A lithium battery separator

Abstract

The present invention belongs to the field of lithium ion technology, and discloses a lithium battery separator, including an inorganic coating, a polyethylene layer and a polypropylene layer, the two sides of the inorganic coating are respectively bonded with polyethylene layers, the side of the polyethylene layer facing away from the inorganic coating is bonded with a polypropylene layer, and a flame retardant capsule is arranged in the polyethylene layer, and the flame retardant capsule includes a polyvinylidene fluoride coating layer and a triphenyl phosphate flame retardant arranged in the polyvinylidene fluoride coating layer. A lithium battery separator provided by the present invention has a polyethylene layer and a polypropylene layer bonded on both sides of the inorganic coating, so that the thickness of the lithium battery separator is increased, and because the inorganic coating is sandwiched in the middle, the safety of the separator is greatly increased, and the effective high-temperature closed-cell mechanism of PP and PE and the embedding of the flame retardant capsule improve the large self-discharge phenomenon of the lithium battery and the safety hazards such as fire and explosion caused by short circuit.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium ions, and in particular relates to a lithium battery separator. Background Art

[0002] With the development of the new energy industry, the importance of lithium batteries has gradually emerged. Lithium metal negative electrode materials have extremely high mass specific capacity, low density and low reduction potential, and are considered to be an ideal negative electrode material for rechargeable batteries. Lithium batteries have high energy storage density, low self-discharge rate, long service life, strong adaptability, and long standby time, but they are costly, have poor safety performance, and are explosive. The positive and negative electrode materials of lithium batteries are both layered structures, and lithium ions are embedded in the layered structure and flow between the positive and negative electrodes. When the lithium battery is in a charging state, lithium ions are deintercalated from the positive electrode material and flow to the negative electrode. Experimental data show that when the charging voltage is higher than the rated maximum charging limit voltage, the number of lithium atoms remaining in the positive electrode material is less than half of the normal value, and the layered storage cells of the positive electrode material begin to collapse. At this time, if charging continues, lithium ions will fill the storage cells of the negative electrode material and accumulate on the surface of the negative electrode material in a dendritic crystal state. These lithium ion crystals pierce the diaphragm, causing the positive and negative electrodes inside the battery to short-circuit. When the charging current is too large, lithium ions will not have time to enter the storage cell, and will gather on the surface of the positive and negative electrode materials. After the aggregation and crystallization, the diaphragm will be punctured to cause a short circuit. The high temperature caused by the short circuit or overcharging causes the electrolyte to decompose and produce gas, which leads to increased internal pressure of the battery, bulging and rupture of the shell, and oxygen enters and reacts with the lithium accumulated on the surface of the electrode, which then explodes. Therefore, the lithium battery diaphragm is one of the most important components of the lithium battery. The preparation of safe, environmentally friendly and low-cost diaphragms is a difficulty in the production process. At present, there are two methods for manufacturing lithium battery diaphragms: wet and dry. Among them, the wet process requires the use of a large amount of organic diluents, the equipment is complex, the cost is high, and it is easy to cause environmental pollution. Compared with the wet process, the dry process has simple equipment, low cost, and no environmental pollution problems. The dry method is mainly divided into dry uniaxial stretching and dry biaxial stretching technology. The microporous membrane prepared by dry biaxial stretching technology has uneven pore formation, mainly because the position and size of the pores formed by polypropylene during the transformation from β crystal to a crystal cannot be controlled. Therefore, the mainstream process of dry diaphragms is now dry uniaxial stretching. After decades of development, this process has become very mature in the United States and Japan. Currently, Celgard Corporation of the United States and UBE Corporation of Japan use this process to produce diaphragms.

[0003] The structure and performance of lithium battery separators are crucial to the safety of lithium batteries: on the one hand, single-layer lithium battery separators are prone to a small number of defects, and the production process is difficult to control, so it is best to cover up the defects of each single-layer separator through a multi-layer separator design; on the other hand, if the separator can be disconnected in time at the initial stage of battery short circuit and temperature rise, further stopping or preventing the short circuit, it can greatly improve the safety of lithium batteries. Existing wet-process separators are usually prepared with ultra-high molecular weight polyethylene, which has a slow melt flow rate, large thermal shrinkage during heating, and cannot be closed in time. The closed-pore temperature of dry-process polypropylene separators is high, and the thermal shrinkage is large during heating, so it cannot respond quickly and protect the safety of the battery. In the dry-process polypropylene (abbreviated PP) / polyethylene (abbreviated PE) / polypropylene three-layer separator, only the polyethylene layer is used as protection, especially when the PE layer is very thin, when the temperature rises further, the membrane will break, resulting in the continuation of the short circuit. In addition, separators with ceramic coatings usually have only one layer of protection, which cannot effectively play a protective role when the accumulated heat causes the temperature to continue to rise. When the battery is in a high temperature environment, the diaphragm will shrink, causing the positive and negative electrodes to contact and short-circuit, resulting in a rapid rise in the battery temperature and even fire, explosion and other hidden dangers. At the same time, when the battery is squeezed or punctured by external force, the positive and negative electrodes are also easy to contact, and the battery will also have an instantaneous short circuit, which not only causes safety hazards, but also causes the battery voltage to drop quickly, resulting in excessive self-discharge of the battery. Existing PP or PE diaphragms are not easy to withstand high temperatures (shrinkage is large at high temperatures, resulting in positive and negative electrodes) and low puncture strength. Therefore, the safety of lithium batteries usually requires higher heat shrinkage resistance and higher puncture strength.

[0004] Micro flame retardant capsules refer to micro containers that use natural or synthetic polymer materials to wrap solid particles or liquid droplets. The diameter of the flame retardant capsules is generally 5-200 microns. The membrane used for wrapping is called the wall material, and the wrapped substance is called the core material. When in use, the prepared micro flame retardant capsules are mixed with polymer materials or other materials, applied to the substrate to be used, and the flame retardant capsule wall is broken by heating, pressurizing or irradiating to release the wrapped substance, producing the required application performance or not destroying the encapsulated membrane. The wrapped material diffuses through the encapsulation membrane and is released outward to produce the effect of controlling performance. The advantage of micro flame retardant capsule technology is that when the flame retardant capsule is formed, the capsule core is wrapped and isolated from the external environment, and its properties can be retained without any influence. Under appropriate conditions, the capsule core can be released when the wall material is destroyed, which brings a lot of convenience to use. Summary of the invention

[0005] In order to solve the above problems existing in the prior art, the present invention aims to provide a lithium battery separator with a flame retardant capsule structure.

[0006] The technical solution adopted by the present invention is:

[0007] A lithium battery separator comprises an inorganic coating, a polyethylene layer and a polypropylene layer. The two sides of the inorganic coating are respectively bonded with polyethylene layers, and the side of the polyethylene layer facing away from the inorganic coating is bonded with a polypropylene layer. A flame retardant capsule is arranged in the polyethylene layer. The flame retardant capsule comprises a polyvinylidene fluoride (abbreviated as PVDF) coating layer and a triphenyl phosphate (abbreviated as TPP) flame retardant arranged in the polyvinylidene fluoride coating layer.

[0008] The flame retardant capsule is composed of a polyvinylidene fluoride coating layer of microfibers with a core-shell structure, in which the triphenyl phosphate flame retardant is the core and the polyvinylidene fluoride coating layer is the shell. The triphenyl phosphate flame retardant is encapsulated in the polyvinylidene fluoride coating layer, which prevents the triphenyl phosphate flame retardant from being directly exposed and dissolved into the electrolyte, preventing its negative impact on the electrochemical performance of the battery. When thermally triggered, the polyvinylidene fluoride coating layer melts and then releases the encapsulated triphenyl phosphate flame retardant into the electrolyte, thereby effectively suppressing the ignition and combustion of the electrolyte.

[0009] Preferably, the weight ratio of the triphenyl phosphate flame retardant to the polyvinylidene fluoride coating layer is (7-9):2.

[0010] Preferably, the inorganic coating is a silicon dioxide layer.

[0011] Preferably, the polypropylene layer is made of polypropylene with an isotacticity greater than or equal to 96%, a melt index of 0.3-5 g / 10 min, and a β-nucleating agent conversion; the porosity of the polypropylene layer is 20%-60%.

[0012] Preferably, the polyethylene layer is made of high-density polyethylene with a melt index of 0.05-5 g / 10 min, and the porosity of the polyethylene layer is 20%-60%.

[0013] Preferably, the porosity of the inorganic coating is 10%-70%.

[0014] Preferably, the thickness of the polyethylene layer is 5-10 μm, the thickness of the polypropylene layer is 5-10 μm, and the thickness of the inorganic coating layer is 0.5-1 μm.

[0015] Preferably, the melting point of the polyethylene layer is 125°C-135°C; the melting point of the polypropylene layer is 155°C-165°C; and the heat-resistant temperature of the inorganic coating is higher than 200°C.

[0016] The beneficial effects of the present invention are:

[0017] 1. The lithium battery separator provided by the present invention has a polyethylene layer and a polypropylene layer respectively bonded on both sides of the inorganic coating, so that the thickness of the lithium battery separator is increased, and because the inorganic coating is sandwiched in the middle, the safety of the separator is greatly increased. The effective high-temperature closed-cell mechanism of PP and PE and the embedding of flame-retardant capsules improve the large self-discharge phenomenon of lithium batteries and the safety hazards such as fire and explosion caused by short circuit.

[0018] 2. The lithium battery separator provided by the present invention has improved β-crystal PP, high-density PE and inorganic coating, all of which have higher puncture strength than conventional PP, PE or PP and PE composite materials, and can significantly reduce the short circuit phenomenon generated during the manufacturing and use of lithium batteries. The layered structure of the lithium battery separator protects the battery in multiple ways. While ensuring that the lithium battery can work efficiently, it also solves a series of battery safety problems caused by flammable electrolytes, ultra-thin separators, hidden dendrites, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0020] Figure 1 It is a schematic structural diagram of the lithium battery separator of the present invention.

[0021] Figure 2 is a plan view of a polyethylene layer of the present invention.

[0022] In the figure: 1-inorganic coating; 2-polyethylene layer; 3-polypropylene layer; 4-flame retardant capsule. DETAILED DESCRIPTION

[0023] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0024] In the description of the present invention, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0025] In the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0026] like Figure 1 and Figure 2 As shown, this embodiment provides a lithium battery separator, including an inorganic coating 1, a polyethylene layer 2 and a polypropylene layer 3, the two sides of the inorganic coating 1 are respectively adhered to the polyethylene layer 2, the side of the polyethylene layer 2 facing away from the inorganic coating 1 is adhered to the polypropylene layer 3, and a flame retardant capsule 4 is provided in the polyethylene layer 2, the flame retardant capsule 4 includes a polyvinylidene fluoride coating layer and a triphenyl phosphate flame retardant provided in the polyvinylidene fluoride coating layer.

[0027] A polyethylene layer 2 and a polypropylene layer 3 are respectively laminated on both sides of the inorganic coating 1, so that the thickness of the lithium battery separator is increased. In addition, because the inorganic coating 1 is sandwiched in the middle, the safety, heat shrinkage resistance and puncture strength of the separator are greatly increased, and the short circuit phenomenon generated during the manufacturing and use of the lithium battery is greatly reduced, the self-discharge phenomenon of the lithium battery is improved, and safety hazards are reduced.

[0028] The inorganic coating 1 can adopt a silicon dioxide layer, which can not only prevent dendrites from puncturing and causing battery short circuits, but also due to the high specific surface area of ​​the silicon dioxide layer and its good wettability to the electrolyte, the lithium battery separator also exhibits a higher porosity and liquid absorption rate. The introduction of the silicon dioxide layer not only greatly improves the thermal stability and mechanical properties of the lithium battery separator, but also enhances the absorption of the electrolyte.

[0029] The air permeability of this lithium battery separator is lower than that of conventional lithium battery separators (PP, PE, PP and PE are compatible), and the battery rate performance is better. Even if the adhesive is corroded due to electrolyte immersion during battery use, the layers will not fall off, so it will not affect the performance of the battery product.

[0030] The triphenyl phosphate flame retardant is encapsulated in the polyvinylidene fluoride coating layer, which prevents the triphenyl phosphate flame retardant from directly dissolving into the electrolyte, otherwise it will have a negative impact on the battery performance. During the thermal runaway process of the lithium-ion battery, the flame retardant capsule 4 in the polyethylene layer 2 is dissolved by heat to release the triphenyl phosphate (TPP) flame retardant. As the temperature rises, the polyvinylidene fluoride coating layer will melt and release the flame retardant, effectively inhibiting the combustion of the flammable electrolyte.

[0031] In this embodiment, the weight ratio of the triphenyl phosphate flame retardant to the polyvinylidene fluoride coating layer is (7-9):2.

[0032] The polyethylene layer 2 is made of high-density polyethylene with a melt index of 0.05-5g / 10min, and the porosity of the polyethylene layer 2 is 20%-60%. The use of high-density polyethylene with a melt index of 0.05-5g / 10min can ensure that the pores can be quickly closed at a lower temperature during the increase in battery temperature. The polypropylene layer 3 is made of polypropylene with an isotacticity greater than or equal to 96%, a melt index of 0.3-5g / 10min, and a β-nucleating agent conversion; the porosity of the polypropylene layer 3 is 20%-60%. This is also to ensure that the pores can be closed secondary during the temperature increase of the battery. β-crystal PP has good toughness and a high heat deformation temperature. Adding a β-nucleating agent is one of the most effective ways to obtain β-crystal PP. By adding a β-crystal modifier with a nucleating effect to PP, the difference in density between different phases of PP is utilized to cause a crystal transformation during the stretching process to form micropores.

[0033] The melting temperature of the polyethylene layer 2 is 125℃-135℃; the melting temperature of the polypropylene layer 3 is 155℃-165℃; the heat-resistant temperature of the inorganic coating 1 is higher than 200℃. The inorganic coating 1 can withstand high temperatures and play a role in high-temperature support. Moreover, due to its own high material hardness, it prevents short circuits caused by dendrite puncture during battery use. When the lithium battery is heated to 130℃ due to a short circuit, the polyethylene layer 2 first melts and closes the circuit to protect the battery. When the temperature continues to rise, the polyethylene of the polyethylene layer 2 will penetrate into the polypropylene pores of the polypropylene layer 3. At the same time, the polypropylene layer 3 will also melt and close the pores to perform a second level of protection, achieving a multi-level safety protection effect.

[0034] The porosity of the inorganic coating 1 is 10% to 70%.

[0035] In the lithium battery separator, the thickness of the polyethylene layer 2 and the thickness of the polypropylene layer 3 are both greater than the thickness of the inorganic coating 1, and the thickness of the polyethylene layer 2 is greater than the thickness of the flame retardant capsule 4. The thickness of the polyethylene layer 2 and the thickness of the polypropylene layer 3 can be the same or different. Specifically, the thickness of the polyethylene layer 2 is 5-10 μm, the thickness of the polypropylene layer 3 is 5-10 μm, and the thickness of the inorganic coating 1 is 0.5-1 μm.

[0036] In different embodiments, the thickness of the polyethylene layer 2, the thickness of the inorganic coating 1 and the thickness of the polypropylene layer 3 can be set as follows:

[0037] (1) The thickness of the polyethylene layer 2 is 7 μm, the thickness of the inorganic coating 1 is 1 μm, the thickness of the polypropylene layer 3 is 7 μm, and the thickness of the flame retardant capsule 4 is 6 μm;

[0038] (2) the thickness of the polyethylene layer 2 is 10 μm, the thickness of the inorganic coating 1 is 0.5 μm, the thickness of the polypropylene layer 3 is 10 μm, and the thickness of the flame retardant capsule 4 is 8 μm;

[0039] (3) The thickness of the polyethylene layer 2 is 10 micrometers, the thickness of the inorganic coating layer 1 is 1 micrometer, the thickness of the polypropylene layer 3 is 6 micrometers, and the thickness of the flame retardant capsule 4 is 5 micrometers.

[0040] In different embodiments, the weight ratio of the triphenyl phosphate flame retardant to the polyvinylidene fluoride coating layer can be set as follows:

[0041] (1) The weight ratio of triphenyl phosphate flame retardant to polyvinylidene fluoride coating layer is 7:2;

[0042] (2) The weight ratio of triphenyl phosphate flame retardant to polyvinylidene fluoride coating layer is 9:2;

[0043] (3) The weight ratio of the triphenyl phosphate flame retardant to the polyvinylidene fluoride coating layer is 8:2.

[0044] The present invention is not limited to the above optional implementations, and anyone can derive other various forms of products under the enlightenment of the present invention. The above specific implementations should not be understood as limiting the scope of protection of the present invention. The scope of protection of the present invention should be based on the definition in the claims, and the description can be used to interpret the claims.

Claims

1. A lithium battery separator, characterized in that: The invention comprises an inorganic coating (1), a polyethylene layer (2) and a polypropylene layer (3), wherein the polyethylene layer (2) is laminated on both sides of the inorganic coating (1), and the polypropylene layer (3) is laminated on the side of the polyethylene layer (2) facing away from the inorganic coating (1). A flame retardant capsule (4) is arranged in the polyethylene layer (2), and the flame retardant capsule (4) comprises a polyvinylidene fluoride coating layer and a triphenyl phosphate flame retardant arranged in the polyvinylidene fluoride coating layer; The weight ratio of the triphenyl phosphate flame retardant to the polyvinylidene fluoride coating layer is (7-9):2; The inorganic coating (1) is a silicon dioxide layer; the porosity of the inorganic coating (1) is 10%-70%; The polypropylene layer (3) is made of polypropylene with an isotacticity greater than or equal to 96%, a melt index of 0.3-5 g / 10 min, and a β-nucleating agent conversion; the porosity of the polypropylene layer (3) is 20%-60%; the polyethylene layer (2) is made of high-density polyethylene with a melt index of 0.05-5 g / 10 min, and the porosity of the polyethylene layer (2) is 20%-60%; the thickness of the polyethylene layer (2) is 5-10 μm, the thickness of the polypropylene layer (3) is 5-10 μm, and the thickness of the inorganic coating (1) is 0.5-1 μm; the melting temperature of the polyethylene layer (2) is 125°C-135°C; the melting temperature of the polypropylene layer (3) is 155°C-165°C; and the heat-resistant temperature of the inorganic coating (1) is higher than 200°C.

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