Positive electrode sheet and battery

Abstract

The present disclosure relates to the field of batteries, and particularly to a positive electrode sheet and a battery comprising the positive electrode sheet. The positive electrode sheet comprises a positive current collector and a positive coating on at least one side surface of the positive current collector, wherein the positive coating comprises a positive active material, a binder and a conductive agent, the conductive agent comprising a first conductive agent and a second conductive agent. A thermogravimetric analysis test curve of the positive electrode sheet comprises at least three weight-loss intervals at 150-600℃, the first weight-loss interval being within 150℃ to 350℃, and the second weight-loss interval and the third weight-loss interval being within 350℃ to 600℃; and the weight-loss percentage of the first weight-loss interval being m1, and the sum of the weight-loss percentage of the second weight-loss interval and the weight-loss percentage of the third weight-loss interval being m2, wherein 0.5％≤m1≤3％, 0.5％≤m2≤5％, and 0.1≤m1 / m2≤4. The positive electrode sheet has a good high-temperature performance and a good high-temperature cycle performance in a high-voltage (the cut-off voltage≥4.45V) system.

Description

Positive electrode and battery Technical Field

[0001] The present disclosure relates to the field of batteries, and in particular to a positive electrode sheet and a battery comprising the positive electrode sheet. Technical Background

[0002] With the development of increasingly thin and lightweight consumer electronic products such as mobile phones and tablets, the energy density of lithium-ion batteries needs to be continuously improved. Lithium cobalt oxide, as a high-energy-density cathode active material, is considered to be an effective way to increase the energy density of lithium-ion batteries by increasing its upper operating voltage and gram capacity. In the future, the demand for lithium cobalt oxide materials will continue to pursue higher voltages, that is, voltages ≥4.5V will become the development direction. However, at high voltages, when the lithium cobalt oxide delithiation content exceeds 75%, an irreversible H1-3 structural phase transition will occur. This irreversible phase transition will cause an increase in the impedance of the positive electrode side. At the same time, lithium cobalt oxide will undergo severe lattice expansion and contraction during high-temperature and high-pressure cycling in a highly delithiated state. The internal stress accumulation of the particles caused by this lattice expansion and contraction can lead to particle cracking, thereby accelerating the deterioration of high-temperature cycling performance.

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

[0004] Summary of the Invention

[0005] The present invention aims to overcome the structural instability of lithium cobalt oxide under high voltage conditions in the prior art and to provide a positive electrode sheet and a battery including the positive electrode sheet. The positive electrode sheet disclosed herein has excellent high-temperature performance, and batteries including the positive electrode sheet disclosed herein have good high-temperature cycling performance in a high-voltage system (cut-off voltage ≥ 4.45V).

[0006] In a first aspect, the present disclosure provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode coating on at least one surface of the positive electrode current collector, wherein the positive electrode coating comprises a positive electrode active material, a binder and a conductive agent, and the conductive agent comprises a first conductive agent and a second conductive agent; a thermogravimetric analysis test curve of the positive electrode sheet comprises at least three weight loss intervals at 150°C-600°C, wherein the first weight loss interval is located at 150°C-350°C, and the second and third weight loss intervals are located at 350°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.

[0007] A second aspect of the present disclosure provides a battery, comprising the positive electrode sheet described in the first aspect of the present disclosure.

[0008] Through the above technical solution, the present disclosure has at least the following advantages over the prior art: the disclosed positive electrode sheet can effectively alleviate the conductive network destruction caused by cracking due to the accumulation of phase change stress during the charge and discharge process of the positive electrode active material lithium cobalt oxide through deep delithiation. This effectively improves the stability of the conductive network, reduces the polarization growth of the positive electrode sheet, improves the structural stability and good conductivity of the positive electrode sheet during the charge and discharge process, and effectively improves the high-temperature performance of the positive electrode sheet. Batteries including this positive electrode sheet have good high-temperature cycling performance in high-voltage systems with a cut-off voltage of 4.45V or higher.

[0009] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG1 shows a thermogravimetric analysis test curve of the positive electrode sheet in one example of the present disclosure.

[0011] FIG2 is a schematic cross-sectional view of the positive electrode sheet according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0012] The following describes the specific embodiments of the present disclosure in detail. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present disclosure and are not intended to limit the present disclosure.

[0013] In a first aspect, the present disclosure provides a positive electrode sheet, comprising a positive electrode current collector and a positive electrode coating on at least one side of the positive electrode current collector, wherein the positive electrode coating comprises a positive electrode active material, a binder, and a conductive agent, wherein the conductive agent comprises a first conductive agent and a second conductive agent; a thermogravimetric analysis test curve of the positive electrode sheet has at least three weight loss intervals at 150°C-600°C, wherein the first weight loss interval is located at 150°C-350°C, and the second weight loss interval and the third weight loss interval are located at 350°C-600°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, and 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 shown in Figure 1, it is a thermogravimetric analysis test curve of the positive electrode sheet described in an example of the present disclosure. It can be seen from the figure that the weight loss rate in the first weight loss interval of 150°C-330°C is 1.19%, and the weight loss rate in the second weight loss interval and the third weight loss interval of 330°C-546°C is 2.22% (wherein, the second weight loss interval is 330°C-431°C, the weight loss rate is 1.22%, and the third weight loss interval is 431°C-546°C, the weight loss rate is 1%).

[0014] The inventors of the present disclosure have discovered that when the thermogravimetric analysis test curve of the positive electrode sheet has a corresponding weight loss change within a specific temperature range, it can effectively improve the conductive network destruction caused by cracking due to deep delithiation of lithium cobalt oxide during the charging and discharging process due to the accumulation of phase change stress, effectively improve the polarization growth caused by the destruction of the conductive network, and thus significantly improve the high-temperature performance of the positive electrode sheet, and improve the high-temperature cycle performance of the battery in a high-voltage system with a cut-off voltage ≥4.45V.

[0015] In one example, 0.5% <m1≤3%,0.5%≤m2≤3%,0.3≤m1 / m2<4。

[0016] In one example, 1%≤m1≤2.5%, 0.8% <m2≤2.5%,0.5≤m1 / m2≤2.5。

[0017] The inventors of the present disclosure discovered 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 charge and discharge 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 a high voltage system.

[0018] In the present disclosure, the positive electrode sheet is subjected to thermogravimetric analysis (TG), and the specific method is as follows: a thermogravimetric analyzer (TGA) is used for testing, the positive electrode coating on the positive electrode sheet is scraped off, the data is weighed and recorded, and then placed in a special crucible and placed inside the instrument, and the test is carried out at a heating rate of 5°C / min, and the test atmosphere is N2.

[0019] In the present disclosure, the positive electrode sheet may include a positive electrode current collector and a positive electrode coating on at least one side of the positive electrode current collector. As shown in Figure 2, a cross-sectional schematic diagram of the positive electrode sheet in an example of the present disclosure is shown, wherein Figure 2(a) shows the case where the positive electrode coating is set on one side; Figures 2(b) and 2(c) show the cases where the positive electrode coating is set 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 of the positive electrode current collector 1; in Figures 2(b) and 2(c), the positive electrode sheet includes a positive electrode current collector 1 and a positive electrode coating 2 on both sides of the positive electrode current collector 1.

[0020] The positive electrode coating may include a positive electrode active material and a conductive agent; the conductive agent may include a first conductive agent and a second conductive agent, the first conductive agent may include carbon nanotubes, and the second conductive agent may include at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite and graphene.

[0021] In the present disclosure, the ratio of the mass of the first conductive agent to the mass of the second conductive agent can be 1:(0.1-20), for example, 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.

[0022] In one example, the ratio of the mass of the first conductive agent to the mass of the second conductive agent is 1:(0.5-10).

[0023] In one example, the ratio of the mass of the first conductive agent to the mass of the second conductive agent is 1:(0.8-8).

[0024] In one example, the conductive agent includes carbon nanotubes and acetylene black.

[0025] In the present disclosure, the mass ratio of the carbon nanotubes to the acetylene black may 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.

[0026] In one example, the mass ratio of the carbon nanotubes to the acetylene black is 1:(0.8-8).

[0027] In the present disclosure, the positive electrode coating may further include a binder. The binder may be selected from binders conventionally used in the art, for example, including at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylate lithium (PAALi), and polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP).

[0028] In the present disclosure, the mass ratio of the binder to 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.

[0029] In one example, the mass ratio of the binder to the conductive agent is (0.3-4):1.

[0030] In one example, the mass ratio of the binder to the conductive agent is (0.5-2.5):1.

[0031] In the present disclosure, based on the total weight of the positive electrode coating, the content of the positive electrode active material can be 70 wt%-99 wt% (for example, 70 wt%, 75 wt%, 80 wt%, 85 wt%, 90 wt%, 95 wt% or 99 wt%), the content of the conductive agent can be 0.5 wt%-15 wt% (for example, 15 wt%, 10 wt%, 5 wt%, 1 wt% or 0.5 wt%), and the content of the binder can be 0.5 wt%-15 wt% (for example, 15 wt%, 10 wt%, 5 wt%, 1 wt% or 0.5 wt%).

[0032] In one example, based on the total weight of the positive electrode coating, the content of the positive electrode active material is 80 wt %-99 wt %, the content of the conductive agent is 0.5 wt %-10 wt %, and the content of the binder is 0.5 wt %-10 wt %.

[0033] In one example, based on the total weight of the positive electrode coating, the content of the positive electrode active material is 95 wt %-98.5 wt %, the content of the conductive agent is 0.7 wt %-3 wt %, and the content of the binder is 0.8 wt %-2 wt %.

[0034] In the present disclosure, the positive electrode active material may include lithium cobalt oxide having 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 may include at least one of Al, Mg and M, M may be selected from at least one of Ti, Zr, Y, La, Ni, Mn, Ce and W, and the shell of the core-shell structure includes a compound containing the element L, L may be selected from at least one of Al, Mg, Ti, Zr, Y, La, Ni and Mn.

[0035] In the present disclosure, the compound containing the element L may include at least one of an oxide, a fluoride, and a phosphate of L.

[0036] Lithium cobalt oxide is a positive electrode active material with a layered structure, in which oxygen ions form a densely packed layer, and cobalt layers and lithium layers are alternately distributed on both sides of the densely packed layer formed by the oxygen ions. As the operating voltage continues to increase (for example, from 4.2V to 4.4V and above), the structure of lithium cobalt oxide (the change in lithium concentration produces structural changes, thereby causing stress to cause microcracks) and the surface (reacting with the electrolyte to cause the dissolution of cobalt) also become more and more unstable, causing local lattice structure collapse and irreversible phase change (including the transformation of layered structure to spinel structure), thereby increasing the impedance of the positive electrode side, thereby affecting the cycle performance of the battery. The inventors of the present disclosure have 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 cobalt oxide 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 may be that: lithium cobalt oxide is doped, and the doping elements can replace cobalt, avoiding the frequent transformation between the layered hexagonal system and the spinel monoclinic system of lithium cobalt oxide during the charge and discharge process, thereby ensuring the stability of the layered structure during the charge and discharge process; further, by controlling the weight loss range and weight loss rate of the thermogravimetric curve of the positive electrode sheet, the impedance of the positive electrode side during the charge and discharge cycle can be effectively reduced, and the polarization generated during the cycle can be effectively reduced, thereby improving the high-temperature cycle performance in the high-voltage system.

[0037] In the present disclosure, the term "high voltage system" has the conventional meaning in the art, and refers to a battery system with a cut-off voltage greater than or equal to 4.45V, such as 4.45V-4.6V.

[0038] In the present disclosure, the doping amount of the doped metal element Al may be 50ppm-20000ppm (e.g., 50ppm, 60ppm, 70ppm, 80ppm, 90ppm, 100ppm, 200ppm, 300ppm, 400ppm, 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 2000ppm, 3000ppm, 4000ppm, 5000ppm, 6000ppm, 7000ppm, 8000ppm, 9000ppm, 10000ppm or 20000ppm), and the doping amount of the doped metal element Mg may be 50ppm-5000ppm. ppm, 600ppm, 700ppm, 80ppm, 90ppm, 100ppm, 200ppm, 300ppm, 400ppm, 500ppm, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 2000ppm, 3000ppm, 4000ppm or 5000ppm), and the total doping amount of the doped metal elements Al, Mg and M can be 4000ppm-30000ppm (for example, 4000ppm, 5000ppm, 6000ppm, 7000ppm, 8000ppm, 9000ppm, 10000ppm, 20000ppm or 30000ppm).

[0039] The inventors of the present disclosure have discovered that when the doping amount of the doped metal element is within a specific 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 specific conditions, a synergistic effect can be produced to further improve the high-temperature performance of the positive electrode sheet.

[0040] In one example, the doping amount of the doped metal element Al is 5000ppm-10000ppm, the doping amount of the doped metal element Mg is 200ppm-3000ppm, and the total doping amount of the doped metal elements Al, Mg and M is 6000ppm-20000ppm.

[0041] In one example, the doping metal element M includes at least one of Ti, Y, and La.

[0042] In the present disclosure, based on the total weight of the lithium cobalt oxide having a core-shell structure, the content of the element L may be 0.05 wt%-2 wt%, for example, 0.05 wt%, 0.1 wt%, 0.15 wt%, 0.18 wt%, 0.25 wt%, 0.5 wt%, 0.7 wt%, 1 wt%, 1.5 wt% or 2 wt%.

[0043] In one example, based on the total weight of the lithium cobalt oxide having a core-shell structure, the content of the element L is 0.1 wt % to 1.5 wt %.

[0044] The inventors of the present disclosure have found that when the ratio of the contents of Al, Mg, and L is within a specific range, the lithium cobalt oxide having a core-shell structure has excellent structural stability.

[0045] In the present disclosure, the ratio of the contents of Al, Mg and L may 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.

[0046] In one embodiment, the ratio of Al, Mg and L is 1:(0.04-0.3):(0.2-1.5).

[0047] In the present disclosure, the types and contents of the doped metal elements and the element L can be obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES).

[0048] In the present disclosure, the specific surface area of ​​the positive electrode active material can be 0.1m 2 / g-0.5m 2 / g (e.g. 0.1m 2 / g, 0.15m 2 / g, 0.2m 2 / g, 0.25m 2 / g, 0.3m 2 / g, 0.35m 2 / g, 0.4m 2 / g, 0.45m 2 / g or 0.5m 2 / g).

[0049] The inventors of the present disclosure have found that the specific surface area of ​​the positive electrode active material is within the above range, which can improve the infiltration of the electrolyte and the conductivity of lithium ions, while reducing the side reaction between the electrolyte and the positive electrode active material under high voltage system.

[0050] In the present disclosure, the particle size Dv50 of the positive active material may be 1 μm-50 μm (e.g., 1 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, or 50 μm).

[0051] In the present disclosure, the specific surface area of ​​the lithium cobalt oxide having a core-shell structure can be 0.1 m 2 / g-0.5m 2 / g (e.g. 0.1m 2 / g, 0.15m 2 / g, 0.2m 2 / g, 0.25m 2 / g, 0.3m 2 / g, 0.35m 2 / g, 0.4m 2 / g, 0.45m 2 / g or 0.5m 2 The particle size Dv50 of the lithium cobalt oxide having a core-shell structure may be 1 μm-50 μm (e.g., 1 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm or 50 μm).

[0052] The thermogravimetric analysis test curve of the positive electrode sheet disclosed herein 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 a high-voltage system with a cut-off voltage ≥4.45V; further, by simultaneously improving the lithium cobalt oxide and the conductive agent (doping and coating the lithium cobalt oxide, and combining conductive agents of different shapes (such as linear conductive agents and point-shaped conductive agents)), the high-temperature cycle performance and rate performance of the battery in a high-voltage system can be improved.

[0053] The present disclosure also provides a method for preparing the lithium cobalt oxide having a core-shell structure, which comprises at least the following steps:

[0054] (A1) mixing a doping metal compound, a cobalt source, and a lithium source and performing a first sintering;

[0055] (A2) The primary sintered particles obtained in step (A1) are mixed with a coating metal compound and subjected to a second sintering.

[0056] The specific selection of materials used in the method and their dosage are the same as those described in the first aspect of the present disclosure and will not be repeated here.

[0057] In the present disclosure, in step (A1), the doping metal compound may include an oxide of a doping metal, wherein the element species of the doping metal include Al, Mg, and M, and M is selected from at least one of Ti, Zr, Y, La, Ni, Mn, Ce, and W. The cobalt source may include Co3O4. The lithium source may include Li2CO3.

[0058] In one example, the dopant metal compound includes Al2O3, MgO, and TiO2.

[0059] In one example, the metal-doped oxide is nano-sized metal-doped oxide.

[0060] In the present disclosure, in step (A1), the ratio of the molar number of Li to the sum of the molar numbers of Co, Al, Mg and M may 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.

[0061] In the present disclosure, in step (A1), the doping amount of the doping metal elements Al, Mg and M may be 4000 ppm-30000 ppm.

[0062] In the present disclosure, in step (A1), the first sintering conditions include heating to 800-1200° C. at a heating rate of 3-10° C. / min and maintaining the temperature for 6-10 hours.

[0063] In the present disclosure, in step (A2), the coating metal compound may include at least one of oxides, fluorides and phosphates of the coating metal; and the element species of the coating metal include at least one of Al, Mg, Ti, Zr, Y, La, Ni and Mn.

[0064] In the present disclosure, the step (A2) may further include crushing and screening the primary sintered particles obtained in the step (1) and then mixing them with the coated metal compound.

[0065] In the present disclosure, in step (A2), the second sintering conditions include: heating to 600-1000° C. at a heating rate of 3-10° C. / min and maintaining the temperature for 6-8 hours.

[0066] In the present disclosure, the step (A2) may further include crushing and screening after the second sintering.

[0067] A second aspect of the present disclosure provides a battery, comprising the positive electrode sheet described in the first aspect of the present disclosure.

[0068] In the present disclosure, the battery can be used in a high voltage system, for example, with a cut-off voltage of 4.45V, 4.47V, 4.48V, 4.5V, 4.52V, 4.53V, 4.55V or 4.6V.

[0069] In the present disclosure, the battery may further include an electrolyte. The electrolyte may include lithium difluorophosphate (LiPO2F2).

[0070] The inventors of the present disclosure have discovered that when the electrolyte includes lithium difluorophosphate, the cycle life of the battery in a high-voltage system can be significantly improved. The reason may be that lithium difluorophosphate can form a surface film with higher ion conductivity on the surface of the negative electrode active material, and can produce a synergistic effect with the positive electrode active material of the present disclosure, thereby improving the stability of the positive electrode sheet interface film, preventing the transition metal of the positive electrode active material (such as lithium cobalt oxide) from dissolving into the negative electrode, improving the structural stability of the positive electrode material, and ensuring good rate performance and cycle performance.

[0071] In the present disclosure, the content of the lithium difluorophosphate can be 0.01 wt%-10 wt%, such as 0.01 wt%, 0.02 wt%, 0.03 wt%, 0.04 wt%, 0.05 wt%, 0.06 wt%, 0.07 wt%, 0.08 wt%, 0.09 wt%, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt% or 10 wt%, based on the total weight of the electrolyte.

[0072] The inventors of the present disclosure have discovered 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.

[0073] In one example, based on the total weight of the electrolyte, the content of the lithium difluorophosphate is 0.5 wt % to 5 wt %.

[0074] The electrolyte may further include an additive, and the additive may include at least one of a nitrile compound and fluoroethylene carbonate.

[0075] The inventors of the present disclosure have discovered that lithium difluorophosphate has poor stability and is more likely to decompose and produce HF. Adding at least one of a nitrile compound and fluoroethylene carbonate as an additive to the electrolyte can improve the problem of poor stability of lithium difluorophosphate. In addition, the combination of the electrolyte and the positive electrode sheet can produce a synergistic effect, which can further improve the high-temperature cycle performance and rate performance of the battery in a high-voltage system. The reason may be that the carbon-nitrogen triple bond in the cyanide group contained in the nitrile compound has a high bond energy and is not easily oxidized. At the same time, the cyanide group has a strong coordination ability and can combine with the high-valent Co ions on the electrode surface to mask the active ions on the surface of lithium cobalt oxide. In addition, the nitrile compound can complex some active high-valent ions on lithium cobalt oxide to form a stable and dense protective film on the surface of the positive electrode, which can inhibit the occurrence of side reactions between the positive electrode and the electrolyte; fluoroethylene carbonate can inhibit the decomposition of the electrolyte and form a stable interface film on the surface of the positive electrode; nitrile compounds, fluoroethylene carbonate and lithium difluorophosphate can produce a synergistic effect, thereby ensuring that the battery has good rate performance and cycle performance.

[0076] In the present disclosure, the nitrile compound may include at least one of 3-methoxypropionitrile, succinonitrile, glutaronitrile, adiponitrile, trans-butenedinitrile, trans-hexenedinitrile, 1,2-bis(cyanoethoxy)ethane, 1,3,6-hexanetrinitrile, and 1,2,3-tris(cyanoethoxy)propane.

[0077] In the present disclosure, based on the total weight of the electrolyte, the content of the nitrile compound can be 1 wt%-10 wt% (e.g., 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt% or 10 wt%), and the content of the fluoroethylene carbonate can be 1 wt%-20 wt% (e.g., 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, 15 wt%, 16 wt%, 17 wt%, 18 wt%, 19 wt% or 20 wt%).

[0078] In one example, based on the total weight of the electrolyte, the content of the nitrile compound is 1 wt % to 10 wt %, and the content of the fluoroethylene carbonate is 1 wt % to 15 wt %.

[0079] In one example, based on the total weight of the electrolyte, the content of the nitrile compound is 2 wt % to 8 wt %, and the content of the fluoroethylene carbonate is 3 wt % to 10 wt %.

[0080] The electrolyte may further include a lithium salt. The lithium salt may include at least one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide.

[0081] In the present disclosure, the electrolyte may further include an organic solvent, and the boiling point of the organic solvent is greater than 100°C.

[0082] In one example, the organic solvent includes at least one of a linear carbonate and a linear carboxylic acid ester 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 carboxylic acid ester includes at least one of ethyl propionate (VP), propyl propionate (PP) and propyl acetate (PA).

[0083] In one example, based on the total volume of the organic solvent, the content of the cyclic carbonate is 20 volume % to 40 volume %, and the content of the linear carbonate and / or the linear carboxylate is 60 volume % to 80 volume %.

[0084] The battery, except for the positive electrode sheet and the electrolyte (such as the negative electrode sheet and the separator, etc.), can be conventionally selected in the art.

[0085] In one example, the battery further includes a negative electrode sheet and a separator.

[0086] The negative electrode sheet may include a negative electrode current collector and a negative electrode coating on at least one side of the negative electrode current collector. The negative electrode coating may include a negative electrode active material, a negative electrode conductor, and a negative electrode binder.

[0087] The negative electrode active material may be a conventional choice 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 oxide.

[0088] The negative electrode conductive agent can be a conventional choice 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 nanotubes, metal powder and carbon fiber.

[0089] The negative electrode binder may be a conventional choice in the art. For example, the negative electrode binder includes at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and polyacrylate lithium (PAALi).

[0090] Based on the total weight of the negative electrode coating, the content of the negative electrode active material can be 70 weight%-99 weight% (for example, 70 weight%, 75 weight%, 80 weight%, 85 weight%, 90 weight%, 95 weight% or 99 weight%), the content of the negative electrode conductor can be 0.5 weight%-15 weight% (for example, 15 weight%, 10 weight%, 5 weight%, 1 weight% or 0.5 weight%), and the content of the negative electrode binder can be 0.5 weight%-15 weight% (for example, 15 weight%, 10 weight%, 5 weight%, 1 weight% or 0.5 weight%).

[0091] In one example, based on the total weight of the negative electrode coating, the content of the negative electrode active material is 80 wt %-98 wt %, the content of the negative electrode conductor is 1 wt %-10 wt %, and the content of the negative electrode binder is 1 wt %-10 wt %.

[0092] The diaphragm can be a conventional choice in the art, for example, the diaphragm includes a substrate layer and a rubber coating layer on at least one side of the substrate layer. The substrate layer may include a polypropylene film. The rubber coating layer may include ceramic particles.

[0093] The battery can be assembled in a conventional manner in the art. For example, the battery preparation method includes the following steps:

[0094] (B1) Preparation of positive electrode sheet: A positive electrode active material, a conductive agent, and a binder are mixed, and a positive electrode solvent is added to form a positive electrode slurry. The positive electrode slurry is coated on a positive electrode current collector, dried, and cold pressed to obtain a positive electrode sheet;

[0095] (B2) Preparation of negative electrode sheet: A negative electrode active material, a negative electrode conductive agent, a negative electrode binder, and a negative electrode thickener are mixed, and a negative electrode solvent is added to form a negative electrode slurry. The negative electrode slurry is coated on a negative electrode current collector, dried, and cold pressed to obtain a negative electrode sheet;

[0096] (B3) Preparation of a battery: The positive electrode sheet and separator obtained in step (B1) and the negative electrode sheet obtained in step (B2) are stacked in order and wound to obtain a bare cell. The bare cell is placed in an outer packaging foil, an electrolyte is injected, and the battery is vacuum-sealed, allowed to stand, formed, and shaped to obtain a battery.

[0097] In the present disclosure, in step (B1), the cathode solvent may include N-methylpyrrolidone.

[0098] In the present disclosure, in step (B1), the solid content of the positive electrode slurry is the conventional positive electrode slurry solid content in the art, for example, 65 wt % to 80 wt %.

[0099] In the present disclosure, in step (B2), the negative electrode solvent may include deionized water.

[0100] In the present disclosure, in step (B2), the solid content of the negative electrode slurry is the conventional solid content of the negative electrode slurry in the art, for example, 40 wt % to 50 wt %.

[0101] In the present disclosure, in step (B1) and step (B2), the drying and the cold pressing are conventional processes in the art.

[0102] In the present disclosure, in step (B3), the separator is placed between the positive electrode sheet and the negative electrode sheet to play a role of isolation.

[0103] It should be noted that the numerical expressions such as "first" and "second" in the present disclosure are only used to distinguish different substances or usage methods, and do not represent a difference in order.

[0104] The present disclosure will be described in detail below through examples. The examples described in this disclosure are only a portion of the examples of the present disclosure, not all of the examples. Based on the examples in this disclosure, all other examples obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this disclosure.

[0105] In the following examples, unless otherwise specified, all materials used were commercially available analytical grade.

[0106] Preparation Example I1

[0107] The positive electrode active material was prepared as follows:

[0108] Co3O4, nano-sized Al2O3, nano-sized MgO and nano-sized Y2O3 were weighed according to a mass ratio of Co:Al:Mg:Y of 6013:75:15:40, and Li2CO3 was added and mixed evenly, wherein the ratio of the molar number of Li to the sum of the molar numbers of Co, Al, Mg and Y was 1.05:1, that is, n(Li):n(Co+Al+Mg+Y)=1.05:1, and a first sintering was performed, wherein the conditions of the first sintering included: heating at 5°C for 10 minutes; / min from room temperature (25 ° C) to 1000 ° C and keep the temperature constant for 8 hours, and naturally cool to obtain primary sintered particles; after the above-mentioned primary sintered particles are crushed and sieved, they are mixed with ZrO2, wherein the mass ratio of Co to Zr is 60.1:1, and a second sintering is carried out, wherein the conditions of the second sintering include: heating from 25 ° C to 800 ° C at a heating rate of 5 ° C / min and keeping the temperature constant for 7 hours, crushing and sieving to obtain positive electrode active material.

[0109] Preparation Example I2

[0110] The positive electrode active material was prepared as follows:

[0111] Co3O4, nano-sized Al2O3, nano-sized MgO and nano-sized TiO2 were weighed according to the mass ratio of Co:Al:Mg:Ti being 6013:50:2:8, and Li2CO3 was added and mixed evenly, wherein the ratio of the molar number of Li to the molar number of the sum of Co, Al, Mg and Ti was 1.05:1, i.e. n(Li):n(Co+Al+Mg+Ti)=1.05:1, and a first sintering was performed, wherein the conditions of the first sintering include: The temperature was raised from room temperature (25°C) to 800°C at a heating rate of 5°C / min and maintained at this temperature for 6 hours, and then naturally cooled to obtain primary sintered particles; the primary sintered particles were crushed and sieved, and then mixed with Y2O3, wherein the mass ratio of Co to Y was 601:1, and a second sintering was performed, wherein the conditions of the second sintering included: raising the temperature from 25°C to 600°C at a heating rate of 5°C / min and maintaining this temperature for 6 hours, and then crushing and sieving to obtain a positive electrode active material.

[0112] Preparation Example I3

[0113] The positive electrode active material was prepared as follows:

[0114] Co3O4, nano-sized Al2O3, nano-sized MgO and nano-sized La2O3 were weighed according to a mass ratio of Co:Al:Mg:La of 601.3:10:3:7, and Li2CO3 was added and mixed uniformly, wherein the ratio of the molar number of Li to the sum of the molar numbers of Co, Al, Mg and La was 1.05:1, that is, n(Li):n(Co+Al+Mg+La)=1.05:1, and a first sintering was performed, wherein the conditions of the first sintering included: heating at 10°C / min from room temperature (25 ° C) to 1200 ° C and maintain the temperature for 10 hours, and naturally cool to obtain primary sintered particles; after the above-mentioned primary sintered particles are crushed and sieved, they are mixed with Al2O3, wherein the mass ratio of Co to Al is 60.1:1.5, and a second sintering is carried out, wherein the conditions of the second sintering include: heating from 25 ° C to 1000 ° C at a heating rate of 10 ° C / min and maintaining the temperature for 8 hours, crushing and sieving, to obtain a positive electrode active material.

[0115] Preparation Examples I4-I6 were prepared in the same manner as Preparation Example I1, except that the doping amounts of Al, Mg, and Y were varied, and the type and content of the coating elements were varied. The specific parameters are shown in Table 1. The particle size Dv50 of the positive electrode active materials prepared in these preparation examples was 1 μm-50 μm, and the specific surface area was 0.1 m 2 / g-0.5m 2 / g.

[0116] Table 1

[0117] Preparation Example II

[0118] Prepare the electrolyte as follows:

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

[0120] Preparation Examples II2-II8 were carried out with reference to Preparation Example II1, except that the mass of LiPO2F2, the mass of the nitrile compound and the mass of fluoroethylene carbonate added to the electrolyte were changed, as shown in Table 2.

[0121] Table 2 Note: “-” in Table 2 means no addition.

[0122] Example

[0123] Prepare the battery as follows:

[0124] (1) Preparation of positive electrode sheet

[0125] The positive electrode active material obtained in Preparation Example I1, the first conductive agent, the second conductive agent, and polyvinylidene fluoride (PVDF) were uniformly mixed in a certain mass ratio, and N-methylpyrrolidone was added to form a positive electrode slurry with a solid content of 75 wt %. The positive electrode slurry was coated on the surface of aluminum foil, dried, and cold pressed to obtain a positive electrode sheet.

[0126] (2) Preparation of negative electrode sheet

[0127] Artificial graphite (particle size Dv50: 13±1 μm, degree of graphitization 94±0.5%, secondary particles mixed with single particles, wherein the mass proportion of secondary particles is 50%), acetylene black, styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC-Na) are uniformly mixed in a mass ratio of 95:2:2:1, deionized water is added, wherein the solid content is 45 wt% to form a negative electrode slurry, and the above negative electrode slurry is coated on the surface of copper foil, dried, and cold pressed to obtain a negative electrode sheet;

[0128] (3) Preparation of batteries

[0129] The positive electrode sheet prepared in step (1), the separator (PE porous polymer film), and the negative electrode sheet prepared in step (2) were stacked in order, with the separator positioned between the positive and negative electrodes to provide isolation. The cells were wound to obtain a bare cell. The bare cell was placed in an outer packaging foil, dried, and then injected with the electrolyte prepared in Group II. The cells were vacuum packaged, allowed to stand, formed, and shaped to obtain a battery. See Table 3 for details.

[0130] Table 3 Note: “-” in Table 3 means no addition.

[0131] Test Case

[0132] (1) Thermogravimetric analysis

[0133] The positive electrode sheets prepared in the examples and comparative examples were subjected to thermogravimetric analysis tests, and the results are recorded in Table 4, wherein the thermogravimetric analysis test curve of Example 2 is shown in FIG1 .

[0134] Table 4 Note: “-” in Table 4 means no result.

[0135] (2) First efficiency test

[0136] The batteries prepared in the examples and comparative examples were charged at 25°C at a constant current of 0.2C to the upper limit voltage (4.5V / 4.6V), and then charged at a constant voltage of 0.05C to the upper limit voltage (4.5V / 4.6V); then discharged at a discharge rate of 0.2C to 3.0V, and the initial charge and discharge capacities were calculated. The initial efficiency = (initial discharge capacity) / (initial charge capacity) × 100%. The test results are recorded in Table 5.

[0137] (3) Gram capacity test

[0138] The batteries prepared in the examples and comparative examples were charged at 25°C at a constant current of 0.2C to the upper limit voltage (4.5V / 4.6V), then charged at a constant voltage of 0.05C to the upper limit voltage (4.5V / 4.6V), and then discharged at a discharge rate of 0.2C to 3.0V. The discharge capacity was calculated as gram capacity = (discharge capacity) / (weight of positive electrode active material). The results are recorded in Table 5.

[0139] (4) 45℃ high temperature cycle test

[0140] The batteries prepared in the examples and comparative examples were placed in a (45±2)°C environment and allowed to stand for 2h-3h. When the battery body reached (45±2)°C, the battery was charged at a constant current of 1C to the upper cut-off voltage (4.5V / 4.6V) with a cut-off current of 0.05C. After the battery was fully charged, it was placed for 5min and then discharged at a constant current of 0.7C to a cut-off voltage of 3.0V. The highest discharge capacity of the first three cycles was recorded as the initial capacity Q. When the cycle reached the required number of times, the last discharge capacity Q1 of the battery was recorded. The capacity retention rate was calculated as follows: Capacity retention rate (%) = Q1 / Q×100%. The results are recorded in Table 5.

[0141] Table 5

[0142] As can be seen from Table 5, compared with the comparative example, the battery prepared by the positive electrode sheet of the present invention significantly improves the cycle capacity retention rate of the battery in a high voltage system while maintaining a relatively high first efficiency and gram capacity.

[0143] The preferred embodiments of the present disclosure are described in detail above, but the present disclosure is not limited thereto. Within the technical concept of the present disclosure, various simple variations of the technical solution of the present disclosure may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed by the present disclosure and fall within the scope of protection of the present disclosure.

Claims

1. A positive electrode sheet, characterized in that: The positive electrode sheet includes a positive electrode collector and a positive electrode coating on at least one side of the positive electrode collector, the positive electrode coating includes a positive electrode active material, a binder and a conductive agent, and the conductive agent includes a first conductive agent and a second conductive agent; the thermogravimetric analysis test curve of the positive electrode sheet includes at least three weight loss intervals at 150°C-600°C, the first weight loss interval is located at 150°C-350°C, and the second weight loss interval and the third weight loss interval are located at 350°C-600°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.

2. The positive electrode sheet according to claim 1, wherein: 0.5% <m1≤3%,0.5%≤m2≤3%,0.3≤m1 / m2<4; Preferably, 1%≤m1≤2.5%, 0.8% <m2≤2.5%,0.5≤m1 / m2≤2.5。 3. The positive electrode sheet according to claim 1 or 2, wherein: 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; Preferably, the conductive agent includes carbon nanotubes and acetylene black.

4. The positive electrode sheet according to any one of claims 1 to 3, wherein: The ratio of the mass of the first conductive agent to the mass of the second conductive agent is 1:(0.1-20); Preferably, the ratio of the mass of the first conductive agent to the mass of the second conductive agent is 1:(0.8-8).

5. The positive electrode sheet according to any one of claims 1 to 4, wherein: The positive electrode active material includes lithium cobalt oxide having a core-shell structure, wherein 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; Preferably, the shell of the core-shell structure comprises a compound containing an element L, and L is selected from at least one of Al, Mg, Ti, Zr, Y, La, Ni and Mn; Preferably, the compound containing element L includes at least one of L oxide, fluoride and phosphate.

6. The positive electrode sheet according to claim 5, wherein: The doping amount of the doped metal element Al is 50ppm-20000ppm, preferably 5000ppm-10000ppm; And / or, the doping amount of the doped metal element Mg is 50ppm-5000ppm, preferably 200ppm-3000ppm.

7. The positive electrode sheet according to claim 5 or 6, wherein: The total doping amount of the doped metal elements Al, Mg and M is 4000ppm-30000ppm, preferably 6000ppm-20000ppm; And / or, the doped metal element M includes at least one of Ti, Y and La.

8. The positive electrode sheet according to any one of claims 5 to 7, wherein: Based on the total weight of the lithium cobalt oxide having a core-shell structure, the content of the element L is 0.05 wt %-2 wt %, preferably 0.1 wt %-1.5 wt %; And / or, the ratio of the content of Al, Mg and L is 1:(0.01-1.5):(0.1-10), preferably 1:(0.04-0.3):(0.2-1.5).

9. The positive electrode sheet according to any one of claims 1 to 8, wherein: Based on the total weight of the positive electrode coating, the content of the positive electrode active material is 70 wt %-99 wt %, the content of the conductive agent is 0.5 wt %-15 wt %, and the content of the binder is 0.5 wt %-15 wt %; Preferably, based on the total weight of the positive electrode coating, the content of the positive electrode active material is 95 wt %-98.5 wt %, the content of the conductive agent is 0.7 wt %-3 wt %, and the content of the binder is 0.8 wt %-2 wt %.

10. The positive electrode sheet according to any one of claims 1 to 9, wherein: The specific surface area of ​​the positive electrode active material is 0.1 m 2 / g-0.5m 2 / g; And / or, the particle size Dv50 of the positive electrode active material is 1 μm-50 μm.

11. A battery, characterized in that: The battery comprises the positive electrode sheet according to any one of claims 1 to 10.

12. The battery according to claim 11, wherein The battery also includes an electrolyte, and the electrolyte includes lithium difluorophosphate; Preferably, based on the total weight of the electrolyte, the content of the lithium difluorophosphate is 0.01 wt%-10 wt%; preferably 0.5 wt%-5 wt%.

13. The battery according to claim 11 or 12, wherein The electrolyte further includes an additive, wherein the additive includes at least one of a nitrile compound and fluoroethylene carbonate; Preferably, the additive comprises a nitrile compound and fluoroethylene carbonate; Preferably, the nitrile compound includes at least one of 3-methoxypropionitrile, succinonitrile, glutaronitrile, adiponitrile, trans-butenedinitrile, trans-hexenedinitrile, 1,2-bis(cyanoethoxy)ethane, 1,3,6-hexanetrinitrile and 1,2,3-tris(cyanoethoxy)propane.

14. The battery according to claim 13, wherein Based on the total weight of the electrolyte, the content of the nitrile compound is 1% to 10% by weight, and the content of the fluoroethylene carbonate is 1% to 20% by weight; Preferably, based on the total weight of the electrolyte, the content of the nitrile compound is 1 wt%-10 wt%, and the content of the fluoroethylene carbonate is 1 wt%-15 wt%; Preferably, based on the total weight of the electrolyte, the content of the nitrile compound is 2-8% by weight, and the content of the fluoroethylene carbonate is 3-10% by weight.

15. The battery according to any one of claims 11 to 14, wherein: The electrolyte includes a lithium salt; Preferably, the lithium salt comprises at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide; And / or, the electrolyte includes an organic solvent, and the boiling point of the organic solvent is greater than 100°C.

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