Positive electrode sheet and battery

Abstract

The present application relates to the field of battery, specifically relates to a positive plate and a battery comprising the same.The thermogravimetric analysis test curve of the positive plate comprises at least three weight loss intervals at 150 DEG C-600 DEG C, the first weight loss interval is located at 150 DEG C-350 DEG C, the second weight loss interval and the third weight loss interval are located at 350 DEG C-600 DEG C;The weight loss rate of the first weight loss interval is m1, the sum of the weight loss rate of the second weight loss interval and the weight loss rate of the third weight loss interval is m2, 0.5% <= m1 <= 3%, 0.5% <= m2 <= 5%, 0.1 <= m1 / m2 <= 4.The positive plate of the present application has excellent high-temperature performance, and the battery comprising the positive plate has good high-temperature cycle performance in the high-voltage (cut-off voltage >= 4.45 V) system.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of batteries, in particular to a positive electrode sheet and a battery comprising the same. BACKGROUND

[0002] With the development of mobile phones, tablets and other consumer electronic products becoming increasingly thin and light, the energy density of lithium ion batteries needs to be continuously improved. As a positive electrode active material with high energy density, lithium cobaltate is considered to be an effective way to improve the energy density of lithium ion batteries by increasing its upper limit operating voltage and gravimetric capacity. In the future, the demand for lithium cobaltate materials will be to continuously pursue the increase of voltage, i.e. ≥4.5V voltage will become the development direction. However, at high voltage, when the delithiation amount of lithium cobaltate is >75%, an irreversible H1-3 structural phase transition occurs. The occurrence of this irreversible phase transition will cause the impedance of the positive electrode side to increase, and the lithium cobaltate will also undergo severe lattice expansion and contraction under high-temperature and high-pressure cycles in a high delithiation state. The stress accumulation within the particles caused by lattice expansion and contraction will cause the particles to crack, thereby accelerating the deterioration of high-temperature cycle performance.

[0003] Therefore, it is very important to improve the structural stability of lithium cobaltate under high voltage conditions. SUMMARY

[0004] The present application aims to overcome the problem of structural instability of lithium cobaltate under high voltage conditions in the prior art, and provides a positive electrode sheet and a battery comprising the same. The positive electrode sheet of the present application has excellent high-temperature performance, and the battery comprising the positive electrode sheet of the present application has good high-temperature cycle performance in a high voltage (cut-off voltage ≥4.45V) system.

[0005] The first aspect of the present application provides a positive electrode sheet, the thermogravimetric analysis test curve of the positive electrode sheet comprising at least three weight loss intervals at 150℃-600℃, the first weight loss interval being located at 150℃-350℃, and the second and third weight loss intervals being located at 350℃-600℃; the weight loss rate of the first weight loss interval being m1, the sum of the weight loss rates of the second and third weight loss intervals being m2, 0.5%≤m1≤3%, 0.5%≤m2≤5%, and 0.1≤m1 / m2≤4.

[0006] The second aspect of the present application provides a battery comprising the positive electrode sheet of the first aspect of the present application.

[0007] Compared with the prior art, the positive electrode sheet has at least the following advantages: the positive electrode sheet can effectively improve the destruction of the conductive network caused by the cracking of lithium cobalt oxide in the deep delithiation process of the positive electrode active material, effectively improve the stability of the conductive network, reduce the polarization growth of the positive electrode sheet, improve the structural stability and good conductivity of the positive electrode sheet in the charging and discharging process, and effectively improve the high-temperature performance of the positive electrode sheet. The battery including the positive electrode sheet has good high-temperature cycle performance in a high-voltage system with a cutoff voltage of 4.45 V or more.

[0008] The endpoints of the ranges and any values claimed herein are not to be understood as being limited to the exact values recited as implicitly split into a narrower range between each pair of endpoints. The range or value should be understood as being given either before or after the respective value. For values having a range, each intervening value, to the tenth of the unit of the lower limit unless the context clearly indicates otherwise, between the upper and lower limit, can be combined with any other claimed or inferred value. BRIEF DESCRIPTION OF DRAWINGS

[0009] Figure 1 A thermogravimetric analysis test curve of the positive electrode sheet is shown.

[0010] Figure 2 A cross-sectional schematic view of the positive electrode sheet is shown. DETAILED DESCRIPTION

[0011] The specific embodiments of the present application are described in detail below. It should be understood that the specific embodiments described herein are merely intended to illustrate and explain the present application, and are not intended to limit the present application.

[0012] The first aspect of the present application provides a positive electrode sheet, the thermogravimetric analysis test curve of the positive electrode sheet has at least three weight loss intervals at 150-600℃, wherein the first weight loss interval is at 150-350℃, the second weight loss interval and the third weight loss interval are at 350-600℃; the weight loss rate of the first weight loss interval is m1, the sum of the weight loss rate of the second weight loss interval and the weight loss rate of the third weight loss interval is m2, 0.5%≤m1≤3% (for example, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9% or 3%), 0.5%≤m2≤5% (for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or 5%), 0.1≤m1 / m2≤4 (for example, 0.1, 0.5, 1, 1.5, 2, 2.5, 3, 3.5 or 4). As Figure 1 The thermogravimetric analysis test curve of the positive electrode sheet in an example of the present application is shown. As can be seen from the figure, the weight loss rate in the first weight loss interval 150-330℃ is 1.19%, and the weight loss rate in the second weight loss interval and the third weight loss interval 330-546℃ is 2.22% (wherein the second weight loss interval is 330-431℃, the weight loss rate is 1.22%, and the third weight loss interval is 431-546℃, the weight loss rate is 1%).

[0013] The inventors of the present application found that when the thermogravimetric analysis test curve of the positive electrode sheet has corresponding weight loss changes in a specific temperature range, the destruction of the conductive network caused by the cracking of lithium cobalt oxide due to phase change stress accumulation during deep delithiation in the charging and discharging process can be effectively improved, the polarization growth caused by the destruction of the conductive network can be effectively improved, and the high temperature performance of the positive electrode sheet can be significantly improved, and the high temperature cycle performance of the battery in the high voltage system with a cutoff voltage≥4.45V can be improved.

[0014] In an example, 0.5%<m1≤3%, 0.5%≤m2≤3%, 0.3≤m1 / m2<4.

[0015] In an example, 1%≤m1≤2.5%, 0.8%<m2≤2.5%, 0.5≤m1 / m2≤2.5.

[0016] The inventors of the present application found that when m1 and m2 are within a specific range, and when m1 / m2 is within a specific range, the impedance of the positive electrode side during the charging and discharging cycle can be effectively reduced, and the lattice expansion and contraction of lithium cobalt oxide in a high temperature and high voltage environment can be further improved, thereby improving the high temperature cycle performance of the battery in the high voltage system.

[0017] In the present application, the positive electrode sheet is subjected to thermal gravimetric analysis (TG), and the specific method is as follows: using a thermal gravimetric analyzer (TGA) for testing, scraping the positive electrode coating on the positive electrode sheet, weighing and recording the data, and then placing it in a special crucible and into the instrument, testing at a temperature rising speed of 5℃ / min, and the testing atmosphere is N2.

[0018] In the present application, the positive electrode sheet can include a positive electrode current collector and a positive electrode coating on at least one side surface of the positive electrode current collector. As shown in Figure 2 which is a cross-sectional schematic diagram of the positive electrode sheet in an example of the present application, wherein Figure 2 (a) is a case where the positive electrode coating is provided on one side; Figure 2 (b) and Figure 2 (c) are cases where the positive electrode coating is provided on both sides. In Figure 2 (a), the positive electrode sheet includes a positive electrode current collector 1 and a positive electrode coating 2 on one side surface of the positive electrode current collector 1; in Figure 2 (b) and Figure 2 (c), the positive electrode sheet includes a positive electrode current collector 1 and a positive electrode coating 2 on both side surfaces of the positive electrode current collector 1.

[0019] The positive electrode coating can include a positive electrode active material and a conductive agent; the conductive agent can include a first conductive agent and a second conductive agent, the first conductive agent can include carbon nanotubes, and the second conductive agent can include at least one of conductive carbon black, acetylene black, ketjen black, conductive graphite, and graphene.

[0020] In the present application, the ratio of the mass of the first conductive agent to the mass of the second conductive agent can be 1:(0.1-20), such as 1:0.1, 1:0.2, 1:0.3, 1:0.4, 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, 1:11, 1:12, 1:13, 1:14, 1:15, 1:16, 1:17, 1:18, 1:19, or 1:20.

[0021] In an example, the ratio of the mass of the first conductive agent to the mass of the second conductive agent is 1:(0.5-10).

[0022] In an example, the ratio of the mass of the first conductive agent to the mass of the second conductive agent is 1:(0.8-8).

[0023] In an example, the conductive agent includes carbon nanotubes and acetylene black.

[0024] In the present application, the mass ratio of the carbon nanotube and the acetylene black can be 1:(0.5-10), for example, 1:0.5, 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, or 1:10.

[0025] In one example, the mass ratio of the carbon nanotube and the acetylene black is 1:(0.8-8).

[0026] In the present application, the positive electrode coating layer can further include a binder. The binder can be selected from the binders conventionally used in the art, for example, including at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), lithium polyacrylate (PAALi), and polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP).

[0027] In the present application, the mass ratio of the binder and the conductive agent can be (0.1-4):1, for example, 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, or 4:1.

[0028] In one example, the mass ratio of the binder and the conductive agent is (0.3-4):1.

[0029] In one example, the mass ratio of the binder and the conductive agent is (0.5-2.5):1.

[0030] In the present application, the content of the positive electrode active material can be 70-99% by weight (for example, 70, 75, 80, 85, 90, 95, or 99% by weight), the content of the conductive agent can be 0.5-15% by weight (for example, 15, 10, 5, 1, or 0.5% by weight), and the content of the binder can be 0.5-15% by weight (for example, 15, 10, 5, 1, or 0.5% by weight), based on the total weight of the positive electrode coating layer.

[0031] In one example, the content of the positive electrode active material is 80-99% by weight, the content of the conductive agent is 0.5-10% by weight, and the content of the binder is 0.5-10% by weight, based on the total weight of the positive electrode coating layer.

[0032] In one example, the content of the positive electrode active material is 95-98.5% by weight, the content of the conductive agent is 0.7-3% by weight, and the content of the binder is 0.8-2% by weight, based on the total weight of the positive electrode coating layer.

[0033] In the present application, the positive active material can include lithium cobaltate with a core-shell structure, the core of the core-shell structure including metal element-doped lithium cobaltate, wherein the doped metal element can include at least one of Al, Mg and M, M can be selected from at least one of Ti, Zr, Y, La, Ni, Mn, Ce and W, and the shell of the core-shell structure including a compound containing element L, L can be selected from at least one of Al, Mg, Ti, Zr, Y, La, Ni and Mn.

[0034] In the present application, the compound containing element L can include at least one of an oxide, a fluoride and a phosphate of L.

[0035] Lithium cobaltate is a positive active material with a layered structure, in which oxygen ions form a layer of close-packed layers, and cobalt layers and lithium layers are alternately distributed on both sides of the close-packed layers formed by oxygen ions. As the operating voltage continues to rise (for example, from 4.2 V to 4.4 V and above), the structure of lithium cobaltate (changes in lithium concentration cause changes in structure, causing stress to cause microcracks to occur) and the surface (reacts with the electrolyte, causing dissolution of cobalt) also become increasingly unstable, causing local lattice structure collapse and irreversible phase transition (including conversion of the layered structure to the spinel structure), thereby increasing the positive side impedance, which in turn affects the cycle performance of the battery. The inventors of the present application found that if the weight loss interval and weight loss rate in the thermogravimetric curve of the positive electrode sheet are controlled, and the lithium cobaltate is doped and coated, the high-temperature cycle performance and rate performance of the battery in a high-voltage system can be effectively improved. The reason can be that the doping of lithium cobaltate can replace the cobalt site, avoiding the frequent conversion between the layered hexagonal system and the spinel monoclinic system of lithium cobaltate during charging and discharging, thereby ensuring the stability of the layered structure during charging and discharging; further, by controlling the weight loss interval and weight loss rate of the thermogravimetric curve of the positive electrode sheet, the impedance of the positive side during the charging and discharging cycle can be effectively reduced, the polarization generated during the cycle can be effectively reduced, and the high-temperature cycle performance in a high-voltage system can be improved.

[0036] In the present application, the term "high-voltage system" has the conventional meaning in the art, referring to a battery system with a cutoff voltage greater than or equal to 4.45 V, for example, 4.45 V-4.6 V.

[0037] In the present application, the doping amount of the doped metal element Al can be 50 ppm-20000 ppm (for example, 50 ppm, 60 ppm, 70 ppm, 80 ppm, 90 ppm, 100 ppm, 200 ppm, 300 ppm, 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 2000 ppm, 3000 ppm, 4000 ppm, 5000 ppm, 6000 ppm, 7000 ppm, 8000 ppm, 9000 ppm, 10000 ppm or 20000 ppm), the doping amount of the doped metal element Mg can be 50 ppm-5000 ppm (for example, 50 ppm, 60 ppm, 70 ppm, 80 ppm, 90 ppm, 100 ppm, 200 ppm, 300 ppm, 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 2000 ppm, 3000 ppm, 4000 ppm or 5000 ppm), and the total doping amount of the doped metal elements Al, Mg and M can be 4000 ppm-30000 ppm (for example, 4000 ppm, 5000 ppm, 6000 ppm, 7000 ppm, 8000 ppm, 9000 ppm, 10000 ppm, 20000 ppm or 30000 ppm).

[0038] The inventors of the present application found that when the doping amount of the doped metal element is within a certain range, the high-temperature stability of the positive electrode active material can be effectively improved, and when the thermogravimetric analysis test curve of the positive electrode sheet meets certain conditions, a synergistic effect can be produced, further improving the high-temperature performance of the positive electrode sheet.

[0039] In an example, the doping amount of the doped metal element Al is 5000 ppm-10000 ppm, the doping amount of the doped metal element Mg is 200 ppm-3000 ppm, and the total doping amount of the doped metal elements Al, Mg and M is 6000 ppm-20000 ppm.

[0040] In an example, the doped metal element M includes at least one of Ti, Y and La.

[0041] In the present application, the content of the element L can be 0.05-2 wt%, for example, 0.05, 0.1, 0.15, 0.18, 0.25, 0.5, 0.7, 1, 1.5 or 2 wt%, based on the total weight of the lithium cobaltate with core-shell structure.

[0042] In one example, the content of the element L is 0.1 to 1.5% by weight, based on the total weight of the lithium cobalt oxide having a core-shell structure.

[0043] The inventors of the present application found that the lithium cobalt oxide having a core-shell structure has excellent structural stability when the ratio of the contents of Al, Mg, and L is within a specific range.

[0044] In the present application, the ratio of the contents of Al, Mg, and L can be 1:(0.01-1.5):(0.1-10), for example, 1:0.01:0.1, 1:0.01:0.5, 1:0.01:1, 1:0.01:5, 1:0.01:10, 1:0.05:0.1, 1:0.05:0.5, 1:0.05:1, 1:0.05:5, 1:0.05:10, 1:0.1:0.1, 1:0.1:0.5, 1:0.1:1, 1:0.1:5, 1:0.1:10, 1:0.5:0.1, 1:0.5:0.5, 1:0.5:1, 1:0.5:5, 1:0.5:10, 1:1:0.1, 1:1:0.5, 1:1:1, 1:1:5, 1:1:10, 1:1.5:0.1, 1:1.5:0.5, 1:1.5:1, 1:1.5:5, or 1:1.5:10.

[0045] In one example, the ratio of the contents of Al, Mg, and L is 1:(0.04-0.3):(0.2-1.5).

[0046] In the present application, the kinds and contents of the doped metal elements and the element L can be obtained by an inductively coupled plasma atomic emission spectrometer (ICP-AES).

[0047] In the present application, the specific surface area of the positive electrode active material can be 0.1 m 2 / g to 0.5 m 2 / g (for example, 0.1 m 2 / g, 0.15 m 2 / g, 0.2 m 2 / g, 0.25 m 2 / g, 0.3 m 2 / g, 0.35 m 2 / g, 0.4 m 2 / g, 0.45 m 2 / g, or 0.5 m 2 / g.

[0048] The inventors of the present application find that the specific surface area of the positive electrode active material in the above range can improve the infiltration of electrolyte, improve the conductivity of lithium ions, and at the same time, can reduce the side reaction between electrolyte and positive electrode active material under high voltage system.

[0049] In the present application, the particle size Dv50 of the positive electrode active material can be 1 μm-50 μm (for example, 1 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm or 50 μm).

[0050] In the present application, the specific surface area of the lithium cobaltate with core-shell structure can be 0.1 m 2 / g-0.5 m 2 / g (for example, 0.1 m 2 / g, 0.15 m 2 / g, 0.2 m 2 / g, 0.25 m 2 / g, 0.3 m 2 / g, 0.35 m 2 / g, 0.4 m 2 / g, 0.45 m 2 / g or 0.5 m 2 / g). The particle size Dv50 of the lithium cobaltate with core-shell structure can be 1 μm-50 μm (for example, 1 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm or 50 μm).

[0051] The thermogravimetric analysis test curve of the positive electrode sheet of the present application meets specific conditions, which can significantly improve the high temperature performance of the positive electrode sheet, and improve the high temperature cycle performance of the battery in the high voltage system with a cutoff voltage ≥ 4.45 V. Further, by simultaneously improving the lithium cobaltate and the conductive agent (doping and coating modification of the lithium cobaltate, and combination of different shapes of conductive agents such as linear conductive agents and point-like conductive agents), the high temperature cycle performance and rate performance of the battery in the high voltage system can be improved.

[0052] The present application also provides a preparation method of the lithium cobaltate with core-shell structure, which at least comprises the following steps:

[0053] (A1) mixing a doping metal compound, a cobalt source and a lithium source, and performing first sintering;

[0054] (A2) mixing the once sintered particles obtained in step (A1) with a coating metal compound, and performing second sintering.

[0055] The specific selection of the materials used in the method and the amount thereof are the same as those described in the first aspect of the present application, and will not be repeated here.

[0056] In the present application, in step (A1), the doped metal compound can comprise an oxide of a doped metal, the elemental species of the doped metal comprising Al, Mg and M, M being selected from at least one of Ti, Zr, Y, La, Ni, Mn, Ce and W. The cobalt source can comprise Co3O4. The lithium source can comprise Li2CO3.

[0057] In an example, the doped metal compound comprises Al2O3, MgO and TiO2.

[0058] In an example, the oxide of the doped metal is a nano-sized oxide of the doped metal.

[0059] In the present application, in step (A1), the ratio of the number of moles of Li to the sum of the number of moles of Co, Al, Mg and M can be (1.01-1.1):1, i.e. n(Li):n(Co+Al+Mg+M)=(1.01-1.1):1, for example 1.01:1, 1.02:1, 1.03:1, 1.04:1, 1.05:1, 1.06:1, 1.07:1, 1.08:1, 1.09:1 or 1.1:1.

[0060] In the present application, in step (A1), the doping amount of the doped metal elements Al, Mg and M can be 4000ppm-30000ppm.

[0061] In the present application, in step (A1), the conditions of the first sintering comprise heating to 800℃-1200℃ at a heating rate of 3℃ / min-10℃ / min and holding for 6h-10h.

[0062] In the present application, in step (A2), the coating metal compound can comprise at least one of an oxide, a fluoride and a phosphate of a coating metal; the elemental species of the coating metal comprising at least one of Al, Mg, Ti, Zr, Y, La, Ni and Mn.

[0063] In the present application, the step (A2) can further comprise mixing the once-sintered particles obtained from step (1) after crushing treatment and sieving treatment with the coating metal compound.

[0064] In the present application, in step (A2), the conditions of the second sintering comprise heating to 600℃-1000℃ at a heating rate of 3℃ / min-10℃ / min and holding for 6h-8h.

[0065] In the present application, the step (A2) can further comprise crushing treatment and sieving treatment after the second sintering.

[0066] The second aspect of the present application provides a battery, the battery comprising the positive electrode sheet according to the first aspect of the present application.

[0067] In the present application, the battery can be used in a high-voltage system, for example, with a cut-off voltage of 4.45 V, 4.47 V, 4.48 V, 4.5 V, 4.52 V, 4.53 V, 4.55 V, or 4.6 V.

[0068] In the present application, the battery can further comprise an electrolyte. The electrolyte can comprise lithium difluorophosphate (LiPO2F2).

[0069] The inventors of the present application have found that when lithium difluorophosphate is included in the electrolyte, the cycle life of the battery in a high-voltage system can be significantly improved, which can be due to the fact that lithium difluorophosphate can form a surface film on the surface of the negative electrode active material with higher ionic conductivity, and can have a synergistic effect with the positive electrode active material of the present application, improving the stability of the positive electrode sheet interface film, preventing the transition metal of the positive electrode active material (such as lithium cobaltate) from dissolving into the negative electrode, improving the structural stability of the positive electrode material, and ensuring good rate performance and cycle performance.

[0070] In the present application, the content of lithium difluorophosphate can be 0.01-10% by weight, for example, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10% by weight, based on the total weight of the electrolyte.

[0071] The inventors of the present application have found that in the electrolyte, the lithium difluorophosphate has a specific content range, which can further improve the cycle life of the battery in a high-voltage system.

[0072] In an example, the content of lithium difluorophosphate is 0.5-5% by weight, based on the total weight of the electrolyte.

[0073] The electrolyte can further comprise an additive, which can comprise at least one of a nitrile compound and a fluoroethylene carbonate.

[0074] The inventors of the present application find that the stability of lithium difluorophosphate is poor and more prone to decomposition to generate HF. Adding at least one of a nitrile compound and fluorinated ethylene carbonate as an additive to the electrolyte can improve the poor stability of lithium difluorophosphate. The electrolyte in combination with the positive electrode sheet can produce a synergistic effect, and can further improve the high-temperature cycle performance and rate performance of the battery under a high-voltage system. The reason may be that the carbon-nitrogen triple bond in the cyano group contained in the nitrile compound has a high bond energy and is not easy to be oxidized. At the same time, the cyano group has strong coordination ability and can combine with high-valence Co ions on the surface of the electrode, thereby playing a role in masking the active ions on the surface of lithium cobaltate. In addition, the nitrile compound can complex some active high-valence ions on the lithium cobaltate, forming a stable and dense protective film on the surface of the positive electrode sheet, which can inhibit the occurrence of side reactions between the positive electrode sheet and the electrolyte. Fluorinated ethylene carbonate can inhibit the decomposition of the electrolyte and form a stable interfacial film on the surface of the positive electrode sheet. The nitrile compound, fluorinated ethylene carbonate and lithium difluorophosphate can produce a synergistic effect, thereby ensuring that the battery has good rate performance and cycle performance.

[0075] In the present application, the nitrile compound can include at least one of 3-methoxypropionitrile, butanedinitrile, pentanedinitrile, hexanedinitrile, trans-butenedinitrile, trans-hexenedinitrile, 1,2-di(cyanoethoxy)ethane, 1,3,6-hexanetricarbonitrile and 1,2,3-tri(cyanoethoxy)propane.

[0076] In the present application, the content of the nitrile compound can be 1-10% by weight (for example, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10% by weight) based on the total weight of the electrolyte, and the content of the fluorinated ethylene carbonate can be 1-20% by weight (for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20% by weight).

[0077] In an example, the content of the nitrile compound is 1-10% by weight and the content of the fluorinated ethylene carbonate is 1-15% by weight based on the total weight of the electrolyte.

[0078] In an example, the content of the nitrile compound is 2-8% by weight and the content of the fluorinated ethylene carbonate is 3-10% by weight based on the total weight of the electrolyte.

[0079] The electrolyte can further include a lithium salt. The lithium salt can include at least one of lithium hexafluorophosphate (LiPF6), lithium bisfluorosulfonimide and lithium bis(trifluoromethanesulfonimide).

[0080] In the present application, the electrolyte can further include an organic solvent having a boiling point greater than 100°C.

[0081] In an example, the organic solvent includes at least one of a linear carbonate and a linear carboxylate, and a cyclic carbonate. The cyclic carbonate includes at least one of ethylene carbonate (EC) and propylene carbonate (PC). The linear carbonate includes at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). The linear carboxylate includes at least one of ethyl propionate (VP), propyl propionate (PP), and propyl acetate (PA).

[0082] In an example, the content of the cyclic carbonate is 20-40% by volume, and the content of the linear carbonate and / or the linear carboxylate is 60-80% by volume, based on the total volume of the organic solvent.

[0083] The electrode, except for the positive electrode sheet and the electrolyte (e.g., the negative electrode sheet and the separator, etc.), can be a conventional selection in the art.

[0084] In an example, the battery further includes a negative electrode sheet and a separator.

[0085] The negative electrode sheet can include a negative electrode current collector and a negative electrode coating on at least one side surface of the negative electrode current collector, and the negative electrode coating can include a negative electrode active material, a negative electrode conductive agent, and a negative electrode binder.

[0086] The negative electrode active material can be a conventional selection in the art, for example, the negative electrode active material includes at least one of natural graphite, artificial graphite, mesocarbon microbeads, hard carbon, lithium titanate, silicon-carbon, and silicon monoxide.

[0087] The negative electrode conductive agent can be a conventional selection in the art, for example, the negative electrode conductive agent includes at least one of conductive carbon black, acetylene black, ketjen black, conductive graphite, conductive carbon fiber, carbon nanotube, metal powder, and carbon fiber.

[0088] The negative electrode binder can be a conventional selection in the art, for example, the negative electrode binder includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyacrylic acid lithium (PAALi).

[0089] The content of the negative electrode active material can be 70-99% by weight (e.g., 70, 75, 80, 85, 90, 95, or 99% by weight), the content of the negative electrode conductive agent can be 0.5-15% by weight (e.g., 15, 10, 5, 1, or 0.5% by weight), and the content of the negative electrode binder can be 0.5-15% by weight (e.g., 15, 10, 5, 1, or 0.5% by weight), based on the total weight of the negative electrode coating.

[0090] In one example, the content of the negative active material is 80-98 wt%, the content of the negative conductive agent is 1-10 wt%, and the content of the negative binder is 1-10 wt%, based on the total weight of the negative electrode coating.

[0091] The separator can be a conventional selection in the art, for example, the separator includes a substrate layer and a rubberized layer on at least one side surface of the substrate layer. The substrate layer can include a polypropylene film. The rubberized layer can include ceramic particles.

[0092] The battery can be assembled in a manner conventional in the art. For example, the method for preparing the battery includes the following steps:

[0093] (B1) Preparation of the positive electrode sheet: mixing the positive active material, the conductive agent, and the binder, adding the positive solvent to form a positive electrode slurry, coating the positive electrode slurry on the positive current collector, drying, and cold pressing to obtain the positive electrode sheet;

[0094] (B2) Preparation of the negative electrode sheet: mixing the negative active material, the negative conductive agent, the negative binder, and the negative thickening agent, adding the negative solvent to form a negative electrode slurry, coating the negative electrode slurry on the negative current collector, drying, and cold pressing to obtain the negative electrode sheet;

[0095] (B3) Preparation of the battery: stacking the positive electrode sheet obtained in step (B1), the separator, and the negative electrode sheet obtained in step (B2) in order, winding to obtain a bare cell, placing the bare cell in an outer packaging foil, injecting an electrolyte, vacuum packaging, standing, formation, shaping, and obtaining the battery.

[0096] In the present application, in step (B1), the positive solvent can include N-methylpyrrolidone.

[0097] In the present application, in step (B1), the solid content of the positive electrode slurry is a conventional solid content of a positive electrode slurry in the art, for example, 65 wt%-80 wt%.

[0098] In the present application, in step (B2), the negative solvent can include deionized water.

[0099] In the present application, in step (B2), the solid content of the negative electrode slurry is a conventional solid content of a negative electrode slurry in the art, for example, 40 wt%-50 wt%.

[0100] In the present application, in step (B1) and step (B2), the drying and the cold pressing are conventional processes in the art.

[0101] In the present application, in step (B3), the separator is placed between the positive electrode sheet and the negative electrode sheet to serve as a separator.

[0102] It should be noted that the "first", "second" and the like in the present application are only used to distinguish different substances or use methods, and do not represent the difference in order.

[0103] The present application will be described in detail by examples. The examples described in the present application are only a part of the examples of the present application, but not all the examples. Based on the examples in the present application, all other examples obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0104] In the following examples, the materials used are commercially available analytical pure without special instructions.

[0105] Preparation Example I1

[0106] The positive electrode active material is prepared according to the following steps:

[0107] Co3O4, nano-sized Al2O3, nano-sized MgO and nano-sized Y2O3 are weighed according to the mass ratio of Co:Al:Mg:Y of 6013:75:15:40, and Li2CO3 is added and mixed uniformly, wherein the ratio of the number of moles of Li to the sum of the number of moles of Co, Al, Mg and Y is 1.05:1, i.e. n(Li):n(Co+Al+Mg+Y) = 1.05:1, and first sintering is performed, wherein the conditions of the first sintering include: heating from room temperature (25℃) to 1000℃ at a heating rate of 5℃ / min and keeping for 8h, and natural cooling, to obtain primary sintered particles; after crushing and sieving treatment of the primary sintered particles, ZrO2 is mixed, wherein the mass ratio of Co to Zr is 60.1:1, and second sintering is performed, wherein the conditions of the second sintering include: heating from 25℃ to 800℃ at a heating rate of 5℃ / min and keeping for 7h, and after crushing and sieving treatment, a positive electrode active material is obtained.

[0108] Preparation Example I2

[0109] The positive electrode active material is prepared according to the following steps:

[0110] Co3O4, nano-Al2O3, nano-MgO and nano-TiO2 are weighed according to a mass ratio of Co:Al:Mg:Ti of 6013:50:2:8, Li2CO3 is added and mixed uniformly, wherein the ratio of the number of moles of Li to the sum of the number of moles of Co, Al, Mg and Ti is 1.05:1, i.e. n(Li):n(Co+Al+Mg+Ti)=1.05:1, first sintering is performed, wherein the conditions of the first sintering include: increasing the temperature from room temperature (25℃) to 800℃ at a temperature increase rate of 5℃ / min and keeping the temperature for 6h, natural cooling, to obtain primary sintered particles; after crushing treatment and sieving treatment of the primary sintered particles, Y2O3 is mixed, wherein the mass ratio of Co to Y is 601:1, second sintering is performed, wherein the conditions of the second sintering include: increasing the temperature from 25℃ to 600℃ at a temperature increase rate of 5℃ / min and keeping the temperature for 6h, crushing treatment and sieving treatment are performed, and the positive electrode active material is obtained.

[0111] Preparation Example I3

[0112] The positive electrode active material is prepared according to the following steps:

[0113] Co3O4, nano-Al2O3, nano-MgO and nano-La2O3 are weighed according to a mass ratio of Co:Al:Mg:La of 601.3:10:3:7, Li2CO3 is added and mixed uniformly, wherein the ratio of the number of moles of Li to the sum of the number of moles of Co, Al, Mg and La is 1.05:1, i.e. n(Li):n(Co+Al+Mg+La)=1.05:1, first sintering is performed, wherein the conditions of the first sintering include: increasing the temperature from room temperature (25℃) to 1200℃ at a temperature increase rate of 10℃ / min and keeping the temperature for 10h, natural cooling, to obtain primary sintered particles; after crushing treatment and sieving treatment of the primary sintered particles, Al2O3 is mixed, wherein the mass ratio of Co to Al is 60.1:1.5, second sintering is performed, wherein the conditions of the second sintering include: increasing the temperature from 25℃ to 1000℃ at a temperature increase rate of 10℃ / min and keeping the temperature for 8h, crushing treatment and sieving treatment are performed, and the positive electrode active material is obtained.

[0114] Preparation Examples I4-I6 are prepared according to Preparation Example I1, except that: the doping amount of Al, Mg and Y is changed; the type and content of the coating element are changed. Among them, the specific parameters are shown in Table 1, and the particle size Dv50 of the positive electrode active material prepared by the preparation example is 1μm-50μm, the specific surface area is 0.1m 2 / g-0.5m 2 / g.

[0115] Table 1

[0116]

[0117] Preparation Example II

[0118] The electrolyte was prepared according to the following steps:

[0119] In an argon atmosphere glove box with water content < 10 ppm, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), propyl propionate (PP) and dimethyl carbonate (DMC) were mixed uniformly in a mass ratio of 20:10:10:10:50 to obtain a non-aqueous solvent; 14.5% LiPF6 based on the total mass of the electrolyte was added to the above non-aqueous solvent, stirred until completely dissolved, 4% 1,3-propane sultone based on the total mass of the electrolyte was added, and then LiPO2F2, a nitrile compound and fluoroethylene carbonate were added to obtain the electrolyte.

[0120] Preparation Examples II1-II8 were prepared according to Preparation Example II1, except that the mass of LiPO2F2, the mass of the nitrile compound and the mass of fluoroethylene carbonate added in the electrolyte were changed, as shown in Table 2.

[0121] Table 2

[0122]

[0123]

[0124] Note: "-" in Table 2 means not added.

[0125] Example

[0126] The battery was prepared according to the following steps:

[0127] (1) Preparation of positive electrode sheet

[0128] The positive electrode active material obtained in Preparation Example I1, the first conductive agent, the second conductive agent and polyvinylidene fluoride (PVDF) were mixed uniformly in a certain mass ratio, and N-methyl pyrrolidone was added, wherein the solid content was 75 wt%, to form a positive electrode slurry; the above positive electrode slurry was coated on the surface of an aluminum foil, dried, cold-pressed to obtain a positive electrode sheet;

[0129] (2) Preparation of negative electrode sheet

[0130] The artificial graphite (particle size Dv50: 13±1 μm, graphitization degree 94±0.5%, secondary particles and single particles mixed, wherein the mass ratio of secondary particles is 50%), acetylene black, butadiene rubber (SBR), carboxymethyl cellulose sodium (CMC-Na) are mixed in a mass ratio of 95:2:2:1, and deionized water is added, wherein the solid content is 45wt%, to form a negative electrode slurry. The above negative electrode slurry is coated on the surface of a copper foil, dried, cold-pressed to obtain a negative electrode sheet.

[0131] (3) Preparation of battery

[0132] The positive electrode sheet prepared in step (1), the separator film (PE porous polymer film) and the negative electrode sheet prepared in step (2) are stacked in order, with the separator film between the positive electrode sheet and the negative electrode sheet to play a separating role, and a bare battery is obtained by winding. The above bare battery is placed in an outer packaging foil, dried, injected with the electrolyte prepared in the preparation example of group II, vacuum packaged, placed, formed, and shaped to obtain a battery. See Table 3 for details.

[0133] Table 3

[0134]

[0135]

[0136] Note: "-" in Table 3 means not added.

[0137] Test Example

[0138] (1) Thermogravimetric analysis

[0139] The positive electrode sheets prepared in the examples and comparative examples were subjected to thermogravimetric analysis test, and the results are shown in Table 4. The thermogravimetric analysis test curve of Example 2 is shown in Figure 1

[0140] Table 4

[0141]

[0142]

[0143] Note: "-" in Table 4 means no result.

[0144] (2) First efficiency test

[0145] ​The batteries prepared from the examples and comparative examples were charged at a charge-discharge rate of 0.2 C to an upper voltage (4.5 V / 4.6 V) at 25°C, and then charged at a charge rate of 0.05 C to the upper voltage (4.5 V / 4.6 V) at constant voltage, and then discharged at a discharge rate of 0.2 C to 3.0 V, and the first charge and discharge capacity was counted, the first efficiency = (first discharge capacity) / (first charge capacity) x 100%, and the test results were recorded in Table 5.

[0146] (3) Gram capacity test

[0147] The batteries prepared from the examples and comparative examples were charged at a charge-discharge rate of 0.2 C to an upper voltage (4.5 V / 4.6 V) at 25°C, and then charged at a charge rate of 0.05 C to the upper voltage (4.5 V / 4.6 V) at constant voltage, and then discharged at a discharge rate of 0.2 C to 3.0 V, and the first charge and discharge capacity was counted, the first efficiency = (first discharge capacity) / (first charge capacity) x 100%, and the test results were recorded in Table 5.

[0148] (4) 45°C high-temperature cycle test

[0149] The batteries prepared from the examples and comparative examples were placed in an environment of (45±2) °C, and stood for 2-3 h, and when the battery body reached (45±2) °C, the batteries were charged at 1 C to an upper cut-off voltage (4.5 V / 4.6 V) at a cut-off current of 0.05 C, and after the batteries were fully charged, they were left for 5 min, and then discharged at 0.7 C to a cut-off voltage of 3.0 V, and the highest discharge capacity of the first 3 cycles was recorded as the initial capacity Q, and when the cycle reached the required number of times, the discharge capacity Q1 of the last cycle of the battery was recorded, and the capacity retention rate was calculated according to the following formula: capacity retention rate (%) = Q1 / Q x 100%, and the results were recorded in Table 5.

[0150] Table 5

[0151]

[0152]

[0153] As can be seen from Table 5, the batteries prepared from the positive electrode sheet of the present application have significantly improved cycle capacity retention rate in the high-voltage system compared with the comparative examples, while maintaining high first efficiency and gram capacity.

[0154] The above describes the preferred embodiments of the present application, 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 the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be considered as disclosed by the present application, and all fall within the protection scope of the present application.

Claims

1. A battery, characterized in that: The battery includes a positive electrode sheet and an electrolyte, wherein a thermogravimetric analysis test curve of the positive electrode sheet includes at least three weight loss intervals at 150° C.-600° C., wherein the first weight loss interval is between 150° C.-350° C., and the second and third weight loss intervals are between 330° C.-600° C.; the weight loss rate in the first weight loss interval is m1, the sum of the weight loss rates in the second and third weight loss intervals is m2, 0.5%≤m1≤3%, 0.5%≤m2≤5%, and 0.1≤m1 / m2≤4; The positive electrode sheet includes a positive electrode current collector and a positive electrode coating on at least one side of the positive electrode current collector, the positive electrode coating includes a positive electrode active material and a conductive agent, the conductive agent includes a first conductive agent and a second conductive agent; the first conductive agent includes carbon nanotubes, and the second conductive agent includes at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, and graphene; The electrolyte includes lithium difluorophosphate.

2. The battery according to claim 1, wherein 0.5% <m1≤3%,0.5%≤m2≤3%,0.3≤m1 / m2<4。 3. The battery according to claim 2, wherein 1%≤m1≤2.5%, 0.8% <m2≤2.5%,0.5≤m1 / m2≤2.5。 4. The battery according to claim 1 or 2, wherein The ratio of the mass of the first conductive agent to the mass of the second conductive agent is 1:(0.1-20).

5. The battery according to claim 4, wherein The ratio of the mass of the first conductive agent to the mass of the second conductive agent is 1:(0.8-8).

6. The battery according to claim 1, wherein The positive electrode active material includes lithium cobalt oxide with a core-shell structure, the core of the core-shell structure includes lithium cobalt oxide doped with a metal element, wherein the doped metal element includes at least one of Al, Mg and M, and M is selected from at least one of Ti, Zr, Y, La, Ni, Mn, Ce and W.

7. The battery according to claim 6, wherein The shell of the core-shell structure includes a compound containing an element L, and L is selected from at least one of Al, Mg, Ti, Zr, Y, La, Ni and Mn.

8. The battery according to claim 7, wherein The compound containing element L includes at least one of L oxide, fluoride and phosphate.

9. The battery according to claim 6, wherein The doping amount of the doped metal element Al is 50ppm-20000ppm; And / or, the doping amount of the doped metal element Mg is 50ppm-5000ppm; And / or, the total doping amount of the doped metal elements Al, Mg and M is 4000ppm-30000ppm; And / or, the doped metal element M includes at least one of Ti, Y and La; And / or, based on the total weight of the lithium cobalt oxide having a core-shell structure, the content of the element L is 0.05-2 wt%; And / or, the ratio of the contents of Al, Mg and L is 1:(0.01-1.5):(0.1-10).

10. The battery according to claim 9, wherein Based on the total weight of the lithium cobalt oxide with a core-shell structure, the content of the element L is 0.1-1.5 wt %.

11. The battery according to claim 1, wherein The positive electrode coating further includes a binder; And / or, based on the total weight of the positive electrode coating, the content of the positive electrode active material is 70-99 weight %, the content of the conductive agent is 0.5-15 weight %, and the content of the binder is 0.5-15 weight %.

12. The battery according to claim 11, wherein Based on the total weight of the positive electrode coating, the content of the positive electrode active material is 95-98.5 weight %, the content of the conductive agent is 0.7-3 weight %, and the content of the binder is 0.8-2 weight %.

13. The battery according to claim 1, wherein The specific surface area of ​​the positive electrode active material is 0.1m 2 / g-0.5m 2 / g; And / or, the particle size Dv50 of the positive electrode active material is 1 μm-50 μm.

14. The battery according to claim 1, wherein The charging cut-off voltage of the battery is 4.55V-4.6V.

15. The battery according to claim 1, wherein Based on the total weight of the electrolyte, the content of the lithium difluorophosphate is 0.01-10 weight %.

16. The battery according to claim 15, wherein Based on the total weight of the electrolyte, the content of the lithium difluorophosphate is 0.5-5% by weight.

17. The battery according to claim 1, wherein The electrolyte further includes an additive, wherein the additive includes at least one of a nitrile compound and fluoroethylene carbonate.

18. The battery according to claim 17, wherein The additives include nitrile compounds and fluoroethylene carbonate.

19. The battery according to claim 17, wherein Based on the total weight of the electrolyte, the content of the nitrile compound is 1-10 weight %, and the content of the fluoroethylene carbonate is 1-20 weight %.

20. The battery according to claim 17, wherein Based on the total weight of the electrolyte, the content of the nitrile compound is 1-10 weight %, and the content of the fluoroethylene carbonate is 1-15 weight %.

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