Electrochemical device

Abstract

An electrochemical device, comprising a first electrode sheet (1), a separator (3) and a second electrode sheet (2) that are sequentially stacked. The separator (3) comprises a porous layer formed of nanofibers; and the density of the nanofibers varies in the thickness direction of the separator (3). A change of the density of the nanofibers in the thickness direction of the separator (3) can ensure that there is sufficient free electrolyte on the side of the separator (3) facing the first electrode sheet (1) or the second electrode sheet (2), thereby facilitating the improvement of rate capability and cycle life.

Description

An electrochemical device

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on July 24, 2023, with application number 202310911087.7 ​​and application name “An Electrochemical Device”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the field of new energy technology, and in particular to an electrochemical device. Background Art

[0003] The diaphragm is one of the core technologies of lithium-ion batteries. The diaphragm itself does not participate in the electrochemical reaction in the battery. Its most basic function is to conduct lithium ions and separate the positive and negative electrodes of the battery to prevent the two electrodes from contacting and causing a short circuit. Batteries without diaphragms can improve the battery energy density to a certain extent. The structure of diaphragm-free batteries currently on the market includes a stacked positive electrode sheet, a porous layer, and a negative electrode sheet. Among them, the porous layer is usually made of cross-linked polymer fibers. Due to the poor liquid absorption rate of polymer fibers, it is impossible to maximize the presence of electrolyte inside them, and it is also impossible to increase the replenishment of free electrolyte during the later cycle of the battery, which is not conducive to the battery's rate performance and cycle life.

[0004] Summary of the Invention

[0005] The present application provides an electrochemical device, which can ensure that there is sufficient free electrolyte on the side of the separator facing the first electrode or the second electrode by changing the density of nanofibers in the thickness direction of the separator, which is beneficial to improving the rate performance and cycle life.

[0006] The electrochemical device provided in the present application comprises a first electrode piece, a separator, and a second electrode piece stacked in sequence; the separator comprises a porous layer formed by nanofibers; and the density of the nanofibers in the separator varies in the thickness direction.

[0007] The electrochemical device as described above, wherein the separator satisfies one of the following characteristics:

[0008] a. The density of the nanofibers increases in the thickness direction from the side close to the first and second pole pieces to the side away from the first and second pole pieces;

[0009] b. The density of nanofibers increases or decreases along the thickness direction.

[0010] The electrochemical device as described above, wherein the separator comprises n porous layers, wherein 2≤n≤10; and the surface density of the porous layers satisfies one of the following characteristics:

[0011] a. The surface density increases layer by layer from the side close to the first pole piece and the second pole piece to the side away from the first pole piece and the second pole piece in the thickness direction;

[0012] b. The surface density increases or decreases layer by layer along the thickness direction.

[0013] In the electrochemical device as described above, the thickness of the separator is greater than 0 and not greater than 50 μm, preferably 5 to 20 μm; and / or the fiber diameter of the porous layer is 100 to 1500 nm.

[0014] The electrochemical device as described above, wherein the separator comprises n porous layers, wherein 2≤n≤10; and the diameter of the nanofibers satisfies one of the following characteristics:

[0015] a. The thickness decreases layer by layer from the side close to the first pole piece and the second pole piece to the side away from the first pole piece and the second pole piece in the thickness direction;

[0016] b. Increase or decrease layer by layer along the thickness direction.

[0017] The electrochemical device as described above, wherein the separator comprises n porous layers, wherein 2≤n≤10; and the diameter of the nanofibers satisfies one of the following characteristics:

[0018] a. The diameter of the side close to the first and second pole pieces is 300-1500 nm, and the diameter of the side away from the first and second pole pieces is 100-1000 nm;

[0019] b. At least one layer has a diameter of 100 to 1000 nm.

[0020] The electrochemical device as described above, wherein the separator satisfies at least one of the following characteristics:

[0021] The total liquid absorption rate η1 of the separator is 90% to 138%; and / or,

[0022] The free liquid absorption rate η2 of the separator is 77% to 117%; and / or,

[0023] The rebound ratio α of the separator is 109% to 150%;

[0024] The porosity of the separator is 30% to 70%;

[0025] The air permeability of the separator is 50 to 200 seconds;

[0026] The puncture resistance of the separator is 70 to 160 gf;

[0027] The average surface density of the separator is 10 to 45 g / m 2Preferably, the average surface density of the separator is 18 to 30 g / m 2 .

[0028] The electrochemical device as described above, wherein the self-discharge level of the electrochemical device is not higher than 0.1 mV / h; and / or,

[0029] The constant current charging ratio of the electrochemical device is not less than 75% at a charging rate of 5C; and / or,

[0030] The electrochemical device has a discharge retention rate of not less than 88% at a discharge rate of 5C; and / or

[0031] The electrochemical device has a constant current charging ratio of not less than 68% at a charging rate of 10C; and / or

[0032] The electrochemical device has a discharge retention rate of not less than 87% at a discharge rate of 10C; and / or

[0033] The capacity retention rate of the electrochemical device after 1000 cycles at 0.7C is not less than 80%.

[0034] The electrochemical device as described above, wherein the nanofibers meet at least one of the following characteristics:

[0035] a. The nanofibers comprise a polymer and inorganic particles by mass, wherein the inorganic particles account for 10% to 40% of the mass of the polymer;

[0036] b. The interior of the nanofiber contains inorganic particles, and the surface of the nanofiber has protrusions formed by the inorganic particles.

[0037] The electrochemical device as described above, wherein the polymer comprises at least one of polyvinylidene fluoride, polyvinylidene fluoride, polyimide, polyamide, polyacrylonitrile, polyethylene glycol, polyethylene oxide, polyphenylene ether, polypropylene carbonate, polymethyl methacrylate, polyethylene terephthalate, poly(vinylidene fluoride-chlorotrifluoroethylene) and derivatives thereof; and / or,

[0038] The material of the inorganic particles includes at least one of aluminum oxide, boehmite, silicon oxide, titanium oxide, aluminum hydroxide, magnesium oxide, and barium sulfate.

[0039] The implementation of this application has at least the following beneficial effects:

[0040] The electrochemical device provided in the present application, by changing the density of the nanofibers in the thickness direction of the separator, is conducive to making the area of ​​the separator close to the first electrode or the second electrode have a loose structure, which can ensure that there is sufficient free electrolyte on the side of the separator facing the first electrode or the second electrode, which is conducive to increasing the absorption of free electrolyte, so that the free electrolyte can be provided to the maximum extent during the cycle process, thereby helping to improve the rate performance and cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG1 is a schematic structural diagram of an electrochemical device according to an embodiment of the present application;

[0042] FIG2 is a schematic structural diagram of an electrochemical device in another embodiment of the present application;

[0043] FIG3 is a schematic structural diagram of an electrochemical device in another embodiment of the present application;

[0044] FIG4 is a SEM image of a partial cross section of the electrochemical device in Example 3 of the present application;

[0045] FIG5 is a SEM image of a partial cross section of the electrochemical device in Example 6 of the present application.

[0046] Explanation of reference numerals: 1 - first pole piece; 2 - second pole piece; 3 - separator; 301 - first porous layer; 302 - second porous layer; 303 - third porous layer; 304 - fourth porous layer. DETAILED DESCRIPTION

[0047] To make the purpose, technical solutions, and advantages of this application more clear, the following will be combined with the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are part of the embodiments of this application, not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0048] The electrochemical device provided in the present application includes a first electrode piece, a separator, and a second electrode piece stacked in sequence; the separator includes a porous layer formed by nanofibers; the density of the nanofibers in the separator varies in the thickness direction.

[0049] The electrochemical device can be a laminated structure or a wound structure.

[0050] Taking a laminated structure as an example, an electrochemical device includes a first electrode sheet, a separator, and a second electrode sheet stacked in sequence. The number of first electrode sheets, separators, and second electrode sheets depends on actual needs. In this case, the electrochemical device includes multiple subassemblies stacked in sequence, with each subassembly including a first electrode sheet, a separator, and a second electrode sheet. The direction in which the first electrode sheet, separator, and second electrode sheet are stacked in sequence is essentially the thickness direction.

[0051] The first electrode and the second electrode have opposite polarities. The first electrode can be a positive electrode or a negative electrode, and correspondingly, the second electrode can be a negative electrode or a positive electrode.

[0052] The first pole piece includes a first current collector, a first active layer and a second active layer, the first current collector includes a first functional surface and a second functional surface arranged opposite to each other, the first active layer is arranged on the first functional surface, and the second active layer is arranged on the second functional surface; the second pole piece includes a second current collector, a third active layer and a fourth active layer, the second current collector includes a third functional surface and a fourth functional surface arranged opposite to each other, the third active layer is arranged on the third functional surface, and the fourth active layer is arranged on the fourth functional surface.

[0053] The separator is located between the first electrode and the second electrode. When the first electrode 1, the separator, and the second electrode are stacked in sequence, the separator can be located between the first active layer and the third active layer, or between the second active layer and the fourth active layer. At this time, as shown in Figures 1 to 3, the separator 3 includes a first surface and a second surface arranged opposite to each other. The first surface of the separator 3 is arranged opposite to the first electrode 1, and the second surface of the separator 3 is arranged opposite to the second electrode 2. The separator is used to conduct lithium ions and separate the first electrode and the second electrode to prevent contact and short circuit. The separator includes a porous layer formed by nanofibers. The porous layer can specifically be a porous layer with pores formed by cross-linking, cross-stacking or spinning of conventional polymer fibers in the field.

[0054] The density of the nanofibers will affect the porosity of the separator, which in turn affects the separator's ability to absorb electrolyte, and also affects the rate charge performance and rate discharge performance of the electrochemical device.

[0055] According to the research of this application, the above electrochemical device has excellent rate performance, long cycle performance and capacity retention rate. This is because the density of the nanofibers in the thickness direction of the separator is varied, and the nanofibers are relatively sparse on the side close to the pole piece, which is conducive to achieving sufficient free electrolyte on the side of the separator facing the first pole piece or the second pole piece, and is conducive to improving the absorption of free electrolyte, so that free electrolyte can be provided to the maximum extent during the cycle, which is conducive to improving rate performance and cycle life; at the same time, there is a dense layer of nanofibers in the separator, which can prevent the solvated lithium ions from freely passing through, from the negative electrode through the separator to the positive electrode side, and then embedded in the positive electrode, resulting in serious self-discharge loss of the electrochemical device and reducing its service life. In addition, for a separator with a constant density of nanofibers, it is impossible to take into account the aforementioned characteristics at the same time. The porosity of the separator is either too large or too small. The larger the porosity of the separator, the more likely it is that the solvated lithium ions will pass through freely and be embedded in the positive electrode sheet, resulting in a decrease in the voltage and capacity of the electrochemical device, which in turn leads to serious self-discharge loss of the electrochemical device and reduces the service life of the electrochemical device. The electrochemical device provided in the present application can avoid the Li ion inside the electrochemical device by setting a separator with a varying density. + The directional movement of the electrochemical device can avoid serious self-discharge losses and thus extend the service life of the electrochemical device.

[0056] In some embodiments, the separator satisfies one of the following characteristics: a. The density of the nanofibers in the thickness direction increases from the side close to the first pole piece and the second pole piece to the side away from the first pole piece and the second pole piece; b. The density of the nanofibers increases or decreases along the thickness direction.

[0057] Among them, the greater the density of the nanofibers, the denser the corresponding part; the smaller the density of the nanofibers, the looser the corresponding part.

[0058] In one embodiment, the density of the nanofibers increases in the thickness direction from the side close to the first and second pole pieces to the side away from the first and second pole pieces. In other words, the density of the nanofibers increases from the side close to the first pole piece to the side away from the first pole piece, and the density of the nanofibers increases from the side close to the second pole piece to the side away from the second pole piece. At this time, the density of the nanofibers first increases and then decreases in the thickness direction, and the distribution of the nanofibers in the separator in the thickness direction is loose on both sides and dense in the middle, which is beneficial to simultaneously improve the rate charging performance and rate discharging performance of the electrochemical device. If the nanofibers first decrease and then increase in the thickness direction, the distribution of the nanofibers in the separator in the thickness direction is dense on both sides and loose in the middle, which is not conducive to the rate performance and electrolyte absorption of the electrochemical device.

[0059] In another embodiment, the density of the nanofibers increases or decreases along the thickness direction. In other words, the nanofibers are gradually dense or loose along the thickness direction. In this case, the separator is divided into two parts along the thickness direction, one part is loose and the other part is dense.

[0060] Compared to the prior art, the density of the nanofibers remains unchanged along the thickness direction, equivalent to no change in porosity, but only increases along the thickness direction. This increase in thickness does not change the self-discharge behavior, resulting in a decrease in the capacity and storage voltage of the electrochemical device. However, by limiting the density of the nanofibers to meet the aforementioned characteristics, the present application facilitates maintaining the porosity of the separator within an appropriate range, ensuring a good self-discharge level for the electrochemical device and improving the rate performance of the electrochemical device. Specifically, when the density of the nanofibers increases or decreases along the thickness direction, one portion of the separator becomes loose and another portion becomes dense. In this case, if the loose portion faces the positive electrode, it can improve rate charging performance, specifically increasing the constant current charge ratio of the electrochemical device from 67% to approximately 83% at a charge rate of 5C to 10C. If the loose portion faces the negative electrode, it can improve rate discharge performance, specifically increasing the discharge retention ratio of the electrochemical device from 78% to approximately 94% at a charge rate of 5C to 10C.

[0061] This application does not limit the number of porous layers in the separator, as long as at least one porous layer is included. For example, in some embodiments, the separator includes n porous layers, where 2 ≤ n ≤ 10. The surface density of the porous layers satisfies one of the following characteristics: a) the surface density increases layer by layer along the thickness direction from the side closest to the first and second pole pieces to the side away from the first and second pole pieces; b) the surface density increases or decreases layer by layer along the thickness direction.

[0062] The areal density of the porous layer is the mass of the porous layer per unit area.

[0063] In one embodiment, the surface density of the porous layer increases layer by layer in the thickness direction from the side close to the first and second electrode pieces to the side away from the first and second electrode pieces. In other words, the surface density of the porous layer increases from the side close to the first electrode piece to the side away from the first electrode piece, and the surface density of the porous layer increases from the side close to the second electrode piece to the side away from the second electrode piece. In this case, it is beneficial to achieve a distribution of nanofibers in the thickness direction of the separator with looseness on both sides and denseness in the middle. For example, when the separator includes three porous layers, along the thickness direction, they are the first electrode piece, the first porous layer, the second porous layer, the third porous layer, and the second electrode piece. At this time, the active layer on the first electrode is arranged opposite to the first porous layer, and the active layer on the second electrode is arranged opposite to the third porous layer. The surface density of the first porous layer is less than the surface density of the second porous layer, and the surface density of the third porous layer is less than the surface density of the second porous layer. This ensures that the active layer on the first electrode and the porous layer close to the active layer on the second electrode are loose structures, which can increase the absorption of free electrolyte. At the same time, there is a dense porous layer in the separator, which avoids serious self-discharge loss of the electrochemical device and improves the service life of the electrochemical device.

[0064] In another embodiment, along the thickness direction, the surface density of the porous layer increases or decreases layer by layer along the thickness direction. That is, when the surface density of the porous layer increases layer by layer along the thickness direction, the distribution state of the nanofibers in the separator gradually becomes denser along the thickness direction; when the surface density of the porous layer decreases layer by layer along the thickness direction, the distribution state of the nanofibers in the separator gradually becomes looser along the thickness direction. For example, when the separator includes two porous layers, as shown in FIG1 , along the thickness direction, they are respectively the first electrode 1, the first porous layer 301, the second porous layer 302, and the second electrode 2. At this time, the active layer on the first electrode 1 is arranged opposite to the first porous layer 301, and the active layer on the second electrode 2 is arranged opposite to the second porous layer 302. The surface density of the first porous layer is greater than the surface density of the second porous layer, or the surface density of the first porous layer is less than the surface density of the second porous layer. This ensures that the porous layer close to the active layer on the first electrode or the active layer on the second electrode is a loose structure, which can increase the amount of liquid absorbed by the free electrolyte. When the separator includes three porous layers, as shown in FIG2 , along the thickness direction, they are respectively the first electrode 1, the first porous layer 301, the second porous layer 302, the third porous layer 303, the second electrode 2, and so on. When the separator includes four porous layers, as shown in FIG3 , along the thickness direction, they are respectively the first electrode 1, the first porous layer 301, the second porous layer 302, the third porous layer 303, the fourth porous layer 304, the second electrode 2, and so on. When the separator includes n porous layers, along the thickness direction, they are respectively the first electrode, the first porous layer, the second porous layer...the nth porous layer, the second electrode. In this case, the active layer on the first electrode is arranged opposite to the first porous layer, and the active layer on the second electrode is arranged opposite to the nth porous layer. The surface density of the first porous layer, the second porous layer...the nth porous layer increases or decreases layer by layer.

[0065] This application does not limit the diameters of the separator and the fibers in the porous layer; these diameters may be determined based on practical needs. For example, in some embodiments, the separator has a thickness greater than 0 μm and no greater than 50 μm, preferably 5 to 20 μm; and / or the fibers in the porous layer have a diameter of 100 to 1500 nm, such as 100 nm, 200 nm, 300 nm, 500 nm, 800 nm, 1000 nm, 1500 nm, or any combination thereof.

[0066] In some embodiments, the porosity of the porous layer increases or decreases layer by layer along the thickness direction.

[0067] The diameter of the nanofibers is related to the density of the separator: smaller nanofiber diameters facilitate a denser structure, while larger nanofiber diameters facilitate a looser structure. In some embodiments, the separator comprises n porous layers, where 2 ≤ n ≤ 10. The nanofiber diameters satisfy one of the following characteristics: a) decreasing layer by layer along the thickness direction from the side closest to the first and second pole pieces to the side further away from the first and second pole pieces; b) increasing or decreasing layer by layer along the thickness direction. These embodiments facilitate varying the density of the nanofibers along the thickness direction.

[0068] In one embodiment, the diameter of the nanofiber decreases layer by layer in the thickness direction from the side close to the first pole piece and the second pole piece to the side away from the first pole piece and the second pole piece. That is to say, the diameter of the nanofiber decreases from the side close to the first pole piece to the side away from the first pole piece, and the diameter of the nanofiber decreases from the side close to the second pole piece to the side away from the second pole piece. At this time, the diameter of the nanofiber first decreases and then increases in the thickness direction, and the distribution state of the nanofibers in the separator in the thickness direction is loose on both sides and dense in the middle, which is beneficial to simultaneously improve the washing performance of the separator and the rate performance of the electrochemical device. If the diameter of the nanofiber first increases and then decreases in the thickness direction, the distribution state of the nanofibers in the separator in the thickness direction is dense on both sides and loose in the middle, which is not conducive to the rate performance of the electrochemical device.

[0069] In another embodiment, the diameter of the nanofiber increases or decreases layer by layer along the thickness direction, that is, when the diameter of the nanofiber increases along the thickness direction, the nanofiber gradually becomes loose along the thickness direction; when the diameter of the nanofiber decreases along the thickness direction, the nanofiber gradually becomes dense along the thickness direction. Compared with the prior art, the diameter of the nanofiber remains unchanged in the thickness direction, which is equivalent to the density of the nanofiber remaining unchanged, the porosity does not change, and only the thickness increases. The increase in thickness cannot change the self-discharge situation, and will lead to a decrease in the capacity and placement voltage of the electrochemical device. The present application satisfies the above-mentioned characteristics by limiting the density of the nanofiber, which is conducive to making the porosity of the separator within an appropriate range, so that the self-discharge of the electrochemical device is at a better level, and at the same time, the rate performance of the electrochemical device is improved to a certain extent.

[0070] This application does not limit the specific diameter of the nanofibers. In some embodiments, the diameter of the nanofibers meets one of the following characteristics: a) the diameter on the side closest to the first and second pole pieces is 300 to 1500 nm, and the diameter on the side away from the first and second pole pieces is 100 to 1000 nm; b) at least one layer has a diameter of 100 to 1000 nm.

[0071] In one embodiment, when the distribution of nanofibers in the separator is loose on both sides and dense in the middle, the diameter of the nanofibers on the side close to the first pole piece and the second pole piece is 300~1500nm, and the diameter on the side away from the first pole piece and the second pole piece is 100nm~1000nm. That is to say, in the thickness direction, the diameter of the nanofibers on both sides facing the pole pieces is 300~1500nm, and the diameter of the nanofibers in the middle is 100nm~1000nm.

[0072] In another embodiment, when the distribution of nanofibers in the separator increases or decreases layer by layer, by limiting the diameter of at least one layer to 100-1000 nm, the maximum surface density of the porous layer is ensured, that is, the minimum porosity of the porous layer that can be achieved, thereby preventing short circuits caused by excessive porosity. In some embodiments, the average surface density of the separator is 14-35 g / m 2 Preferably, the average surface density of the separator is 20 to 28 g / m 2 When the separator includes multiple porous layers, the average surface density of the separator refers to the ratio of the total mass to the area of ​​the multiple porous layers after stacking.

[0073] In this application, after the electrochemical device is filled with electrolyte, the electrolyte not only soaks the electrode sheets but also resides within the separator, facilitating the replenishment of free electrolyte during later cycles. In some embodiments, the total liquid absorption rate η1 of the separator is 90% to 138%; and / or the free liquid absorption rate η2 of the separator is 77% to 117%; and / or the rebound ratio α of the separator is 109% to 150%.

[0074] Wherein, the total liquid absorption rate of the separator η1 = (W2-W1) / W1×100%, where W1 is the weight of the separator before immersion in the electrolyte at 25°C±3°C, humidity <50% RH, and atmospheric pressure, and W2 is the weight of the separator after immersion in the electrolyte for 24 hours. In the specific implementation process of this application, at 25°C±3°C, humidity <50% RH, and atmospheric pressure, a separator with a length of a cm, a width of b cm, and a thickness of c cm is weighed as W1. The separator is then immersed in the electrolyte, sealed and stored for 24 hours, removed, and the electrolyte on the outside is wiped off, and the weight is weighed as W2.

[0075] The free liquid absorption rate of the separator is η2 = W3 / W1×100%, where W1 is the weight of the separator before being immersed in the electrolyte at 25°C ± 3°C, humidity <50% RH, and normal pressure. The separator weighing W2 is placed under a pressure plate (the pressure of the pressure plate is 1.2MPa) to press out the absorbed electrolyte, and the pressed electrolyte is caught in a container. The weight of the pressed electrolyte is then weighed as W3. W3 is essentially the weight of the electrolyte that can be absorbed by the interlayer or interlayer pores of the separator. The size of W3 can also be considered as the electrolyte stored in the separator or the free electrolyte directly supplied to the electrochemical device. This part of the electrolyte can be used to improve the rate performance of the electrochemical device.

[0076] Rebound ratio α = h2 / h1 × 100%, where h1 is the thickness of the separator after being immersed in the electrolyte for 24 hours and applying a pressure of 1.2 MPa to the separator, and h2 is the thickness of the separator after applying a pressure of 1.2 MPa and allowing it to stand at 25°C, 45% RH to 65% RH for 4 hours. In the specific implementation of the present application, a fresh electrochemical device can be disassembled and the separator can be soaked. The separator is then placed between parallel plates and a certain pressure (for example, a pressure of 1.2 MPa) is applied to the parallel plates to obtain the thickness h1 of the separator. The separator is then allowed to stand in a normal temperature and humidity environment (25°C, 45% RH to 65% RH) for 4 hours, and then the thickness of the removed separator is tested to obtain the thickness h2 of the separator.

[0077] In the prior art, due to the limitation of the porous layer itself, the internal Li + The directional movement leads to serious self-discharge loss and shortens the service life. The electrochemical device of the present application is not only conducive to improving the replenishment of free electrolyte in the later cycle process, but also can maintain the self-discharge level to no more than 0.1mV / h to the maximum extent.

[0078] The porosity of the separator affects the amount of electrolyte stored in the separator. In some embodiments, the porosity of the separator is 30% to 70%. By limiting the porosity of the separator, the rate performance of the electrochemical device can be further improved.

[0079] In this application, the permeability of the porous layer is 50-200s. Such a porous layer can act as a separator to isolate the positive and negative electrodes, so that the self-discharge level of the electrochemical device can be kept at a low level. The permeability value refers to the area of ​​6.45cm2 that 100mL of air can pass through under an applied pressure of 1.21kPa. 2 The time required for the porous layer. The applied pressure of 1.21kPa is a constant pressure, and the 6.45cm 2 The area is fixed.

[0080] In an electrochemical device, to prevent burrs on the first and second electrode sheets from piercing the separator and causing a short circuit in the electrochemical device, in some embodiments, the separator has a puncture resistance of 70 to 160 gf. Limiting the puncture resistance to this range helps improve the safety of the electrochemical device.

[0081] The electrochemical device of the present application can improve the replenishment of free electrolyte in the later cycle process, thereby improving the cycle life. In addition, it is also beneficial to improve the rate performance of the electrochemical device. In some embodiments, the constant current charging ratio of the electrochemical device is not less than 75% at a charge rate of 5C; and / or, the discharge retention rate of the electrochemical device is not less than 88% at a discharge rate of 5C; and / or, the constant current charging ratio of the electrochemical device is not less than 68% at a charge rate of 10C; and / or, the discharge retention rate of the electrochemical device is not less than 87% at a discharge rate of 10C; and / or, the capacity retention rate of the electrochemical device after 1000 cycles at 0.7C is not less than 80%.

[0082] In some embodiments, the nanofibers satisfy at least one of the following characteristics: a. The nanofibers contain polymers and inorganic particles in mass fraction, and the inorganic particles are 10% to 40% of the mass ratio of the polymer; b. The interior of the nanofibers contains inorganic particles, and the surface of the nanofibers has protrusions formed by the inorganic particles.

[0083] Specifically, the nanofibers forming the porous layer contain polymers and inorganic particles. The inorganic particles are effectively embedded into the interior of the nanofibers using electrostatic spinning technology. The inorganic particles form protrusions on the nanofibers. The inorganic particles may not penetrate or partially penetrate the nanofibers, and a porous layer with high puncture resistance can be obtained. The effective puncture resistance of the porous layer can be 5 to 8 times higher than that of the porous layer without adding inorganic particles. It can effectively prevent foreign objects from piercing the separator, thereby achieving the purpose of preventing short circuits between the positive and negative electrodes. This is because the inorganic particles can produce a pulling effect on the nanofibers, causing a certain amount of tearing in the porous layer, thereby improving the puncture resistance of the porous layer, and thus making the separator have a higher puncture resistance.

[0084] Furthermore, the inorganic particles account for 10% to 40% of the polymer mass. As the proportion of inorganic particles in the porous layer increases, the puncture resistance of the porous layer first increases and then decreases. This is mainly because the inorganic particles used have a large specific surface area, and the area that can be connected to the nanofibers increases. As the mass of the inorganic particles increases to about 25% of the polymer, the tearing effect on the nanofibers during the spinning process is the best, and the puncture resistance is the highest at this time, which can reach 5 to 8 times the original puncture resistance. When the content of inorganic particles is low, the effect of pulling the nanofibers is weakened, resulting in a less obvious increase in puncture resistance. However, as the mass of the inorganic particles increases to 40% to 50% of the polymer, the effective specific surface area is not sufficiently covered by the polymer, the nanofibers cannot be fully torn, and the excessive amount of inorganic particles will lead to a decrease in the toughness of the nanofibers, and the puncture resistance will also decrease.

[0085] The present application does not make too many restrictions on the specific materials of polymers and inorganic particles. For example, polymers include polyvinylidene fluoride, polyvinylidene fluoride, polyimide, polyamide, polyacrylonitrile, polyethylene glycol, polyethylene oxide, polyphenylene ether, polypropylene carbonate, polymethyl methacrylate, polyethylene terephthalate, poly (vinylidene fluoride - chlorotrifluoroethylene) and its derivatives at least one. The melting points of these polymers are significantly higher than polyethylene (PE) finished products. Therefore, relative to the electrochemical device containing the prior art PE diaphragm, the furnace temperature safety of the electrochemical device of the present application is at a higher level and can pass the furnace temperature test at a pressure drop level within 500mV. The material of the inorganic particles includes at least one of aluminum oxide, boehmite, silicon oxide, titanium oxide, aluminum hydroxide, magnesium oxide, and barium sulfate.

[0086] This application does not limit the specific parameters of the inorganic particles. For example, the specific surface area of ​​the inorganic particles is 5 to 50 m 2 / g, preferably 18 to 32 m 2 / g, the larger the specific surface area of ​​the inorganic particles, the larger the effective area in contact with the polymer, further increasing the pulling effect of the polymer in the spinning process, and making the puncture ability of the porous layer greater. When the inorganic particles are 20% to 30% of the polymer mass ratio, the puncture resistance of the porous layer can reach 5 to 8 times the original puncture resistance strength, and at the same time increase the air permeability of the porous layer to 50 to 200s.

[0087] In some embodiments, the molecular weight of the polymer is 7×10 5 ~10×10 5 , crystallinity is 48-52%, and melting point is 151-153℃.

[0088] This application does not limit the specific preparation process of each porous layer, and it can be prepared using conventional spinning equipment such as electrospinning machines in the field. The porous layer can be prepared by a method including the following steps: dissolving a polymer in a solvent to obtain a spinning solution, wherein the solvent includes at least one of acetone and N,N-dimethylformamide, and the mass ratio of the polymer to the solvent is (10:90) to (20:80); electrospinning the spinning solution to obtain an intermediate layer; and baking the intermediate layer to obtain a porous layer, wherein the drying temperature is 60 to 100°C and the drying time is 0.5 to 3 hours.

[0089] This application does not limit the spinning parameters of electrospinning. For example, in some embodiments, the following parameters are used for spinning: the reciprocating speed of the spinning needle is 200-320 cm / min, the needle propulsion speed is 1.0-1.6 mL / h, the needle is 22-28G, the high voltage of the electrostatic is +5-25.7 KV, the negative high voltage is -1-15.4 KV, the receiving disk speed is 500-800 r / min, and the effective spinning time is 18 min-1 h. The receiving disk can be copper foil or other foil materials as a carrier that can carry the porous layer. The electrode can also be used directly as a carrier for spinning. When the electrode is used as a carrier, the active layer of the electrode needs to be the receiving surface.

[0090] By using different spinning speeds and needle slurry propulsion speeds, porous layers with different surface densities can be obtained. Adjusting the reciprocating speed of the spinning needle and the rotation speed of the receiving disk can help make the porous layer more uniform. The separator of the present application can be obtained by stacking the layers in a sequence of increasing and then decreasing surface density, or increasing or decreasing layer by layer.

[0091] The present application does not limit the specific type of the electrochemical device, which may be a lithium-ion battery, for example.

[0092] The present application will be further described below through specific examples and comparative examples.

[0093] Example 1

[0094] 1. Preparation of separators

[0095] PVDF, acetone, and DMF are mixed in a mass ratio of 3:5:12, and then inorganic particles are added to the above solution in a mass ratio of PVDF:inorganic particles = 4:1, and the spinning solution is obtained after stirring;

[0096] The spinning solution was spun using a spinning device according to the following spinning parameters: the reciprocating speed of the spinning needle was 200 cm / min, the needle propulsion speed was 1.0 mL / h, a 22G needle was used, the high voltage of the electrostatic discharge was +15.7 kV, the negative high voltage was -5.4 kV, the receiving disk speed was 500 r / min, and the effective spinning time was 1 h. The first porous layer was obtained, and its surface density was 14.6 g / m 2 ;

[0097] Then the parameters of the spinning equipment were adjusted to obtain the second porous layer, whose surface density was 21.9 g / m 2 ;

[0098] Adjust the parameters of the spinning equipment to obtain the third porous layer, whose surface density is 32.9g / m 2 ;

[0099] Adjust the parameters of the spinning equipment to obtain the fourth porous layer, whose surface density is 49.3g / m 2 ;

[0100] The density of the nanofibers in the porous layer is evenly distributed in the thickness direction.

[0101] The first porous layer and the second porous layer are arranged along the thickness direction and baked at 85°C for 1 hour to obtain the separator comprising the first porous layer and the second porous layer of this embodiment, wherein the average surface density of the separator is 18.3 g / m 2 The total thickness of the separator is 20 μm, and the thickness of the first porous layer and the second porous layer is about 10 μm.

[0102] 2. Preparation of positive electrode

[0103] The positive electrode active material LiCoO2, conductive carbon black, and polyvinylidene fluoride (PVDF) are fully stirred and mixed in N-methylpyrrolidone solvent at a mass percentage ratio of 97.8:1.1:1.1 to obtain a positive electrode slurry;

[0104] The positive electrode slurry is coated on the functional surface of the aluminum foil, and the positive electrode sheet is obtained after drying, rolling and cutting.

[0105] 3. Preparation of negative electrode sheet

[0106] The negative electrode active material natural graphite, the conductive agent carbon nanotubes, the adhesive SBR, and the dispersant sodium carboxymethyl cellulose / CMC are mixed in a mass percentage ratio of 96.6:0.6:1.3:1.2 and dispersed in deionized water to form a negative electrode slurry. The negative electrode slurry is coated on a copper foil, and the negative electrode sheet is obtained after drying, rolling and cutting.

[0107] 4. Assembly of lithium-ion batteries

[0108] The separator can be prepared separately, and then the positive electrode sheet, separator, and negative electrode sheet are stacked in sequence; or the separator can be directly spun on the electrode sheet. The first porous layer is arranged opposite to the negative electrode sheet. In this case, the negative electrode sheet, the first porous layer, the second porous layer, and the positive electrode sheet are stacked in sequence, and then the electrode assembly is obtained by lamination or winding. After welding the electrode ears, packaging, baking, liquid injection, formation, secondary sealing, sorting, and OCV, a lithium-ion battery is obtained.

[0109] The electrolyte includes ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and LiPF6, wherein the mass ratio of EC:EMC:DEC is 2:5:3, and the LiPF6 is 1.2 mol / L.

[0110] Example 2

[0111] The preparation process is basically the same as that of Example 1, except that the separator is different. The preparation process of the separator of this embodiment is as follows: the first porous layer, the second porous layer, and the third porous layer of Example 1 are arranged along the thickness direction and baked at 85°C for 1h to obtain a separator comprising the first porous layer, the second porous layer, and the third porous layer, wherein the average surface density of the separator is 23.2g / m 2 The total thickness of the separator is 20 μm, and the thicknesses of the first porous layer, the second porous layer, and the third porous layer are approximately 6.7 μm.

[0112] The separator of Example 1 is replaced with the separator of this example, and the negative electrode sheet, the first porous layer, the second porous layer, the third porous layer, and the positive electrode sheet are stacked in sequence, while other conditions remain unchanged, to obtain the lithium ion battery of this example.

[0113] Example 3

[0114] The preparation process is basically the same as that of Example 1, except that the separator is different. The preparation process of the separator of this embodiment is as follows:

[0115] The first porous layer, the second porous layer, the third porous layer and the fourth porous layer are arranged along the thickness direction and baked at 85°C for 1 hour to obtain a separator comprising the first porous layer, the second porous layer, the third porous layer and the fourth porous layer, wherein the average surface density of the separator is 29.7 g / m 2 The total thickness of the separator is 20 μm, and the thicknesses of the first porous layer, the second porous layer, the third porous layer, and the fourth porous layer are approximately 5 μm.

[0116] The separator of Example 1 is replaced by the separator of this embodiment, and the negative electrode sheet, the first porous layer, the second porous layer, the third porous layer, the fourth porous layer, and the positive electrode sheet are stacked in sequence, while other conditions remain unchanged, to obtain the lithium ion battery of this embodiment.

[0117] Example 4

[0118] The preparation process is basically the same as that of Example 1, except that the separator is different. The preparation process of the separator of this embodiment is as follows: the fourth porous layer and the third porous layer are arranged along the thickness direction and baked at 85°C for 1 hour to obtain a separator comprising the fourth porous layer and the third porous layer, wherein the average surface density of the separator is 41.1 g / m 2 The total thickness of the separator is 20 μm, and the thickness of the fourth porous layer and the third porous layer is about 10 μm.

[0119] The separator of Example 1 is replaced with the separator of this example, and the negative electrode sheet, the fourth porous layer, the third porous layer, and the positive electrode sheet are stacked in sequence, while other conditions remain unchanged, to obtain the lithium ion battery of this example.

[0120] Example 5

[0121] The preparation process is basically the same as that of Example 1, except that the separator is different. The preparation process of the separator of this embodiment is as follows:

[0122] The fourth porous layer, the third porous layer, and the second porous layer are arranged along the thickness direction and baked at 85°C for 1 hour to obtain a separator comprising the fourth porous layer, the third porous layer, and the second porous layer, wherein the average surface density of the separator is 34.7 g / m 2 The total thickness of the separator is 20 μm, and the thicknesses of the fourth porous layer, the third porous layer, and the second porous layer are approximately 6.7 μm.

[0123] The separator of Example 1 is replaced by the separator of this embodiment, and the negative electrode sheet, the fourth porous layer, the third porous layer, the second porous layer, and the positive electrode sheet are stacked in sequence, while other conditions remain unchanged, to obtain the lithium ion battery of this embodiment.

[0124] Example 6

[0125] The preparation process is basically the same as that of Example 1, except that the separator is different. The preparation process of the separator of this embodiment is as follows:

[0126] The fourth porous layer, the third porous layer, the second porous layer, and the first porous layer are arranged along the thickness direction and baked at 85°C for 1 hour to obtain a separator comprising the fourth porous layer, the third porous layer, the second porous layer, and the first porous layer, wherein the average surface density of the separator is 29.7 g / m 2 The total thickness of the separator is 20 μm, and the thicknesses of the fourth porous layer, the third porous layer, the second porous layer, and the first porous layer are approximately 5 μm.

[0127] The separator of Example 1 is replaced by the separator of this embodiment, and the negative electrode sheet, the fourth porous layer, the third porous layer, the second porous layer, the first porous layer, and the positive electrode sheet are stacked in sequence, while other conditions remain unchanged, to obtain the lithium ion battery of this embodiment.

[0128] Example 7

[0129] The preparation process is basically the same as that of Example 1, except that the separator is different. The preparation process of the separator of this embodiment is as follows:

[0130] The first porous layer, the fourth porous layer, and the first porous layer are arranged along the thickness direction and baked at 85°C for 1 hour to obtain a separator comprising the first porous layer, the fourth porous layer, and the first porous layer, wherein the average surface density of the separator is 26.2 g / m 2 The total thickness of the separator is 20 μm, and the thicknesses of the first porous layer, the fourth porous layer, and the first porous layer are approximately 6.7 μm.

[0131] The separator of Example 1 is replaced with the separator of this embodiment, and the negative electrode sheet, the first porous layer, the fourth porous layer, the first porous layer, and the positive electrode sheet are stacked in sequence, while other conditions remain unchanged, to obtain the lithium ion battery of this embodiment.

[0132] Example 8

[0133] The preparation process is basically the same as that of Example 1, except that the spinning solution and the separator are different. The preparation process of this embodiment is as follows:

[0134] PVDF, acetone, and DMF were mixed in a mass ratio of 3:5:12, and then inorganic particles were added to the above solution in a mass ratio of PVDF:inorganic particles = 9:1, and the spinning solution of the present embodiment was obtained after stirring; the spinning solution of the present embodiment was replaced by the spinning solution of the present embodiment, and the spinning parameters were the same as those of the present embodiment. The first porous layer of the present embodiment (surface density 12.87 g / m 2 ), the fourth porous layer (surface density 43.44g / m 2 ), the first porous layer of this embodiment (surface density 12.87g / m 2 ), the fourth porous layer (surface density 43.44g / m 2 ), the first porous layer (surface density 12.87g / m 2 ) is arranged along the thickness direction and baked at 85°C for 1 hour to obtain a separator comprising a first porous layer, a fourth porous layer, and a first porous layer, wherein the average surface density of the separator is 23.1 g / m 2 The total thickness of the separator is 20 μm, and the thicknesses of the first porous layer, the fourth porous layer, and the first porous layer are approximately 6.7 μm.

[0135] The separator of Example 1 is replaced with the separator of this embodiment, and the negative electrode sheet, the first porous layer, the fourth porous layer, the first porous layer, and the positive electrode sheet are stacked in sequence, while other conditions remain unchanged, to obtain the lithium ion battery of this embodiment.

[0136] Example 9

[0137] The preparation process is basically the same as that of Example 1, except that the spinning solution and the separator are different. The preparation process of this embodiment is as follows:

[0138] PVDF, acetone, and DMF were mixed in a mass ratio of 3:5:12, and then inorganic particles were added to the above solution in a mass ratio of PVDF: inorganic particles = 5:2, and the spinning solution of the present embodiment was obtained after stirring; the spinning solution of the present embodiment was replaced by the spinning solution of the present embodiment, and the spinning parameters were the same as those of the present embodiment. The first porous layer of the present embodiment (surface density 16.38 g / m2) was prepared by using the spinning parameters of the first and fourth porous layers prepared in the present embodiment. 2 ), the fourth porous layer (surface density 55.28g / m 2 ), the first porous layer of this embodiment (surface density 16.38g / m 2 ), the fourth porous layer (surface density 55.28g / m 2 ), the first porous layer (surface density 16.38g / m 2 ) is arranged along the thickness direction and baked at 85°C for 1 hour to obtain a separator comprising a first porous layer, a fourth porous layer, and a first porous layer, wherein the average surface density of the separator is 29.3 g / m 2 The total thickness of the separator is 20 μm, and the thicknesses of the first porous layer, the fourth porous layer, and the first porous layer are approximately 6.7 μm.

[0139] The separator of Example 1 is replaced with the separator of this embodiment, and the negative electrode sheet, the first porous layer, the fourth porous layer, the first porous layer, and the positive electrode sheet are stacked in sequence, while other conditions remain unchanged, to obtain the lithium ion battery of this embodiment.

[0140] Example 10

[0141] The preparation process is basically the same as that of Example 1, except that the spinning solution and the separator are different. The preparation process of this embodiment is as follows:

[0142] PVDF, acetone, and DMF were mixed in a mass ratio of 3:5:12, and then inorganic particles were added to the above solution in a mass ratio of PVDF: inorganic particles = 20:1, and the spinning solution of the present embodiment was obtained after stirring; the spinning solution of the present embodiment was replaced by the spinning solution of the present embodiment, and the spinning parameters were the same as those of the present embodiment. The first porous layer of the present embodiment (surface density 10.28-14.28 g / m2) was prepared by using the spinning parameters of the first and fourth porous layers prepared in the present embodiment. 2 ), the fourth porous layer (surface density 39.46 ~ 43.46g / m 2 ), the first porous layer of this embodiment (surface density 12.28g / m 2 ), the fourth porous layer (surface density 41.46g / m 2 ), the first porous layer (surface density 12.28g / m 2 ) is arranged along the thickness direction and baked at 85°C for 1h to obtain a separator comprising a first porous layer, a fourth porous layer, and a first porous layer, wherein the average surface density of the separator is 22.0g / m 2 The total thickness of the separator is 20 μm, and the thicknesses of the first porous layer, the fourth porous layer, and the first porous layer are approximately 6.7 μm.

[0143] The separator of Example 1 is replaced with the separator of this embodiment, and the negative electrode sheet, the first porous layer, the fourth porous layer, the first porous layer, and the positive electrode sheet are stacked in sequence, while other conditions remain unchanged, to obtain the lithium ion battery of this embodiment.

[0144] Example 11

[0145] The preparation process is basically the same as that of Example 1, except that the spinning solution and the separator are different. The preparation process of this embodiment is as follows:

[0146] PVDF, acetone, and DMF were mixed in a mass ratio of 3:5:12, and then inorganic particles were added to the above solution in a mass ratio of PVDF:inorganic particles = 2:1, and the spinning solution of the present embodiment was obtained after stirring; the spinning solution of the present embodiment was replaced by the spinning solution of the present embodiment, and the spinning parameters were the same as those of the present embodiment. The first porous layer of the present embodiment (surface density 17.55 g / m2) was prepared by using the spinning parameters of the first and fourth spinning layers prepared in the present embodiment. 2 ), the fourth porous layer (surface density 5.23 ~ 59.23g / m 2 ), the first porous layer (surface density 17.55g / m2) and the fourth porous layer (surface density 59.23g / m 2 ), the first porous layer (surface density 17.55g / m 2) is arranged along the thickness direction and baked at 85°C for 1 hour to obtain a separator comprising a first porous layer, a fourth porous layer, and a first porous layer, wherein the average surface density of the separator is 31.4 g / m 2 The total thickness of the separator is 20 μm, and the thicknesses of the first porous layer, the fourth porous layer, and the first porous layer are approximately 6.7 μm.

[0147] The separator of Example 1 is replaced with the separator of this embodiment, and the negative electrode sheet, the first porous layer, the fourth porous layer, the first porous layer, and the positive electrode sheet are stacked in sequence, while other conditions remain unchanged, to obtain the lithium ion battery of this embodiment.

[0148] Example 12

[0149] The preparation process is basically the same as that of Example 1, except that the spinning solution and the separator are different. The preparation process of this embodiment is as follows:

[0150] PVDF, acetone, and DMF were mixed in a mass ratio of 3:5:12 and stirred to obtain the spinning solution of the present embodiment. The spinning solution of Example 1 was replaced by the spinning solution of the present embodiment, and the spinning parameters were the same as those of Example 1. The spinning parameters of the first and fourth porous layers of Example 1 were used to prepare the first porous layer of the present embodiment (surface density 11.7 g / m 2 ), the fourth porous layer (surface density 39.5g / m 2 ), the first porous layer of this embodiment (surface density 11.7g / m 2 ), the fourth porous layer (surface density 39.5g / m 2 ), the first porous layer (surface density 11.7g / m 2 ) is arranged along the thickness direction and baked at 85°C for 1 hour to obtain a separator comprising a first porous layer, a fourth porous layer, and a first porous layer, wherein the average surface density of the separator is 20.9 g / m 2 The total thickness of the separator is 20 μm, and the thicknesses of the first porous layer, the fourth porous layer, and the first porous layer are approximately 6.7 μm.

[0151] The separator of Example 1 is replaced with the separator of this embodiment, and the negative electrode sheet, the first porous layer, the fourth porous layer, the first porous layer, and the positive electrode sheet are stacked in sequence, while other conditions remain unchanged, to obtain the lithium ion battery of this embodiment.

[0152] Example 13

[0153] The preparation process is basically the same as that of Example 1, except that the separator is different. The preparation process of the separator of this embodiment is as follows:

[0154] The first porous layer, the second porous layer, and the first porous layer are arranged along the thickness direction and baked at 85°C for 1 hour to obtain a separator comprising the first porous layer, the second porous layer, and the first porous layer, wherein the average surface density of the separator is 17.1 g / m 2 The total thickness of the separator is 20 μm, and the thicknesses of the first porous layer, the fourth porous layer, and the first porous layer are approximately 6.7 μm.

[0155] The separator of Example 1 is replaced with the separator of this embodiment, and the negative electrode sheet, the first porous layer, the fourth porous layer, the first porous layer, and the positive electrode sheet are stacked in sequence, while other conditions remain unchanged, to obtain the lithium ion battery of this embodiment.

[0156] Comparative Example 1

[0157] The preparation process is basically the same as that of Example 1, except that the spinning parameters are different without adding inorganic particles and separators: the spinning equipment is used according to the following parameters: the reciprocating speed of the spinning needle is 200 cm / min, the needle propulsion speed is 1.0 mL / h, the needle is 22G, the high voltage of the electrostatic is +15.7 kV, the negative high voltage is -5.4 kV, the receiving disk speed is 500 r / min, and the effective spinning time is 2 h to obtain the intermediate layer;

[0158] The middle layer was baked at 85°C for 1 hour to obtain a separator, wherein the average surface density of the separator was 11.7 g / m 2 , the total thickness of the separator is 20 μm.

[0159] The separator of Example 1 was replaced by the separator of this comparative example, and other conditions remained unchanged to obtain the lithium ion battery of this example.

[0160] Comparative Example 2

[0161] The preparation process was basically the same as that in Example 1, except that the inorganic particles and separators were not added. The spinning parameters were different: the spinning equipment was used according to the following parameters: the reciprocating speed of the spinning needle was 320 cm / min, the needle propulsion speed was 1.0 mL / h, a 28G needle was used, the high voltage of the electrostatic was +15.7 kV, the negative high voltage was -5.4 kV, the receiving disk speed was 800 r / min, and the effective spinning time was 2 h to obtain the intermediate layer;

[0162] The middle layer was baked at 85°C for 1 hour to obtain a separator, wherein the average surface density of the separator was 39.5 g / m 2 , the total thickness of the separator is 20 μm.

[0163] The separator of Example 1 was replaced by the separator of this comparative example, and other conditions remained unchanged to obtain the lithium ion battery of this example.

[0164] Comparative Example 3

[0165] The preparation process is basically the same as that of Example 1, except that the spinning parameters of the separator are adjusted to obtain a three-layer porous surface density of 17.6 g / m 2 , where the average surface density of the separator is 17.6 g / m 2 , the total thickness of the separator is 20 μm.

[0166] The separator of Example 1 was replaced by the separator of this comparative example, and other conditions remained unchanged to obtain the lithium ion battery of this example.

[0167] Test example

[0168] 1. Surface density, total liquid absorption rate, free liquid absorption rate, rebound ratio

[0169] Surface density measurement: Spin the porous layer onto the corresponding foil, then remove the porous layer from the foil and cut it into a porous layer with a length a and a width b. Then, place the porous layer on a scale with an accuracy of ten thousandths and weigh it to obtain the mass m. According to the surface density ρ = m / a / b, the surface density value can be obtained.

[0170] The separators of the embodiment and comparative example and the PP / PE / PP diaphragm were placed in the electrolyte for weighing and soaking. The total liquid absorption rate, free liquid absorption rate and bulk liquid absorption rate were obtained by wiping, weighing, flat pressing and weighing.

[0171] Wherein, the total liquid absorption rate η1 = (W2-W1) / W1×100%, the free liquid absorption rate η2 = W3 / W1×100%, W1 is the weight before immersion in the electrolyte at 25℃±3℃, humidity <50%RH, and normal pressure, W2 is the weight after immersion in the electrolyte for 24 hours, and W3 is the weight of the electrolyte squeezed out after the separator is immersed in the electrolyte for 24 hours and a pressure of 1.2MPa is applied to the separator;

[0172] The rebound ratio α=h2 / h1×100%, h1 is the thickness of the separator after being immersed in the electrolyte for 24 hours and applying a pressure of 1.2 MPa to the separator, and h2 is the thickness of the separator after applying a pressure of 1.2 MPa and standing at 25°C, 45%RH~65%RH for 4 hours.

[0173] 2. Rate performance

[0174] Rate discharge: Place the battery in a temperature environment of 25℃±3℃, then discharge the battery at 0.2C to 3.0V, let it stand for 10 minutes, then charge it at 0.7C to the system cut-off voltage, with a cut-off current of 0.025C, then let it stand for 10 minutes, and then discharge it at different rates. The rates used are as follows: 0.5C / 1.0C / 2.0C / 3.0C / 4.0C / 5.0C / 6.0C / 7.0C / 8.0C / 9.0C / 10.0C, and the discharge capacity Q will be obtained. 放 Then we can get different rate discharge holding ratios α, with α=Q 放 / Q 0.5C × 100%. Where Q 0.5C Refers to the capacity value at a rate of 0.5C.

[0175] Rate charging: Place the battery in a temperature environment of 25℃±3℃, then discharge the battery at 0.2C to 3.0V, let it stand for 10 minutes, then charge it at different rates to the system cut-off voltage, the cut-off current is 0.025C, then let it stand for 10 minutes, and then charge it at different rates. The rates used are as follows: 0.5C / 1.0C / 2.0C / 3.0C / 4.0C / 5.0C / 6.0C / 7.0C / 8.0C / 9.0C / 10.0C. Charge at the above rates until the system cut-off voltage, and the capacity charged is Q 充 At this time, switch to constant voltage charging until the charging cut-off current reaches 0.025C, let it stand for 10 minutes, and then discharge the battery to 3.0V at a current rate of 0.5C to obtain the battery's discharge capacity Q 0.5C The constant current charging ratio β of different charging rates can be obtained, then β=Q 充 / Q 0.5C ×100%.

[0176] 3. Self-discharge test

[0177] Charge the battery with a certain current (for example 0.7C, C is the nominal capacity) to the system stable voltage (generally 3.85V~4.0V) and set the cut-off current to 0.025C. Place the battery in a high temperature environment (45℃±3℃) and store it for 24~36h. When the time comes, take it out and place it at room temperature (25℃±2℃) for 24 hours to measure the battery voltage U1. The time is recorded as t1. Then put it aside for a certain period of time and continue to test the voltage U2 of the same battery. The time is recorded as t2. The self-discharge K value of the battery is recorded as: K=(U1-U2) / (t2-t1), and the unit is generally mV / h. The voltage system of the above test is 4.50V and the volume energy density is 770Wh / L.

[0178] 4. 1000T cycle capacity retention rate

[0179] Place the battery in a temperature environment of 25℃±3℃, let it stand for 4 hours, and then repeat the cycle test according to the 0.7C / 0.7C capacity test. After the cycle is 1000T, the battery is discharged in the last step and the battery power is Q 1000 , the three earliest maximum initial capacity values ​​Q max , we can get the 100T capacity retention rate α, then the capacity retention rate after 1000T α=Q 1000 / Q max ×100%.

[0180] 5. Puncture resistance test

[0181] Under normal temperature and humidity conditions, spread the separator sample flat in a fixture and clamp it. Use a 1.0 mm diameter puncture needle to pierce the separator at a rate of 100±10 mm / min. Measure the force used during puncture. Repeat the test 4 times for each group of samples and take the average value.

[0182] The test results are shown in Table 1.

[0183] Table 1

[0184] Figures 4 and 5 are SEM images of partial cross-sections of the electrochemical devices in Examples 3 and 6, respectively. Figures 4 and 5 show that the nanofibers of the separator are more sparsely distributed in the region near the active layer and more densely distributed in the region away from the active layer.

[0185] As can be seen from Table 1, the use of a multi-layer structure to assemble into a separator can maintain the self-discharge level of the battery in a usable state, and at the same time can maximize the liquid absorption capacity and improve the rate performance of the battery. Although the self-discharge level of Comparative Example 2 is low, its free liquid absorption rate is not high, which is not conducive to the subsequent cycle of the battery. It cannot maximize the free liquid absorption rate and cannot maintain the cycle capacity for a long time; the free liquid absorption rate in Comparative Example 1 is the highest, but the self-discharge level of the assembled battery is high. The battery's own self-discharge during the discharge process greatly aggravates the battery's capacity decay, resulting in the worst rate discharge and rate charge levels of the battery.

[0186] The above describes in detail the preferred embodiments and experimental verifications of the present application. It should be understood that ordinary skill in the art can make numerous modifications and variations based on the concepts of the present application without inventive effort. Therefore, any technical solutions that can be derived by those skilled in the art based on the concepts of the present application through logical analysis, reasoning, or limited experimentation on the basis of the existing technology should be within the scope of protection defined by the claims.

Claims

1. An electrochemical device, wherein: It includes a first pole piece, a separator, and a second pole piece which are stacked in sequence; The separator comprises a porous layer formed of nanofibers; The density of the nanofibers in the separator varies in the thickness direction.

2. The electrochemical device according to claim 1, wherein: The separator meets one of the following characteristics: a. The density of the nanofibers increases in the thickness direction from the side close to the first pole piece and the second pole piece to the side away from the first pole piece and the second pole piece; b. The density of nanofibers increases or decreases along the thickness direction.

3. The electrochemical device according to claim 1 or 2, wherein: The separator comprises n porous layers, wherein 2≤n≤10; and the surface density of the porous layers satisfies one of the following characteristics: a. The surface density increases layer by layer from the side close to the first pole piece and the second pole piece to the side away from the first pole piece and the second pole piece in the thickness direction; b. The surface density increases or decreases layer by layer along the thickness direction.

4. The electrochemical device according to any one of claims 1 to 3, wherein: The thickness of the separator is greater than 0 and not greater than 50 μm; preferably 5 to 20 μm; and / or the fiber diameter of the porous layer is 100 to 1500 nm.

5. The electrochemical device according to any one of claims 1 to 4, wherein: The separator comprises n porous layers, wherein 2≤n≤10; and the diameter of the nanofibers satisfies one of the following characteristics: a. The thickness decreases gradually from the side close to the first pole piece and the second pole piece to the side away from the first pole piece and the second pole piece in the thickness direction; b. Increase or decrease layer by layer along the thickness direction.

6. The electrochemical device according to any one of claims 1 to 5, wherein: The separator comprises n porous layers, wherein 2≤n≤10; and the diameter of the nanofibers satisfies one of the following characteristics: a. The diameter of the side close to the first pole piece and the second pole piece is 300-1500nm, and the diameter of the side away from the first pole piece and the second pole piece is 100-1000nm; b. At least one layer has a diameter of 100 to 1000 nm.

7. The electrochemical device according to any one of claims 1 to 6, wherein: The separator meets at least one of the following characteristics: The total liquid absorption rate η1 of the separator is 90% to 138%; The free liquid absorption rate η2 of the separator is 77% to 117%; The rebound ratio α of the separator is 109% to 150%; The porosity of the separator is 30% to 70%; The air permeability of the separator is 50 to 200 s; The puncture resistance of the separator is 70 to 160 gf; The average surface density of the separator is 10 to 45 g / m 2 .

8. The electrochemical device according to claim 7, wherein: The average surface density of the separator is 18 to 30 g / m 2 .

9. The electrochemical device according to any one of claims 1 to 8, wherein: The self-discharge level of the electrochemical device is no higher than 0.1 mV / h; and / or, The constant current charging ratio of the electrochemical device is not less than 75% when the charging rate is 5C; and / or, The electrochemical device has a discharge retention rate of not less than 88% at a discharge rate of 5C; and / or, The constant current charging ratio of the electrochemical device is not less than 68% when the charging rate is 10C; and / or, The electrochemical device has a discharge retention rate of not less than 87% at a discharge rate of 10C; and / or, The capacity retention rate of the electrochemical device after 1000 cycles at 0.7C is not less than 80%.

10. The electrochemical device according to any one of claims 1 to 9, wherein: The nanofibers satisfy at least one of the following characteristics: a. The nanofibers contain polymers and inorganic particles according to mass fraction, and the inorganic particles account for 10% to 40% of the mass proportion of the polymer; b. The interior of the nanofiber contains inorganic particles, and the surface of the nanofiber has protrusions formed by the inorganic particles.

11. The electrochemical device according to claim 10, wherein: The polymer includes at least one of polyvinylidene fluoride, polyvinylidene fluoride, polyimide, polyamide, polyacrylonitrile, polyethylene glycol, polyethylene oxide, polyphenylene ether, polypropylene carbonate, polymethyl methacrylate, polyethylene terephthalate, poly(vinylidene fluoride-chlorotrifluoroethylene) and derivatives thereof; and / or, The material of the inorganic particles includes at least one of aluminum oxide, boehmite, silicon oxide, titanium oxide, aluminum hydroxide, magnesium oxide, and barium sulfate.

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