Microporous polyolefin separator

Currently, all commercially available, spirally wound lithium-ion cells use microporous polyolefin separators. In particular, separators are made from polyethylene, polypropylene, or some combination of the two. Polyolefins provide excellent mechanical properties and chemical stability at a reasonable cost. A number of manufacturers produce microporous polyolefin separators (Table 1.)... 

Lithium secondary batteries can be classified into three types, a liquid type battery using liquid electrolytes, a gel type battery using gel electrolytes mixed with polymer and liquid, and a solid type battery using polymer electrolytes. The types of separators used in different types of secondary lithium batteries are shown in Table 1. The liquid lithium-ion cell uses microporous polyolefin separators while the gel polymer lithium-ion cells either use a PVdF separator (e.g. PLION cells) or PVdF coated microporous polyolefin separators. The PLION cells use PVdF loaded with silica and plasticizer as separator. The microporous structure is formed by removing the plasticizer and then filling with liquid electrolyte. They are also characterized as plasticized electrolyte. In solid polymer lithium-ion cells, the solid electrolyte acts as both electrolyte and separator. 

Abraham et al. were the first ones to propose saturating commercially available microporous polyolefin separators (e.g., Celgard) with a solution of lithium salt in a photopolymerizable monomer and a nonvolatile electrolyte solvent. The resulting batteries exhibited a low discharge rate capability due to the significant occlusion of the pores with the polymer binder and the low ionic conductivity of this plasticized electrolyte system. Dasgupta and Ja-cobs patented several variants of the process for the fabrication of bonded-electrode lithium-ion batteries, in which a microporous separator and electrode were coated with a liquid electrolyte solution, such as ethylene—propylenediene (EPDM) copolymer, and then bonded under elevated temperature and pressure conditions. This method required that the whole cell assembling process be carried out under scrupulously anhydrous conditions, which made it very difficult and expensive. 

The microporous polyolefin separator has been used extensively in lithium-ion batteries, since it is difficult for most other conventional separator materials to satisfy the characteristics required in lithium-ion batteries. In lithium-ion batteries two layers of separators are sandwiched between positive and negative electrodes and then spirally wound together in cylindrical and prismatic configurations. The pores of the separator are filled with an ionicaUy conductive liquid electrolyte. 

Lower-density E-plastomers have found alternate use in cast film processes to make elastic film laminates with good breathability which contain laminates of liquid impermeable extensible polymeric films with extensible-thermoplastic-polymer-fiber nonwovens and nonwoven webs of polyethylene-elastomer fibers as the intermediate layers. The development relates to a breathable film including an E-plastomer and filler that contributes to pore formation after fabrication and distension of the film. The method and extent of distension is designed to produce a breathable film by stretching the film to form micropores by separation of the film of the E-plastomer from the particulate solids. This film is useful for manufacture of absorbent personal-care articles, such as disposable diapers and sanitary napkins and medical garments. In detail, these constructions comprise a liquid impermeable extensible film comprising polyolefins. The outer layer contains extensible-thermoplastic-polymer-fiber nonwovens, and an elastic intermediate layer contains nonwoven webs of fiber E-plastomers. The intermediate layer is bonded to the film layer and the outer... 

They are fabricated from a variety of inorganic, organic, and naturally occurring materials and generally contain pores that are greater than 50—100 A in diameter. Materials such as nonwoven fibers (e.g. nylon, cotton, polyesters, glass), polymer films (e.g. polyethylene (PE), polypropylene (PP), poly(tetrafluo-roethylene) (PTFE), poly (vinyl chloride) (PVC)), and naturally occurring substances (e.g. rubber, asbestos, wood) have been used for microporous separators in batteries that operate at ambient and low temperatures (<100 °C). The microporous polyolefins (PP, PE, or laminates of PP and PE) are widely used in lithium based nonaqueous batteries (section 6.1), and filled polyethylene separators in lead-acid batteries (section 7.3), respectively. 

Recently Serenyl used a flexible alkaline separator (FAS) in Silver—Zinc cells, which consists of a microporous polyolefin film, with inorganic filler. This can be folded around the silver and/or zinc electrodes to form conventional U wraps or heat sealed bags. They showed that the FAS was not attacked by the electrolyte and helps in inhibiting the shape change of zinc electrode. 

Conventional shutdown temperatures are around 130°C. However, a microporous polyolefin battery separator with a shutdown temperature of 95-110°C and a melt integrity of more than 165°C can be made from a basic UHMWPE formulation (38). 

LIB separators must provide electrical insulation between the positive and negative electrodes while permitting imi transport between them. They are microporous polyolefin films 10-30 pm thick with pores of 0.01-0.1 pm diameter. Most LIB separators are made of high-density polyethylene, although polypropylene is also used to a certain extent... 

Polyolefin materials used for the battery separator are based on a homopolymer or a blend of polyethylene (PE) and polypropylene (PP) in a number of combinations between high density polyethylene (HOPE) and ultrahigh molecular weight polyethylene (UHMWPE). The methods for manufacturing the microporous polyolefin membranes can be divided into the dry process and wet process. Both processes contain an extmsion step to produce a thin film, and employ one or more orientation steps to impart porosity and increase tensile strength. The membranes made by diy process show a distinct slit-pore microstmcmre, while those by wet process feamre interconnected spherical or elliptical pores. 

As a favorable separator, the microporous polyolefin membranes are required to absorb liquid electrolyte quickly in the electrolyte filling operation and retain the liquid electrolyte permanently for long-term operation of battery. Unfortunately, considerable difference in the polarity (dielectric constant) between the polyolefin... 

Most practices for the physical modification are the coating of a polymer or a polymer composite onto one or two surfaces of the microporous polyolefin membrane. In order to enhance the interfacial contact between the separator and electrode, a thin polymer layer that can be gelled by the liquid electrolyte can be coated onto the... 

Lithium-Ion Cel Polymer Battery The Hthium-ion gel polymer batteries offer better performance than that of solid polymer electrolyte batteries. The gel electrolyte is a polymer matrix swollen with a liquid electrolyte, and the batteries that employ gel electrolyte are known as gel polymer batteries. The detailed information on this type of battery can be found in other chapters of this book and in a review [33]. Most of the gel electrolytes have been made employing PEO, poly(acrylonitrile) (PAN) [34], poly(methyl methacrylate) (PMMA) [35, 36], and PVdF [37, 38]. The poor mechanical properties of polymer and gel polymer electrolytes have led to an alternative approach where microporous membranes are impregnated with gel polymer electrolytes [39-42]. The process builds upon the work of Abraham et al. who saturated commercial polyolefin separators with a solution of lithium salt in a photopolymerizable monomer and a nonvolatile... 

Phase Separation. Microporous polymer systems consisting of essentially spherical, intercoimected voids, with a narrow range of pore and ceU-size distribution have been produced from a variety of thermoplastic resins by the phase-separation technique (127). If a polyolefin or polystyrene is insoluble in a solvent at low temperature but soluble at high temperatures, the solvent can be used to prepare a microporous polymer. When the solutions, containing 10—70% polymer, are cooled to ambient temperatures, the polymer separates as a second phase. The remaining nonsolvent can then be extracted from the solid material with common organic solvents. These microporous polymers may be useful in microfiltrations or as controlled-release carriers for a variety of chemicals. 

These types of separators consist of a solid matrix and a liquid phase, which is retained in the microporous structure by capillary forces. To be effective for batteries, the liquid in the microporous separator, which generally contains an organic phase, must be insoluble in the electrolyte, chemically stable, and still provide adequate ionic conductivity. Several types of polymers, such as polypropylene, polysulfone, poly(tetrafluoroethylene), and cellulose acetate, have been used for porous substrates for supported-liquid membranes. The PVdF coated polyolefin-based microporous membranes used in gel—polymer lithium-ion battery fall into this category. Gel polymer... 

All lithium based batteries use nonaqueous electrolytes because of the reactivity of lithium in aqueous solution and because of the electrolyte s stability at high voltage. The majority of these cells use microporous membranes made of polyolefins. In some cases, nonwovens made of polyolefins are either used alone or with microporous separators. This section will mainly focus on separators used in secondary lithium batteries followed by a brief summary of separators used in lithium primary batteries. 

Asahi Chemical Industry carried out an exploratory investigation to determine the requirements for cellulose based separators for lithium-ion batteries. In an attempt to obtain an acceptable balance of lithium-ion conductivity, mechanical strength, and resistance to pinhole formation, they fabricated a composite separator (39—85 /cellulosic fibers (diameter 0.5—5.0 /pore diameter 10—200 nm) film. The fibers can reduce the possibility of separator meltdown under exposure to heat generated by overcharging or internal short-circuiting. The resistance of these films was equal to or lower than the conventional polyolefin-based microporous separators. The long-term cycling performance was also very comparable. 

The dry and wet processes are two main manufacturing methods to prepare microporous polymeric membranes. Both methods are conducted through an extruder and a stretching process to increase the porosity and improve the tensile strength. Generally, separators made by dry process exhibit distinct slit-pore and straight microstructures, whereas those made by wet process show intercormected spherical or elliptical pores. Both methods use cheap polyolefin materials, so the microporous polymeric membranes are not expensive. 

To prepare multilayer membranes, another irradiation method to prepare cross-linked microporous multilayer membranes with enhanced thermal stability has been developed. It is realized by two steps. First, the polymer-blended layers, such as poly(ethylene glycol) diacrylate/poly(ethylene glycol) methyl ether acrylate are coated onto polyolefin microporous membranes. Second, the resultant membranes are irradiated to form chemically cross-linked membranes. They exhibited higher thermal and electrochemical properties compared to conventional separators. TOth the increase of irradiation dose, the thermal stability of the resultant membranes increases accordingly. By using the microporous multilayer membranes, the advantages of each component layers are well combined. 

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