Microporous separator materials

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.) 

Nonwoven materials have not been able to compete with microporous films, most probably because of the difficulty in mak ing thin (25pm) nonwovens with acceptable physical properties (for example, 

Hoechst Celanese Corp. Tonen Corp. Asahi Chemical Industries Mitsubishi Ube Industries Ltd. Pall RAI Celgard membranes made of polypropylene, polyethylene, and combinations Setela membranes made of polyethylene HiPore membranes made of polyethylene Exepol membranes made of polyethylene Polypropylene membranes Polyethylene membranes 

Wet processes involve mixing a hydrocarbon liquid or some other low-molecular -weight substance with a polyolefin resin, heating and melting the mixture, extruding the melt into a sheet, orientating the sheet either in the machine direction or biaxi-ally, and then extracting the liquid with a volatile solvent [6-8]. 

Dry processes involve melting a polyolefin resin, extruding it into a film, thermal annealing, orientation at a low temperature to form micropore initiators, and then orientation at a high temperature to form micropores [9, 10]. The dry process involves no solvent handling, and therefore is inherently simpler than the wet process. The dry process involves only virgin polyolefin resins and so presents little possibility of battery contamination. 

Traditionally, lithium-ion separators were made from PE, PP, or some combination of the two, because these polyolefins provide excellent mechanical projjerties and chemical stability at a reasonable cost. Recently, ceramic materials and aramid polymers have been introduced as a means to improve the thermal stabihty of separators to temperatures of 200 °C and above. 

Nonwoven materials have to date not been able to compete with microporous films, most probably because of the difficulty in making thin ( 25 pm) nonwovens with acceptable physical properties (for example, gauge uniformity, puncture strength). However, nonwovens are used in button cells and bobbin cells when thicker separators and low discharge rates are acceptable. More recently. 

Excluding nonwovens, the processes for manufacturing microporous membranes can be broadly divided into wet processes and dry processes. Both processes usually employ one or more orientation steps to impart porosity and/or increase tensile strength. 

Separators made by the dry process are available from Polypore Corporation (Celgard Division) and Ube. Dry process separators ranging from 12 to 38 xm are available as both single and multi-layer Aims. Dry process separators are available with various coatings such surfactants or adhesive polymers like polyvinylidene 

2) Polypore produced both dry process separators through their Celgard division and 

Hoechst Celanese Corp. Celgard membranes made of polypropylene, polyethylene, and combinations 

Tonen Corp. Setela membranes made of polyethylene 

Asahi Chemical Industries HiPore membranes made of polyethylene 

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The pores of tire separating membrane are to be most uniformly distributed and of minimum size to avoid deposition of metallic particles and thus electronic bridging. One distinguishes between macroporous and microporous separators, the latter having to show pore diameters below I micron (/urn ), i.e., below one-thousandth of a millimeter. Thus the risk of metal particle deposition and subsequent shorting is quite low, since active materials in storage batteries usually have particle diameters of several microns. 

A mixture of powdered poly(vinyl chloride), cyclohexanone as solvent, silica, and water is extruded and rolled in a calender into a profiled separator material. The solvent is extracted by hot water, which is evaporated in an oven, and a semiflexible, microporous sheet of very high porosity ( 70 percent) is formed [19]. Further developments up to the 75 percent porosity have been reported [85,86], but these materials suffer increasingly from brittleness. The high porosity results in excellent values for acid displacement and electrical resistance. For profiles, the usual vertical or diagonal ribs on the positive side, and as an option low ribs on the negative side, are available [86],... 

Table l. Commercially available microporous membrane materials used as separators in lithium-ion batteries. 

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. 

A novel microporous separator using polyolefins has been developed and used extensively in lithium-ion batteries since it is difficult for 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 ionically conductive liquid electrolyte. 

In certain forms of the material, the microporous polymer creates exactly two distinct, interwoven but disconnected porespace labyrinths, separated by a continuous polymeric dividing wall. This opens up the possibility of performing enzymatic, catalytic or photosyndietic reactions in controlled, ultrafinely microporous polymeric materials with the prevention of recombination of the reaction products by their division into the two labyrinths. These features combine with specific surface areas for reaction on the order of lO -lO square meters per gram, and with the possibility of readily controllable chirality and porewall surface characteristics of the two labyrinths. 

In a related study, Farmer et al. studied the destruction of benzene by Ag(II) [50]. Unlike in the case of ethylene glycol, a maximum conversion of 60% was achieved after 5 h of electrolysis at 336 mA. The authors attributed the failure to achieve 100% conversion to the volatilization of benzene or one of its intermediates from the anolyte. Of the three separator materials, porous ceramic, Vycor microporous glass, and Nafion 117, which were investigated, the latter two were found to be effective barriers to HNO2 (formed by the reduction of HNO3 at the cathode) migration from the catholyte to the anolyte. 

Solid electrolytes for lithium-ion batteries are expected to offer several advantages over traditional, nonaqueous liquid electrolytes. A solid electrolyte would give a longer shelf life, along with an enhancement in specific energy density. A solid electrolyte may also eliminate the need for a distinct separator material, such as the polypropylene or polyethylene microporous separators commonly used in contemporary liquid electrolyte-based batteries. Solid electrolytes are also desirable over liquid electrolytes in certain specialty applications where bulk lithium-ion batteries as weU as thin-film lithium-ion batteries are needed for primary and backup power supplies for systems, devices, and individual integrated circuit chips. 

Microporous carbon materials arc widely used in adsorption processes for separating gaseous and liquid components [1], Fossil peat and coal [2], polymers and resins [3], wood pulp and other plant raw materials [4] are widely used as raw to produce microporous carbon-containing materials. 

Molecular design and rational synthesis of inorganic microporous crystalline materials are frontier subjects in the fields of zeolites science and molecular engineering. Zeolite synthesis is an active field of research because zeolites with uniform micropores are important in many industrial processes in catalysis, adsorption, and separation, and are finding new applications in electronics, magnetism, chemical sensors, and medicine, etc.12 91 Synthesis of such materials typically involves crystallization from a gel medium under hydrothermal/solvothermal conditions in the presence of organic amines as... 

More promising for reactive separations involving gas phase reactions appears to be the development and use in such applications of microporous zeolite and carbon molecular sieve (Itoh and Haraya [2.25] Strano and Foley [2.26]) membranes. Zeolites are crystalline microporous aluminosilicate materials, with a regular three-dimensional pore structure, which are relatively stable to high temperatures, and are currently used as catalysts or catalyst supports for a number of high temperature reactions. One of the earliest mentions of the preparation of zeolite membranes is by Mobil workers (Haag and Tsikoyiannis... 

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