Lithium cobalt oxide positive electrode material and lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium cobalt oxide positive electrode material and a lithium ion secondary battery comprising the lithium cobalt oxide positive electrode material. The lithium cobalt oxide positive electrode material has an R-3m layered structure; the lithium cobalt oxide positive electrode material comprises elements Ni and Mn, and the mass ratio of the element Mn to the element Ni of the lithium cobalt oxide positive electrode material is 1.2-10; the mass content of the element Ni of the lithium cobalt oxide positive electrode material is 100 ppm to 3000 ppm; the mass content of the element Mn of the lithium cobalt oxide positive electrode material is 125 ppm to 5000 ppm; the lithium cobalt oxide positive electrode material comprises lithium cobalt oxide particles; the lithium cobalt oxide particles have grain boundaries, and the grain boundaries comprise the elements Ni and Mn. The positive electrode material disclosed by the invention can maintain relatively good structural stability and relatively high gram capacity under the condition of high voltage (charging cut-off voltage is greater than or equal to 4.5 V); the battery provided by the invention has excellent cycle performance and rate capability.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a lithium cobalt oxide cathode material and a lithium ion secondary battery comprising the same. BACKGROUND

[0002] With the rapid development of electric vehicles and portable electronic devices, the requirements for energy density, cycle life and rate performance of lithium ion batteries are also increasing. As an important lithium battery cathode material, lithium cobalt oxide (LiCoO2) has high theoretical specific capacity (274 mAh / g), excellent cycle performance and mature preparation process. Its stable layered structure enables lithium ions to be reversibly inserted and extracted during charging and discharging, ensuring the cycle life of the battery. Currently, it has become the mainstream cathode material for consumer electronic batteries.

[0003] However, under high voltage conditions (for example, the charging cutoff voltage is greater than or equal to 4.5V), lithium cobalt oxide is deeply delithiated, which causes irreversible phase transition of the crystal structure, resulting in local distortion of the crystal lattice, accompanied by volume change and stress concentration. With the continuous extraction of lithium ions, the internal stress of the crystal gradually accumulates. When the stress exceeds the bearing limit of the crystal itself, cracks will occur at the grain boundaries, exposing new interfaces, intensifying the electrolyte side reaction, leading to poor structural stability of lithium cobalt oxide, significantly increasing the interfacial impedance of the cathode, and significantly reducing the cycle performance and rate performance of the battery. Therefore, it is necessary to improve the cycle performance and structural stability of lithium cobalt oxide under high voltage conditions. SUMMARY

[0004] The present application aims to overcome the problems of poor capacity performance and poor structural stability of lithium cobalt oxide under high voltage conditions in the prior art, which leads to poor cycle stability and rate performance of the battery. A lithium cobalt oxide cathode material and a lithium ion secondary battery comprising the same are provided. The lithium ion secondary battery (hereinafter referred to as the battery) of the present application modifies lithium cobalt oxide, effectively improves the capacity performance of lithium cobalt oxide cathode material, suppresses the irreversible phase transition of lithium cobalt oxide, and significantly improves the structural stability of lithium cobalt oxide cathode material and the cycle stability and rate performance of the battery under high voltage conditions.

[0005] In the prior art, in order to further improve the capacity performance of LiCoO2, the working voltage is usually increased (for example, higher than or equal to 4.5V) to promote more lithium ions to be extracted and improve the capacity. 3+ The charge compensation mechanism depends on the oxidation reaction of Co 4+ to Co 4+With the increase of the ratio, especially in the case of significant polarization effect, the lithium cobaltate particles exhibit non-uniform delithiation, i.e. the delithiation amount of the surface layer is much higher than that of the internal body. This delithiation non-uniformity promotes the generation of a large number of grain boundaries in the lithium cobaltate particles. As the transition region of different oriented crystals, the grain boundaries have poor compatibility with deformation and become a weak region for stress accumulation, which is prone to produce cracks, aggravate the electrolyte side reaction, and further destroy the crystal structure stability of LiCoO2, causing particle cracking, crushing, serious capacity decay and other problems, affecting the energy density and cycle stability of the battery.

[0006] Based on the above reasons, the inventors have conducted a large number of targeted researches and found that improving the lithium cobaltate can effectively solve the above problems, specifically: First, the doping of elements Ni and Mn in the lithium cobaltate positive electrode material can synergistically improve the tap density performance and crystal structure stability of lithium cobaltate. Among them, the doping of element Ni can promote the tap density performance by improving the charge compensation, promote more lithium ions to be deintercalated, and improve the tap density performance of lithium cobaltate; the doping of element Mn can effectively reduce the Li / Ni mixing phenomenon, reduce the lattice disorder to enhance the crystal structure stability. At the same time, appropriate Mn doping can regulate the degree of polycrystallization of lithium cobaltate particles to avoid the generation of a large number of grain boundaries due to particle agglomeration; in addition, the presence of elements Ni and Mn at the grain boundaries can enhance the bonding strength of the transition metal layer, improve the structure stability of the grain boundary region, optimize the anisotropy of the crystal, promote the uniformity of lithium ion deintercalation, and reduce the generation of cracks caused by excessive stress accumulation in the grain boundary region; in addition, it can also reduce the particle size without affecting the positive electrode compaction density, shorten the lithium ion transmission path, and further improve the rate performance and cycle stability of the battery.

[0007] Secondly, by regulating the mass ratio of element Mn and element Ni in the lithium cobalt oxide positive electrode material within a suitable range, the Ni element can fully participate in the charge compensation process, the lithium cobalt oxide positive electrode material can improve the gram capacity, the Mn element can stabilize the crystal structure, the irreversible phase change under high voltage can be inhibited, the gram capacity of the positive electrode material can be further optimized, the structure stability can be improved, the cycle stability and energy density of the battery under high voltage condition can be effectively improved. At the same time, it is helpful to reduce the stress concentration of the grain boundary region in the cycle process, prevent the particle from cracking, improve the diffusion rate of lithium ions in the positive electrode material, and further improve the rate performance of the battery. When the mass ratio of element Mn and element Ni is too small (for example, <1.2), the amount of Mn doping is insufficient, a large amount of element Ni doping will cause Li / Ni mixing, lithium ion deintercalation in the cycle process will cause lattice distortion to be aggravated, the lithium cobalt oxide crystal structure stability will be reduced, and the first particle polycrystallization degree is low, the lithium ion transmission path is long, which affects the rate performance of the battery; when the mass ratio of element Mn and element Ni is too large (for example, >10), a large amount of element Mn will cause the lithium cobalt oxide to be severely self-aggregated, the polycrystallization degree will be intensified, the lattice distortion will be caused, a large number of grain boundaries will be generated, cracks will be generated at the grain boundaries, new interfaces will be exposed, the electrolyte side reaction will be aggravated, the crystal structure stability will be destroyed, and the cycle performance of the battery will be affected.

[0008] Based on this, the inventors of the present application propose the following scheme: The present application provides a lithium cobalt oxide positive electrode material in a first aspect. The lithium cobalt oxide positive electrode material has an R-3m layered structure. The lithium cobalt oxide positive electrode material includes elements Ni and Mn. The mass ratio of element Mn to element Ni in the lithium cobalt oxide positive electrode material is 1.2-10. The mass content of element Ni in the lithium cobalt oxide particle is 100ppm-3000ppm. The mass content of element Mn in the lithium cobalt oxide particle is 125ppm-5000ppm. The lithium cobalt oxide positive electrode material includes lithium cobalt oxide particles. The lithium cobalt oxide particles have grain boundaries, and the grain boundaries contain elements Ni and Mn.

[0009] The present application provides a lithium ion secondary battery in a second aspect. The lithium ion secondary battery includes the lithium cobalt oxide positive electrode material described in the first aspect of the present application.

[0010] The above technical solution has at least the following advantages compared with the prior art: (1) The lithium cobalt oxide positive electrode material of the present application can maintain good structure stability and high gram capacity under high voltage (charge cut-off voltage ≥4.5V); (2) The battery of the present application has excellent cycle performance and rate performance.

[0011] The endpoints of the ranges and any values disclosed herein are not limited to the precise values recited as implicitly split by the language of the specification. The described ranges should be construed as being encompassing both the endpoints and the individual points within the ranges. For ranges including both endpoints, any intervening values between the declared endpoints are intended to be included. For ranges excluding either or both endpoints, any intervening values including the excluded endpoint are intended to be included. For ranges as defined by two Minimum value and / or two Maximum value, any intervening value including the Minimum value or the Maximum value is intended to be included. The same applies to ranges having endpoints that are not integers. The disclosure as found in the Art section is included herein by reference in its entirety. The following examples further illustrate the application. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 A schematic diagram of a first particle in an example of the application is shown.

[0013] Figure 2 A scanning electron microscope (SEM) image of a lithium cobalt oxide particle in an example of the application is shown.

[0014] Figure 3 A schematic diagram of a second particle in an example of the application is shown.

[0015] Figure 4 An X-ray diffraction pattern (XRD) of a lithium cobalt oxide cathode material in an example of the application is shown.

[0016] Figure 5 A schematic diagram I of a lithium ion secondary battery in an example of the application is shown.

[0017] Figure 6 A schematic diagram II of a lithium ion secondary battery in an example of the application is shown. DETAILED DESCRIPTION

[0018] The specific embodiments of the present application will now be described in detail below. It should be appreciated that the detailed description of specific embodiments is intended to be illustrative only and is not intended to limit the present application.

[0019] The first aspect of the present application provides a lithium cobalt oxide cathode material, the lithium cobalt oxide cathode material having an R-3m layered structure; the lithium cobalt oxide cathode material comprising elements Ni and Mn; the mass ratio of the element Mn to the element Ni in the lithium cobalt oxide cathode material being 1.2-10 (for example, 1.2, 2, 3, 4, 5, 6, 7, 8, 9, or 10); the lithium cobalt oxide cathode material comprising lithium cobalt oxide particles, the lithium cobalt oxide particles having grain boundaries, the grain boundaries comprising elements Ni and Mn; the mass content of the element Ni in the lithium cobalt oxide cathode material being 100 ppm-3000 ppm (for example, 100 ppm, 500 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, or 3000 ppm), and the mass content of the element Mn in the lithium cobalt oxide cathode material being 125 ppm-5000 ppm (for example, 125 ppm, 500 ppm, 1000 ppm, 2000 ppm, 3000 ppm, 4000 ppm, or 5000 ppm).

[0020] In an example, the mass ratio of the element Mn to the element Ni in the lithium cobalt oxide cathode material is 1.2-4.

[0021] In an example, the mass content of the element Ni in the lithium cobalt oxide cathode material is 400 ppm-2000 ppm.

[0022] In an example, the mass content of the element Ni in the lithium cobalt oxide cathode material is 700 ppm-1500 ppm.

[0023] In an example, the mass content of the element Mn in the lithium cobalt oxide cathode material is 600 ppm-3500 ppm.

[0024] In an example, the mass content of the element Mn in the lithium cobalt oxide cathode material is 1500 ppm-2500 ppm.

[0025] In the present application, the lithium cobalt oxide particles have grain boundaries. The “grain boundary” refers to a transition region with a certain thickness formed at the interface of adjacent lithium cobalt oxide particles when the adjacent lithium cobalt oxide particles agglomerate with different crystal orientations (i.e., the lattice fringes on both sides of the grain boundary can be clearly seen to change under HRTEM observation). The grain boundary is a key channel for lithium ion transport inside the lithium cobalt oxide particle, and is also a weak area where side reactions occur and ion migration is hindered at high voltage. The grain boundary comprises elements Ni and Mn, which can improve the structural stability of the grain boundary region, inhibit the generation of cracks caused by excessive stress accumulation, optimize the lithium ion transport channel, improve the ion diffusion efficiency of the battery during charging and discharging, and help to improve the cycle stability and rate performance of the battery.

[0026] In an example, the mass content of the element Mn in the grain boundary is greater than the mass content of the element Ni. Specifically, the position of the grain boundary is determined by scanning electron microscopy (SEM) cross-section analysis, the mass content of the elements Mn and Ni is measured at 10 different points on the cross-section of the grain boundary at random by using energy dispersive spectrometer (EDS) line scanning, and the average value is calculated to obtain the mass content of the elements Mn and Ni in the grain boundary, respectively. The results show that the mass content of the element Mn is greater than the mass content of the element Ni. The element Mn is more appropriate than the element Ni, which can reduce Li / Ni mixing, reduce lattice disorder, enhance the structural stability and ion transport efficiency of the lithium cobaltate particles under the premise of ensuring the capacity of the lithium cobaltate particles by promoting the deintercalation of lithium ions through electron transfer; meanwhile, the excessive element Mn can also moderately control the polycrystallization of the lithium cobaltate particles, avoid the agglomeration of the particles to generate a large number of grain boundaries, shorten the lithium ion transport path, and further improve the rate performance and cycle stability of the battery.

[0027] The elements Ni and Mn in the lithium cobaltate particles mainly occupy the Co site (judged by the continuous lattice fringes observed by high-resolution transmission electron microscopy HRTEM and no obvious heterophase diffraction peak), which can inhibit lattice distortion and enhance interlayer bonding to provide a stable channel for the deintercalation of lithium ions. The mass ratio of the element Mn to the element Co is greater than the mass ratio of the element Ni to the element Co, a small amount of Ni and Mn randomly occupies the Li site (the proportion of Li atoms occupied by Ni and Mn atoms is 0.001%-0.1%), and the proportion of Li atoms occupied by Ni atoms is higher than the proportion of Li atoms occupied by Mn atoms.

[0028] When the mass content of the element Ni in the lithium cobaltate positive electrode material is too low (for example, less than 100 ppm), the amount of electron contribution will be reduced, the deintercalation dynamics of lithium ions will be insufficient, the capacity of the lithium cobaltate will be significantly reduced, the lithium ion transport channel will be narrowed, and the rate performance of the battery will be affected. When the mass content of the element Ni in the lithium cobaltate positive electrode material is too high (for example, greater than 3000 ppm), Li / Ni mixing will be intensified, lattice disorder will be caused, interlayer sliding and volume expansion caused by the deintercalation of lithium ions during the cycle process will be more obvious, which is easy to cause particle cracking, promote the dissolution of metal Co, and accelerate the decomposition of electrolyte, resulting in the attenuation of battery capacity and the reduction of cycle stability. When the mass content of the element Mn in the lithium cobaltate positive electrode material is too low (for example, less than 125 ppm), Li / Ni mixing and the enhancement of crystal structure stability cannot be effectively inhibited, and the polycrystallization effect of the lithium cobaltate particles is weakened, the particle size is large, the lithium ion transport path is lengthened, and the rate performance of the battery is reduced. When the mass content of the element Mn in the lithium cobaltate positive electrode material is too high (for example, greater than 5000 ppm), it will excessively occupy the Co site, weaken the charge compensation effect of Ni, reduce the capacity of the lithium cobaltate, and excessive polycrystallization will reduce the compaction density of the positive electrode sheet and the energy density of the battery, hinder the transport of lithium ions, and affect the cycle performance of the battery.

[0029] In the present application, the mass content of element Ni and element Mn in the lithium cobalt oxide cathode material can be obtained by testing by conventional methods in the art, for example, discharging the battery to 0% SOC (for example, discharging the battery to 3V), disassembling and taking out the cathode sheet, soaking in dimethyl carbonate (DMC) solvent for 12h, then rinsing with DMC solvent to remove the lithium salt attached to the cathode sheet, calcining the cathode sheet in air at 450 degrees for 2-4 hours, then scraping the lithium cobalt oxide cathode material from the surface of the cathode sheet with a ceramic knife, and testing using inductively coupled plasma-optical emission spectrometer (ICP-OES) to obtain the mass content of element Ni and element Mn in the lithium cobalt oxide particles.

[0030] In the present application, the lithium cobalt oxide particles include first particles, the first particles include a first substrate and a first protrusion on the surface of the first substrate, the first protrusion at least partially covers the outer surface of the first substrate; the first substrate includes a first region and a second region on the surface of the first region, extending in a direction towards the center of the surface of the first substrate, the second region is a region with a depth of 300nm from the surface of the first substrate, and the first region is the remaining region of the first substrate excluding the second region. As shown in Figure 1 The structure of the first particle in an example of the present application is shown in the figure, as can be seen from the figure, the first particle includes a first substrate and a first protrusion (3) on the surface of the first substrate, and the first substrate includes a first region (1) and a second region (2).

[0031] In an example, the first region includes continuous lattice fringes observed in HRTEM.

[0032] In the present application, the first region includes elements Ni, Mn, Al and Mg, the second region includes elements Ni, Mn, Al and Mg and a first element group, and the first element group includes at least one of elements Ti, La, Y, Zr and S.

[0033] In an example, the mass ratio of element Ti to element Co in the second region is 0.0001-0.001 (for example, 0.0001, 0.0002, 0.0004, 0.0006, 0.0008 or 0.001).

[0034] In an example, the mass ratio of element Ti to element Co in the second region is 0.0002-0.0005.

[0035] In an example, the mass ratio of element La to element Co in the second region is 0.0001-0.0006 (for example, 0.0001, 0.0002, 0.0003, 0.0004, 0.0005 or 0.0006).

[0036] In one example, the mass ratio of element La to element Co in the second region is 0.0001-0.0004.

[0037] In one example, the mass ratio of element Y to element Co in the second region is 0.0001-0.001 (for example, 0.0001, 0.0002, 0.0004, 0.0006, 0.0008 or 0.001).

[0038] In one example, the mass ratio of element Y to element Co in the second region is 0.0001-0.0005.

[0039] In one example, the mass ratio of element Zr to element Co in the second region is 0.0001-0.0005 (for example, 0.0001, 0.0002, 0.0003, 0.0004 or 0.0005).

[0040] In one example, the mass ratio of element Zr to element Co in the second region is 0.0001-0.0003.

[0041] When the second region contains element Ti, La, Y, Zr or Al, the structural stability of the first particle can be enhanced, the interface reaction between the positive electrode and the electrolyte can be optimized, and the performance decay during the cycle can be inhibited. The element mainly occupies the Co site, and among them, element Ti and Zr as high-valence ions (for example, Ti 4+ and Zr 4+ ) can enhance the bond energy of the layered lattice after occupying the Co site, reduce the lattice distortion and avoid the collapse of the particle structure; element La and Y as rare earth elements (for example, La 3+ and Y 3+ ) can improve the interface electronic conductivity, and their large atomic radius can increase the lattice spacing of the second region, providing a wider channel for the rapid transmission of lithium ions; Al 3+ occupying the Co site can construct a dense "Al-O" bond network, which can inhibit the reaction between PF6 - in the electrolyte and the surface Co 3+ , reduce the dissolution of Co element, hinder the penetration of electrolyte decomposition products (such as HF) into the bulk phase, and reduce the interface impedance.

[0042] In an example, the mass ratio of the elements in the second region satisfies: (Ti+La+Y+Zr+Al) / Co>(Ni+Mn) / Co. The balance of "strong surface protection and attenuation resistance, and high capacity and performance of the inner layer" can be achieved while maintaining the "contribution capacity and stable structure" of the first region. In the first region, element Ni improves the specific capacity of the positive electrode material, and element Mn inhibits Li / Ni mixing and improves the stability of the crystal structure; the second region can effectively inhibit lattice distortion, prevent HF in the electrolyte from corroding and metal Co from dissolving out, reduce the side reaction of active substances and electrolyte, and the elements in the second region will not diffuse inward to occupy the Co sites occupied by Ni and Mn. Through the synergistic effect of the elements, the cycle stability and rate performance of the battery under high voltage conditions can be effectively improved.

[0043] In an example, the mass ratio of element Al to element Co in the second region is greater than the mass ratio of element Al to element Co in the first region.

[0044] Since the second region can directly contact the electrolyte, deep delithiation will occur under high voltage conditions, resulting in a decrease in the Co-O bond force in the lattice and a dramatic contraction of the interlayer spacing, which causes irreversible phase transition of the crystal structure. When a high content of element Al occupies the Co site, it can form a strong "Al-O" bond, inhibit lattice distortion, and a dense "Al-O" network can effectively inhibit the corrosion of HF in the electrolyte on the particles and the dissolution of metal Co, thereby strengthening the interface stability of the second region and reducing the degree of phase transition of the second region. The lower content of Al in the first region avoids occupying the Co sites of Ni and Mn, and can further enhance the structural stability of the first particle without affecting the lithium ion deintercalation efficiency and specific capacity.

[0045] In the present application, the mass ratio of elements Ti, La, Y, Zr, Al and element Co in the second region can be obtained by conventional methods in the art, for example, polishing the cross section of the first particle using an argon ion grinder, and then observing under a transmission electron microscope (TEM); using an argon ion grinder CP laser to cut the first particle, and then using an energy dispersive spectrometer (EDS) to scan the second region, randomly measuring the mass ratio of elements Ti, La, Y, Zr, Al and element Co at 10 different points on the cross section of the second region and calculating the average value; at least 10 first particles are selected, and the average value is finally obtained to obtain the mass ratio of elements Ti, La, Y, Zr, Al and element Co in the second region.

[0046] In the present application, the valence state of element Co in the lithium cobaltate positive electrode material includes +2 and +3, the valence state of element Ni includes +2 and +3, and the valence state of element Mn includes +3 and +4.

[0047] In an example, the atomic percentage of ions of different valence states in the lithium cobaltate positive electrode material satisfies: Mn 4+ / Co 3+ >Ni 2+ / Co 3+ >Ni 3+ / Co 3+ >Mn 3+ / Co 3+ . Among them, Mn 4+ has a high proportion, which can effectively inhibit Li / Ni mixing and enhance the lattice stability; Ni 2+ dominates the capacity contribution, which can improve the capacity per gram of the positive electrode material; and Mn 3+ has a low content, which can reduce the impedance rise and capacity decay caused by disproportionation reaction, and maintain the lithium cobaltate to have high capacity per gram and structural stability.

[0048] In the present application, the atomic percentage of ions of different valence states in the lithium cobaltate positive electrode material can be obtained by conventional methods in the art, for example, by transmission electron microscopy-electron energy loss spectroscopy (TEM-EELS) test.

[0049] In an example, in the cross-sectional X-ray electron energy spectrum of the lithium cobaltate particles (with Al Kα as the excitation source, a beam spot of 400 um, a working voltage of 12 kV, and C1s as the binding energy of 284.8 eV for energy standard for charge correction), Co2p will appear two or more peaks in the range of 770 eV-815 eV, wherein the characteristic peak range of Co 2+ is 785.7 eV-786.7 eV, and the characteristic peak range of Co 3+ is 781.5 eV-782.5 eV.

[0050] In the present application, the first particles contain element S, and a small amount of S can form SO4 - polyanion conductors, which have good stability at high voltage.

[0051] In an example, the mass content of element S in the first particles is 10 ppm-300 ppm (for example, 10 ppm, 50 ppm, 100 ppm, 150 ppm, 200 ppm, 250 ppm, or 300 ppm).

[0052] In an example, the mass content of element S in the first particles is 10 ppm-100 ppm.

[0053] In an example, the grain boundary contains element S, and the mass content of element S in the grain boundary is greater than the mass content of element S in the second region. The grain boundary is a key channel for lithium ion transmission inside the lithium cobalt oxide particle, and is also a weak region where side reactions occur and ion migration is blocked at high voltage. The appropriate enrichment of element S at the grain boundary and the formation of more SO4 - The polyanion conductor has good stability at high voltage, can inhibit the dissolution of transition metal ions at the grain boundary, reduce the interface side reaction, and ensure the rapid transmission of lithium ions. The second region containing a small amount of element S can improve the interface stability, and if the content of element S in the second region is too high, it will affect the lithium ion deintercalation and reduce the ion transmission efficiency.

[0054] In the present application, the test method of the mass content of element S in the first particle is the same as the test method of the mass content of elements Ni and Mn in the lithium cobalt oxide particle, which will not be repeated here.

[0055] In the present application, the mass content of element S in the grain boundary and the second region can be tested by conventional methods in the art, for example, after polishing the cross section of the first particle using an argon ion grinder, observing in a transmission electron microscope (TEM); using an argon ion grinder CP laser to cut the first particle, and then using an energy dispersive spectrometer (EDS) to scan the cross section of the grain boundary and the second region respectively, the area of the face scan should not be less than 50% of the respective cross section, and the scanning range should be completely within the cross section, and the mass content of element S in the grain boundary and the second region is calculated. At least 10 first particles are selected for measurement, and the average value is taken.

[0056] In the present application, the particle size Dn10, Dn50 and Dn90 of the lithium cobalt oxide particle satisfy: (Dn90-Dn50) > 1.5xDn10. Wherein, the particle size Dn10, Dn50 and Dn90 can be tested by conventional methods in the art, for example, by a laser particle size analyzer.

[0057] In the present application, the lithium cobalt oxide particle further comprises a second particle, and the average particle size of the first particle is greater than the average particle size of the second particle. As Figure 2 The scanning electron microscope (SEM) image of the lithium cobalt oxide particle in an example of the present application is shown in the figure, from which it can be seen that the lithium cobalt oxide particle comprises a first particle and a second particle, and the average particle size of the first particle is greater than the average particle size of the second particle.

[0058] In one example, the average particle size of the first particles is 12 μm-22 μm (e.g., 12 μm, 14 μm, 16 μm, 18 μm, or 22 μm), and the average particle size of the second particles is 0.5 μm-6.5 μm (e.g., 0.5 μm, 2 μm, 4 μm, or 6.5 μm). The average particle size of the first particles and the second particles can be tested by methods conventional in the art, for example, by measuring the particle size of at least 10 first particles and second particles, respectively, in a scanning electron microscope (SEM) image of the lithium cobalt oxide positive electrode material, and taking the average.

[0059] The lithium cobalt oxide containing the first particles and the second particles can further improve the rate performance and the charge-discharge efficiency of the battery, prolong the cycle life of the battery, and maintain the stability of the capacity of the battery. The first particles, as the main carrier, occupy the main space and play a role of skeleton in the electrode, and can maintain a relatively stable structure during the charge-discharge process to provide stable support for the electrode. Meanwhile, the first particles have a high compaction density, which can improve the volume energy density of the battery. However, the specific surface area of the first particles is relatively limited, which leads to a relatively small contact area with the electrolyte, and is not conducive to the rapid transmission of lithium ions at the interface between the electrode and the electrolyte. Moreover, due to the large particle size of the first particles, lithium ions need to pass through a long path when diffusing in the first particles, which increases the diffusion resistance and time, resulting in poor rate performance of the battery. The second particles have a small size, which can be filled in the gaps between the first particles, so that the positive electrode active material is more closely packed, and the compaction density of the positive electrode sheet is improved. Moreover, the presence of particles of different sizes can construct a more hierarchical and reasonable pore structure. The large pores formed by the first particles are beneficial to the rapid penetration and storage of the electrolyte, and the relatively small pores formed around the second particles provide more transmission channels for lithium ions. The two cooperate with each other to ensure that the electrolyte is in full contact with the active material, and the diffusion of lithium ions in the positive electrode sheet is more smooth, which further improves the rate performance and the charge-discharge efficiency of the battery.

[0060] In the present application, the second particle includes a second substrate and a second protrusion on the surface of the second substrate, and the second protrusion at least partially covers the outer surface of the second substrate; the second substrate includes a third region and a fourth region on the surface of the third region, which extends in the direction of the center of the surface of the second substrate, the fourth region is a region with a depth of 300 nm from the surface of the second substrate, and the third region is the remaining region of the second substrate excluding the fourth region. As shown in the structure schematic diagram of the second particle in one example of the present application, it can be seen from the figure that the second particle includes a second substrate and a second protrusion (6) on the surface of the second substrate, and the second substrate includes a third region (4) and a fourth region (5). Figure 3 As shown in the structure schematic diagram of the second particle in one example of the present application, it can be seen from the figure that the second particle includes a second substrate and a second protrusion (6) on the surface of the second substrate, and the second substrate includes a third region (4) and a fourth region (5).

[0061] In the present application, the third region comprises elements Al and Mg, and the fourth region comprises elements Al, Mg and a second element group, wherein the second element group comprises at least one of Zr, Ti, La and Y.

[0062] In an example, the mass ratio of element Mg to element Co in the first region is greater than the mass ratio of element Mg to element Co in the third region.

[0063] The first particles have a large volume, and the lattice volume change caused by lithium ion deintercalation during charge and discharge cycles is more significant, which is prone to stress concentration and causes particle cracking. The high content of Mg in the first region can stabilize the crystal structure of the first particles by occupying the Co site, relieve volume expansion and lattice stress, and reduce the risk of cracking. The second particles have a small size and weak stress concentration effect, and the lower content of Mg in the third region can meet the basic structural stability requirement, while not affecting the rapid transport of lithium ions in the second particles. By differentiating the element Mg, the first particles and the second particles can both achieve the balance of "structural stability-capacity development".

[0064] In an example, the mass ratio of element Zr to element Co in the fourth region is greater than the mass ratio of element Zr to element Co in the second region.

[0065] The second particles have a large specific surface area, and the contact interface between the surface layer and the electrolyte is wide, which is prone to side reactions. The first particles have a small specific surface area, and the surface layer interface pressure is relatively low. Therefore, the mass ratio of element Zr to element Co in the fourth region is greater than the mass ratio of element Zr to element Co in the second region to adapt to the size difference between the particles. 4+ When occupying the Co site, Zr can enhance the Co-O bond energy. The high content of Zr in the fourth region can fully strengthen the lattice structure, resist the erosion of HF in the electrolyte, reduce the dissolution of transition metals, and inhibit the irreversible phase transition at high voltage.

[0066] In an example, the mass ratio of element Ti to element Co in the fourth region is 0.7-1.5 (for example, 0.7, 0.9, 1.1, 1.3 or 1.5), the mass ratio of element La to element Co is 0.7-1.5 (for example, 0.7, 0.9, 1.1, 1.3 or 1.5), the mass ratio of element Y to element Co is 0.7-1.5 (for example, 0.7, 0.9, 1.1, 1.3 or 1.5), the mass ratio of element Zr to element Co is 0.7-1.5 (for example, 0.7, 0.9, 1.1, 1.3 or 1.5), and the mass ratio of element Al to element Co is 0.7-1.5 (for example, 0.7, 0.9, 1.1, 1.3 or 1.5). The test method is the same as that for the mass ratio of elements Ti, La, Y and Zr to element Co in the second region, which is not repeated here.

[0067] In an example, the mass ratio of element Al to element Co in the fourth region is greater than the mass ratio of element Al to element Co in the third region. The fourth region can directly contact the electrolyte and is prone to deep delithiation under high-voltage conditions, triggering irreversible phase transition of the crystal structure, resulting in poor crystal structure stability. A higher content of Al can enhance the structural stability of the second particle and inhibit the interface side reaction between the positive plate and the electrolyte; a low content of Al doped in the third region can maintain a high electrical conductivity and lithium ion extraction capacity to ensure the capacity, so that the battery has excellent cycle performance and electrochemical performance.

[0068] In an example, the second particle comprises element Ni and / or Mn.

[0069] In the present application, the first protrusion and the second protrusion each independently comprises at least one of element Al, Ti, La, Y and Zr.

[0070] In the present application, the height of the first protrusion is 10-300 nm (for example, 10 nm, 100 nm, 200 nm or 300 nm), and the width of the orthographic projection of the first protrusion on the surface of the first particle is 10-300 nm (for example, 10 nm, 100 nm, 200 nm or 300 nm); the height of the second protrusion is 10-300 nm (for example, 10 nm, 100 nm, 200 nm or 300 nm), and the width of the orthographic projection of the second protrusion on the surface of the second particle is 10-300 nm (for example, 10 nm, 100 nm, 200 nm or 300 nm). The area ratio of the orthographic projection of the first protrusion on the surface of the first particle is 30-90% (for example, 30%, 50%, 70% or 90%); the area ratio of the orthographic projection of the second protrusion on the surface of the second particle is 30-90% (for example, 30%, 50%, 70% or 90%). The height of the first protrusion or the second protrusion and the width of the orthographic projection of the first protrusion or the second protrusion on the surface of the first particle or the second particle can be tested by conventional methods in the art, for example, using an atomic force microscope to measure the height of the protrusion; using a transmission electron microscope to measure the projection width of the protrusion; at least 20 first protrusions or second protrusions are selected, the height and projection width of each first protrusion or second protrusion are measured, and the average value is taken.

[0071] In an example, the mass percentage of element Al in the first protrusion is 0.0012-0.0021 (for example, 0.0012, 0.0014, 0.0016, 0.0018, or 0.0021), the mass percentage of element Ti is 0.0007-0.0015 (for example, 0.0007, 0.0009, 0.0011, 0.0013, or 0.0015), the mass percentage of element La is 0.0005-0.0009 (for example, 0.0005, 0.0006, 0.0007, 0.0008, or 0.0009), the mass percentage of element Y is 0.0013-0.0021 (for example, 0.0013, 0.0015, 0.0017, 0.0019, or 0.0021), and the mass percentage of element Zr is 0.0003-0.001 (for example, 0.0003, 0.0005, 0.0007, 0.0009, or 0.001), taking the total weight of element Co in the lithium cobalt oxide positive electrode material as 1.

[0072] In an example, the mass percentage of element Al in the second protrusion is 0.0012-0.0021 (for example, 0.0012, 0.0014, 0.0016, 0.0018, or 0.0021), the mass percentage of element Ti is 0.0007-0.0015 (for example, 0.0007, 0.0009, 0.0011, 0.0013, or 0.0015), the mass percentage of element La is 0.0005-0.0009 (for example, 0.0005, 0.0006, 0.0007, 0.0008, or 0.0009), the mass percentage of element Y is 0.0013-0.0021 (for example, 0.0013, 0.0015, 0.0017, 0.0019, or 0.0021), and the mass percentage of element Zr is 0.0003-0.001 (for example, 0.0003, 0.0005, 0.0007, 0.0009, or 0.001), taking the total weight of element Co in the lithium cobalt oxide positive electrode material as 1.

[0073] The synergistic effect of different elements in the first protrusion and the second protrusion can strengthen the interface protection of the positive electrode material. Among them, La can be adsorbed on the surface defect sites of the particles, inhibit particle agglomeration and interface side reactions, and improve the cross-section stability; Ti and Zr cooperatively form a Ti-Zr-O passivation layer to block electrolyte corrosion and inhibit Co dissolution; Mg can optimize the lattice structure to inhibit side reactions and Co dissolution; element Al can construct a dense “Al-O” network, adjust the interface charge balance through electronic compensation, help to broaden the lithium ion transmission channel, and improve the interface ion conductivity; Y can adsorb PF6 -, reduce the decomposition barrier of LiPF6, promote the F-P bond to break to generate a LiF-rich CEI film. The protrusions are point-like discontinuous coverage, which can form a "nanobarrier" to repair surface vacancies, relieve stress, and also retain lithium ion channels, reduce direct contact with electrolyte, inhibit side reactions and Co dissolution, and improve battery cycle stability and rate performance.

[0074] In the present application, the total weight of element Co in the lithium cobaltate positive electrode material is 1, and the mass percentage of elements Al, Ti, La, Y and Zr in the first protrusion and the second protrusion can be obtained by conventional methods in the art, for example, discharging the battery to 0% SOC (for example, discharging the battery to 3V), disassembling and taking out the positive electrode sheet, using argon ion grinder CP laser to cut the positive electrode sheet, determining the position of the first protrusion and the second protrusion by scanning electron microscope (SEM), using energy dispersive spectrometer (EDS) to scan the cross section of the first protrusion and the second protrusion, the area of the scan should not be less than 50% of the cross section, and the scanning range should be completely within the cross section, to obtain the mass content of elements Al, Ti, La, Y and Zr in the first protrusion and the second protrusion; the mass content of element Co in the lithium cobaltate positive electrode material is obtained by ICP test; the mass percentage of elements Al, Ti, La, Y and Zr in the first protrusion and the second protrusion is obtained by calculation, taking the total weight of element Co in the lithium cobaltate positive electrode material as 1. At least 10 first protrusions and second protrusions on 5 first particles and second particles are randomly selected for measurement, and the average value is finally taken.

[0075] In the present application, the crystal structure of the first particle and the second particle independently includes at least one of a layered hexagonal structure, a spinel phase, a rutile phase, a monoclinic phase, a tetragonal phase and a rock salt phase. The oxides of different elements in the first particle and the second particle correspond to different crystal structures. Among them, the existence form of the oxide of Ti (such as TiO2, Li4Ti5O 12 , Li2CoTiO4 or Li2CoTi3O8, etc.) includes a spinel phase or a rutile phase; the existence form of Li2ZrO3 includes a monoclinic phase or a tetragonal phase; the existence form of the oxide of metal elements Al, Co, Mg and Ti includes a rock salt phase or a spinel phase.

[0076] In an example, the crystal structure of the first particle includes a layered hexagonal structure.

[0077] In the present application, the X-ray diffraction pattern of the lithium cobaltate positive electrode material has a characteristic peak one at 18°-20° and a characteristic peak two at 44°-46°; the peak intensity of the characteristic peak one is greater than the peak intensity of the characteristic peak two. As Figure 4The X-ray diffraction pattern (XRD) of the lithium cobaltate positive electrode material in an example of the present application is shown in the figure, and it can be seen from the figure that there is a characteristic peak one at 18°-20°, and a characteristic peak two at 44°-46°, and the peak strength of the characteristic peak one is greater than that of the characteristic peak two. Among them, the characteristic peak one is attributed to the (003) crystal face, corresponding to the interlayer stacking in the c-axis direction; the characteristic peak two is attributed to the (104) crystal face, corresponding to the composite crystal face in the interlayer and in the plane; the peak strength of the characteristic peak one is greater than that of the characteristic peak two, indicating that the crystal of the first particle tends to grow along the c-axis direction, forming a layered hexagonal structure material, and the crystal quality is high, the crystallinity is good, and the crystal structure is more stable in the charging and discharging process, which helps to improve the cycle life of the battery.

[0078] The present application also provides a method for preparing the lithium cobaltate positive electrode material, comprising at least the following steps: S1, uniformly mixing a first Co source and a first Li source with specific element Al, Mg, Mn and Ni contents, and performing first sintering in a dry air atmosphere; S2, uniformly mixing a second Co source and a second Li source with specific element Al and Mg contents, and performing second sintering in a dry air atmosphere; S3, uniformly mixing the material prepared in step S1 with the material prepared in step S2, and adding optional first Al source, first Ti source, first La source, first Y source and second Zr source for third sintering.

[0079] In the present application, "optional" means that it can be added or not.

[0080] In the present application, the particle size Dv50 of the first Co source is 10-20 μm, for example, 10 μm, 12.5 μm, 15 μm or 20 μm; the particle size Dv50 of the second Co source is 1-5 μm, for example, 1 μm, 2 μm, 3 μm, 4 μm or 5 μm.

[0081] In the present application, the temperature of the first sintering is 900-1050℃, for example, 900℃, 950℃, 1000℃ or 1050℃; the time of the first sintering is 8-12 h, for example, 8 h, 10 h or 12 h; the heating rate of the first sintering is 1-5℃ / min, for example, 1℃ / min, 3℃ / min or 5℃ / min.

[0082] In the present application, step (1) further comprises naturally cooling to room temperature after the first sintering.

[0083] In the present application, the temperature of the second sintering is 900-1050℃, for example, 900℃, 950℃, 1000℃ or 1050℃; the time of the second sintering is 5-10h, for example, 5h, 7h or 10h; and the temperature rising speed of the second sintering is 1-5℃ / min, for example, 1℃ / min, 3℃ / min or 5℃ / min.

[0084] In the present application, after the second sintering, the material is naturally cooled to room temperature.

[0085] In the present application, in step (3), the material prepared in step S1 and the material prepared in step S2 can be mixed in any mass ratio.

[0086] In the present application, the temperature of the third sintering is 700-950℃, for example, 700℃, 800℃ or 950℃; the time of the third sintering is 1-5h, for example, 1h, 2h, 3h, 4h or 5h.

[0087] In the present application, after the third sintering, the material is naturally cooled to room temperature.

[0088] In the present application, the first Co source and the second Co source each independently comprises at least one of tricobalt tetroxide, cobalt hydroxide and cobalt carbonate; the first Li source and the second Li source each independently comprises lithium carbonate and / or lithium fluoride; the first Al source comprises at least one of Al2O3, Al(OH)3, Al2(SO4)3, Al2(CO3)3 and Al(NO3)3; the first Ti source comprises at least one of TiO2, TiF4, TiCl3, Ti(OH)4, Ti(CO3)2 and Ti(SO4)2; the first La source comprises at least one of La2O3, La(OH)3, La(CO3)3, La2(SO4)3, LaTiO3 and LaZrO3; the first Y source comprises at least one of Y2O3, Y(OH)3, Y2(CO3)3, Y2(SO4)3 and Y(NO3)3; and the first Zr source and the second Zr source each independently comprises zirconium oxide and / or zirconium hydroxide.

[0089] In an example, the first Al source comprises Al2(SO4)3.

[0090] In the present application, the specific amount of the material used is as described above, which is not repeated here.

[0091] Since the charging and discharging process of the battery is an electrochemical process involving multiple components such as the positive electrode, the negative electrode, and the electrolyte. Therefore, only improving the kinetic performance and structural stability of the positive electrode can only improve the embedding, de-embedding, and transmission speed of lithium ions on the positive electrode side. However, the charging and discharging reaction requires the negative electrode to simultaneously and efficiently embed and de-embed lithium ions, and to build a smooth ion transmission channel between the two through the electrolyte. Therefore, the negative electrode and the electrolyte are further improved in the present application, effectively improving the kinetic performance of the battery.

[0092] The second aspect of the present application provides a lithium ion secondary battery, which comprises a positive electrode sheet, a negative electrode sheet and a separator, wherein the positive electrode sheet comprises the positive electrode material according to the first aspect of the present application; and the charging cut-off voltage of the lithium ion secondary battery is ≥4.5V (for example, 4.5V, 4.52V, 4.54V, 4.55V, 4.56V, 4.58V, 4.6V or 4.8V).

[0093] In one example, the charging cut-off voltage of the lithium ion secondary battery is ≥4.55V.

[0094] In the present application, the compaction density of the positive electrode sheet is 3.2g / cm 3 -4.5g / cm 3 , and the compaction density of the negative electrode sheet is 1.5g / cm 3 -2g / cm 3 .

[0095] In the present application, the negative electrode sheet comprises a negative electrode material, and the negative electrode material comprises a silicon-carbon material. The silicon-carbon material comprises a porous carbon matrix and a silicon material located in the internal pores of the porous carbon matrix.

[0096] In the present application, the silicon-carbon material comprises first silicon-carbon particles and / or second silicon-carbon particles.

[0097] In one example, the sphericity of the first silicon-carbon particles is S1, and 0.7≤S1≤1 (for example, 0.7, 0.8, 0.9 or 1); and the sphericity of the second silicon-carbon particles is S2, and 0.3≤S2<0.7 (for example, 0.3, 0.4, 0.5, 0.6 or 0.65).

[0098] In one example, the average particle size of the first silicon-carbon particles is 1μm-15μm (for example, 1μm, 5μm, 10μm or 15μm); and the average particle size of the second silicon-carbon particles is 6μm-15μm (for example, 6μm, 10μm, 12μm or 15μm).

[0099] In one example, the average particle size of the first silicon-carbon particles is 1μm-10μm.

[0100] In an example, the first silicon-carbon particles have a mass content of element Si of 25-70% (e.g. 25%, 40%, 60% or 70%); and the second silicon-carbon particles have a mass content of element Si of 20-60% (e.g. 20%, 30%, 40%, 50% or 60%).

[0101] The first silicon-carbon particles have high sphericity, which helps to alleviate volume expansion, uniformly disperse stress, and reduce the risk of membrane puncture, but the adhesion is poor and the long-term cycle is prone to failure; the second silicon-carbon particles can improve the compaction density, but the irregular shape and severe expansion can easily lead to material pulverization, puncture the separator and cause battery short circuit, and the poor electrical conductivity will hinder the transmission of lithium ions. The present application synergistically uses the above two kinds of silicon-carbon particles to optimize the bulk density, reduce the volume expansion of silicon, improve the compaction density and volume energy density of the negative electrode, and improve the lithium ion diffusion kinetics and the rate performance of the battery.

[0102] In the present application, the sphericity s1 of the first silicon-carbon particles and the sphericity s2 of the second silicon-carbon particles can be tested by conventional methods in the art, for example, after disassembling the battery, the negative electrode sheet is taken out, washed with dimethyl carbonate (DMC) and dried, the cross section of the negative electrode sheet is polished with an argon ion grinder, and observed in a SEM device using backscattering imaging mode; find the first silicon-carbon particles and the second silicon-carbon particles with continuous and smooth outline, connect any two points on the edge of the particles to form a straight line segment inside the particles, select the longest straight line segment inside the particles, and its length is recorded as Z1; take the midpoint of the longest straight line segment, draw a straight line through the midpoint to form a straight line segment with endpoints on the edge of the particles, select the shortest straight line segment, and its length is recorded as Z2, then the sphericity of the particle is Z2 / Z1. At least 10 first silicon-carbon particles and second silicon-carbon particles are selected respectively, and the sphericity is measured and then averaged.

[0103] In the present application, the average particle size of the first and second silicon-carbon particles can be tested by conventional methods in the art, for example, after discharging the battery to 0% SOC, the negative electrode sheet is taken out, soaked in DMC solvent for 12 h, then rinsed with DMC solvent to remove the lithium salt attached to the negative electrode sheet, or directly take the negative electrode sheet before soaking in electrolyte, use argon ion grinder CP to laser cut the negative electrode sheet, then use scanning electron microscope (SEM) to observe (use backscattering imaging mode), in this mode, the contrast of silicon-carbon material is brighter (can be used to distinguish carbon-based materials and conductive agents in the negative electrode active layer), measure at 5K times, randomly select at least 20 first and second silicon-carbon particles respectively, measure the particle size of each silicon-carbon particle, take the average value. If the number of particles is less than 20 at 5K times, take another mirror picture until 20 particles are measured. When the particles in the mirror picture are regular circles, the particle size is the diameter of the regular circle; when the particles in the mirror picture are not "regular circles", connect any two points on the edge of the particle to form a straight line segment inside the particle, and select the longest straight line segment inside the particle as the particle size.

[0104] In the present application, the mass content of element Si in the first and second silicon-carbon particles can be tested by conventional methods in the art, for example, discharge the battery to 0% SOC, take out the negative electrode sheet, or directly take the negative electrode sheet before soaking in electrolyte, polish the cross section of the negative electrode sheet with argon ion grinder, then observe the silicon-carbon material in SEM equipment using backscattering imaging mode to maximize the magnification; use energy dispersive spectrometer (EDS) to scan the cross section of the first and second silicon-carbon particles, the scanning area should not be less than 50% of the cross section of the particle, and the scanning range should be completely within the cross section of the particle, and the mass content of element Si is calculated. At least 10 particles are selected for measurement respectively, and the average value is taken.

[0105] In the present application, the negative electrode sheet includes 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 includes the silicon-carbon material; the negative electrode active layer further includes single-walled carbon nanotubes. By constructing a continuous conductive network with single-walled carbon nanotubes, not only can the volume change caused by the continuous expansion and contraction of the negative electrode Si in the charging and discharging process be inhibited, but also the poor conductivity of the negative electrode material (such as graphite) can be effectively improved, the transmission impedance of electrons and lithium ions can be reduced, more fast transmission channels for lithium ions can be provided, and the rate performance of the battery can be improved.

[0106] In an example, the diameter of the single-walled carbon nanotube is 1 nm-10 nm (for example, 1 nm, 4 nm, 8 nm, or 10 nm); the length of the single-walled carbon nanotube is greater than 3 μm (for example, 5 μm, 10 μm, 100 μm, 500 μm, or 1000 μm).

[0107] In one example, the single-walled carbon nanotube has a diameter of 1 nm to 5 nm; and a length greater than 5 μm.

[0108] In the present application, the lithium ion secondary battery further includes an electrolyte, and the electrolyte includes a carbonate-based solvent.

[0109] In one example, the carbonate-based solvent includes at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and fluoroethylene carbonate (FEC).

[0110] In one example, the mass content of the carbonate-based solvent in the electrolyte is 10% to 70% (e.g., 10%, 20%, 30%, 40%, 50%, 60%, or 70%).

[0111] In one example, the mass content of the carbonate-based solvent in the electrolyte is 20% to 50%.

[0112] In one example, the carbonate-based solvent includes DMC, DEC, and FEC.

[0113] In one example, the mass content of FEC in the electrolyte is 5% to 20% (e.g., 5%, 8%, 10%, 12%, 15%, 18%, or 20%).

[0114] In the present application, the electrolyte includes a carboxylate-based solvent. The carboxylate-based solvent includes 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), ethyl butyrate (EB), and fluoroethyl acetate.

[0115] In one example, the carboxylate-based solvent includes at least one of PP, ethyl 2,2-difluoroacetate, and 2,2-difluoroethyl acetate (DFEA).

[0116] In one example, the mass content of PP in the electrolyte is 10% to 50% (e.g., 10%, 20%, 30%, 40%, or 50%).

[0117] In one example, the mass content of fluoroethyl acetate in the electrolyte is 10% to 65% (e.g., 10%, 20%, 30%, 40%, 50%, 60%, or 65%).

[0118] In the present application, the electrolyte further comprises an additive, the additive comprising at least one of 1,3,6-hexanetricarbonitrile (HTCN), vinylene carbonate, 1,3-propylene sulfite, vinyl ethylene carbonate, vinyl sulfate, butanedinitrile, pentanedinitrile, hexanedinitrile, heptanedinitrile, octanedinitrile, sunflower dinitrile, glycerol trinitrile, and 1,2-bis(2-cyanoethoxy)ethane.

[0119] In an example, the mass content of HTCN in the electrolyte is 0.5%-8% (for example, 0.5%, 2%, 3%, 4%, 5%, 6%, 7% or 8%).

[0120] In an example, the electrolyte comprises at least one of DMC, DEC, FEC, PP, DFEA and HTCN.

[0121] The carbonate solvent in the electrolyte can fully dissolve and dissociate lithium ions, providing sufficient free lithium ions to improve the kinetic performance of the battery; FEC can generate a stable interface protection film at the negative electrode, reduce side reactions, accelerate lithium ion shuttling, and improve the charge and discharge speed of the battery; DFEA can inhibit the oxidative decomposition of the electrolyte at high voltage, ensuring stable operation of the battery at high voltage; and the low viscosity of HTCN helps to improve the ion transport efficiency and rate performance, and can form a CEI film at the positive electrode to protect the positive electrode structure.

[0122] In the present application, the mass content of the carbonate solvent, DFEA, PP or HTCN in the electrolyte can be obtained by testing by conventional methods in the art, for example, by gas chromatography (GC) or gas chromatography-mass spectrometry (GCMS).

[0123] In the present application, the electrolyte further comprises an electrolyte salt, the electrolyte salt comprising lithium hexafluorophosphate (LiPF6) and other lithium salts, the other lithium salts comprising at least one of lithium bisfluorosulfonylimide, lithium bistrifluoromethylsulfonylimide, lithium difluorophosphate, lithium hexafluoroantimonate, lithium hexafluoroarsenate, lithium bis(pentafluoroethylsulfonyl)imide and lithium tris(trifluoromethylsulfonyl)methide.

[0124] In the present application, the battery can further comprise a separator, the separator comprising a substrate layer and a coating layer located on at least one side surface of the substrate layer, the coating layer being arranged opposite to the positive electrode sheet.

[0125] In an example, the substrate layer comprises a base film, the base film 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.

[0126] In one example, the coating comprises nitrogen-containing particles containing nitrogen-containing functional groups, including at least one of cyano, isocyano, isocyanate, and amino and imino groups found in melamine derivatives. The nitrogen-containing particles exhibit excellent thermal stability and mechanical strength, suppressing membrane thermal shrinkage at high temperatures, reducing positive and negative electrode contact, inhibiting lithium dendrite growth, and reducing capacity decay during battery cycling.

[0127] In one example, the coating thickness is 0.5 μm to 3 μm (e.g., 0.5 μm, 1 μm, 2 μm, or 3 μm). The nitrogen content in the coating is 10% to 60% by mass (e.g., 10%, 30%, 50%, or 60%).

[0128] In this invention, the lithium-ion secondary battery includes a wound structure or a stacked structure; the lithium-ion secondary battery includes an aluminum-plastic film or a steel shell.

[0129] In one example, the positive electrode, the separator, and the negative electrode are sequentially stacked to form a core. The lithium-ion secondary battery includes a steel casing, which contains an inner cavity in which the core is located. Figure 5 The figure shows a schematic diagram of the structure of a lithium-ion secondary battery in an example of the present invention. As can be seen from the figure, the lithium-ion secondary battery has a steel shell 7, an inner cavity 8 and tabs 9; the length of the inner cavity L1 = the length of the steel shell L1 - the top edge width L3 - the bottom edge width L4 - the inner cavity wall thickness × 2; the width of the inner cavity w1 = the width of the steel shell w2 - 2 × the side edge width w3 - the inner cavity wall thickness × 2.

[0130] like Figure 6 The figure shows a schematic diagram II of the structure of a lithium-ion secondary battery in an example of the present invention. As can be seen from the figure, the lithium-ion secondary battery has an inner cavity 8, tabs 9 and stacked cores 10; the thickness of the inner cavity = the thickness of the stacked cores in the half-charge state + the thickness margin h between the inner cavity and the stacked cores, where 0μm < h ≤ 200μm (for example, 50μm, 100μm, 150μm or 200μm).

[0131] If h is too small (e.g., 0μm), the volume expansion caused by the delithiation / intercalation of active materials during charge-discharge cycles will lack buffer space, resulting in huge stress between the stack and the inner cavity. This will squeeze the stack, causing electrode deformation, active material shedding, or separator damage, leading to increased battery internal resistance and poor cycle stability. In severe cases, it may even cause an internal short circuit. If h is too large (e.g., greater than 200μm), it will reduce the utilization rate of the battery's internal space, resulting in a decrease in the battery's volumetric energy density. It may also cause the stack to loosen in the cavity, making it prone to problems such as tab breakage and separator scratches during transportation or use due to collisions and vibrations, affecting the overall performance of the battery.

[0132] In the present application, the "half-electric state" refers to a state after the battery is cycled to 50% state of charge (i.e. 50% SOC) of its theoretical charge-discharge capacity and the lithium intercalation / deintercalation degree of the positive and negative active materials corresponds to 50% of the rated capacity, and the battery is left to stand until the thickness of the stacked core stabilizes at an ambient temperature of 23±2°C.

[0133] The assembly of the battery can be performed in a conventional manner in the art.

[0134] It should be noted that the "first", "second" and the like numerical designations in the present application are only used to distinguish different substances or usage manners, and do not represent the difference in order.

[0135] 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 a person of ordinary skill in the art without making creative efforts fall within the scope of protection of the present application.

[0136] In the following examples, the materials used are commercially available analytical pure unless otherwise specified.

[0137] The following preparation examples are used to prepare the lithium cobalt oxide cathode material of the present application Preparation Example 1 The lithium cobalt oxide cathode material is prepared according to the following method: (1) Co3O4 and Li2CO3 with specific element Al, Mg, Mn, Ni content are weighed and mixed uniformly in a molar ratio of Co to Li of 1:1.03, and placed in a muffle furnace, heated to 1000°C at a rate of 3°C / min in a dry air atmosphere, and naturally cooled to room temperature after high-temperature calcination for 10h; (2) Co3O4 and Li2CO3 with specific element Al, Mg content are weighed and mixed uniformly in a molar ratio of Co to Li of 1:1.03, and placed in a muffle furnace, heated to 1000°C at a rate of 3°C / min in a dry air atmosphere, and naturally cooled to room temperature after high-temperature calcination for 8h; (3) The material prepared in step (1) and the material prepared in step (2) are mixed uniformly in a mass ratio of 4:1, Al2O3, TiO2, La2O3, Y2O3 and ZrO2 are added and mixed uniformly, and placed in a muffle furnace, heated to 800°C at a rate of 3°C / min in a dry air atmosphere, and naturally cooled to room temperature after high-temperature calcination for 3h, to prepare the first and second particles; In the step (1), the mass content of element Al in the Co3O4 is 8500 ppm, the mass content of element Mg is 1200 ppm, the mass content of element Mn is 2400 ppm, and the mass content of element Ni is 1200 ppm; in the step (2), the mass content of element Al in the Co3O4 is 8500 ppm, and the mass content of element Mg is 1200 ppm; the mass ratio of element Mn to element Ni in the lithium cobaltate positive electrode material is 2.25; the mass content of element Ni in the lithium cobaltate positive electrode material is 1016 ppm, and the mass content of element Mn is 2284 ppm; in the second region, the mass ratio of element Ti to element Co is 0.0003, the mass ratio of element La to element Co is 0.0002, the mass ratio of element Y to element Co is 0.0003, and the mass ratio of element Zr to element Co is 0.0002; In the second region, the mass ratio of elements satisfies: (Ti+La+Y+Zr+Al) / Co>(Ni+Mn) / Co; in the lithium cobaltate positive electrode material, the atomic percentage of different valence state ions satisfies: Mn 4+ / Co 3+ >Ni 2+ / Co 3+ >Ni 3+ / Co 3+ >Mn 3+ / Co 3+ ; the particle size Dn10 of the lithium cobaltate positive electrode material is 2.7 μm, the particle size Dn50 is 3.9 μm, and the particle size Dn90 is 8.8 μm; the mass content of element S in the first particle is 62 ppm; the mass content of element S in the grain boundary is greater than the mass content of element S in the second region; the mass ratio of element Al to element Co in the second region is greater than the mass ratio of element Al to element Co in the first region; the mass ratio of element Al to element Co in the fourth region is greater than the mass ratio of element Al to element Co in the third region; the mass ratio of element Mg to element Co in the first region is greater than the mass ratio of element Mg to element Co in the third region; and the mass ratio of element Zr to element Co in the fourth region is greater than the mass ratio of element Zr to element Co in the second region; The height of the first protrusion is 176 nm, the width of the orthographic projection of the first protrusion on the surface of the first particle is 168 nm, the height of the second protrusion is 158 nm, and the width of the orthographic projection of the second protrusion on the surface of the second particle is 162 m; the area ratio of the orthographic projection of the first protrusion on the surface of the first particle is 60%, and the area ratio of the orthographic projection of the second protrusion on the surface of the second particle is 58%; the total weight of the element Co in the lithium cobaltate positive electrode material is 1, the mass percentage of the element Al in the first protrusion is 0.0016, the mass percentage of the element Ti is 0.0012, the mass percentage of the element La is 0.0007, the mass percentage of the element Y is 0.0016, and the mass percentage of the element Zr is 0.0006; the mass content of the element Al in the second protrusion is 0.0018, the mass content of the element Ti is 0.0012, the mass content of the element La is 0.0007, the mass content of the element Y is 0.0017, and the mass content of the element Zr is 0.0007; the lithium cobaltate positive electrode material has an R-3m layered structure; in the X-ray diffraction pattern of the lithium cobaltate positive electrode material, the peak intensity of characteristic peak one is greater than the peak intensity of characteristic peak two.

[0138] Preparation Example 2 The lithium cobaltate positive electrode material is prepared by the following method: (1) Co3O4 and Li2CO3 with specific element Al, Mg, Mn, and Ni contents are weighed and uniformly mixed in a molar ratio of Co to Li of 1:1.03, and then placed in a muffle furnace and heated to 1000°C at a rate of 3°C / min in a dry air atmosphere, and then naturally cooled to room temperature after high-temperature calcination for 10 h; (2) Co3O4 and Li2CO3 with specific element Al and Mg contents are weighed and uniformly mixed in a molar ratio of Co to Li of 1:1.03, and then placed in a muffle furnace and heated to 1000°C at a rate of 3°C / min in a dry air atmosphere, and then naturally cooled to room temperature after high-temperature calcination for 8 h; (3) The material prepared in step (1) and the material prepared in step (2) are mixed uniformly in a mass ratio of 4:1, and then Al2O3, TiO2, La2O3, Y2O3, and ZrO2 are added and uniformly mixed, and then placed in a muffle furnace and heated to 800°C at a rate of 3°C / min in a dry air atmosphere, and then naturally cooled to room temperature after high-temperature calcination for 3 h, to prepare the first particles and the second particles; In the step (1), the mass content of element Al in the Co3O4 is 8500 ppm, the mass content of element Mg is 1200 ppm, the mass content of element Mn is 1800 ppm, and the mass content of element Ni is 1500 ppm; in the step (2), the mass content of element Al in the Co3O4 is 8500 ppm, and the mass content of element Mg is 1200 ppm; the mass ratio of element Mn to element Ni in the lithium cobaltate positive electrode material is 1.24; the mass content of element Ni in the lithium cobaltate positive electrode material is 1285 ppm, and the mass content of element Mn is 1598 ppm; in the second region, the mass ratio of element Ti to element Co is 0.0002, the mass ratio of element La to element Co is 0.0001, the mass ratio of element Y to element Co is 0.0001, and the mass ratio of element Zr to element Co is 0.0001; In the second region, the mass ratio of elements satisfies: (Ti+La+Y+Zr+Al) / Co>(Ni+Mn) / Co; in the lithium cobaltate positive electrode material, the atomic percentage of different valence state ions satisfies: Mn 4+ / Co 3+ >Ni 2+ / Co 3+ >Ni 3+ / Co 3+ >Mn 3+ / Co 3+ ; the particle size Dn10 of the lithium cobaltate positive electrode material is 2.3 μm, the particle size Dn50 is 3.7 μm, and the particle size Dn90 is 8.4 μm; the mass content of element S in the first particle is 15 ppm; the mass content of element S in the grain boundary is greater than the mass content of element S in the second region; the mass ratio of element Al to element Co in the second region is greater than the mass ratio of element Al to element Co in the first region; the mass ratio of element Al to element Co in the fourth region is greater than the mass ratio of element Al to element Co in the third region; the mass ratio of element Mg to element Co in the first region is greater than the mass ratio of element Mg to element Co in the third region; and the mass ratio of element Zr to element Co in the fourth region is greater than the mass ratio of element Zr to element Co in the second region; The height of the first protrusion is 18 nm, the width of the orthographic projection of the first protrusion on the surface of the first particle is 15 nm, the height of the second protrusion is 14 nm, and the width of the orthographic projection of the second protrusion on the surface of the second particle is 16 nm; the area ratio of the orthographic projection of the first protrusion on the surface of the first particle is 35%, and the area ratio of the orthographic projection of the second protrusion on the surface of the second particle is 32%; the total weight of the element Co in the lithium cobaltate positive electrode material is 1, the mass percentage of the element Al in the first protrusion is 0.0012, the mass percentage of the element Ti is 0.0007, the mass percentage of the element La is 0.0005, the mass percentage of the element Y is 0.0013, and the mass percentage of the element Zr is 0.0003; the mass percentage of the element Al in the second protrusion is 0.0012, the mass percentage of the element Ti is 0.0007, the mass percentage of the element La is 0.0005, the mass percentage of the element Y is 0.0013, and the mass percentage of the element Zr is 0.0003; the lithium cobaltate positive electrode material has an R-3m layered structure; in the X-ray diffraction pattern of the lithium cobaltate positive electrode material, the peak intensity of characteristic peak one is greater than the peak intensity of characteristic peak two.

[0139] Preparation Example 3 The lithium cobaltate positive electrode material is prepared by the following method: (1) Co3O4 and Li2CO3 with specific element Al, Mg, Mn, and Ni contents are weighed and uniformly mixed in a molar ratio of Co to Li of 1:1.03, and then placed in a muffle furnace and heated to 1000°C at a rate of 3°C / min in a dry air atmosphere, and then naturally cooled to room temperature after high-temperature calcination for 10 h; (2) Co3O4 and Li2CO3 with specific element Al and Mg contents are weighed and uniformly mixed in a molar ratio of Co to Li of 1:1.03, and then placed in a muffle furnace and heated to 1000°C at a rate of 3°C / min in a dry air atmosphere, and then naturally cooled to room temperature after high-temperature calcination for 8 h; (3) The material prepared in step (1) and the material prepared in step (2) are mixed uniformly in a mass ratio of 4:1, and then Al2O3, TiO2, La2O3, Y2O3, and ZrO2 are added and uniformly mixed, and then placed in a muffle furnace and heated to 800°C at a rate of 3°C / min in a dry air atmosphere, and then naturally cooled to room temperature after high-temperature calcination for 3 h, to prepare the first particles and the second particles; In the step (1), the mass content of element Al in the Co3O4 is 8500 ppm, the mass content of element Mg is 1200 ppm, the mass content of element Mn is 2500 ppm, and the mass content of element Ni is 705 ppm; in the step (2), the mass content of element Al in the Co3O4 is 8500 ppm, and the mass content of element Mg is 1200 ppm; the mass ratio of element Mn to element Ni in the lithium cobaltate positive electrode material is 3.56; the mass content of element Ni in the lithium cobaltate positive electrode material is 701 ppm, and the mass content of element Mn is 2498 ppm; in the second region, the mass ratio of element Ti to element Co is 0.0005, the mass ratio of element La to element Co is 0.0004, the mass ratio of element Y to element Co is 0.0005, and the mass ratio of element Zr to element Co is 0.0003; In the second region, the mass ratio of elements satisfies: (Ti+La+Y+Zr+Al) / Co>(Ni+Mn) / Co; in the lithium cobaltate positive electrode material, the atomic percentage of different valence state ions satisfies: Mn 4+ / Co 3+ >Ni 2+ / Co 3+ >Ni 3+ / Co 3+ >Mn 3+ / Co 3+ ; the particle size Dn10 of the lithium cobaltate positive electrode material is 3.1 μm, the particle size Dn50 is 4.3 μm, and the particle size Dn90 is 9.1 μm; the mass content of element S in the first particle is 96 ppm; the mass content of element S in the first grain boundary is greater than the mass content of element S in the second region; the mass ratio of element Al to element Co in the second region is greater than the mass ratio of element Al to element Co in the first region; the mass ratio of element Al to element Co in the fourth region is greater than the mass ratio of element Al to element Co in the third region; the mass ratio of element Mg to element Co in the first region is greater than the mass ratio of element Mg to element Co in the third region; and the mass ratio of element Zr to element Co in the fourth region is greater than the mass ratio of element Zr to element Co in the second region; The height of the first protrusion is 285 nm, the width of the orthographic projection of the first protrusion on the surface of the first particle is 294 nm, the height of the second protrusion is 289 nm, and the width of the orthographic projection of the second protrusion on the surface of the second particle is 293 nm; the area ratio of the orthographic projection of the first protrusion on the surface of the first particle is 86%, and the area ratio of the orthographic projection of the second protrusion on the surface of the second particle is 88%; the total weight of the element Co in the lithium cobalt oxide positive electrode material is 1, the mass percentage of the element Al in the first protrusion is 0.0021, the mass percentage of the element Ti is 0.0015, the mass percentage of the element La is 0.0009, the mass percentage of the element Y is 0.0021, and the mass percentage of the element Zr is 0.001; the mass percentage of the element Al in the second protrusion is 0.0021, the mass percentage of the element Ti is 0.0015, the mass percentage of the element La is 0.0009, the mass percentage of the element Y is 0.0021, and the mass percentage of the element Zr is 0.001; the lithium cobalt oxide positive electrode material has an R-3m layered structure; and the peak intensity of characteristic peak one is greater than the peak intensity of characteristic peak two in the X-ray diffraction spectrum of the lithium cobalt oxide positive electrode material.

[0140] Preparation Example 4 group The preparation examples in this group are prepared by referring to Preparation Example 1, except that the mass ratio of the element Mn to the element Ni in the lithium cobalt oxide positive electrode material is adjusted by changing the mass content of the element Mn in the lithium cobalt oxide positive electrode material, and the details are as follows: Preparation Example 4a, the mass content of the element Mn in the lithium cobalt oxide positive electrode material is 6825 ppm, and the mass ratio of the element Mn to the element Ni in the lithium cobalt oxide positive electrode material is 6.72; Preparation Example 4b, the mass content of the element Mn in the lithium cobalt oxide positive electrode material is 9886 ppm, and the mass ratio of the element Mn to the element Ni in the lithium cobalt oxide positive electrode material is 9.73.

[0141] Preparation Example 5 group The preparation examples in this group are prepared by referring to Preparation Example 1 or Preparation Example 3, except that the mass content of the element Ni or Mn in the lithium cobalt oxide positive electrode material is changed, and the details are as follows: Preparation Example 5a, referring to Preparation Example 1, the mass content of the element Ni in the lithium cobalt oxide positive electrode material is 583 ppm, and the mass ratio of the element Mn to the element Ni is 3.92; Preparation Example 5b, referring to Preparation Example 1, the mass content of the element Ni in the lithium cobalt oxide positive electrode material is 1887 ppm, and the mass ratio of the element Mn to the element Ni is 1.21; Preparation Example 5c, referring to Preparation Example 3, the mass content of the element Mn in the lithium cobalt oxide positive electrode material is 851 ppm, and the mass ratio of the element Mn to the element Ni is 1.21; Preparation Example 5d, referring to Preparation Example 1, the mass content of element Mn in the lithium cobalt oxide positive electrode material is 3496 ppm, and the mass ratio of element Mn to element Ni is 3.44; Preparation Example 5e, referring to Preparation Example 1, the mass content of element Ni in the lithium cobalt oxide positive electrode material is 108 ppm, the mass content of element Mn is 135 ppm, and the mass ratio of element Mn to element Ni is 1.25; Preparation Example 5f, referring to Preparation Example 1, the mass content of element Ni in the lithium cobalt oxide positive electrode material is 2947 ppm, the mass content of element Mn is 4969 ppm, and the mass ratio of element Mn to element Ni is 1.75.

[0142] Preparation Example 6 group The preparation examples in this group are prepared by referring to Preparation Example 1, except that the mass ratio of elements Ti, La, Y, Zr to element Co in the second region is regulated by changing the adding amount of TiO2, La2O3, Y2O3 and ZrO2 in step (3), which is as follows: Preparation Example 6a, the mass ratio of element Ti to element Co in the second region is 0.001, and the mass ratio of element Zr to element Co in the second region is 0.0005; Preparation Example 6b, the mass ratio of element Ti to element Co in the second region is 0, and the mass ratio of element Zr to element Co in the second region is 0; the first protrusion and the second protrusion do not contain elements Ti and Zr; Preparation Example 6c, the mass ratio of element La to element Co in the second region is 0.0006, and the mass ratio of element Y to element Co in the second region is 0.001; Preparation Example 6d, the mass ratio of element La to element Co in the second region is 0, and the mass ratio of element Y to element Co in the second region is 0; the first protrusion and the second protrusion do not contain elements La and Y.

[0143] Preparation Example 7 The preparation example is prepared by referring to Preparation Example 1, except that Al2O3 is not added in step (3), the mass ratio of element Al to element Co in the second region is less than the mass ratio of element Al to element Co in the first region; the mass ratio of element Al to element Co in the fourth region is less than the mass ratio of element Al to element Co in the third region.

[0144] Preparation Example 8 group The preparation examples in this group are prepared by referring to Preparation Example 1, except that the composition of the lithium cobalt oxide particles is changed, which is as follows: Preparation Example 8a, the preparation process does not contain step (2), and the lithium cobalt oxide particles only contain the first particles; Preparation Example 8b, the preparation process does not contain step (1), and the lithium cobalt oxide particles only contain the second particles.

[0145] Preparation Example 9 group The preparation examples in this group are prepared according to Preparation Example 1, except that the type of elements in the third region is regulated by changing the addition amount of elements Al and Mg in the second Co source in step (2), as follows: Preparation Example 9a, the third region does not contain element Al; Preparation Example 9b, the third region does not contain element Mg.

[0146] Preparation Example 10 The preparation example is prepared according to Preparation Example 1, except that the mass content of element Mg in Co3O4 in step (1) is 512 ppm, and the mass content of element Mg in Co3O4 in step (2) is 2005 ppm; the mass ratio of element Mg to element Co in the first region is less than the mass ratio of element Mg to element Co in the third region.

[0147] Comparative Preparation Example 1 group The comparative preparation examples in this group are prepared according to Preparation Example 1, except that the mass content of elements Ni or Mn in the lithium cobaltate positive electrode material is changed, as follows: Comparative Preparation Example 1a, the mass content of element Ni in the lithium cobaltate positive electrode material is 58 ppm, the mass content of element Mn is 75 ppm, and the mass ratio of element Mn to element Ni is 1.3; Comparative Preparation Example 1b, the mass content of element Ni in the lithium cobaltate positive electrode material is 4618 ppm, the mass content of element Mn is 6145 ppm, and the mass ratio of element Mn to element Ni is 1.3.

[0148] Comparative Preparation Example 2 group The comparative preparation examples in this group are prepared according to Preparation Example 1, except that the type of elements in the lithium cobaltate positive electrode material is regulated by changing the addition amount of elements Ni and Mn in Co3O4 in step (1), as follows: Comparative Preparation Example 2a, the lithium cobaltate positive electrode material does not contain element Mn; Comparative Preparation Example 2b, the lithium cobaltate positive electrode material does not contain element Ni; Comparative Preparation Example 2c, the lithium cobaltate positive electrode material does not contain elements Ni and Mn.

[0149] Comparative Preparation Example 3 group The preparation examples in this group are prepared according to Preparation Example 1, except that the mass ratio of element Mn to element Ni in the lithium cobaltate positive electrode material is regulated by changing the mass content of element Mn in the lithium cobaltate positive electrode material, as follows: The mass content of element Mn in the lithium cobalt oxide positive electrode material is 614 ppm, and the mass ratio of element Mn to element Ni is 0.6, compared with Preparation Example 3a. The mass content of element Mn in the lithium cobalt oxide positive electrode material is 15246 ppm, and the mass ratio of element Mn to element Ni is 15, compared with Preparation Example 3b.

[0150] The following examples are used to illustrate the battery of the present application Example 1 The battery is prepared according to the following method: (1) Preparation of positive electrode sheet The positive electrode material prepared in Preparation Example 1, conductive carbon black and polyvinylidene fluoride are mixed in a mass ratio of 97:1.5:1.5, N-methyl pyrrolidone is added, and stirring is carried out under the action of a vacuum stirrer until uniform mixing to obtain a positive electrode slurry with a solid content of 95%; the positive electrode slurry is uniformly coated on both sides of the aluminum foil surface, placed in an oven at 100°C for 12h, and then subjected to rolling and cutting to obtain a positive electrode sheet; (2) Preparation of negative electrode sheet The negative electrode material (mass ratio of artificial graphite to silicon-carbon material is 8:2), carboxymethyl cellulose sodium, styrene-butadiene rubber, conductive carbon black and single-walled carbon nanotubes are mixed in a mass ratio of 94.5:2.5:1.5:1:0.5, deionized water is added, and a negative electrode slurry is prepared; the negative electrode slurry is uniformly coated on both sides of the copper foil surface, placed in an oven at 100°C for 4h, and then subjected to rolling and cutting to obtain a negative electrode sheet. The silicon-carbon material includes a porous carbon matrix and a silicon material located in the internal pores of the porous carbon matrix; S1 is 0.85 and S2 is 0.45; the average particle size of the first silicon-carbon particles is 6.4μm, and the average particle size of the second silicon-carbon particles is 10.6μm; the mass content of element Si in the first silicon-carbon particles is 50%, and the mass content of element Si in the second silicon-carbon particles is 45%; the diameter of the single-walled carbon nanotubes is 2.5nm, and the length is 18μm; (3) Preparation of battery The positive electrode sheet prepared in step (1), a separator (a polyethylene film with a thickness of 8 μm, and a boehmite-containing ceramic layer with a thickness of 2 μm coated on one side surface of the polyethylene film), and the negative electrode sheet prepared in step (2) are sequentially stacked to form a core, with the separator between the positive electrode sheet and the negative electrode sheet to prevent short circuiting; the core is placed in a steel can with an inner cavity, and an electrolyte (lithium hexafluorophosphate is dissolved in a mixed solution of DMC / DEC with a volume ratio of 1:1, and PP, FEC, DFEA, and HTCN are added) is injected into the dried bare battery, and the lithium ion battery is obtained through vacuum packaging, standing, formation, shaping, sorting, and other processes. In the electrolyte, the mass content of PP is 20%, the mass content of FEC is 15%, the mass content of DFEA is 30%, the mass content of 1,3,6-hexanetricarbonitrile is 4.5%, and the concentration of lithium hexafluorophosphate is 1 mol / L; the thickness allowance h between the inner cavity of the battery and the core is 152 μm.

[0151] Example 2 The battery is prepared as follows: (1) Preparation of a positive electrode sheet The positive electrode material prepared in Preparation Example 2, conductive carbon black, and polyvinylidene fluoride are mixed in a mass ratio of 97:1.5:1.5, N-methylpyrrolidone is added, and stirring is performed under the action of a vacuum stirrer until the mixture is uniform to obtain a positive electrode slurry with a solid content of 75%; the positive electrode slurry is uniformly coated on both side surfaces of an aluminum foil, and the aluminum foil is placed in an oven at 100°C for 12 h, and then subjected to rolling and cutting to obtain a positive electrode sheet. (2) Preparation of a negative electrode sheet The negative electrode material (artificial graphite and silicon-carbon material in a mass ratio of 8:2), carboxymethyl cellulose sodium, styrene-butadiene rubber, conductive carbon black, and single-walled carbon nanotubes are mixed in a mass ratio of 94.5:2.5:1.5:1:0.5, and deionized water is added to prepare a negative electrode slurry; the negative electrode slurry is uniformly coated on both side surfaces of a copper foil, and the copper foil is placed in an oven at 100°C for 4 h, and then subjected to rolling and cutting to obtain a negative electrode sheet. The silicon-carbon material comprises a porous carbon matrix and a silicon material in the internal pores of the porous carbon matrix; S1 is 0.7 and S2 is 0.3; the average particle size of the first silicon-carbon particles is 1.2 μm, and the average particle size of the second silicon-carbon particles is 6.5 μm; the mass content of element Si in the first silicon-carbon particles is 25%, and the mass content of element Si in the second silicon-carbon particles is 20%; the diameter of the single-walled carbon nanotubes is 1.6 nm, and the length is 5.3 μm. (3) Preparation of a battery The positive electrode sheet prepared in step (1), a separator (a polyethylene film with a thickness of 8 μm, and a boehmite-containing ceramic layer with a thickness of 2 μm coated on one side surface of the polyethylene film), and the negative electrode sheet prepared in step (2) are sequentially stacked to form a core, with the separator between the positive electrode sheet and the negative electrode sheet to prevent short circuiting; the core is placed in a steel can with an inner cavity, and an electrolyte (lithium hexafluorophosphate is dissolved in a mixed solution of DMC / DEC with a volume ratio of 1:1, and PP, FEC, DFEA, and HTCN are added) is injected into the dried bare battery, and the lithium ion battery is obtained through vacuum packaging, standing, formation, shaping, sorting, and other processes. In the electrolyte, the mass content of PP is 15%, the mass content of FEC is 20%, the mass content of DFEA is 35%, the mass content of 1,3,6-hexanetricarbonitrile is 1.5%, and the concentration of lithium hexafluorophosphate is 1 mol / L; the thickness allowance h between the inner cavity of the battery and the core is 52 μm.

[0152] Example 3 The battery is prepared according to the following method: (1) Preparation of a positive electrode sheet The positive electrode material prepared in Preparation Example 3, conductive carbon black, and polyvinylidene fluoride are mixed in a mass ratio of 97:1.5:1.5, N-methylpyrrolidone is added, and stirring is performed under the action of a vacuum stirrer until the mixture is uniform to obtain a positive electrode slurry with a solid content of 75%; the positive electrode slurry is uniformly coated on both side surfaces of an aluminum foil, and the aluminum foil is placed in an oven at 100°C for 12 h, and then subjected to rolling and cutting to obtain a positive electrode sheet. (2) Preparation of a negative electrode sheet The negative electrode material (artificial graphite and silicon-carbon material in a mass ratio of 8:2), carboxymethyl cellulose sodium, styrene-butadiene rubber, conductive carbon black, and single-walled carbon nanotubes are mixed in a mass ratio of 94.5:2.5:1.5:1:0.5, and deionized water is added to prepare a negative electrode slurry; the negative electrode slurry is uniformly coated on both side surfaces of a copper foil, and the copper foil is placed in an oven at 100°C for 4 h, and then subjected to rolling and cutting to obtain a negative electrode sheet. The silicon-carbon material comprises a porous carbon matrix and a silicon material in the internal pores of the porous carbon matrix; S1 is 0.95 and S2 is 0.6; the average particle size of the first silicon-carbon particles is 9.8 μm, and the average particle size of the second silicon-carbon particles is 14.6 μm; the mass content of element Si in the first silicon-carbon particles is 68%, and the mass content of element Si in the second silicon-carbon particles is 60%; the diameter of the single-walled carbon nanotubes is 4.6 nm, and the length is 25 μm. (3) Preparation of a battery The positive electrode sheet prepared in step (1), a separator (a polyethylene film with a thickness of 8 μm, and a boehmite-containing ceramic layer with a thickness of 2 μm coated on one side surface of the polyethylene film), and the negative electrode sheet prepared in step (2) are sequentially stacked with the separator between the positive electrode sheet and the negative electrode sheet to form a core, and then the core is placed in a steel can having an inner cavity. An electrolyte (lithium hexafluorophosphate is dissolved in a mixed solution of DMC / DEC in a volume ratio of 1:1, and PP, FEC, DFEA, and HTCN are added) is injected into the dried bare cell. After vacuum packaging, standing, formation, shaping, sorting, and other processes, a lithium ion battery is obtained. In the electrolyte, the mass content of PP is 30%, the mass content of FEC is 8%, the mass content of DFEA is 20%, the mass content of 1,3,6-hexanetricarbonitrile is 8%, and the concentration of lithium hexafluorophosphate is 1 mol / L. The thickness allowance h between the inner cavity of the battery and the core is 189 μm.

[0153] Examples 4-10 and Comparative Examples 1-3 are performed with reference to Example 1, except that the lithium cobaltate positive electrode material prepared in Preparation Example 1 is replaced with an equal amount of lithium cobaltate positive electrode material, as shown in Table 1.

[0154] Example 11 group This group of examples is performed with reference to Example 1, except that the composition of the silicon-carbon material is changed, as follows: Example 11a, the silicon-carbon material contains only the first silicon-carbon particles; Example 11b, the silicon-carbon material contains only the second silicon-carbon particles.

[0155] Example 12 This example is performed with reference to Example 1, except that the single-walled carbon nanotubes are not contained in the negative electrode active layer.

[0156] Example 13 group This group of examples is performed with reference to Example 1, except that the composition of the electrolyte is changed, as follows: Example 13a, the electrolyte does not contain DFEA; Example 13b, the mass content of HTCN in the electrolyte is 0.5%; Example 13c, the mass content of FEC in the electrolyte is 0.5%.

[0157] Test Example (1) Gram capacity test The gram capacity test is performed on the positive electrode materials prepared in the preparation examples and the comparative preparation examples, and the specific test method is as follows: The positive electrode material, acetylene black and polyvinylidene fluoride are uniformly mixed in a mass ratio of 94:3:3, and dispersed with NMP solvent to form a slurry; the slurry is uniformly coated on an aluminum foil and dried at 80℃ for 12h, and the dried electrode sheet is cut into a round sheet and placed in a glove box for standby; the above-prepared electrode sheet is used as a positive electrode sheet, a lithium sheet is used as a negative electrode, a microporous polypropylene film is used as a separator, and 1mol / L of LiPF6 (EC:DMC:EMC=1:1:1) is used as an electrolyte to assemble a button cell; The button cell prepared above is placed at 25℃±2℃ for 10min; discharged at 0.2C to 3V and placed for 10min; charged at 0.2C to 4.55V under a 25℃ constant temperature room, and stopped at 0.05C, and placed for 10min; discharged at 0.2C to 3V, and the discharge gram capacity is calculated, and the result is recorded in Table 1.

[0158] (2) Cycle test The batteries prepared in the examples and the comparative examples are subjected to cycle test, and the specific test method is as follows: Placed at 45℃±2℃ for 10min, discharged at 0.2C to 3V and placed for 10min; charged at 0.7C to the upper limit of the cut-off voltage 4.55V, and stopped at 0.05C, and placed for 10min; discharged at 0.5C to 3V and placed for 10min to obtain the initial capacity C0; charged at 0.7C to the cut-off voltage 4.55V, and stopped at 0.05C, and placed for 10min; discharged at 0.5C to 3V and placed for 10min; repeat the above full charge and full discharge steps until the capacity decays to 80% and stops, and the number of repeated cycles is the cycle number, so as to evaluate the high-temperature cycle performance of the battery, and the result is recorded in Table 1.

[0159] (3) Discharge rate test The batteries prepared in the examples and the comparative examples are subjected to discharge rate test, and the specific test method is as follows: Placed at 25℃±2℃ for 10min; discharged at 0.2C to 3V and placed for 10min; charged at 0.7C to the cut-off voltage 4.55V under a 25℃ constant temperature room, and stopped at 0.025C, and placed for 10min; discharged at 2.5C to 3V under a 25℃ constant temperature room or a constant temperature box environment, and placed for 10min; the 2.5C rate discharge capacity is divided by the 0.2C rate discharge capacity to obtain the rate capacity retention rate, and the result is recorded in Table 1.

[0160] Table 1 As can be seen from Table 1, the present application is beneficial to improve the gram capacity of the positive electrode material compared with the comparative examples, and the present application significantly improves the rate performance and cycle stability of the battery.

[0161] The preferred embodiments of the present application are described in detail above, but the present application is not limited thereto. Within the technical concept of the present application, various simple modifications can be made to the technical solutions of the present application, including that each technical feature is combined in any other suitable manner. These simple modifications and combinations should also be considered as disclosed by the present application and fall within the protection scope of the present application.

Claims

1. A lithium cobalt oxide cathode material, characterized in that, The lithium cobalt oxide cathode material has an R-3m layered structure; The lithium cobalt oxide cathode material includes elements Ni and Mn, and the mass ratio of element Mn to element Ni in the lithium cobalt oxide cathode material is 1.2-10; The mass content of Ni in the lithium cobalt oxide cathode material is 100ppm-3000ppm; The mass content of Mn in the lithium cobalt oxide cathode material is 125ppm-5000ppm; The lithium cobalt oxide cathode material comprises lithium cobalt oxide particles; the lithium cobalt oxide particles have grain boundaries, and the grain boundaries contain the elements Ni and Mn.

2. The lithium cobalt oxide cathode material according to claim 1, wherein, The mass content of element Mn in the grain boundary is greater than the mass content of element Ni. And / or, the mass ratio of element Mn to element Ni in the lithium cobalt oxide cathode material is 1.2-4; And / or, the mass content of Ni element in the lithium cobalt oxide cathode material is 400ppm-2000ppm; preferably 700ppm-1500ppm; And / or, the mass content of Mn element in the lithium cobalt oxide cathode material is 600ppm-3500ppm; preferably 1500ppm-2500ppm; And / or, the lithium cobalt oxide particles include the elements Ni and Mn; And / or, the lithium cobalt oxide particles include a first particle, the first particle including a first substrate and a first protrusion located on the surface of the first substrate, the first protrusion at least partially covering the outer surface of the first substrate; the first substrate including a first region and a second region located on the surface of the first region extending in a direction towards the center along the surface of the first substrate, the second region being a region with a depth of 300 nm from the surface of the first substrate, the first region being the region remaining in the first substrate excluding the second region; the first region including elements Ni, Mn, Al and Mg, the second region including elements Ni, Mn, Al, Mg and a first element group, the first element group including at least one of elements Ti, La, Y, Zr and S.

3. The lithium cobalt oxide cathode material according to claim 2, wherein, In the second region, the mass ratio of element Ti to element Co is 0.0001-0.001; preferably 0.0002-0.0005. And / or, the mass ratio of element La to element Co in the second region is 0.0001-0.0006; preferably 0.0001-0.0004; And / or, the mass ratio of element Y to element Co in the second region is 0.0001-0.001; preferably 0.0001-0.0005; And / or, the mass ratio of element Zr to element Co in the second region is 0.0001-0.0005; preferably 0.0001-0.0003; And / or, the mass ratio of the elements in the second region satisfies: (Ti+La+Y+Zr+Al) / Co>(Ni+Mn) / Co.

4. The lithium cobalt oxide cathode material according to claim 1 or 2, wherein, The lithium cobalt oxide cathode material has the following valence states: Co (+2 and +3), Ni (+2 and +3), and Mn (+3 and +4). Preferably, the lithium cobalt oxide cathode material satisfies the following condition: Mn 4+ / Co 3+ >Ni 2+ / Co 3+ >Ni 3+ / Co 3+ >Mn 3+ / Co 3+ ; Preferably, in the cross-sectional X-ray electron spectrum of the lithium cobalt oxide particles, Co 2+ The characteristic peak range is 785.7 eV-786.7 eV, Co 3+ The characteristic peak range is 781.5 eV-782.5 eV; And / or, the first particle contains element S; Preferably, the mass content of element S in the first particle is 10ppm-300ppm; more preferably, it is 10ppm-100ppm. And / or, the grain boundary contains element S, and the mass content of element S in the grain boundary is greater than the mass content of element S in the second region; And / or, the mass ratio of element Al to element Co in the second region is greater than the mass ratio of element Al to element Co in the first region.

5. The lithium cobalt oxide cathode material according to claim 2, wherein, The particle sizes Dn10, Dn50, and Dn90 of the lithium cobalt oxide cathode material satisfy the following condition: (Dn90-Dn50) > 1.5 × Dn10; And / or, the lithium cobalt oxide particles further include a second particle, the average particle size of the first particle being larger than the average particle size of the second particle; the second particle includes a second matrix and a second protrusion located on the surface of the second matrix, the second protrusion at least partially covering the outer surface of the second matrix; the second matrix includes a third region and a fourth region located on the surface of the third region, extending in a direction towards the center along the surface of the second matrix, the fourth region being a region with a depth of 300 nm from the surface of the second matrix, the third region being the region remaining in the second matrix excluding the fourth region; the third region includes elements Al and Mg, the fourth region includes elements Al, Mg and a second element group, the second element group including at least one of Zr, Ti, La and Y; Preferably, the mass ratio of Mg to Co in the first region is greater than the mass ratio of Mg to Co in the third region; Preferably, the mass ratio of element Zr to element Co in the fourth region is greater than the mass ratio of element Zr to element Co in the second region; Preferably, the mass ratio of element Al to element Co in the fourth region is greater than the mass ratio of element Al to element Co in the third region; Preferably, in the fourth region, the mass ratio of element Ti to element Co is 0.7-1.5, the mass ratio of element La to element Co is 0.7-1.5, the mass ratio of element Y to element Co is 0.7-1.5, the mass ratio of element Zr to element Co is 0.7-1.5, and the mass ratio of element Al to element Co is 0.7-1.

5.

6. The lithium cobalt oxide cathode material according to claim 5, wherein, The first protrusion and the second protrusion each independently contain at least one of the elements Al, Ti, La, Y and Zr; And / or, the height of the first protrusion is 10nm-300nm, and the width of the orthographic projection of the first protrusion onto the surface of the first particle is 10nm-300nm; the height of the second protrusion is 10nm-300nm, and the width of the orthographic projection of the second protrusion onto the surface of the second particle is 10nm-300nm. And / or, the area of ​​the first protrusion projected onto the surface of the first particle accounts for 30%-90%; the area of ​​the second protrusion projected onto the surface of the second particle accounts for 30%-90%; Preferably, with the total weight of element Co in the lithium cobalt oxide cathode material being 1, the mass percentage of element Al in the first protrusion is 0.0012-0.0021, the mass percentage of element Ti is 0.0007-0.0015, the mass percentage of element La is 0.0005-0.0009, the mass percentage of element Y is 0.0013-0.0021, and the mass percentage of element Zr is 0.0003-0.

001. Preferably, with the total weight of element Co in the lithium cobalt oxide cathode material being 1, the mass percentage of element Al in the second protrusion is 0.0012-0.0021, the mass percentage of element Ti is 0.0007-0.0015, the mass percentage of element La is 0.0005-0.0009, the mass percentage of element Y is 0.0013-0.0021, and the mass percentage of element Zr is 0.0003-0.

001. And / or, the crystal structures of the first particle and the second particle each independently include at least one of layered hexagonal structure, spinel phase, rutile phase, monoclinic phase, tetragonal phase and rock salt phase; And / or, the X-ray diffraction pattern of the lithium cobalt oxide cathode material has a characteristic peak one in the 18°-20° range and a characteristic peak two in the 44°-46° range; the peak intensity of the first characteristic peak is greater than that of the second characteristic peak.

7. A lithium-ion secondary battery, characterized in that, The lithium-ion secondary battery includes a positive electrode, a negative electrode, and a separator, wherein the positive electrode comprises the lithium cobalt oxide positive electrode material according to any one of claims 1-6; Preferably, the charging cutoff voltage of the lithium-ion secondary battery is ≥4.5V; Preferably, the negative electrode sheet comprises a negative electrode material, and the negative electrode material comprises silicon-carbon material.

8. The lithium-ion secondary battery according to claim 7, wherein, The silicon-carbon material includes a porous carbon matrix and silicon material located in the internal channels of the porous carbon matrix; And / or, the silicon-carbon material comprises first silicon-carbon particles and / or second silicon-carbon particles; Preferably, the sphericity of the first silicon-carbon particle is S1, where 0.7 ≤ S1 ≤ 1. The sphericity of the second silicon-carbon particle is S2, where 0.3 ≤ S2 < 0.7; Preferably, the average particle size of the first silicon-carbon particles is 1 μm-15 μm; more preferably, it is 1 μm-10 μm. The average particle size of the second silicon-carbon particles is 6μm-15μm; Preferably, the mass content of elemental Si in the first silicon-carbon particles is 25%-70%, and the mass content of elemental Si in the second silicon-carbon particles is 20%-60%. And / or, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on at least one side surface of the negative electrode current collector, the negative electrode active layer including the silicon-carbon material; the negative electrode active layer further includes single-walled carbon nanotubes; Preferably, the diameter of the single-walled carbon nanotube is 1 nm to 10 nm.

9. The lithium-ion secondary battery according to claim 7, wherein, The lithium-ion secondary battery further includes an electrolyte, which comprises a carbonate solvent; preferably, the carbonate solvent comprises at least one selected from ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and fluoroethylene carbonate; preferably, the carbonate solvent in the electrolyte has a mass content of 10%-70%; more preferably, the carbonate solvent comprises propylene carbonate, ethylene carbonate, and fluoroethylene carbonate; even more preferably, the fluoroethylene carbonate has a mass content of 5%-20% in the electrolyte. And / or, the electrolyte includes a carboxylic acid ester solvent; Preferably, the carboxylic acid ester solvent includes propyl propionate and / or ethyl fluoroacetate; More preferably, the propyl propionate in the electrolyte has a mass content of 10%-50%; More preferably, the mass content of ethyl fluoroacetate in the electrolyte is 10%-65%; More preferably, ethyl fluoroacetate includes ethyl 2,2-difluoroacetate and / or ethyl 2,2-difluoroacetate; And / or, the electrolyte further includes 1,3,6-hexanetrionitrile; preferably, the mass content of 1,3,6-hexanetrionitrile in the electrolyte is 0.5%-8%.

10. The lithium-ion secondary battery according to claim 7, wherein, The separator includes a substrate layer and a coating located on at least one surface of the substrate layer, the coating being disposed facing the positive electrode sheet; Preferably, the coating comprises nitrogen-containing particles containing nitrogen-containing functional groups, the nitrogen-containing functional groups including cyano, isocyano, isocyanate, and at least one of amino, isocyanate, isocyanate, and imino groups contained in melamine derivatives; Preferably, the thickness of the coating is 0.5 μm-3 μm; Preferably, the nitrogen content in the coating is 10%-60% by mass.

Images