Battery

Abstract

The invention relates to the technical field of batteries, in particular to a battery, which comprises a positive plate, a negative plate and a diaphragm, the diaphragm comprises a porous carrier, a first heat-resistant layer and a second heat-resistant layer, the first heat-resistant layer and the second heat-resistant layer are respectively positioned on two oppositely arranged functional surfaces of the porous carrier, the first heat-resistant layer comprises organic filler particles, and the second heat-resistant layer comprises organic filler particles. The organic filler particles comprise an element N and / or an element P, the second heat-resistant layer comprises heat-absorbing microspheres, the diaphragm is located between the positive plate and the negative plate, the positive plate is opposite to one side of the first heat-resistant layer, and the negative plate is opposite to one side of the second heat-resistant layer. The battery provided by the invention comprises the diaphragm, and the diaphragm can effectively inhibit the thermal runaway phenomenon of the battery and can improve the thermal stability of the positive electrode material.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a battery. Background Art

[0002] With the rapid development of high-energy-density batteries, the issue of thermal stability in high-temperature applications has become a serious challenge facing the industry. Currently, widely used polyolefin-based separators (such as PE and PP) are prone to thermal shrinkage deformation under high-temperature conditions, which may lead to safety hazards such as electrode contact short circuits. Mainstream cathode material systems (such as nickel-cobalt-manganese ternary NCM and lithium cobalt oxide (LCO)) are prone to crystal structure distortion during high-temperature charging and discharging, often leading to irreversible capacity decay. Moreover, when the battery is thermally abused during use, the internal temperature rises to 120°C or above, and the positive, negative electrodes and electrolyte react violently, releasing a large amount of heat and forming hot gas internally. The temperature will also rise sharply within seconds, causing the battery to burn or even explode.

[0003] In the existing technology, thermal stability is mainly improved by preparing inorganic ceramic coating composites or organic composite membranes. However, conventional membrane modification technology is difficult to achieve the active regulation function of the internal heat distribution of the battery, and has little effect on improving the thermal runaway phenomenon of the battery. In addition, an effective inhibition mechanism has not yet been established for the structural degradation problem caused by the high-temperature phase change of the positive electrode material. Summary of the Invention

[0004] In view of this, in order to solve the above-mentioned technical problems existing in the prior art, the present invention provides a battery, which includes a positive electrode sheet, a negative electrode sheet and a separator. The separator of the present invention can effectively inhibit thermal runaway of the battery and improve the thermal stability of the positive electrode material.

[0005] The present invention provides a battery, the battery comprising a positive electrode sheet, a negative electrode sheet and a separator, the separator comprising a porous carrier, a first heat-resistant layer and a second heat-resistant layer, the first heat-resistant layer and the second heat-resistant layer being respectively located on two functional surfaces of the porous carrier arranged opposite to each other, the first heat-resistant layer comprising organic filler particles, the composition of the organic filler particles comprising element N and / or element P, the second heat-resistant layer comprising heat-absorbing microspheres, the separator being located between the positive electrode sheet and the negative electrode sheet, the positive electrode sheet being opposite to one side of the first heat-resistant layer, and the negative electrode sheet being opposite to one side of the second heat-resistant layer On the other hand, there are three endothermic characteristic peaks in the differential scanning calorimetry spectrum of the diaphragm, namely the first characteristic peak, the second characteristic peak and the third characteristic peak. The peak temperature of the first characteristic peak is 90°C-120°C, the peak temperature of the second characteristic peak is 125°C-180°C, and the peak temperature of the third characteristic peak is 200°C-450°C. The diaphragm also satisfies the following relationship: J3≥J1+3×J2, J2≥5×J1, wherein J1 is the area of the first characteristic peak, J2 is the area of the second characteristic peak, and J3 is the area of the third characteristic peak.

[0006] Through the above technical solution, the present invention has at least the following advantages compared with the prior art:

[0007] The present invention realizes an active regulation mechanism of the internal heat distribution of the battery by constructing a composite diaphragm structure including a porous carrier, a first heat-resistant layer and a second heat-resistant layer: first, by arranging a second heat-resistant layer including heat-absorbing microspheres on the functional surface of the porous carrier opposite to the negative electrode sheet, it can effectively absorb the heat generated in the initial stage of thermal runaway, and can effectively inhibit the heat release of the side reaction between the negative electrode and the electrolyte, and slow down the reaction process of thermal runaway; secondly, the porous carrier will produce closed pores under high temperature environment, which can effectively reduce the electrochemical reaction rate of the battery under high temperature environment, block the electron penetration and internal short-circuit current between the positive and negative electrodes, directly cut off the chain reaction process of thermal runaway, and further reduce the risk of thermal runaway of the battery; finally, by arranging a first heat-resistant layer including organic filler particles on the functional surface of the porous carrier opposite to the positive electrode sheet, it can improve the thermal stability of the positive electrode material, inhibit the secondary heat generation caused by the lattice oxygen precipitation of the active material at high temperature, thereby systematically reducing the risk of thermal runaway of the battery.

[0008] Other features and advantages of the present invention will be described in detail in the following detailed description.

[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] Figure 1 Shown is a schematic diagram of the diaphragm structure provided by an example of the present invention.

[0011] Figure 2 Shown is a differential scanning calorimetry spectrum of a membrane provided by an embodiment of the present invention.

[0012] Figure 3 Shown is a differential scanning calorimetry spectrum of a separator provided by a comparative example of the present invention.

[0013] Description of the reference numerals: porous carrier 1 , first heat-resistant layer 2 , second heat-resistant layer 3 . DETAILED DESCRIPTION

[0014] The specific embodiments of the present invention are described in detail below. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention. In this article, unless otherwise specified, data ranges include endpoints.

[0015] 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.

[0016] The present invention provides a battery, the battery comprising a positive electrode sheet, a negative electrode sheet and a separator, the separator comprising a porous carrier, a first heat-resistant layer and a second heat-resistant layer, the first heat-resistant layer and the second heat-resistant layer being respectively located on two functional surfaces of the porous carrier arranged opposite to each other, the first heat-resistant layer comprising organic filler particles, the composition of the organic filler particles comprising element N and / or element P, the second heat-resistant layer comprising heat-absorbing microspheres, the separator being located between the positive electrode sheet and the negative electrode sheet, the positive electrode sheet being opposite to one side of the first heat-resistant layer, and the negative electrode sheet being opposite to one side of the second heat-resistant layer On the other hand, there are three endothermic characteristic peaks in the differential scanning calorimetry spectrum of the diaphragm, namely the first characteristic peak, the second characteristic peak and the third characteristic peak. The peak temperature of the first characteristic peak is 90°C-120°C, the peak temperature of the second characteristic peak is 125°C-180°C, and the peak temperature of the third characteristic peak is 200°C-450°C. The diaphragm also satisfies the following relationship: J3≥J1+3×J2, J2≥5×J1, wherein J1 is the area of the first characteristic peak, J2 is the area of the second characteristic peak, and J3 is the area of the third characteristic peak.

[0017] like Figure 1 The figure shows a schematic diagram of the diaphragm structure provided by an example of the present invention, wherein the diaphragm includes a porous carrier 1, a first heat-resistant layer 2, and a second heat-resistant layer 3. The first heat-resistant layer 2 and the second heat-resistant layer 3 are respectively located on two functional surfaces of the porous carrier 1 that are opposite to each other. The functional surface can be understood as the surface with the largest area in the porous carrier 1, which is located on two opposite sides of the porous carrier 1. The first heat-resistant layer 2 is located on one functional surface of the opposite side, and the second heat-resistant layer 3 is located on the other functional surface of the opposite side. The first heat-resistant layer 2 and the second heat-resistant layer 3 are connected to the porous carrier 1 through the functional surface. In addition, it should be noted that the thickness of the first heat-resistant layer 2 and the second heat-resistant layer 3 can be the same or different.

[0018] The separator is located between the positive electrode sheet and the negative electrode sheet, with the positive electrode sheet facing one side of the first heat-resistant layer, and the negative electrode sheet facing one side of the second heat-resistant layer. Specifically, the positive electrode sheet directly faces the side of the separator with the first heat-resistant layer (i.e., the side away from the porous carrier), while the negative electrode sheet directly faces the side of the separator with the second heat-resistant layer. This means that from the positive electrode sheet to the negative electrode sheet, the positive electrode sheet, the first heat-resistant layer 2, the porous carrier 1, the second heat-resistant layer 3, and finally the negative electrode sheet are in the order of the positive electrode sheet, the separator, and the negative electrode sheet together form the battery structure of the present invention.

[0019] There are three endothermic characteristic peaks in the differential scanning calorimetry (DSC) spectrum of the membrane, such as Figure 2As shown, they are the first characteristic peak, the second characteristic peak and the third characteristic peak, respectively. The peak temperature of the first characteristic peak is 90°C-120°C (for example, 90°C, 100°C, 110°C or 120°C), the peak temperature of the second characteristic peak is 125°C-180°C (for example, 125°C, 130°C, 140°C, 150°C, 160°C, 170°C or 180°C), the peak temperature of the third characteristic peak is 200°C-450°C (200°C, 210°C, 220°C, 230°C, 240°C, 25 0℃, 260℃, 270℃, 280℃, 290℃, 300℃, 310℃, 320℃, 330℃, 340℃, 350℃, 360℃, 370℃, 380℃, 390℃, 400℃, 410℃, 420℃, 430℃, 440℃ or 450℃), the diaphragm simultaneously satisfies the following relationship: J3≥J1+3×J2, J2≥5×J1, wherein J1 is the area of the first characteristic peak, J2 is the area of the second characteristic peak, and J3 is the area of the third characteristic peak.

[0020] In the present invention, the differential scanning calorimetry (DSC) spectrum refers to a curve of the heat flow rate of the membrane during the programmed temperature increase process recorded by a differential scanning calorimeter as a inverse of the temperature. The heat flow rate indicates the rate at which the membrane absorbs or releases heat. The heat flow rate is the ordinate of the differential scanning calorimetry (DSC) spectrum, and the unit is milliwatt (mW). The abscissa of the differential scanning calorimetry (DSC) spectrum is the temperature, and the unit is degrees Celsius (°C). There are three endothermic characteristic peaks in the differential scanning calorimetry (DSC) spectrum of the membrane, indicating that the membrane has three temperature ranges of energy absorption during the heating process. The heat flow rate of the membrane in this temperature range is It will increase to a certain peak and then drop, indicating that the diaphragm undergoes physical or chemical changes during the heating process and generates heat, which are the first characteristic peak, the second characteristic peak and the third characteristic peak respectively. The peak temperature of the first characteristic peak is 90℃-120℃, that is, when the heat flow rate of the diaphragm increases to the first peak, the corresponding diaphragm temperature distribution range is within 90℃-120℃; the meaning of the peak temperature of the second characteristic peak and the third characteristic peak is the same; the peak temperature range of the three characteristic peaks can reflect the diaphragm's participation in the thermal reaction process of the battery at the corresponding temperature stage and the change in the state of the diaphragm material, such as the melting, film rupture, and thermal decomposition of the diaphragm. Moreover, the area J1 of the first characteristic peak, the area J2 of the second characteristic peak, and the area J3 of the third characteristic peak all satisfy the following relationship: J3≥J1+3×J2, J2≥5×J1. The areas of the characteristic peaks are obtained through calculus and are proportional to the heat absorbed by the diaphragm during phase change or other changes. They can be used to quantitatively analyze the heat changes in the three temperature ranges corresponding to the three characteristic peaks. Therefore, the relationship between the peak areas of the three characteristic peaks can also be understood as the relationship between the heat change values of the diaphragm in different temperature ranges.

[0021] In the present invention, the steps of the differential scanning calorimetry (DSC) measurement method of the diaphragm are as follows: First, use an Open Al crucible as a sample crucible, stabilize the sample crucible at room temperature and clear it, then take it out, disassemble the diaphragm from the battery cell, add 5 mg of the diaphragm sample to the above crucible, and buckle the edge of the crucible; then, gently place the crucible containing the sample on the stand, call the latest calibration baseline, enter the sample number, sample name, sample weight according to the software process, edit the heating rate and heating range, save the file name, confirm that the input parameters are correct, and click Start to start the heating test of the diaphragm sample using a differential scanning calorimeter. Among them, the instrument sets the heating rate to 10 ° C / min, the test temperature is 25 ° C to 600 ° C, and the peak temperature, characteristic peak area and other parameters in the differential scanning calorimetry (DSC) can be obtained by analyzing the measured differential scanning calorimetry spectrum using software technology.

[0022] The present invention realizes an active regulation mechanism of internal heat distribution of the battery by constructing a composite diaphragm structure comprising a porous carrier, a first heat-resistant layer and a second heat-resistant layer: first, when the diaphragm is heated to about 105°C, thermal runaway enters the initial stage, and the heat-absorbing microspheres contained in the second heat-resistant layer enter a molten state, and absorb the heat of the diaphragm by undergoing phase change, thereby reducing the overall temperature of the battery in the initial stage, and the heat-absorbing microspheres release anionic groups when the phase change occurs. The second heat-resistant layer comprising the heat-absorbing microspheres is arranged on a functional surface opposite to the negative electrode sheet on one side of the porous carrier, so that these anionic groups can preferentially establish an elastic insulating layer at the negative electrode sheet-diaphragm interface, thereby inhibiting the heat release of the side reaction between the negative electrode and the electrolyte, and the insulating layer can block the local diffusion of lithium ions, and has a certain blocking effect on lithium ions in a high temperature state, thereby effectively slowing down the reaction process of thermal runaway. At this time, the air permeability value of the diaphragm is ≥250s, and this process corresponds to the first characteristic peak in the differential scanning calorimetry (DSC) spectrum; secondly When the diaphragm is heated to 125°C or above, micropores in the porous carrier begin to close, which will cause a significant increase in the lithium ion conduction impedance, thereby effectively reducing the electrochemical reaction rate of the diaphragm in a high temperature environment, blocking the electron penetration and internal short-circuit current between the positive and negative electrodes, and directly cutting off the chain reaction process of thermal runaway, further reducing the risk of thermal runaway of the battery. This process corresponds to the second characteristic peak in the differential scanning calorimetry (DSC) spectrum. Finally, when the diaphragm is heated to 200°C or above, a first heat-resistant layer containing organic filler particles is provided on the functional surface opposite to the positive electrode sheet on the porous carrier. The groups of element N and / or element P contained in the organic filler particles contained in the first heat-resistant layer will begin to diffuse to the surface of the positive electrode material in the positive electrode sheet, thereby improving the stability of the positive electrode material and inhibiting the secondary heat generation caused by the lattice oxygen precipitation of the active material at high temperature, thereby systematically reducing the risk of thermal runaway of the battery. This process corresponds to the third characteristic peak in the differential scanning calorimetry (DSC) spectrum. The composite diaphragm structure of the porous carrier, the first heat-resistant layer and the second heat-resistant layer of the present invention respectively targets the thermal reaction process of the diaphragm material at different temperatures, and synergistically slows down the thermal runaway reaction process in the initial stage, cuts off the occurrence of subsequent chain reactions, and can simultaneously improve the stability of the positive electrode material, control the continued heat generation of the active substance at high temperature, and systematically slow down the reaction process of the battery from heating to thermal runaway, which can effectively inhibit the thermal runaway phenomenon of the battery and improve the thermal stability of the positive electrode material.

[0023] Furthermore, the present invention can effectively block the degree of thermal reaction at different stages of battery thermal runaway by controlling the peak temperature of the endothermic characteristic peaks generated in the above three temperature intervals within an appropriate range, significantly prolong the time from heat rise to combustion of the battery, and delay the occurrence of battery thermal runaway; and, by further regulating the relationship between the peak areas of the three endothermic characteristic peaks so that J3≥J1+3×J2 and J2≥5×J1 are satisfied at the same time, the heat generation and heat state of the diaphragm at different stages can be different. When this range relationship is met, the thermal stability of the positive electrode material can be maximized, the heat source from the electrolyte and the positive electrode reaction can be effectively reduced, and the occurrence of thermal runaway can be further alleviated.

[0024] In this invention, by constructing a composite separator structure comprising a porous support, a first heat-resistant layer, and a second heat-resistant layer, an active regulation mechanism for the internal heat distribution of the battery is achieved. Compared with existing technologies, this can effectively suppress thermal runaway in the battery and improve the thermal stability of the positive electrode material. To further enhance the effect, one or more of these technical features may be further optimized.

[0025] In one embodiment, there is a fourth characteristic peak in the differential scanning calorimetry spectrum of the diaphragm, such as Figure 2 As shown, the peak temperature of the fourth characteristic peak is 450° C.-550° C., which is the decomposition peak of the carbonized layer of the organic filler particles in the diaphragm.

[0026] In one embodiment, the thickness of the first heat-resistant layer is greater than the thickness of the second heat-resistant layer. By designing the first heat-resistant layer to be thicker than the second heat-resistant layer, the heat conduction rate of the second heat-resistant layer on the negative electrode side of the separator can be improved at high temperatures, further suppressing the exothermic side reaction between the negative electrode and the electrolyte, thereby effectively slowing the progression of thermal runaway reactions.

[0027] In a specific embodiment, the diaphragm satisfies the following relationship at the same time: 2×D2≤D1≤4×D2 (D1 can be 2×D2, 2.1×D2, 2.2×D2, 2.3×D2, 2.4×D2, 2.5×D2, 2.6×D2, 2.7×D2, 2.8×D2, 2.9×D2, 3×D2, 3.1×D2, 3.2×D2, 3.3×D2, 3.4×D2, 3.5×D2, 3.6×D2, 3.7×D2, 3.8×D2 D2, 3.9×D2 or 4×D2), 0.5≤H1-H2≤1.5 (H1-H2 can be 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4 or 1.5), wherein D1 is the average particle size of the organic filler particles, in μm, D2 is the average particle size of the heat-absorbing microspheres, in μm, H1 is the thickness of the first heat-resistant layer, in μm, and H2 is the thickness of the second heat-resistant layer, in μm.

[0028] The average particle size of organic filler particles and the average particle size of heat-absorbing microspheres can be tested and obtained by the following method: taking the average particle size of heat-absorbing microspheres as an example, the diaphragm is disassembled from the battery cell, and a scanning image of the surface of the second heat-resistant layer of the diaphragm is obtained by SEM. On the obtained scanning image, a square or rectangle with the smallest area that completely surrounds one heat-absorbing microsphere is depicted, that is, a square or rectangle with the edge of the heat-absorbing microsphere connected to the four sides of the square or rectangle is depicted, and the length of one side of the square or the length of the long side of the rectangle is the particle size of the heat-absorbing microsphere. In an arbitrarily selected 100μm*100μm area on the surface of the second heat-resistant layer, the particle size of any 100 heat-absorbing microspheres is measured, and the number average is the average particle size; repeat the above operation 5 times, and the average value is the average particle size of the heat-absorbing microspheres. It should be noted that when 100 endothermic microspheres can be observed in the captured image, the number average of the particle sizes of any 100 endothermic microspheres in the image is defined as the average particle size of the endothermic microspheres. If 100 endothermic microspheres are not observed in the image, multiple images are captured and the number average of the particle sizes of a total of 100 first endothermic microspheres is defined as the average particle size. The scanned image can be obtained by observing the surface of the second heat-resistant layer using an electrolytic emission scanning electron microscope (S-3400N manufactured by Hitachi, Ltd.).

[0029] The thickness of the first heat-resistant layer and the thickness of the second heat-resistant layer can be tested by the following method: disassemble the diaphragm from the battery cell, cut the diaphragm with an argon ion grinder, and make a CP cross-section sample. The cross-sectional height of the diaphragm heat-resistant layer can be observed under a microscope, and the average value after measuring 5 points is taken, which is the thickness of the heat-resistant layer.

[0030] The design of asymmetric thickness of heat-resistant layers on both sides of the diaphragm and the thickness difference range of heat-resistant layers of different thicknesses, combined with the appropriate combined particle size of inorganic filler particles and heat-absorbing microspheres, can regulate the porosity distribution of the heat-resistant layers on both sides of the diaphragm in the thickness direction, thereby affecting the wetting effect of the electrolyte, increasing the transmission impedance of lithium ions in the diaphragm, and increasing the swelling of the porous carrier material at high temperature and melting and closing the pores due to heat, further increasing the ion transmission impedance in the diaphragm, reducing the speed of lithium ion conduction in the diaphragm under high temperature environment (≥105℃), thereby inhibiting the occurrence of thermal runaway.

[0031] In addition, when the internal reaction temperature of the battery rises above 150°C, due to the diffusion of material groups in the first heat-resistant layer and the second heat-resistant layer, the molecular thermal motion caused by the temperature increase will lead to an increase in the activity of the material group side chains, resulting in a weakening of the adhesion between the first heat-resistant layer, the second heat-resistant layer and the porous carrier, thereby causing the heat-resistant layer to peel off from the porous carrier. The local gap where the heat-resistant layer is peeled off is conducive to the hot flow gas passing through the battery cell to break open the top seal ear glue, thereby inhibiting the continuation of the thermal reaction chain; the present invention can optimize the above-mentioned peeling process by controlling the difference in thickness of the heat-resistant layers on different sides and the combination of different particle sizes, effectively inhibiting the fire phenomenon caused by thermal runaway of the lithium-ion battery, and effectively delaying the time when thermal runaway occurs in the lithium-ion battery.

[0032] Specifically, when D1 is less than 2×D2, the increase in the transfer impedance of lithium ions in the separator at high temperatures is insignificant, and there is no significant slowing effect on the reaction process of thermal runaway. When D1 is greater than 4×D2, the diffusion rate of lithium ions in the first heat-resistant layer is reduced, which is not conducive to the cycle stability of the positive electrode material. Therefore, by controlling the particle size relationship between the inorganic filler particles and the heat-absorbing microspheres, the risk of thermal runaway can be further improved and the cycle stability of the positive electrode material can be enhanced. When H1-H2 is less than 0.5, the porosity distribution difference in the thickness direction of the heat-resistant layer is small, and it is impossible to achieve the regulation of the electrolyte distribution and infiltration effect. When H1-H2 is greater than 1.5, the ion conduction speed is low, which cannot meet the charging requirements and easily leads to lithium reduction on the negative electrode surface. Therefore, by controlling the thickness relationship between the first and second heat-resistant layers, the electrolyte infiltration effect can be optimized, the ion transfer impedance in the separator can be increased, the occurrence of thermal runaway can be suppressed, and other performance problems of the negative electrode can be avoided.

[0033] In one specific embodiment, the average particle size of the organic filler particles is 0.5 μm-2 μm, for example, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm or 2 μm.

[0034] In one embodiment, the average particle size of the heat-absorbing microspheres is 0.2 μm-1.5 μm, for example, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm or 1.5 μm.

[0035] In a specific embodiment, the thickness of the first heat-resistant layer is 1 μm-3.5 μm, for example, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, 3 μm, 3.1 μm, 3.2 μm, 3.3 μm, 3.4 μm or 3.5 μm.

[0036] In a specific embodiment, the thickness of the second heat-resistant layer is 0.5 μm-2 μm, for example, 0.5 μm, 1.0 μm, 1.5 μm or 2 μm.

[0037] In one embodiment, the area of the heat-absorbing microspheres within a 10 μm*10 μm area on the surface of the second heat-resistant layer is 10%-60%, for example, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, or 60%. The area ratio of the heat-absorbing microspheres in the second heat-resistant layer is determined by removing the separator from the battery cell, taking a 10K magnification SEM image of the second heat-resistant layer of the separator, adjusting the contrast to 50%, capturing the SEM image, and analyzing the captured SEM image using ImageJ software. An area of 10 μm*10 μm on the surface of the second heat-resistant layer is randomly selected, and the orthographic projection area of the heat-absorbing microspheres on the porous carrier surface is calculated. The orthographic projection area of the heat-absorbing microspheres relative to the total area of the porous carrier is then calculated, i.e., the area ratio of the heat-absorbing microspheres in the second heat-resistant layer. The above operation is repeated five times, and the average value is calculated.

[0038] In another specific embodiment, the heat-absorbing microspheres account for 15%-45% of the area within an area of 10 μm*10 μm on the surface of the second heat-resistant layer.

[0039] In a specific embodiment, the weight content of the heat-absorbing microspheres in the second heat-resistant layer is 10wt%-55wt%, for example, 10wt%, 15wt%, 20wt%, 25wt%, 30wt%, 35wt%, 40wt%, 45wt%, 50wt% or 55wt%.

[0040] In a specific embodiment, the second heat-resistant layer further includes an inorganic filler, and the components of the inorganic filler include one or more of aluminum oxide, boehmite, magnesium oxide, magnesium hydroxide, barium sulfate, barium titanate, zinc oxide, calcium oxide and silicon dioxide.

[0041] In a specific embodiment, the weight content of the inorganic filler in the second heat-resistant layer is 40wt%-85wt%, for example, 40wt%, 45wt%, 50wt%, 55wt%, 60wt%, 65wt%, 70wt%, 75wt%, 80wt% or 85wt%.

[0042] In a specific embodiment, the organic filler particles are composed of one or more of melamine, melamine cyanurate, melamine polyphosphate, diethylaluminum hypophosphite, and modified melamine salts.

[0043] In a specific embodiment, the weight content of the organic filler particles in the first heat-resistant layer is 92wt%-98wt%, for example, 92wt%, 93wt%, 94wt%, 95wt%, 96wt%, 97wt% or 98wt%.

[0044] In one embodiment, the thermal conductivity of the organic filler particles is less than the thermal conductivity of the heat-absorbing microspheres.

[0045] In a specific embodiment, the thermal conductivity of the organic filler particles is 0.1W / (m·K)-0.5W / (m·K), for example, 0.12W / (m·K), 0.13W / (m·K), 0.14W / (m·K), 0.15W / (m·K), 0.16W / (m·K), 0.17W / (m·K), 0.18W / (m·K), 0.19W / (m·K), 0.2W / (m·K), 0.21W / (m·K), 0.22W / (m·K), 0.23 W / (m·K), 0.24W / (m·K), 0.25W / (m·K), 0.26W / (m·K), 0.27W / (m·K), 0.28W / (m·K), 0.29W / (m·K), 0.3W / (m·K), 0 .31W / (m·K), 0.32W / (m·K), 0.33W / (m·K), 0.34W / (m·K), 0.35W / (m·K), 0.4W / (m·K), 0.45W / (m·K) or 0.5W / (m·K).

[0046] In one embodiment, the thermal conductivity of the heat-absorbing microspheres is 0.3W / (m·K)-0.5W / (m·K), for example, 0.3W / (m·K), 0.31W / (m·K), 0.32W / (m·K), 0.33W / (m·K), 0.34W / (m·K), 0.35W / (m·K), 0.36W / (m·K), 0.37W / (m·K), 0.38W / (m·K), 0.39W / (m·K), 0.4W / (m·K), 0.41W / (m·K), 0.42W / (m·K), 0.43W / (m·K), 0.44W / (m· K), 0.45W / (m·K), 0.46W / (m·K), 0.47W / (m·K), 0.48W / (m·K), 0.49W / (m·K) or 0.5W / (m·K).

[0047] The thermal conductivity of the organic filler particles in the first heat-resistant layer and the heat-absorbing microspheres in the second heat-resistant layer are measured using the hot wire method. Specifically, a thin metal wire is embedded in the sample, and the thermal conductivity is calculated by measuring the temperature change when an electric current passes through the wire. The specific steps are as follows: First, the metal wire is embedded in the center of the sample whose thermal conductivity needs to be measured; second, the metal wire is heated by electricity and the temperature change over time is recorded; then, the thermal conductivity is calculated using the formula: K = (P / U) * (ln(L / d) / A). Here, P is power, U is voltage, L is the wire length, D is the wire diameter, and A is the cross-sectional area of the sample.

[0048] The thermal conductivity of the organic filler particles in the first heat-resistant layer is lower than that of the heat-absorbing microspheres in the second heat-resistant layer, which allows the battery to store more heat on the positive electrode side at low temperatures, making the temperature of the positive electrode side slightly higher than that of the negative electrode side, helping to maintain the temperature of the positive electrode within a certain range, promoting the diffusion of lithium ions on the positive electrode side under low temperatures, increasing the lithium insertion and extraction rate of the positive electrode active material under low temperatures, facilitating the transmission of lithium ions in the thickness direction of the battery, and improving the low-temperature rate performance of the battery.

[0049] However, the difference in thermal conductivity between the organic filler particles in the first heat-resistant layer and the heat-absorbing microspheres in the second heat-resistant layer can easily lead to a mismatch or large difference in the lithium ion insertion and extraction rates on the positive and negative electrode sides, making the negative electrode prone to lithium plating problems.

[0050] Based on this, the present invention can effectively improve the lithium ion transfer impedance on the negative electrode side by further adjusting the components in the electrolyte, thereby avoiding the occurrence of negative electrode lithium plating. In a specific embodiment, the battery further includes an electrolyte, and the electrolyte further includes a cyclic sulfur-containing compound and / or fluoroethylene carbonate (FEC). Since the temperature on the negative electrode side is relatively low, the corrosion effect of FEC on the negative electrode current collector is weakened. Therefore, by appropriately increasing the amount of FEC added to the electrolyte, FEC and the cyclic sulfur-containing compound can produce a synergistic effect to generate a denser, more stable, low-impedance SEI film, thereby improving the lithium insertion and extraction rate of the negative electrode active material, thereby alleviating the lithium plating problem caused by the accumulation of lithium ions on the negative electrode surface, and improving the low-temperature performance of the battery.

[0051] In a specific embodiment, the weight content of the cyclic sulfur-containing compound in the electrolyte is 0.5%-1%, for example, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% or 1%.

[0052] In a specific embodiment, the weight content of the fluoroethylene carbonate in the electrolyte is 10%-30%, for example, 10%, 15%, 20%, 25% or 30%.

[0053] In another specific embodiment, the weight content of the fluoroethylene carbonate in the electrolyte is 15%-25%.

[0054] In one embodiment, the cyclic sulfur-containing compound comprises one or more of the following structures:

[0055]

[0056] In one embodiment, the composition of the heat-absorbing microspheres includes a first polymer, the main chain of the first polymer is a saturated alkane, at least some of the carbon atoms on the main chain are connected to a group R, and the group R is *-(CC-R1) x1 -(CC-R2) x2 -(CC-R3) x3 -(CC-R4) x4 -(CC-R5) x5 , x1, x2, x3, x4, x5 are the same or different and are positive integers of 0-10 and x1, x2, x3, x4, x5 are not 0 at the same time, R1, R2, R3, R4, R5 are each independently selected from *-halogen, *-COOH, *-T2COOT1, T1 and T2 are each independently selected from methyl, ethyl, n-propyl, and isopropyl, and * represents a connecting end.

[0057] The main chain of the first polymer can be a saturated alkane, which is a carbon-carbon single bond (-CC-). It is understood that the main chain of the first polymer is a carbon-carbon single bond. At least some (≥1) carbon atoms on the main chain are connected to a group R, and among the carbon atoms on the main chain, at least some (≥1) carbon atoms are connected to a group R. It is understood that the first polymer includes ≥1 (for example, 1, 5, 10, 15, 20, 25, 30, 35 or 40) groups R. When the first polymer includes >1 group R, R1, R2, R3, R4, and R5 in any two groups R may be the same or different, and x1, x2, x3, x4, and x5 in any two groups R may be the same or different.

[0058] In one embodiment, the group R is *-(CC-R1) x1 -(CC-R2) x2 -(CC-R3) x3 -(CC-R4) x4 -(CC-R5) x5 .

[0059] x1, x2, x3, x4, and x5 may be the same or different and be positive integers of 0-10 (e.g., 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10), and x1, x2, x3, x4, and x5 are not all 0 at the same time.

[0060] The number of hydrogen atoms in the group R is not expressed. The number of hydrogen atoms can be determined based on the specific structure of the group R. However, it should be noted that the carbon atoms in the group R satisfy four single bonds. For example, when x1 is 1, and x2, x3, x4, and x5 are all 0, the group R is *-CH2-CH2-R1; when x1 is 3, x2 is 1, x3 is 5, and x4 and x5 are all 0, the group R is *-(CH2-CH-R1)3-(CH2-CH-R2)-(CH2-CH-R3)4-CH2-CH2-R3.

[0061] R1, R2, R3, .....R5 may be the same or different and are independently selected from *-halogen, *-COOH, *-T2COOT1. * indicates the connection end. There are two connecting ends in the formula, and the two connecting ends are connected to two different adjacent carbon atoms. For example, R1 is Then the group R is

[0062]

[0063] In one embodiment, T1 and T2 are each independently selected from methyl, ethyl, n-propyl, and isopropyl.

[0064] In one embodiment, the halogen is selected from one or more of F, Cl and Br.

[0065] In one embodiment, the number average molecular weight of the first polymer is 1200-7000, for example, 1200, 1500, 2000, 3000, 4000, 5000, 5500, 5900, 6000, 6500 or 7000. The number average molecular weight of the first polymer can be measured by gel permeation chromatography.

[0066] In one embodiment, x1+x2+x3+x4+x5≤40 (e.g., 1, 5, 10, 15, 20, 25, 30, 35, or 40).

[0067] In a specific embodiment, no emulsifier and pH regulator are added during the preparation of the first polymer, and the pH regulator is one or more of KOH and NaOH.

[0068] In a specific embodiment, the first polymer is polyethylene or polypropylene with a modified group, and the modified group is one or more of a carbonyl group (*-CO), a carboxyl group (*-COOH), and an ester group (*-COO-*).

[0069] The components of the heat-absorbing microspheres of the present invention include a first polymer, the main chain of the first polymer is a saturated alkane, and at least some of the carbon atoms on the main chain are connected to a group R. Based on the structure of the first polymer, when the battery is in a high temperature state (for example, 80°C-140°C), the heat-absorbing microspheres including the first polymer melt from a solid state to a liquid state. This process requires the absorption of sufficient heat. At this time, the heat-absorbing microspheres can slow down the temperature rise rate of the battery by absorbing latent heat, thereby reducing the temperature of the negative electrode side, avoiding side reactions between the negative electrode material and the electrolyte at high temperature, delaying the time for thermal runaway, and providing a buffer for the thermal runaway reaction of the battery, thereby improving the thermal stability of the battery under high temperature conditions and enhancing the safety performance of the battery. At the same time, the molten heat-absorbing microspheres have a certain fluidity. When a diaphragm near the heat-absorbing microspheres is punctured (for example, the diaphragm is punctured by small particles on the surface of the negative electrode sheet), the interface defects between the diaphragm and the electrode sheet can be dynamically repaired. This dynamic repair effect can reduce the gaps and defects at the interface, thereby reducing the interfacial impedance and achieving a breakthrough optimization of the thermo-electrochemical performance; and the group R in the first polymer contains a polar group, which has a strong electronegativity and usually contains a lone pair of electrons in the atomic orbital. When it melts and diffuses at high temperature, it can repel the polar groups in the electrolyte (taking *-COOH as an example, *-COOH can be at least partially deprotonated in the electrolyte to form a negatively charged carboxylate (-COO-), the oxygen atom of the carboxylate carries a lone pair of electrons, which can weaken the physical complexation between substances containing polar groups such as nitrile additives in the electrolyte and the negative electrode active material). This mutual repulsion can reduce the physical complexation between substances containing polar groups in the electrolyte and the negative electrode active particles, reduce the side reactions between the negative electrode active material and the electrolyte under high temperature conditions, cut off the chain reaction that causes thermal runaway of the battery, and further improve the thermal stability of the battery. The polar group R in the first polymer can also reduce the surface energy of the heat-absorbing microspheres, thereby enhancing the interfacial interaction between the diaphragm and the electrode. In addition, the polar groups R have an electrostatic repulsion effect, which can improve the dispersion stability of the heat-absorbing microspheres, inhibit agglomeration, achieve monodisperse morphology control, make the thickness of the diaphragm more uniform, improve the consistency of the diaphragm performance, facilitate the transmission of lithium ions, and improve the cycle stability of the battery.

[0070] In one embodiment, the weight content of impurity elements in the heat-absorbing microspheres is no more than 20 ppm, and the impurity elements include one or more of Al, Cr, Cu, K, Mg, Mn, Ni, Pb, Ca, and Na. During the preparation process of the heat-absorbing microspheres, impurity elements are inevitably introduced. These impurity elements may increase the weight of the battery and affect the battery's heat dissipation performance. Moreover, these impurity elements are metallic elements and have magnetism when subjected to electric current. Magnetic impurities may generate eddy currents within the battery, causing the battery temperature to rise and affecting battery stability. In addition, certain impurity elements (for example, chromium (Cr) may form chromium oxide or release chromium vapor at high temperatures; lead (Pb) may evaporate at high temperatures to form lead vapor; and manganese (Mn) may form manganese oxide or release manganese vapor at high temperatures) may release harmful gases at high temperatures, affecting the safe use of the battery. Therefore, the present invention controls the weight content of impurity elements in the heat-absorbing microspheres to be no more than 20 ppm (for example, 20 ppm, 18 ppm, 15 ppm, 13 ppm, 10 ppm, 8 ppm, 5 ppm, 3 ppm, 1 ppm, 0.5 ppm, 0.1 ppm or 0), and avoids the introduction of impurity elements as much as possible during the preparation process of the heat-absorbing microspheres. On the one hand, it can avoid that when the heat-absorbing microspheres are melted, the impurity elements diffuse along with the liquid heat-absorbing microspheres to affect the stability of the crystal structure of the positive electrode material, causing the positive electrode material to evolve oxygen and generate heat. On the other hand, it can also avoid the side reaction of impurity elements with the electrolyte at high temperature, producing gas and releasing heat, aggravating the thermal runaway of the battery, and affecting the safety performance of the battery.

[0071] In one embodiment, the porous support comprises a polyolefin, and the polyolefin comprises polypropylene and / or polyethylene.

[0072] In one embodiment, the porous support has a thickness of 3 μm to 12 μm, for example, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, or 12 μm. The thickness of the porous support can be measured by referring to the thickness measurement method of the first heat-resistant layer and the second heat-resistant layer.

[0073] In one embodiment, the porosity of the separator is 25%-55%, for example, 25%, 30%, 35%, 40%, 45%, 50%, or 55%. The separator porosity is tested by removing the separator from the battery cell and taking a separator sample of any size. The apparent volume of the separator sample is calculated as follows: measured sample thickness * sample length * sample width. The porosity of the separator sample is calculated as follows: (sample apparent volume - sample true volume) / sample apparent volume * 100%. The apparent volume of the separator sample can be measured and calculated using a thickness gauge and a steel ruler, while the true volume of the separator sample is calculated using a Micron density meter (AccuPycⅡ1340) from the United States.

[0074] In one embodiment, the average pore size of the diaphragm is 25 nm to 50 nm, for example, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, or 50 nm. The average pore size of the diaphragm can be measured using a mercury porosimeter from PMI. Based on the Laplace equation, r = γ / (√P), where r is the pore radius, γ is the surface tension of mercury, and P is the pressure at the time of mercury injection, the pore radius of the diaphragm can be calculated. The number of pores within different pore size ranges is counted, and the average pore size value, i.e., the average pore size of the diaphragm, can be calculated.

[0075] By controlling the porosity and average pore size of the diaphragm within the above range, the present invention can maintain the normal ion transmission efficiency of the diaphragm, and effectively achieve the thermal closure effect of the diaphragm under high temperature conditions, thereby inhibiting the progress of the thermal runaway reaction chain and reducing the risk of thermal runaway.

[0076] In one embodiment, the separator further comprises a rubber coating layer, the rubber coating layer being located on the surface of the first heat-resistant layer and / or the second heat-resistant layer. The rubber coating layer may be located only on the surface of the first heat-resistant layer, only on the surface of the second heat-resistant layer, or on the surfaces of both the first heat-resistant layer and the second heat-resistant layer.

[0077] In a specific embodiment, the coating layer comprises a second polymer, and the second polymer comprises one or more of polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene modified and copolymers thereof, polyacrylonitrile, polymethyl methacrylate, polyacrylic acid, polyvinyl alcohol and copolymer-modified polyvinyl alcohol thereof, polyvinyl acetate, polyacrylamide, phenolic resin, epoxy resin, ethylene-vinyl acetate copolymer, polyacrylic acid copolymer, lithium polystyrene sulfonate, polyvinylidene fluoride-trichloroethylene, polyvinylidene fluoride-chlorotrifluoroethylene, polyethylene oxide and cyanoethyl polyvinyl alcohol;

[0078] In a specific embodiment, the thickness of the rubber coating layer is 1 μm-10 μm, for example, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm. The thickness of the rubber coating layer refers to the total thickness of the rubber coating layer. For example, when there is only one rubber coating layer on the surface of the first heat-resistant layer, the thickness of the rubber coating layer is only the thickness of one layer. When there are rubber coating layers on the surfaces of the first heat-resistant layer and the second heat-resistant layer, the thickness of the rubber coating layer is the sum of the thicknesses of the rubber coating layers on the surfaces of the two heat-resistant layers. The method for measuring the thickness of the rubber coating layer can refer to the method for measuring the thickness of the first heat-resistant layer and the second heat-resistant layer.

[0079] In a specific embodiment, the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer located on one side or both sides of the positive electrode current collector, the positive electrode active layer includes a positive electrode active material, and the positive electrode active material includes one or more of a lithium cobalt oxide material, a nickel-cobalt-manganese ternary material, a nickel-cobalt-aluminum ternary material and a lithium iron phosphate material, and the weight content of the positive electrode active material in the positive electrode active layer is ≥96%.

[0080] In one example, the positive electrode active layer further includes a positive electrode conductor and a positive electrode binder.

[0081] In one example, the positive electrode conductive agent includes at least one of conductive carbon black (Super P), acetylene black, graphite, graphene, carbon nanotubes, metal conductive powder, carbon nanofibers, and Ketjen black.

[0082] In one example, the positive electrode binder includes at least one of polyvinylidene fluoride (PVDF), acrylic acid-modified PVDF, carboxylic acid-modified PVDF, styrene-butadiene rubber, acrylic acid-modified styrene-butadiene rubber, polymethyl methacrylate, and polyimide.

[0083] In one example, based on the total weight of the positive electrode active layer, the weight content of the positive electrode conductor is 0.5%-1.5%, and the weight content of the positive electrode binder is 1%-2.5%.

[0084] In a specific embodiment, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on one side or both sides of the negative electrode current collector, the negative electrode active layer includes a negative electrode active material, and the negative electrode active material includes one or more of graphite material, hard carbon, elemental silicon, silicon alloy, silicon oxide material and silicon carbon material. The weight content of the negative electrode active material in the negative electrode active layer is ≥96.5%, and the weight content of silicon element in the negative electrode active layer is ≤50%.

[0085] In one example, the negative electrode active layer further includes a negative electrode conductor and a negative electrode binder.

[0086] In one example, the negative electrode conductive agent includes at least one of conductive carbon black (Super P), carbon fiber, activated carbon, acetylene black, graphene, and carbon nanotubes.

[0087] In one example, the negative electrode binder includes at least one of styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose (CMC-Na), lithium carboxymethyl cellulose (CMC-Li), potassium carboxymethyl cellulose (CMC-K), polyacrylic acid (salt), polyvinyl alcohol, polyacrylonitrile, polyacrylamide, polymethacrylate, alginic acid (salt), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF).

[0088] In one example, based on the total weight of the negative electrode active layer, the weight content of the negative electrode conductive agent is 0.5%-1.5%, and the weight content of the negative electrode binder is 1%-2.5%.

[0089] In the present invention, the electrolyte may contain other additives contained in conventional electrolytes in the art, for example, lithium salts, other organic solvents and additives.

[0090] In one embodiment, the battery is a lithium-ion secondary battery.

[0091] The present invention will be described in detail below through examples. The examples described in the present invention are only some examples of the present invention, not all examples. All other examples obtained by persons of ordinary skill in the art based on the examples of the present invention without creative work are within the scope of protection of the present invention.

[0092] Example 1

[0093] (1) Preparation of positive electrode sheet

[0094] Lithium cobalt oxide, binder polyvinylidene fluoride (PVDF 500), and conductive carbon material (Super P: carbon nanotubes = 2:1) were mixed in N-methylpyrrolidone (NMP) solvent at a weight ratio of 97.5:2:2.5. A blender was used to continuously stir the mixture into a uniform, fluid cathode slurry. The slurry was then coated onto 10μm-thick aluminum foil with a 2% elongation and dried in a 120°C vacuum oven for 6 hours. The resulting cathode sheets were then rolled and slit.

[0095] (2) Preparation of negative electrode sheet

[0096] Graphite, silicon carbon, conductive carbon black (Super P), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber were mixed in an aqueous solvent at a weight ratio of 91:7:1:0.5:0.5 and continuously stirred in a blender to form a uniform, fluid negative electrode slurry. The slurry was then coated onto a 10μm-thick current collector copper foil to form the negative electrode active layer. The slurry was then dried in a 120°C vacuum oven for 6 hours, followed by rolling and slitting to produce the desired negative electrode sheets. The silicon content in the negative electrode active layer was 15% by weight.

[0097] (3) Preparation of electrolyte

[0098] In an argon-filled glove box (moisture <1ppm, oxygen <1ppm), ethylene carbonate, propylene carbonate, propyl propionate, and ethyl propionate solvents were mixed in a mass ratio of 15:15:50:20 to form a uniform solvent, and 15.5wt% of LiPF6, 2wt% of 1,3-propane sultone, and 3wt% of 1,3,6-hexane trinitrile were slowly added and stirred evenly to obtain the desired lithium-ion battery electrolyte.

[0099] (4) Preparation of diaphragm

[0100] 1) First heat-resistant layer: An organic filler, a first polymer binder, a first thickener, a first wetting agent, and deionized water are blended to obtain a ceramic slurry with a solid content of 35%. The weight ratio of the organic filler, the first polymer binder, the first thickener, and the first wetting agent in the first heat-resistant layer is 94.5:5:0.4:0.1, calculated based on 100% solid mass. After sufficient stirring and dispersion, the slurry is coated on one surface of a porous carrier base membrane using a gravure roller and dried in a multi-section oven at 60°C to form a first heat-resistant layer with a thickness H1 of 1.5 μm. The organic filler is melamine cyanurate (C6H9N9O3) with an average particle size D1 of 1.2 μm. The first polymer binder is polyacrylate, the first thickener is CMC, and the first wetting agent is sodium dodecylbenzene sulfonate. The porous carrier base membrane is selected from a polyethylene microporous membrane, the substrate thickness is 5 μm, the porosity of the diaphragm is 40%, and the average pore size of the diaphragm is 35 nm.

[0101] 2) Second heat-resistant layer: Inorganic particles, heat-absorbing microspheres, a second polymer binder, a second thickener, a second wetting agent and deionized water are blended to obtain a ceramic slurry with a solid content of 30%, wherein the weight ratio of inorganic particles: heat-absorbing microspheres: second polymer binder: second thickener: second wetting agent in the second heat-resistant layer is 59.5:35:5:0.4:0.1, calculated based on 100% solid mass. After sufficient stirring and dispersion, the slurry is coated on the other side surface of the above-mentioned porous carrier base membrane through a gravure roller, and dried in a multi-section oven at 60°C to form a second heat-resistant layer with a thickness H2 of 1 μm. The inorganic particles use aluminum oxide with an average particle size of 0.5 μm. The group R on the main chain of the first polymer in the heat-absorbing microspheres includes *-COOH and *-CH2CH2COOCH3. The number average molecular weight of the first polymer is 3265. The average particle size D2 of the heat-absorbing microspheres is 0.5 μm. The specific components of the second polymer binder, the second thickener and the second wetting agent are the same as above.

[0102] 3) Glue coating: PVDF and DMAC were mixed and thoroughly stirred to dissolve, and then alumina was added and stirred to disperse evenly to obtain a mixed slurry with a solid content of 10%. The slurry had a solid mass ratio of 100% PVDF:alumina = 0.4:0.6. The mixed slurry was coated on the other side of the first and second heat-resistant layers using a gravure roller. After drying in a multi-section oven at 60°C, a double-sided glue coating was formed to obtain the diaphragm of this embodiment. The total thickness of the glue coating on both sides was 9 μm.

[0103] The differential scanning calorimetry (DSC) spectrum of the membrane in this embodiment is as follows: Figure 2 As shown, there are four endothermic characteristic peaks:

[0104] The peak temperature of the first characteristic peak is 109.71 °C, and the area J1 of the first characteristic peak is 31.94 mJ;

[0105] The peak temperature of the second characteristic peak is 139.12 °C, and the area J2 of the second characteristic peak is 170.67 mJ;

[0106] The peak temperature of the third characteristic peak is 396.67 °C, and the area J3 of the third characteristic peak is 794.50 mJ;

[0107] The peak temperature of the fourth characteristic peak is 482.74 °C, and the area J4 of the fourth characteristic peak is 614.10 mJ;

[0108] Satisfy J3≥J1+3×J2, J2≥5×J1.

[0109] (5) Preparation of lithium-ion batteries

[0110] The positive electrode sheet, separator, and negative electrode sheet prepared above are wound to prepare a bare cell; the bare cell is then placed in an aluminum-plastic film (as shown in Table 1), and the prepared electrolyte is injected into the dried bare cell. After vacuum packaging, room temperature standing, and high-temperature formation, the desired lithium-ion battery is obtained.

[0111] The details are shown in Table 1 and Table 2.

[0112] Example 2 group

[0113] The preparation method of the lithium-ion battery in this embodiment refers to Example 1, with the only difference being that the average particle sizes D1 and D2 of the organic filler and the heat-absorbing microspheres are changed, as shown in Tables 1 and 2.

[0114] Example 3 group

[0115] The preparation method of the lithium-ion battery in this embodiment refers to that in Example 1, the only difference is that the thicknesses H1 and H2 of the first heat-resistant layer and the second heat-resistant layer are changed, as shown in Tables 1 and 2.

[0116] Example 4 Group

[0117] The preparation method of the lithium-ion battery in this embodiment refers to Example 1, with the only difference being that the specific selection of the first polymer in the heat-absorbing microspheres and the specific composition of the organic filler particles are changed. Specifically:

[0118] In Example 4-1, the groups R on the main chain of the first polymer in the heat-absorbing microspheres include *-COOH, *-CH2CH2COOCH3, and *-F, and the number average molecular weight of the first polymer is 2672; the specific component of the organic filler particles is melamine;

[0119] In Example 4-2, the groups R on the main chain of the first polymer in the heat-absorbing microspheres include *-COOH, and *-CH2CH2COOCH3, the number average molecular weight of the first polymer is 5967; the specific component of the organic filler particles is melamine polyphosphate.

[0120] Example 5 group

[0121] The preparation method of the lithium-ion battery in this embodiment refers to Example 1, with the only difference being that fluoroethylene carbonate and / or a cyclic sulfur-containing compound are added to the electrolyte. Specifically:

[0122] In Example 5-1, 20 wt% of fluoroethylene carbonate and 0.8 wt% of a cyclic sulfur-containing compound of formula (I-1) were added to the electrolyte;

[0123] In Example 5-2, 30 wt% of fluoroethylene carbonate and 0.5 wt% of a cyclic sulfur-containing compound of formula (I-2) were added to the electrolyte;

[0124] In Example 5-3, 35 wt% of fluoroethylene carbonate and 1 wt% of a cyclic sulfur-containing compound of formula (I-1) were added to the electrolyte;

[0125] In Example 5-4, 30 wt % of fluoroethylene carbonate and 2 wt % of a cyclic sulfur-containing compound of formula (I-1) were added to the electrolyte;

[0126] In Example 5-5, 10 wt% of fluoroethylene carbonate was added to the electrolyte.

[0127] Comparative Example 1

[0128] The preparation method of the lithium-ion battery in this comparative example refers to Example 1, with the only difference being that the first heat-resistant layer is not provided, and only a rubber coating layer is provided on the positive electrode side of the separator, as shown in Table 1.

[0129] Comparative Example 2

[0130] The preparation method of the lithium-ion battery in this comparative example refers to Example 1, with the only difference being that no second heat-resistant layer is provided, and only a rubber coating layer is provided on the negative electrode side of the separator, as shown in Table 1.

[0131] Comparative Example 3

[0132] The preparation method of the lithium-ion battery in this comparative example refers to Example 1, with the only difference being that the first heat-resistant layer no longer corresponds to the positive electrode side, and the second heat-resistant layer no longer corresponds to the negative electrode layer. The positions of the two layers are swapped, with the first heat-resistant layer corresponding to the negative electrode side and the second heat-resistant layer corresponding to the positive electrode layer, as shown in Table 1.

[0133] Comparative Example 4

[0134] The preparation method of the lithium-ion battery in this comparative example refers to Example 1, with the only difference being that the first heat-resistant layer and the second heat-resistant layer are not provided on both sides of the porous carrier base membrane of the diaphragm, but a conventional ceramic layer is provided on the positive electrode side of the porous carrier base membrane, one side of the glue layer is provided on the surface of the conventional ceramic layer, and the other side is provided on the surface of the porous carrier base membrane, and the composition and ratio of the conventional ceramic layer are: aluminum oxide, accounting for 94.5% by mass, wherein the differential scanning calorimetry (DSC) spectrum of the diaphragm is as follows Figure 3 As shown, there is only one endothermic characteristic peak, the peak temperature of which is 138.37 °C and the area of which is 207.56 mJ.

[0135] Comparative Example 5

[0136] The preparation method of the lithium-ion battery in this comparative example refers to Example 1, with the only difference being that the thickness H1 of the first heat-resistant layer is changed, as shown in Tables 1 and 2.

[0137] Comparative Example 6

[0138] The preparation method of the lithium-ion battery in this comparative example refers to Example 1, with the only difference being that the weight content of the heat-absorbing microspheres in the second heat-resistant layer is changed. Specifically, calculated based on 100% solid mass, the weight content ratio of inorganic particles: heat-absorbing microspheres: second polymer binder: second thickener: second wetting agent in the second heat-resistant layer is 37.5:57:5:0.4:0.1.

[0139] Table 1

[0140]

[0141]

[0142] In Table 1, “ / ” indicates that the endothermic characteristic peak does not appear in this temperature range. Therefore, the peak temperature and peak area are represented by “ / ”. When the peak area cannot be tested, Formula 1 and Formula 2 cannot be calculated, so they are also represented by “ / ”.

[0143] Table 2

[0144]

[0145] Test Case

[0146] (1) Furnace temperature test

[0147] The fully charged cells obtained after charging and discharging the lithium-ion batteries prepared in the above embodiments and comparative examples were placed in an oven, and the oven was heated at 5°C / min. The batteries to be tested in each embodiment and comparative example were divided into three groups, and the three groups were heated to different temperatures (130°C / 132°C / 135°C) and kept warm for 1 hour. The pass rate of the batteries in each group passing the oven temperature test was obtained. The judgment standard for a single battery passing the oven temperature test was: the battery cell was considered to have passed if it did not catch fire or explode. 20 batteries were tested at each temperature for each group of embodiments and comparative examples. The oven temperature test pass rate = number of batteries passing the test / total number of batteries tested. The test results are recorded in Table 3.

[0148] (2) 10℃ cycle test

[0149] The lithium-ion batteries prepared in the above examples and comparative examples were placed in an environment of 25°C ± 2°C, charged at a constant current of 0.5C to the upper limit voltage (4.53V), then charged at a constant voltage of 4.53V to 0.05C, and allowed to stand for 5 minutes. The initial fully charged thickness P0 was recorded, and then discharged at a constant current of 0.2C to 3.0V, with the initial discharge capacity recorded as C0. The battery was allowed to stand for 5 minutes, and then charged at a constant current and constant voltage of 0.7C to the upper limit voltage, and then charged at a constant voltage of 4.53V to 0.05C, and allowed to stand for 5 minutes. The lithium-ion batteries were then transferred to an environment of 10°C ± 2°C, allowed to stand for 60 minutes, and then discharged at a constant current of 0.7C to 3V, and allowed to stand for 5 minutes.

[0150] Cycling system: 0.7C constant current and constant voltage charging to the upper limit voltage, then charging to 0.05C at 4.53V constant voltage, standing for 5min, and then discharging at 0.5C constant current to 3.0V. This is one charge and discharge cycle. After 500T of cycling, 0.7C constant current and constant voltage charging to the upper limit voltage, then charging to 0.05C at 4.53V constant voltage, standing for 5min, recording the final full-charge thickness P1 of each group of examples and comparative examples, then 0.2C constant current discharge to 3.0V, recording the final discharge capacity C1 of each group of examples and comparative examples, and calculating the 10°C cycle capacity retention rate of the lithium-ion battery prepared in each group of examples and comparative examples: C=C1 / C0*100%, thickness expansion rate: P=(P1-P0) / P0*100%, and the test results are recorded in Table 3.

[0151] Table 3

[0152]

[0153]

[0154] It can be seen from the above table that, by comparing the comparative example and the embodiment, it can be seen that the battery of the embodiment passes the hot box better, has a higher cycle capacity retention rate, and a lower thickness expansion rate, indicating that by controlling the endothermic characteristic peaks and the relationship between the characteristic peaks in the differential scanning calorimetry spectrum of the diaphragm, the thermal stability of the battery is improved, and the safety performance and cycle performance of the battery are improved.

[0155] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention.

Claims

1. A battery, characterized in that: The battery includes a positive electrode sheet, a negative electrode sheet and a separator, the separator includes a porous carrier, a first heat-resistant layer and a second heat-resistant layer, the first heat-resistant layer and the second heat-resistant layer are respectively located on two functional surfaces of the porous carrier that are opposite to each other, the first heat-resistant layer includes organic filler particles, the components of the organic filler particles include element N and / or element P, the second heat-resistant layer includes heat-absorbing microspheres, the separator is located between the positive electrode sheet and the negative electrode sheet, the positive electrode sheet is opposite to one side of the first heat-resistant layer, the negative electrode sheet is opposite to one side of the second heat-resistant layer, and the positive electrode sheet is opposite to one side of the second heat-resistant layer. There are three endothermic characteristic peaks in the differential scanning calorimetry spectrum of the diaphragm, namely the first characteristic peak, the second characteristic peak and the third characteristic peak. The peak temperature of the first characteristic peak is 90℃-120℃, the peak temperature of the second characteristic peak is 125℃-180℃, and the peak temperature of the third characteristic peak is 200℃-450℃. The diaphragm also satisfies the following relationship: J3≥J1+3×J2, J2≥5×J1, wherein J1 is the area of the first characteristic peak, J2 is the area of the second characteristic peak, and J3 is the area of the third characteristic peak.

2. The battery according to claim 1, wherein The diaphragm satisfies the following relationship at the same time: 2×D2≤D1≤4×D2, 0.5≤H1-H2≤1.5, wherein D1 is the average particle size of the organic filler particles, in μm, D2 is the average particle size of the heat-absorbing microspheres, in μm, H1 is the thickness of the first heat-resistant layer, in μm, and H2 is the thickness of the second heat-resistant layer, in μm; and / or, the average particle size of the organic filler particles is 0.5 μm-2 μm; and / or, the average particle size of the heat-absorbing microspheres is 0.2 μm-1.5 μm; and / or, the thickness of the first heat-resistant layer is greater than the thickness of the second heat-resistant layer; and / or, the thickness of the first heat-resistant layer is 1 μm-3.5 μm; And / or, the thickness of the second heat-resistant layer is 0.5 μm-2 μm.

3. The battery according to claim 1, wherein The heat-absorbing microspheres account for 10% to 60% of the surface area of the second heat-resistant layer within an area of 10 μm*10 μm, preferably 15% to 45%; And / or, the second heat-resistant layer further comprises an inorganic filler, wherein the composition of the inorganic filler comprises one or more of aluminum oxide, boehmite, magnesium oxide, magnesium hydroxide, barium sulfate, barium titanate, zinc oxide, calcium oxide, lithium aluminum titanium phosphate and silicon dioxide; And / or, the components of the organic filler particles include one or more of melamine, melamine cyanurate, melamine polyphosphate, diethyl aluminum hypophosphite and modified melamine salt.

4. The battery according to claim 1, wherein The thermal conductivity of the organic filler particles is less than the thermal conductivity of the heat-absorbing microspheres; and / or, the thermal conductivity of the organic filler particles is 0.1 W / (m·K)-0.5 W / (m·K); and / or, the thermal conductivity of the heat-absorbing microspheres is 0.3 W / (m·K)-0.5 W / (m·K); And / or, the battery further comprises an electrolyte, and the electrolyte further comprises a cyclic sulfur-containing compound and / or fluoroethylene carbonate.

5. The battery according to claim 4, wherein The weight content of the cyclic sulfur-containing compound in the electrolyte is 0.5%-1%; and / or, the weight content of the fluoroethylene carbonate in the electrolyte is 10%-30%, preferably 15%-25%; And / or, the cyclic sulfur-containing compound includes one or more of the following structures:

6. The battery according to any one of claims 1 to 5, wherein The composition of the heat-absorbing microspheres includes a first polymer, the main chain of the first polymer is a saturated alkane, at least some of the carbon atoms on the main chain are connected to a group R, and the group R is *-(CC-R1) x1 -(CC-R2) x2 -(CC-R3) x3 -(CC-R4) x4 -(CC-R5) x5 , x1, x2, x3, x4, x5 are the same or different and are positive integers of 0-10 and x1, x2, x3, x4, x5 are not 0 at the same time, R1, R2, R3, R4, R5 are each independently selected from *-halogen, *-COOH, *-T2COOT1, T1 and T2 are each independently selected from methyl, ethyl, n-propyl, isopropyl, and * represents a connecting end; Preferably, the halogen is selected from one or more of F, Cl and Br; Preferably, the number average molecular weight of the first polymer is 1200-7000; Preferably, x1+x2+x3+x4+x5≤40.

7. The battery according to any one of claims 1 to 5, wherein There is also a fourth characteristic peak in the differential scanning calorimetry spectrum of the diaphragm, and the peak temperature of the fourth characteristic peak is 450°C-550°C.

8. The battery according to any one of claims 1 to 5, wherein The porous carrier comprises a polyolefin, and the polyolefin comprises polypropylene and / or polyethylene; and / or, the thickness of the porous support is 3 μm-12 μm; and / or, the porosity of the diaphragm is 25%-55%; And / or, the average pore size of the diaphragm is 25 nm-50 nm.

9. The battery according to any one of claims 1 to 5, wherein The diaphragm further comprises a rubber coating layer, and the rubber coating layer is located on the surface of the first heat-resistant layer and / or the second heat-resistant layer; Preferably, the coating layer comprises a second polymer, and the second polymer comprises one or more of polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene modified and copolymers thereof, polyacrylonitrile, polymethyl methacrylate, polyacrylic acid, polyvinyl alcohol and copolymer-modified polyvinyl alcohol thereof, polyvinyl acetate, polyacrylamide, phenolic resin, epoxy resin, ethylene-vinyl acetate copolymer, polyacrylic acid copolymer, lithium polystyrene sulfonate, polyvinylidene fluoride-trichloroethylene, polyvinylidene fluoride-chlorotrifluoroethylene, polyethylene oxide and cyanoethyl polyvinyl alcohol; Preferably, the thickness of the adhesive layer is 1 μm-10 μm.

10. The battery according to any one of claims 1 to 5, wherein The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer located on one side or both sides of the positive electrode current collector, wherein the positive electrode active layer includes a positive electrode active material, wherein the positive electrode active material includes one or more of a lithium cobalt oxide material, a nickel-cobalt-manganese ternary material, a nickel-cobalt-aluminum ternary material, and a lithium iron phosphate material, and the weight content of the positive electrode active material in the positive electrode active layer is ≥96%; And / or, the negative electrode sheet includes a negative electrode current collector and a negative electrode active layer located on one side or both sides of the negative electrode current collector, the negative electrode active layer includes a negative electrode active material, and the negative electrode active material includes one or more of graphite material, hard carbon, elemental silicon, silicon alloy, silicon oxide material and silicon carbon material, the weight content of the negative electrode active material in the negative electrode active layer is ≥96.5%, and the weight content of silicon element in the negative electrode active layer is ≤50%.

Images