CR2450 battery.Research and development of lithium-ion battery separators

　　Research and development of lithium-ion battery separators

　　1Lithium-ion battery separator materials

　　Because polyethylene and polypropylene microporous membranes have higher porosity, lower resistance, higher tear resistance, better acid and alkali resistance, good elasticity and retention of aprotic solvents, lithium It was used as its separator material in the early stages of ion battery research and development. So far, the separator materials of commercial lithium-ion batteries mainly use polyethylene and polypropylene microporous films.

　　However, polyethylene and polypropylene separators have the disadvantage of poor affinity for electrolytes. In this regard, many scholars have done a lot of modification work, such as grafting hydrophilic monomers on the surface of polyethylene and polypropylene microporous membranes. body or change the organic solvent in the electrolyte, etc. Some people also use other materials as lithium-ion battery separators. For example, research by I. Kuribayashi and others found that cellulose composite membrane materials have good lithium ion conductivity and mechanical strength, and can be used as lithium-ion battery separator materials.

　　In recent years, the use of polymer electrolytes in lithium-ion batteries has reached the level of commercialization. Polymer electrolytes can be divided into pure polymer electrolytes and colloidal polymer electrolytes. Pure polymer electrolytes are difficult to commercialize due to their low room temperature conductivity. Colloidal polymer electrolytes use liquid electrolyte molecules fixed in a polymer network with suitable microstructure to achieve ion conduction. They have the stability of solid polymers and the high ion conductivity of liquid electrolytes, showing good application prospects. Tab.1 is a polymer electrolyte that has been actively researched in recent years. Colloidal polymer electrolytes can be used both as electrolytes in lithium-ion batteries and as separators. However, they are difficult to put into practical use due to their poor mechanical properties, complex preparation processes, or poor conductivity at room temperature, and colloidal polymer electrolytes are thermodynamic in nature. If the system is unstable and stored in an open environment or for a long time, the solvent will seep out of the surface, resulting in a decrease in conductivity. Therefore, there are still many problems that need to be solved when colloidal polymer electrolytes completely replace polyethylene and polypropylene films as separators for lithium-ion batteries.

　　Recently, there have been many reports on polymer lithium-ion battery separators composed of polymer electrolytes together with polyethylene and polypropylene films. Colloidal polymers are covered or filled in microporous membranes, which are comparable to polymer electrolyte lithium-ion batteries without separators. Compared to Mechanical properties and thermal stability. It can be considered that due to their special structures and properties, polyethylene and polypropylene membranes will not be shaken in their status as ion battery separators unless real liquid-free polymer electrolytes appear.

　　2 Preparation method of lithium-ion battery separator

　　There are two main methods for preparing lithium-ion battery separators: melt stretching (MSCS) and thermally induced phase separation (TIPS). Since the MSCS method does not include any phase separation process, its process is relatively simple and there is no pollution during the production process. Currently, most of the world uses this method for production, such as Ube, Mitsubishi, Tonen in Japan and Celanese in the United States. The TIPS method is more complex than the MSCS method and requires adding and removing diluents, so the production cost is relatively high and may cause secondary pollution. Currently, companies in the world that use this method to produce separators include Asahi Kasei of Japan, Akzo of the United States, and 3M. wait.

　　2.1 Melt extrusion/stretching/heat setting method

　　The preparation principle of the melt extrusion/stretching/heat setting method is that the polymer melt crystallizes under a high stress field to form a lamellar structure perpendicular to the extrusion direction and arranged in parallel, and then undergoes heat treatment to obtain a so-called hard elastic material. After the hard elastic polymer film is stretched, the lamellae separate and a large number of microfibers appear, thus forming a large number of microporous structures. The microporous film is then heat-set.

　　There are many patents describing the preparation process of polyolefin microporous films. The stretching temperature is higher than the glass transition temperature of the polymer and lower than the crystallization temperature of the polymer. For example, the polypropylene film formed by blow molding and extrusion is heat-treated to obtain a hardened film. The elastic film is first cold drawn by 6% to 30%, then hot stretched by 80% to 150% between 120°C and 150°C, and then heat set to obtain a microporous film with high stability.

　　U.S. Patent] introduces another stretching process for film making. The stretching is performed at extremely low temperatures such as -198°C to -70°C, and then heat-fixed at a temperature of 5°C to 60°C lower than the melting temperature of the polymer. , and then stretched at a speed of 10%/min at 10°C to 60°C below the polymer melting temperature to prepare a microporous membrane.

　　The melt extrusion/stretching/heat setting method is a simple and pollution-free process and is a common method for preparing lithium-ion battery separators. However, this method has shortcomings such as difficult control of pore size and porosity.

　　2.2 Adding nucleating agent to co-extrusion/stretching/heat fixing method

　　A nucleating agent is added and co-extruded to form a film containing solid additives. The solid additives are evenly distributed in the polymer phase with sub-micron particle sizes. Phase separation occurs due to stress concentration during stretching to form a microporous film, Xu Mao introduced a method for making a polypropylene microporous membrane by biaxially stretching a polypropylene membrane containing a large amount of B crystal form and then heat-fixing it. The pore diameter is 0.02 μm ~ 0.08 μm, and the porosity is 30% ~ 40 %, the strength of the film is consistent in all directions, about 60MPa~70MPa. Since the polypropylene form of the B crystalline form is composed of lamellae that grow in bundles, and the spherulites are less dense, the amorphous areas between the wafer bundles can easily be pulled apart to form micro-striations or micropores.

　　After adding the nucleating agent, the crystal structure becomes loose, which makes it easy to form holes during stretching and is pollution-free. This method is worth noting.

　　2.3 Thermal induced phase separation method

　　The thermally induced phase separation method is a method developed in recent years to prepare microporous membranes. It uses polymers and certain small molecular compounds with high boiling points to react at higher temperatures (generally higher than the melting temperature Tm of the polymer). When the temperature is lowered, a homogeneous solution is formed, and solid-liquid or liquid-liquid phase separation occurs when the temperature is lowered. In this way, the polymer-rich phase contains the additive phase, and the additive-rich phase contains the polymer phase. After stretching, the low content is removed. Molecular substances can be made into interconnected microporous membrane materials.

　　The thermally induced phase separation method can better control the pore size and porosity. The disadvantage is that it requires the use of solvents, which may cause pollution and increase costs.

　　3Structure and performance of lithium-ion battery separator

　　3.1 Porosity

　　Fig.1 is a porosity diagram of a typical commercial membrane. It can be seen that the porosity of most lithium-ion battery separators is between 40% and 50%. Some commercial separators (such as surface treated with surfactant) have low porosity. More than 30%, and some separators have higher porosity, up to about 60%.

　　High-performance lithium-ion batteries rely primarily on the ionic conductivity of the liquid electrolyte filled in the separator. The ionic conductivity of the non-aqueous liquid electrolyte of lithium-ion batteries is generally in the range of 10-2S/cm~10-3S/cm. Although the separator can effectively prevent the short circuit between the positive and negative electrodes, it can also reduce the short circuit between the positive and negative electrodes. distance, thereby correspondingly reducing the impedance of the battery. However, its existence causes the effective ion conductivity in the electrolyte solution to decrease and increases the impedance of the battery. Some separators can even cause the ion conductivity to decrease by one to two orders of magnitude.

　　In principle, for a certain electrolyte, separators with high porosity can reduce the impedance of the battery, but the higher the porosity, the better. The higher the porosity, the worse their mechanical properties and anti-pore resistance will be. Even if the porosity and thickness are the same, the impedance may be different due to differences in the penetration of the pores.

　　3.2 Pore size and distribution

　　G.Venugopal et al. used a capillary flow poremeter (CFP) and Povewick (a non-volatile fluorine-containing organic liquid) as the medium to measure the relationship between the pressure and gas flow rate of different commercial lithium-ion battery separators. The measurement results show that the pore diameter of commercial membranes is generally 0.03μm~0.05μm or 0.09μm~0.12μm. It is also believed that the difference between the maximum pore diameter and the average pore size distribution of most commercial membranes is less than 0.01μm, which indicates that the pore size distribution is narrower. Getting thinner, such as 25μm or even lower (because thinner separators can increase energy density and reduce battery impedance), submicron pores are extremely important to prevent short circuits of the positive and negative electrodes of lithium batteries.

　　The size and distribution of the pore size are related to the preparation method of the microporous membrane. In the melt extrusion/stretching/heat setting/method, it has a great relationship with the temperature, stress, cooling conditions and stretching conditions of the melt extrusion. In the thermally induced phase separation method, the size and distribution of the pore size are related to the amount of the second component added, the extrusion temperature and the stretching conditions. In the melt extrusion (adding nucleating agent)/stretching/heat setting method, in addition to the process conditions, it is obviously also related to the type and quantity of the nucleating agent added.

　　3.3 Air permeability

　　Air permeability is an important physical and chemical index of breathable membranes. It is determined by the pore size, pore size distribution, porosity, etc. of the membrane. G. Venagopa et al. used a pressure drop meter to measure the air permeability of commercial lithium-ion battery separators. The faster the pressure drop decreases with time, the higher the air permeability of the membrane is, and vice versa. Generally speaking, the lower the porosity, the slower the pressure drop decreases and the lower the air permeability. The air permeability of double-layer or multi-layer membranes is generally lower than that of single-layer membranes of the same material. For the same material, the porosity is the same and the air permeability is similar. Even if the porosity of different materials is similar, due to the difference in pore diameter, the air permeability is lower. Rates also vary greatly.

　　Tab.2 is the measured pressure drop time when the pressure drop of the commercial diaphragm drops from 0.69MPa to 0.14MPa. It can be seen that the porosity increases or the penetration of the pore diameter increases, the air permeability increases and the pressure drop time decreases.

　　The Gurley method can also be used to characterize air permeability, which refers to the time required for a certain volume of gas to pass through the membrane under a certain pressure, that is, the Gurley index.

　　3.4 Thermal performance and automatic shutdown mechanism

　　Like most batteries, the components in the battery will undergo exothermic reactions above a certain temperature, causing self-heating. In addition, overcharging often occurs due to charger failure, safety current failure, etc. When overcharging, lithium-ion batteries Heat is generated, and the self-closing nature of the separator in lithium-ion batteries is an effective way for lithium-ion batteries to limit temperature rise and prevent short circuits.

　　When the temperature approaches the melting point of the polymer, the porous ion-conducting polymer membrane becomes a non-porous insulating layer, and the micropores close to produce a self-closing phenomenon. At this time, the impedance rises significantly and the current through the battery is also limited, thus preventing explosions caused by overheating.

　　Most polyolefin separators have a melting temperature lower than 200°C. For example, the self-closing temperature of polyethylene separators is 130°C to 140°C, while the self-closing temperature of polypropylene separators is about 170°C. Of course, in some cases, even Already self-closing, the temperature of the battery may continue to rise, so the separator is required to withstand higher temperatures and have high enough strength. The PP and PE laminated films developed in recent years, because the PP/PE/PP multi-layer separator provides lower The self-closing temperature, such as 80℃~120℃, while maintaining its strength, is safer than using only a single layer of film. Composite multi-layer has become a hot spot in current research and development.

　　Fig.2 shows the relationship between the impedance and temperature of a lithium-ion battery containing a polyolefin separator when it heats up. (a) in the figure shows the impedance of a lithium-ion battery using a single-layer PP separator at a temperature of 165°C.

　　It is obviously increased by about 2 orders of magnitude, but its impedance is still not high. In this case, it is still possible to continue charging and cause safety problems; (b) is a PE separator with a self-closing temperature of 135°C. At this time, the impedance rises by about It is 3 orders of magnitude higher. It can be seen that PE has a lower closing temperature and high impedance; (c) is a PP/PE/PP multi-layer separator. It can be seen that its self-closing temperature is wide and the impedance is high when self-closing. It is safer to use in lithium-ion batteries, so the multi-layer composite separator has a certain strength and a low self-closing temperature, and is more suitable as a lithium-ion battery separator. It is worth pointing out that not all separators have the same closing behavior, and their closing ability is related to the molecular weight, crystallinity, processing history, etc. of the polymer.

　　3.5 Mechanical properties

　　The requirements for the strength of lithium-ion battery separators are relatively high. Generally speaking, the porosity is high. Although its impedance is low, its strength will decrease. When uniaxial stretching is used, the strength of the membrane in the stretching direction is different from that in the vertical stretching direction. , a typical lithium battery separator has a strength of ~50N in the tensile direction and a strength of ~5N in the transverse direction. The strength of the separator prepared by biaxial stretching is basically the same in both directions. For example, the separator of Japan Toran Company is prepared by biaxial stretching.

　　In summary, it can be seen that multi-layer separators have both a certain strength and a low self-shutdown temperature, and are more suitable as lithium-ion battery separators. Solid polymer electrolyte not only serves as an electrolyte in lithium-ion batteries, but also functions as a separator. It is a promising separator material for lithium-ion batteries.

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