carbon nanotube has a significant influence on its performance. The thinner the diameter of the carbon nanotube and the longer the length, the higher the heat exchange performance along the length direction, and the better the conductive performance. A suitable diameter ratio can promote the infiltration and uniform and rapid penetration of the electrolyte in the positive electrode sheet, improve the lithium ion migration rate, and improve the positive electrode charge and discharge efficiency and service life. When the aspect ratio of the carbon nanotube is too small (for example, < 50), the length of the carbon nanotube is short, it is difficult to form a continuous conductive channel, the storage space and transportation channel of the lithium ion are reduced, the internal resistance of the battery is increased, and the rate performance of the battery is reduced. When the aspect ratio of the carbon nanotube is too large (for example, > 10000), the length of the carbon nanotube is too long, it is easy to intertwine and agglomerate, forming a bundle structure that is difficult to disperse, resulting in uneven electrode conductivity, affecting the charge and discharge performance of the battery. At the same time, the electrolyte is difficult to enter the tube, resulting in poor infiltration effect of the electrolyte on the carbon nanotube, affecting the diffusion path and rate of lithium ions in the material, and further affecting the rate performance of the battery.

[0037] In the present application, the diameter of the carbon nanotube can be tested by conventional methods in the art, for example, by SEM, at least 20 carbon nanotubes are selected in the electron microscope image, the diameter of each carbon nanotube is measured, and the average value is taken. The length of the carbon nanotube can be tested by conventional methods in the art, for example, by SEM, at least 20 carbon nanotubes are selected in the electron microscope image, the length of each carbon nanotube is measured, and the average value is taken.

[0038] In the present application, the positive electrode active material further comprises elements Al and Ti. The content of element Al is 800 ppm-3500 ppm (for example, 800 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm or 3500 ppm) based on the total weight of the positive electrode active material. The content of element Ti is 500 ppm-3000 ppm (500 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm or 3000 ppm).

[0039] In an example, the content of element Al is 800 ppm-3000 ppm.

[0040] In the present invention, the matrix includes a bulk region and a surface region, the mass content of element Al in the surface region is greater than the mass content of element Al in the bulk region, and the mass content of element Ti in the surface region is greater than the mass content of element Ti in the bulk region. Figure 2 The figure shows a schematic structural diagram of the substrate in an example of the present invention. It can be seen from the figure that the surface area is the area formed by the points 0%-10% away from the surface of the substrate along the direction from the surface of the substrate to the core; the main body area of ​​the substrate is the area of ​​the substrate excluding the surface area.

[0041] In one example, the ratio of the mass content of element Al in the surface region to the mass content of element Al in the bulk region is (1.2-15):1, for example, 1.2:1, 2:1, 3:1, 4:1, 5:1, 6:1, 8:1, 10:1 or 15:1.

[0042] In one example, the ratio of the mass content of element Al in the surface region to the mass content of element Al in the bulk region is (1.5-5):1.

[0043] In one example, the ratio of the mass content of elemental Ti in the surface region to the mass content of elemental Ti in the bulk region is (1.5-200):1, for example, 1.5:1, 5:1, 10:1, 50:1, 100:1, 150:1 or 200:1.

[0044] In one example, the ratio of the mass content of element Ti in the surface region to the mass content of element Ti in the bulk region is (10-100):1.

[0045] The increase of nickel content will aggravate the sensitivity of the positive active material to temperature, especially when the nickel content exceeds 90%, the lithium ion battery prepared therefrom is prone to release oxygen and structural collapse of the positive active material in an environment exceeding 115 DEG C, thereby causing the risk of thermal runaway. The present application can improve the structural stability of the positive active material by introducing elements Al and Ti into the positive active material and regulating the difference between the mass content of the element Al or the element Ti in the surface layer region and the mass content of the element Al or the element Ti in the bulk region. The reason is that the introduction of elements Al and Ti will occupy the Ni sites of the transition metal layer in the structure of the positive active material, thereby reducing the Li / Ni misarrangement defects. At the same time, it can also disperse the phase transition stress, especially during high-voltage charging and discharging, it can inhibit the formation of non-uniform phase, reduce the severity of H2-H3 phase transition, thereby reducing stress accumulation, relieving lattice distortion and micro-crack. By controlling the lower content of element Al or element Ti in the bulk region, the lattice distortion can be reduced, and the capacity of the positive active material can be ensured. The higher content of element Al / Ti in the surface layer region can form a protective film on the surface of the positive active material, prevent the direct contact of the positive material with the electrolyte, reduce the interface side reaction, and improve the cycle stability of the battery.

[0046] In the present application, the mass content of the element Al or the element Ti in the surface layer region and the bulk region can be obtained by testing by the conventional method in the art, for example, discharging the battery to 0% SOC, disassembling and taking out the positive plate, soaking in DMC solvent for 12h, then rinsing with DMC solvent to remove the lithium salt attached to the positive plate, calcining the plate in air at 450 DEG C for 3 hours, then scraping the positive active material from the positive plate with a ceramic knife, then etching the positive active material with dilute aqua regia (volume ratio 1:1 of aqua regia and water) for different time, then combining inductively coupled plasma-optical emission spectrometer (ICP-OES) measurement to obtain the mass content of the element Al or the element Ti in the surface layer region and the bulk region.

[0047] As Figure 3 The structure of the positive plate and the tab in an example of the present application is shown in the schematic diagram, from which it can be seen that the positive plate further comprises a plurality of positive tabs extending from the positive current collector, and the number of the positive tabs is greater than or equal to 2. Since the thermal runaway temperature of the high-nickel positive active material lithium ion battery is lower than that of the low-nickel positive active material, reducing the heat generation of the heat generating components in the battery can effectively improve its safety. Compared with the traditional single-tab wound battery, the present application adopts a multi-tab or laminated battery process to disperse the current, which can greatly reduce the temperature rise and the risk of thermal runaway caused by the large current at the tab.

[0048] In the present application, the electrolyte further comprises lithium bisfluorosulfonylimide (LiFSI), and the content of lithium bisfluorosulfonylimide is 3%-15% (for example, 3%, 5%, 10% or 15%) based on the total mass of the electrolyte.

[0049] In the present application, the charging cut-off voltage of the lithium ion secondary battery is greater than or equal to 4.2V.

[0050] Under high voltage conditions, the Li / Ni mixing phenomenon of high-nickel positive electrode active materials is intensified, which can destroy the crystal structure of the material, cause the lithium ion deintercalation channel to become unsmooth, and further affect the charge and discharge performance and cycle stability of the battery. LiFSI has excellent thermal stability (decomposition temperature > 200℃) and electrical conductivity. Compared with traditional electrolyte salts, LiFSI is less likely to decompose at high voltage (for example, > 4.2V), which can reduce electrolyte gas production and oxidation side reactions, thereby improving the safety of the battery. Moreover, LiFSI itself has high electrical conductivity, which helps to improve the overall conductivity of the electrolyte, reduces the resistance of lithium ions in the transmission process under high voltage conditions, and thus improves the charge and discharge performance of the battery. At the same time, the strong coordination ability of LiFSI can also enhance the structural stability of the positive electrode active material, reduce the reduction of Ni 3 + in the material and Li+ / Ni 2 + mixing, and thus alleviate the capacity decay problem caused by phase change. In addition, LiFSI also has a certain protective effect on the negative electrode. For example, the decomposition products of LiFSI in the electrolyte (such as LiF) can participate in the formation of the negative electrode SEI film, reduce the direct contact between the negative electrode sheet and the electrolyte, reduce the interface side reaction, and further stabilize the interface.

[0051] In the present application, the content of lithium bisfluorosulfonylimide can be obtained by conventional methods in the art, for example, by ICP testing.

[0052] In the present application, the lithium ion secondary battery further comprises a negative electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer located on at least one side surface of the negative electrode current collector, and the negative electrode active layer comprises a silicon-carbon material.

[0053] In an example, the silicon-carbon material comprises a porous carbon matrix and a silicon material located in the internal pores of the porous carbon matrix; more preferably, the mass content of silicon elements in the silicon-carbon material is 30%-80% (for example, 30%, 40%, 50%, 60%, 70% or 80%).

[0054] In an example, the mass content of silicon elements in the negative electrode active layer is 2%-50% (for example, 2%, 5%, 10%, 15%, 20%, 30%, 40% or 50%).

[0055] In one example, the mass content of silicon element in the negative active layer is 5%-30%.

[0056] In the present application, the N / P ratio of the lithium ion secondary battery is 1.03-1.25 (for example, 1.03, 1.1, 1.15, 1.2 or 1.25).

[0057] When the mass content of silicon element in the negative active layer is too high, it will cause the negative active material to expand in volume during the cycle process, reduce the stability, and further cause the accumulation of internal stress, particle breakage, and increase of side reactions with electrolyte, etc., ultimately leading to a significant reduction in the cycle stability of the battery. Therefore, in order to meet the needs, the present application further improves the cycle stability of the battery by adjusting the silicon content within a suitable range.

[0058] The N / P ratio refers to the ratio of the lithium ion storage capacity of the negative active material to the lithium ion storage capacity of the positive active material in the battery, which reflects the relative matching degree between the positive and negative capacities of the battery, and has an important influence on the charge-discharge performance, cycle life, etc. of the battery. When the N / P ratio is too high (for example, >1.25), the negative redundant capacity is too large, and the number of lithium insertion into the positive electrode is too small, which affects the capacity development; when the N / P ratio is too low (for example, <1.03), the negative redundant capacity is too small, which may cause lithium ions to be precipitated on the negative side, causing safety accidents. Therefore, by reasonably adjusting the N / P ratio, the capacity of the negative and positive electrodes can be matched, the excessive migration of lithium ions between the electrodes can be reduced, and the volume expansion can be reduced. On this basis, the silicon-carbon material with rich pore structure can provide sufficient buffer space for the volume expansion of silicon.

[0059] In the present application, the content of silicon element in the silicon-carbon material can be tested by conventional methods in the art, for example, using EDS, as follows: the battery is discharged to 0% SOC, the negative electrode sheet is taken out, soaked in dimethyl carbonate (DMC) solvent for 12 h, then washed with DMC to remove the lithium salt attached to the negative electrode sheet, dried, and then treated using an argon ion cutting machine (for example, Japan Electronics IB-19530CP argon ion cross-section polisher). The sample after cutting is quickly transferred to the sample chamber of the scanning electron microscope for observation. At least 5 points on the surface of the silicon-carbon material are measured by EDS point scanning mode to obtain the content of silicon element at each point, and the average value is calculated.

[0060] In the present application, the mass content of silicon element in the negative electrode active layer can be obtained by testing with conventional methods in the art, for example, after discharging the battery to 0% SOC, disassembling and taking out the negative electrode sheet, soaking in DMC solvent for 12h, then rinsing with DMC solvent to remove the lithium salt attached to the negative electrode sheet, drying, then high-temperature treatment of the negative electrode sheet at 400℃ in an inert atmosphere for 2h (for example, in a tube furnace, under nitrogen or argon atmosphere), the negative electrode active layer can be peeled off from the negative electrode current collector, and the negative electrode active layer is collected as a test sample. Using a thermal gravimetric analyzer (for example, TGA 550 thermal gravimetric analyzer), the test sample amount is 5mg-15mg, under air or oxygen atmosphere, the temperature is raised from room temperature (25℃) to 900℃ at a rate of 10℃ / min, and kept at 900℃ for 40min, so that the non-silicon components in the negative electrode active layer volatilize while the silicon is fully oxidized to silicon dioxide. The residual material is the ash of the negative electrode active layer, and the mass content of silicon element in the negative electrode active layer can be calculated according to the mass of the ash, the calculation formula is as follows: mass content of silicon element in the negative electrode active layer = 7 x mass of ash / (15 x mass of test sample).

[0061] In the present application, the negative electrode active layer comprises a first active coating layer and a second active coating layer arranged along the thickness direction of the negative electrode sheet, and the first active coating layer is located between the negative electrode current collector and the second active coating layer; the first active coating layer comprises a first carbon-based material, and the second active coating layer comprises a second carbon-based material and the silicon-carbon material.

[0062] In an example, the first carbon-based material comprises at least one of artificial graphite, natural graphite, mesocarbon microbeads, hard carbon and soft carbon.

[0063] In an example, the second carbon-based material comprises at least one of artificial graphite, natural graphite, mesocarbon microbeads, hard carbon and soft carbon.

[0064] In an example, the average particle size of the first carbon-based material is D1, and 6μm≤D1≤30μm, for example, 6μm, 10μm, 15μm, 20μm, 25μm or 30μm; the average particle size of the second carbon-based material is D2, and 2μm≤D2≤10μm, for example, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm or 10μm.

[0065] Figure 4 The structure of the negative electrode current collector in an example of the present application is shown, and the tensile strength of the negative electrode current collector in the width direction is σ, and 300MPa≤σ≤850MPa (for example, 300MPa, 400MPa, 500MPa, 600MPa, 700MPa, 800MPa or 850MPa).

[0066] In the present application, the particle size Dv10 of the silicon-carbon material is 0.5-6 μm (for example, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm or 6 μm), the Dv50 is 3-15 μm (for example, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm or 15 μm), and the Dv90 is 9-30 μm (for example, 9 μm, 15 μm, 20 μm, 25 μm or 30 μm).

[0067] The use of zoned coating in the thickness direction of the negative electrode sheet can further improve the negative electrode kinetics and the swelling problem. The negative electrode active layer uses zoned coating, and the silicon-carbon material and the second carbon-based material are used in the negative electrode active layer far from the negative electrode current collector. This layer of coating is closer to the positive electrode sheet, and the electrolyte is also more fully infiltrated. The distance of lithium ion migration is relatively short, and the number of lithium ions that can contact the silicon-carbon material most quickly is greatly increased. Therefore, the rapid lithium ion demand pressure generated when the high-nickel positive electrode material is delithiated at high voltage is effectively alleviated, the polarization phenomenon is reduced, and the high specific capacity characteristics of the high-nickel positive electrode are more efficiently exhibited. The negative electrode active layer close to the negative electrode current collector does not add the silicon-carbon material, and the number of lithium ions that migrate a long distance is reduced, which is conducive to the infiltration of the electrolyte and reduces the capacity decay of the carbon-based material in the negative electrode active layer close to the negative electrode current collector during the cycle process. At the same time, a gradient protection layer is formed by using the stability of the first carbon-based material, the heat generation rate is reduced, and the thermal sensitivity of the high-nickel positive electrode is complementary, which can further inhibit the occurrence of battery thermal runaway.

[0068] In the high-rate charging and discharging process, the volume of the negative electrode with high silicon content changes dramatically. After the negative electrode current collector is subjected to frequent and large volume changes, the electrical contact between the active material and the current collector is poor, which easily leads to powder shedding. In the present application, the tensile strength of the negative electrode current collector in the width direction is adjusted, which can ensure that the current collector better adapts to the volume change of the silicon-carbon material and reduces the active material shedding phenomenon caused by the volume expansion and contraction of the silicon-carbon material, thereby prolonging the cycle life of the battery.

[0069] In the present application, the tensile strength of the negative electrode current collector in the width direction can be obtained by conventional methods in the art. For example, after the battery is discharged to 0% SOC, the negative electrode sheet is disassembled and taken out, soaked in DMC solvent for 12 h, then rinsed with DMC to remove lithium salts attached to the sheet, and the negative electrode sheet is cut into a sample to be tested with a width of 15 mm and a length of more than 50 mm using a knife. A WD-D3 electronic universal testing machine (precision of 0.5 level, accuracy of ±1% of the indicated value) is used to set the gauge length to 50 mm and the speed to 10 mm / min. The sample to be tested is subjected to tensile testing, the maximum tensile stress of the negative electrode sheet is measured, and the tensile strength is the maximum tensile stress / thickness of the negative electrode current collector. The tensile strength of the negative electrode current collector in the width direction is tested.

[0070] In the present application, the particle size Dv10, Dv50 and Dv90 of the silicon-carbon material can be tested by conventional methods in the art, for example, by a laser particle size analyzer.

[0071] In the present application, the lithium ion secondary battery further comprises a separator, the separator comprising a substrate layer, a ceramic layer located on at least one side surface of the substrate layer and a glue layer located on the outer surface of both sides of the separator, the ceramic layer at least facing the positive electrode sheet.

[0072] In an embodiment, the ceramic layer is only located on one side surface of the substrate and faces the positive electrode sheet.

[0073] In an example, the ceramic layer comprises particulate alumina and / or boehmite.

[0074] In an example, the substrate layer comprises a matrix, which can comprise at least one of polyethylene, polyvinyl chloride, polyethylene oxide, polypropylene, nylon, glass fiber, polyethylene terephthalate (PET), polyimide (PI), aramid fiber, cellulose and non-woven fabric.

[0075] In an example, the glue layer comprises at least one of polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), sodium carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose (CMC-Na), lithium carboxymethyl cellulose (CMC-Li) and polyvinylpyrrolidone (PVP).

[0076] In an example, the thickness of the separator is 3-12 μm (for example, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm or 12 μm).

[0077] In an example, the adhesion of the separator to the positive electrode sheet is 0.1-30 N / m (for example, 0.1 N / m, 1 N / m, 5 N / m, 10 N / m, 15 N / m, 20 N / m, 25 N / m or 30 N / m).

[0078] With the increase of nickel content, the thermal stability of the positive electrode active material decreases, especially when the R value is large and the molar content of element Co is higher than that of element Mn, the thermal stability of the positive electrode active material will decrease significantly, which is easy to cause thermal runaway under high temperature or overcharge condition. The ceramic separator can inhibit the internal short circuit and thermal runaway chain reaction of the battery through multiple mechanisms such as physical barrier, chemical stability enhancement and thermodynamic performance optimization, thereby improving the thermal runaway temperature of the battery. Specifically, the ceramic separator (such as ZrO2, Al2O3 coating) can still maintain structural integrity in a high temperature environment. In the early stage of thermal runaway, the battery needs to accumulate a lot of heat to reach the thermal runaway temperature, and before that, the ordinary separator will soften and melt, causing local direct discharge or breakdown discharge between the positive and negative electrodes, thereby generating local high temperature and triggering thermal runaway. Among the ceramic separator, the ceramic particles and the polymer base film form a rigid skeleton structure, which can significantly reduce the thermal shrinkage rate of the separator, effectively prevent internal short circuit caused by the shrinkage of the separator, thereby reducing the probability of thermal runaway and improving the thermal runaway temperature.

[0079] In the present application, the thickness of the separator can be tested by conventional methods in the art, for example, taking 10 different points on the separator respectively, measuring the thickness of each point by a conventional thickness gauge, and taking the average value.

[0080] In the present application, the adhesion of the separator to the positive electrode sheet can be tested by conventional methods in the art, for example, disassembling and removing the separator in the dry cell with tab and the positive electrode sheet (positive electrode active material coated on the current collector aluminum foil) from the battery discharged to 0% SOC, respectively clamping the separator and the positive electrode sheet in the two tension clamps of the universal material testing machine, one for fixing the positive electrode sheet (usually fixed on the fixed beam of the testing machine), the other for clamping the separator (usually fixed on the moving beam of the testing machine). Install the sample on the testing machine, ensure that the separator is peeled off from the positive electrode sheet surface at an angle of 180° (i.e. the separator is folded backward in line with the unpeeled part), start the test at a speed of 50 mm / min, and record the data. The error of 3 tests is not more than 10%, and the average value is taken.

[0081] The positive electrode active material surface has a coating layer, which can effectively isolate the direct contact between the positive electrode material and the electrolyte, and reduce the side reaction. When the coating layer covers a small area, the area of the uncovered region on the surface of the positive electrode active material is large, the exposed active sites on the surface of the positive electrode active material increase, the contact area with the electrolyte increases, which leads to a significant increase in side reactions (such as transition metal dissolution and electrolyte decomposition), a large amount of electrolyte is consumed, and then the performance and life of the battery are affected. Adding an appropriate amount of nitrile additive can preferentially oxidize and decompose on the surface of the positive electrode, form a dense CEI film rich in inorganic components such as LiF and Li3PO4, and effectively fill the area not covered by the coating layer, thereby reducing the corrosion of the electrolyte on the positive electrode material. When the non-covered area is relatively large compared to the nitrile additive, the interface side reaction cannot be effectively inhibited, which leads to a decrease in the cycle performance and safety of the battery. On the contrary, when the content of the nitrile additive is relatively too much, the problem of too thick CEI film leading to an increase in lithium ion migration impedance will occur.

[0082] In the present application, the positive electrode active layer further comprises a positive electrode binder. The positive electrode binder can include at least one of polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose, butadiene rubber, polytetrafluoroethylene, polyethylene oxide, polyacrylic acid, and derivatives of the above substances. The content of the positive electrode material can be 80%-98% (for example, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, or 98%) based on the total weight of the active material layer, the content of the positive electrode conductive agent can be 1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1%), and the content of the positive electrode binder can be 1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1%).

[0083] In the present application, the negative electrode sheet can further include a negative electrode conductive agent and a negative electrode binder. The negative electrode conductive agent can include at least one of super P, acetylene black, carbon black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers; and the negative electrode binder can include at least one of polyvinylidene fluoride, carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, styrene butadiene rubber, polyacrylic acid, and polytetrafluoroethylene. The content of the negative electrode material can be 80-98% (for example, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, or 98%) based on the total weight of the negative electrode active material layer, the content of the negative electrode conductive agent can be 1-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1%), and the content of the negative electrode binder can be 1-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1%).

[0084] In the present application, the electrolyte further includes a carboxylic acid ester-based solvent and a conductive lithium salt, and the conductive lithium salt can be selected from at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPF2O2), and lithium bis-trifluoromethanesulfonimide. The carboxylic acid ester-based solvent can be selected from at least one of methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), and ethyl butyrate (EB).

[0085] The assembly of the battery can be performed in a manner conventional in the art.

[0086] It should be noted that the "first", "second", and the like in the present application are only used to distinguish different substances or usage manners, and do not represent the difference in order.

[0087] The present application will be described in detail below through examples. The examples described in the present application are only a part of the examples of the present application, rather than all the examples. Based on the examples in the present application, all other examples obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0088] In the following examples, the materials used are commercially available analytical pure unless otherwise specified.

[0089] The following examples are used to illustrate the lithium ion secondary battery of the present application.

[0090] Example 1

[0091] A battery was prepared according to the following method

[0092] (1) Preparation of a positive electrode sheet

[0093] The positive electrode active material (X is 90%, R is 1.2), polyvinylidene fluoride, carbon nanotubes and conductive carbon black were mixed in a mass ratio of 97:1.5:0.5:1, N-methylpyrrolidone was added, and stirring was performed under the action of a vacuum stirrer until the mixed system became a positive electrode slurry with uniform fluidity; the above-mentioned positive electrode slurry was uniformly coated on an aluminum foil with a thickness of 9 μm, the coated aluminum foil was baked in an oven with 5 different temperature gradients, then it was dried in an oven at 120°C for 8 h, and then it was subjected to rolling and slitting to obtain the required positive electrode sheet.

[0094] The positive electrode active material includes single crystal particles and polycrystal particles (mass ratio of 1:1), the average particle size of the single crystal particles is 2.3 μm, and the average particle size of the polycrystal particles is 6.2 μm; the particle size Dv10 is 0.5 μm, the particle size Dv50 is 1.2 μm, and the particle size Dv90 is 10.3 μm; the mass content of element Zr in the positive electrode active material is 1250 ppm, the mass content of element Al is 815 ppm, and the mass content of element Ti is 512 ppm; the ratio of the mass content of element Al in the surface layer region to the mass content of element Al in the bulk region is 2.5:1, and the ratio of the mass content of element Ti in the surface layer region to the mass content of element Ti in the bulk region is 50:1; the tube diameter of the carbon nanotubes is 10 nm, and the ratio of the tube length of the carbon nanotubes to the tube diameter of the carbon nanotubes is 4000; and the number of positive electrode tabs is 15.

[0095] (2) Preparation of a negative electrode sheet

[0096] The artificial graphite (D1 is 6.8 μm), single-walled carbon nanotubes, conductive carbon black, sodium carboxymethyl cellulose and butadiene-styrene rubber were mixed in a mass ratio of 96.9:0.1:0.9:0.8:1.3 to prepare a first negative electrode slurry by a wet process; the artificial graphite (D2 is 2.3 μm), silicon-carbon material (Dv10 is 1.5 μm, Dv50 is 3.3 μm, and Dv90 is 10.2 μm), single-walled carbon nanotubes, conductive carbon black, sodium carboxymethyl cellulose and butadiene-styrene rubber were mixed in a mass ratio of 64.6:32.3:0.1:0.9:0.8:1.3 to prepare a second negative electrode slurry by a wet process; the above-mentioned first negative electrode slurry was uniformly coated on both surfaces of a copper foil, and was dried; then the second negative electrode slurry was coated on both surfaces of the copper foil coated with the first negative electrode slurry, and was then transferred to a 80°C oven for drying for 10 h, and then was subjected to rolling and die cutting to obtain a negative electrode sheet.

[0097] The mass content of silicon in the silicon-carbon material is 45%, and the mass content of silicon in the negative active layer is 15%; the tensile strength σ of the negative current collector in the width direction is 562 MPa.

[0098] (3) Preparation of electrolyte

[0099] In an argon-filled glove box (moisture <10 ppm, oxygen <1 ppm), 13% LiPF6 based on the total mass of the electrolyte was slowly added to a mixed solution containing 3% nitrile-based additive (including benzyl cyanide and butanedinitrile at a mass ratio of 1:1), 40% chain carbonate (DMC), 25% cyclic carbonate (FEC), and 5% lithium bisfluorosulfonylimide, and propyl propionate (PP) was added, and the mixture was stirred to obtain the electrolyte. The ratio of the content of the chain carbonate to the content of the cyclic carbonate is 1.6.

[0100] (5) Preparation of battery

[0101] The positive electrode sheet prepared in step (1), the separator (a polyethylene film with a thickness of 7.6 μm, a boehmite ceramic layer with a thickness of 2 μm on one side surface of the polyethylene film, a mixed adhesive layer of polyvinylidene fluoride and polymethyl methacrylate with a thickness of 1 μm on the outer surface of the boehmite ceramic layer, and a polymethyl methacrylate adhesive layer with a thickness of 1 μm on the other side surface of the polyethylene film), and the negative electrode sheet prepared in step (2) were wound to obtain a bare cell without liquid injection; the bare cell was placed in an outer packaging foil, and the electrolyte prepared in step (3) was injected into the dried bare cell, and the battery was obtained after vacuum packaging, standing, formation, shaping, sorting, and other processes.

[0102] The N / P ratio is 1.14, and the adhesion between the separator and the positive electrode sheet is 2.5 N / m.

[0103] Example 2

[0104] The battery was prepared according to the following method

[0105] (1) Preparation of positive electrode sheet

[0106] The positive electrode active material (X is 91%, R is 2), polyvinylidene fluoride, carbon nanotubes, and conductive carbon black were mixed in a mass ratio of 97:1.5:0.5:1, N-methylpyrrolidone was added, and the mixture was stirred under the action of a vacuum stirrer until the mixture became a homogeneous positive electrode slurry; the above-mentioned positive electrode slurry was uniformly coated on an aluminum foil with a thickness of 9 μm, the coated aluminum foil was baked in an oven with five different temperature gradients, then dried in an oven at 120°C for 8h, and then rolled, cut to obtain the desired positive electrode sheet.

[0107] The positive active material comprises single-crystal particles and polycrystal particles (mass ratio 1:1), the average particle size of the single-crystal particles is 4.5 μm, the average particle size of the polycrystal particles is 13.2 μm; the particle size Dv10 is 2.4 μm, the Dv50 is 8.6 μm, and the Dv90 is 25.3 μm; the mass content of element Zr in the positive active material is 1050 ppm, the mass content of element Al is 2015 ppm, and the mass content of element Ti is 1755 ppm; the ratio of the mass content of element Al in the surface layer region to the mass content of element Al in the bulk region is 2:1, and the ratio of the mass content of element Ti in the surface layer region to the mass content of element Ti in the bulk region is 100:1; the carbon nanotube has a tube diameter of 20 nm, the ratio of the tube length of the carbon nanotube to the tube diameter of the carbon nanotube is 2000; and the number of the positive electrode tabs is 15.

[0108] (2) Preparation of the negative electrode sheet

[0109] The artificial graphite (D1 is 18.6 μm), single-walled carbon nanotubes, conductive carbon black, sodium carboxymethyl cellulose and butadiene rubber are mixed in a mass ratio of 96.9:0.1:0.9:0.8:1.3 to prepare a first negative electrode slurry by a wet process; the artificial graphite (D2 is 6.2 μm), silicon-carbon material (Dv10 is 3.8 μm, Dv50 is 7.5 μm, and Dv90 is 20.1 μm), single-walled carbon nanotubes, conductive carbon black, sodium carboxymethyl cellulose and butadiene rubber are mixed in a mass ratio of 67.9:29:0.1:0.9:0.8:1.3 to prepare a second negative electrode slurry by a wet process; the first negative electrode slurry is uniformly coated on both surfaces of a copper foil, and dried; then the second negative electrode slurry is coated on both surfaces of the copper foil coated with the first negative electrode slurry after drying, and then transferred to a 80 ℃ oven for drying for 10 h, followed by rolling and die cutting to obtain a negative electrode sheet.

[0110] The mass content of silicon element in the silicon-carbon material is 50%, and the mass content of silicon element in the negative active layer is 15%; the tensile strength σ of the negative current collector in the width direction is 312 MPa.

[0111] (3) Preparation of the electrolyte

[0112] In an argon-filled glove box (moisture <10 ppm, oxygen <1 ppm), 13% of LiPF6 based on the total mass of the electrolyte is slowly added to a mixed solution containing 2% of a nitrile additive (succinonitrile), 35% of a chain carbonate (DEC), 30% of a cyclic carbonate (PC) and 8% of lithium bisfluorosulfonylimide, and then propyl propionate (PP) is added, and stirred uniformly to obtain an electrolyte. The ratio of the content of the chain carbonate to the content of the cyclic carbonate is 1.2.

[0113] (5) Preparation of the battery

[0114] The positive electrode sheet prepared in step (1), a separator (a polyethylene film with a thickness of 3.1 μm, a boehmite ceramic layer with a thickness of 2 μm on one side surface of the polyethylene film, a mixed adhesive layer of polyvinylidene fluoride and polymethyl methacrylate with a thickness of 1 μm on the outer surface of the boehmite ceramic layer, and a polymethyl methacrylate adhesive layer with a thickness of 1 μm on the other side surface of the polyethylene film), and the negative electrode sheet prepared in step (2) are wound to obtain an un-liquid-injected bare battery cell; the bare battery cell is placed in an outer packaging foil, and the electrolyte prepared in step (3) is injected into the dried bare battery cell, and the battery is obtained after vacuum packaging, standing, formation, shaping, sorting, and other processes.

[0115] The N / P ratio is 1.03, and the adhesion between the separator and the positive electrode sheet is 15.5 N / m.

[0116] Example 3

[0117] The battery is prepared according to the following method

[0118] (1) Preparation of a positive electrode sheet

[0119] The positive electrode active material (X is 90%, and R is 4), polyvinylidene fluoride, carbon nanotubes, and conductive carbon black are mixed in a mass ratio of 97:1.5:0.5:1, N-methylpyrrolidone is added, and stirring is performed under the action of a vacuum stirrer until the mixed system becomes a homogeneous positive electrode slurry; the above-mentioned positive electrode slurry is uniformly coated on an aluminum foil with a thickness of 9 μm, the coated aluminum foil is baked in an oven with 5 different temperature gradients, then dried in an oven at 120°C for 8 h, and then subjected to rolling and cutting to obtain the required positive electrode sheet.

[0120] The positive electrode active material includes single crystal particles and polycrystal particles (in a mass ratio of 1:1), the average particle size of the single crystal particles is 5.8 μm, and the average particle size of the polycrystal particles is 16.5 μm; the particle size Dv10 is 4.8 μm, the particle size Dv50 is 14.7 μm, and the particle size Dv90 is 39.8 μm; the mass content of element Zr in the positive electrode active material is 1495 ppm, the mass content of element Al is 2855 ppm, and the mass content of element Ti is 2985 ppm; the ratio of the mass content of element Al in the surface layer region to the mass content of element Al in the bulk region is 1.5:1, and the ratio of the mass content of element Ti in the surface layer region to the mass content of element Ti in the bulk region is 10:1; the tube diameter of the carbon nanotubes is 30 nm, the ratio of the tube length of the carbon nanotubes to the tube diameter of the carbon nanotubes is 1200; and the number of positive electrode tabs is 15.

[0121] (2) Preparation of a negative electrode sheet

[0122] The artificial graphite (D1 is 28.5 μm), single-walled carbon nanotubes, conductive carbon black, sodium carboxymethyl cellulose and styrene-butadiene rubber are mixed in a mass ratio of 96.9:0.1:0.9:0.8:1.3 to prepare a first negative electrode slurry by a wet process; the artificial graphite (D2 is 9.6 μm), silicon-carbon material (Dv10 is 5.6 μm, Dv50 is 11.6 μm, and Dv90 is 29.8 μm), single-walled carbon nanotubes, conductive carbon black, sodium carboxymethyl cellulose and styrene-butadiene rubber are mixed in a mass ratio of 70.4:26.5:0.1:0.9:0.8:1.3 to prepare a second negative electrode slurry by a wet process; the above first negative electrode slurry is uniformly coated on both surfaces of a copper foil, and dried; then the second negative electrode slurry is coated on both surfaces of the copper foil coated with the first negative electrode slurry after drying, and then transferred to a 85 ℃ oven for drying for 5 h, followed by rolling and die cutting to obtain a negative electrode sheet.

[0123] wherein the mass content of silicon element in the silicon-carbon material is 55%, and the mass content of silicon element in the negative electrode active layer is 15%; the tensile strength σ of the negative electrode current collector in the width direction is 845 MPa.

[0124] (3) Preparation of electrolyte

[0125] In an argon-filled glove box (moisture <10 ppm, oxygen <1 ppm), 13% LiPF6 based on the total mass of the electrolyte is slowly added to a mixed solution containing 4% nitrile-based additive (including benzyl cyanide, butanedinitrile and fluorobenzyl cyanide in a mass ratio of 2:1:1), 45% chain carbonate (including DMC and DEC in a mass ratio of 2:1), 15% cyclic carbonate (EC) and 12% lithium bisfluorosulfonylimide, and then propyl propionate (PP) is added and stirred uniformly to obtain the electrolyte. The ratio of the content of the chain carbonate to the content of the cyclic carbonate is 3.

[0126] (5) Preparation of battery

[0127] The positive electrode sheet prepared in step (1), the separator (a polyethylene film with a thickness of 11.6 μm, a boehmite ceramic layer with a thickness of 2 μm on one side surface of the polyethylene film, a mixed layer of polyvinylidene fluoride and polymethyl methacrylate with a thickness of 1 μm on the outer surface of the boehmite ceramic layer, and a polymethyl methacrylate adhesive layer with a thickness of 1 μm on the other side surface of the polyethylene film) and the negative electrode sheet prepared in step (2) are wound to obtain a bare cell without liquid injection; the bare cell is placed in an outer packaging foil, and the electrolyte prepared in step (3) is injected into the dried bare cell, and then vacuum packaging, standing, formation, shaping, sorting and other processes are performed to obtain the battery.

[0128] wherein the N / P ratio is 1.25; and the adhesion between the separator and the positive electrode sheet is 28.5 N / m.

[0129] Example 4

[0130] to verify the effect of the change in the ratio R of the molar content of the elements Co and Mn.

[0131] Example 1 is followed, except that R is 5.

[0132] Example 5 group

[0133] This group of examples is used to verify the effect of the change in the mass content of the element Zr in the positive electrode active material.

[0134] This group of examples follows Example 1, except that the mass content of the element Zr in the positive electrode active material is changed, as follows:

[0135] Example 5a, the mass content of the element Zr in the positive electrode active material is 200 ppm;

[0136] Example 5b, the mass content of the element Zr in the positive electrode active material is 2450 ppm.

[0137] Example 6 group

[0138] This group of examples is used to verify the effect of the change in the mass content G of the nitrile additive in the electrolyte.

[0139] This group of examples follows Example 1, except that G is changed, as follows:

[0140] Example 6a, G is 0.5%;

[0141] Example 6b, G is 8%.

[0142] Example 7

[0143] to verify the effect of the change in the ratio of the mass content of the chain carbonate to the mass content of the cyclic carbonate.

[0144] Example 1 is followed, except that the mass content of the chain carbonate and the mass content of the cyclic carbonate are changed, as follows: the mass content of the chain carbonate (DMC) is 60%, the mass content of the cyclic carbonate (FEC) is 5%; the ratio of the content of the chain carbonate to the content of the cyclic carbonate is 12.

[0145] Example 8 group

[0146] This group of examples is used to verify the effect of the change in the mass content of lithium bisfluorosulfonylimide in the electrolyte.

[0147] This group of examples was performed in accordance with Example 1, except that the mass content of lithium bisfluorosulfonylimide was changed, as follows:

[0148] Example 8a, the mass content of lithium bisfluorosulfonylimide was 0%.

[0149] Example 8b, the mass content of lithium bisfluorosulfonylimide was 15%.

[0150] Example 9 group

[0151] This group of examples was performed in order to verify the effects of changes in the "crystal morphology of the positive electrode active material".

[0152] This group of examples was performed in accordance with Example 1, except that the crystal morphology of the positive electrode active material was changed, as follows:

[0153] Example 9a, the positive electrode active material included only single-crystal particles.

[0154] Example 9b, the positive electrode active material included only polycrystal particles.

[0155] Example 10

[0156] This group of examples was performed in order to verify the effects of changes in the "kind of element in the positive electrode active material".

[0157] This group of examples was performed in accordance with Example 1, except that the positive electrode active material did not include the element Ti.

[0158] Example 11

[0159] This group of examples was performed in order to verify the effects of changes in the "ratio of the mass content of element Al in the surface layer region to the mass content of element Al in the bulk region".

[0160] This group of examples was performed in accordance with Example 1, except that the ratio of the mass content of element Al in the surface layer region to the mass content of element Al in the bulk region was 1:1.

[0161] Example 12 group

[0162] This group of examples was performed in order to verify the effects of changes in the "ratio of the tube length to the tube diameter of the carbon nanotube".

[0163] This group of examples was performed in accordance with Example 1, except that the ratio of the tube length to the tube diameter of the carbon nanotube was changed, as follows:

[0164] Example 12a, the ratio of the tube length to the tube diameter of the carbon nanotube was 50.

[0165] Example 12b, the ratio of the tube length to the tube diameter of the carbon nanotube was 10000.

[0166] Example 13

[0167] to verify the influence of the change in the mass content of element Si in the silicon-carbon material and the mass content of element Si in the negative active layer.

[0168] Example 1 was followed, except that the mass content of element Si in the silicon-carbon material was 30% and the mass content of element Si in the negative active layer was 2%.

[0169] Comparative Example 1 Group

[0170] Example 1 was followed, except that the molar content ratio R of element Co and element Mn in the positive active material was changed, specifically as follows:

[0171] Comparative Example 1a, R was 0.67;

[0172] Comparative Example 1b, R was 9.

[0173] Comparative Example 2 Group

[0174] Example 1 was followed, except that the mass content of element Zr in the positive active material was changed, specifically as follows:

[0175] Comparative Example 2a, the mass content of element Zr in the positive active material was 0 ppm;

[0176] Comparative Example 2b, the mass content of element Zr in the positive active material was 4000 ppm.

[0177] Comparative Example 3 Group

[0178] Example 1 was followed, except that the mass content G of nitrile-based additive in the electrolyte was changed, specifically as follows:

[0179] Comparative Example 3a, G was 0%;

[0180] Comparative Example 3b, G was 15%.

[0181] Comparative Example 4

[0182] Example 1 was followed, except that the ratio of the content of chain carbonate to the content of the cyclic carbonate was 0.2, wherein the content of chain carbonate (DMC) was 10% and the content of cyclic carbonate (FEC) was 50%.

[0183] Test Example

[0184] (1) Cycle Test

[0185] The batteries prepared from the examples and comparative examples were subjected to cycle tests, and the specific test method was as follows:

[0186] The battery was charged at a current density of 1.2 C to 4.3 V at 45 °C, then charged at 4.3 V, the cutoff current was 0.05 C, then rested for 10 min, then discharged at a current density of 0.5 C to 3.0 V, then rested for 10 min, and the discharge capacity of the battery at this time was recorded as the initial capacity; the above charging and discharging process was repeated until the 500th cycle constant voltage charging process ended and rested for 10 min, then discharged at a current density of 0.5 C to 3.0 V, rested for 10 min, and the discharge capacity of the battery at this time was recorded as the capacity after cycling. The cycle capacity retention rate = (capacity after cycling / initial capacity) x 100%, and the cycle capacity retention rate was recorded in Table 1.

[0187] (2) Rate test

[0188] The batteries prepared in the examples and comparative examples were subjected to rate test, and the specific test method was as follows:

[0189] The battery was charged at a current density of 1.2 C to 4.3 V at 25 °C, then charged at 4.3 V, the cutoff current was 0.05 C, then rested for 10 min, then discharged at a current density of 0.1 C to 3.0 V, then rested for 10 min, and the above charging and discharging process was repeated 3 times, and the discharge capacity of the battery in the above 3 times was recorded as the discharge capacity at 0.1 C; similarly, after the same charging process, the battery reached 4.3 V, and then discharged at a current density of 0.2 C, 0.5 C and 1 C to 3.0 V, respectively, and 3 discharges were carried out at each different rate to obtain the average value as the discharge capacity at that rate;

[0190] The rate retention rate = (discharge capacity at a specific rate / discharge capacity at 0.1 C rate) x 100%, and the results were recorded in Table 1.

[0191] (3) Thermal runaway test

[0192] The batteries prepared in the examples and comparative examples were subjected to thermal runaway test, and the specific test method was as follows:

[0193] The battery was charged at a current density of 1.2 C to 4.3 V at 25 °C, then charged at 4.3 V, the cutoff current was 0.05 C, then rested for 10 min. Then placed in a constant temperature oven (temperature sensitivity of 0.1 °C), and a temperature sensor was tightly attached to the surface of the battery body with iron fluoride. The constant temperature oven was adjusted to a temperature rising rate of 2 °C / min to 100 °C, and after reaching the temperature, it was kept constant for 30 min, then the temperature was raised at a rate of 0.1 °C / min until the battery burned out or the temperature of the constant temperature oven and the battery body reached 160 °C, the heating power was turned off and disconnected, and the battery was naturally cooled. The temperature of the temperature sensor during the whole process was recorded, and when the temperature sensor reached 60 °C / min for the first time, it was recorded as the ignition point. The results were recorded in Table 1.

[0194] (4) Gram capacity test

[0195] At (25±2)°C, the batteries prepared in the Examples and Comparative Examples were charged at a standard constant current of 0.1C to 4.3V, then charged at a constant voltage of 0.05C to 4.3V. The batteries were allowed to rest for 10 minutes and then discharged at a standard constant current of 0.1C to a discharge cutoff voltage of 3V. The discharge capacity of the batteries at this point was recorded as the initial gram capacity of the positive electrode. The results are shown in Table 1.

[0196] Table 1

[0197]

[0198]

[0199] As can be seen from Table 1, compared with the comparative example, the battery of the present invention significantly improves the thermal stability, rate performance and cycle stability of the battery at 45°C while ensuring high initial gram capacity of the positive electrode active material.

[0200] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention.

Claims

1. A lithium-ion secondary battery, characterized by comprising: The lithium ion secondary battery comprises a positive electrode sheet and an electrolyte, the positive electrode sheet comprises a positive electrode active material, the positive electrode active material comprises a lithium metal transition oxide, the lithium metal transition oxide comprises elements Ni, Co and Mn; the content X of element Ni is 90%-98% based on the total moles of elements Ni, Co and Mn, and the ratio R of the contents of element Co and element Mn is 1.2-5; the positive electrode active material further comprises at least one of elements Al, Mg, Ti, Zr, Y, La, Te, W, P and B; the positive electrode active material comprises a matrix and a coating layer located on the surface of the matrix, the coating layer comprises element Zr, and the mass content of element Zr in the positive electrode active material is 200 ppm-2500 ppm; The electrolyte comprises a nitrile additive and a carbonate solvent, the carbonate solvent comprises a chain carbonate and a cyclic carbonate, and the content G of the nitrile additive is 0.3%-6% based on the total mass of the electrolyte; the ratio of the content of the chain carbonate to the content of the cyclic carbonate is >1.

2. The lithium ion secondary battery according to claim 1, wherein the positive electrode active material comprises single-crystal particles and polycrystal particles; the average particle size of the single-crystal particles is 2 μm-6 μm, and the average particle size of the polycrystal particles is 6 μm-17 μm; And / or, the nitrile additive comprises at least one of benzonitrile, succinonitrile, fluorobenzonitrile, adiponitrile, 1,3,6-hexanetricarbonitrile, glycerol trinitrate, 1,4-dicyano-2-butene and ethylene glycol bis(propionitrile) ether; And / or, the chain carbonate comprises at least one of dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate and methyl propyl carbonate; And / or, the content of the chain carbonate is 30%-65% based on the total mass of the electrolyte; And / or, the cyclic carbonate comprises at least one of propylene carbonate, ethylene carbonate and fluorinated ethylene carbonate; And / or, the content of the cyclic carbonate is 15%-35% based on the total mass of the electrolyte.

3. The lithium-ion secondary battery according to claim 1 or 2, wherein The particle size Dv10 of the positive electrode active material is 0.5 μm-5 μm, the particle size Dv50 is 1 μm-15 μm, and the particle size Dv90 is 10 μm-40 μm.

4. The lithium-ion secondary battery according to claim 1 or 2, wherein The positive electrode sheet comprises a positive electrode current collector and a positive electrode active layer located on at least one side surface of the positive electrode current collector, and the positive electrode active layer further comprises a positive electrode conductive agent, the positive electrode conductive agent comprises carbon nanotubes and conductive carbon black; the tube diameter of the carbon nanotubes is 1 nm-100 nm, and the ratio of the tube length of the carbon nanotubes to the tube diameter of the carbon nanotubes is 50-10000.

5. The lithium-ion secondary battery according to claim 1 or 2, wherein The positive electrode active material further comprises element Al and element Ti, and the content of element Al is 800 ppm-3500 ppm, preferably 800 ppm-3000 ppm, and the content of element Ti is 500 ppm-3000 ppm based on the total weight of the positive electrode active material; Preferably, the substrate comprises a bulk region and a surface layer region, the mass content of element Al in the surface layer region is greater than that in the bulk region, and the mass content of element Ti in the surface layer region is greater than that in the bulk region; wherein the surface layer region is a region formed by a point 0%-10% away from the surface of the substrate in the direction from the surface to the core of the substrate, and the bulk region of the substrate is a region of the substrate excluding the surface layer region; More preferably, the ratio of the mass content of element Al in the surface layer region to that in the bulk region is (1.2-15):1; and further preferably (1.5-5):1; and the ratio of the mass content of element Ti in the surface layer region to that in the bulk region is (1.5-200):1; and further preferably (10-100):

1.

6. The lithium-ion secondary battery according to claim 4, wherein The positive electrode sheet further comprises a plurality of positive electrode tabs extending from the positive electrode current collector, and the number of the positive electrode tabs is greater than or equal to 2.

7. The lithium-ion secondary battery according to claim 1 or 2, wherein The electrolyte further comprises lithium bisfluorosulfonylimide, and the content of lithium bisfluorosulfonylimide is 3%-15% based on the total mass of the electrolyte. And / or, the charging cutoff voltage of the lithium ion secondary battery is greater than or equal to 4.2 V.

8. The lithium-ion secondary battery according to claim 1 or 2, wherein The lithium ion secondary battery further comprises a negative electrode sheet, and the negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer located on at least one side surface of the negative electrode current collector, and the negative electrode active layer comprises a silicon-carbon material. Preferably, the silicon-carbon material comprises a porous carbon substrate and a silicon material located in the internal pores of the porous carbon substrate; and more preferably, the mass content of silicon in the silicon-carbon material is 30%-80%. Preferably, the mass content of silicon in the negative electrode active layer is 2%-50%. And / or, the N / P ratio of the lithium ion secondary battery is 1.03-1.

25.

9. The lithium-ion secondary battery according to claim 8, wherein The negative electrode active layer comprises a first active coating layer and a second active coating layer arranged in the thickness direction of the negative electrode sheet, and the first active coating layer is located between the negative electrode current collector and the second active coating layer; the first active coating layer comprises a first carbon-based material, and the second active coating layer comprises a second carbon-based material and the silicon-carbon material. Preferably, the first carbon-based material comprises at least one of artificial graphite, natural graphite, mesocarbon microbeads, hard carbon, and soft carbon. The second carbon-based material comprises at least one of artificial graphite, natural graphite, mesocarbon microbeads, hard carbon, and soft carbon. Preferably, the average particle size of the first carbon-based material is D1, and 1.6 μm≤D1≤30 μm, and the average particle size of the second carbon-based material is D2, and 2 μm≤D2≤10 μm. And / or, the tensile strength of the negative electrode current collector in the width direction is σ, and 300 MPa≤σ≤850 MPa. And / or, the particle size Dv10 of the silicon-carbon material is 0.5 μm-6 μm, the particle size Dv50 is 3 μm-15 μm, and the particle size Dv90 is 9 μm-30 μm.

10. The lithium-ion secondary battery according to claim 1 or 2, wherein The lithium ion secondary battery further comprises a separator, the separator comprising a substrate layer, a ceramic layer on at least one side surface of the substrate layer, and a glue layer on the outer surfaces of both sides of the separator, the ceramic layer at least facing the positive electrode sheet; Preferably, the thickness of the separator is 3-12 μm; Preferably, the adhesion between the separator and the positive electrode sheet is 0.1-30 N / m.

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