Micropores, separators

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. 

The thin backweb, typically 0.2 mm thick with a porosity of 60 percent yields excellent electrical resistance values of 50 rafl cm2, permitting further optimization of high-performance battery constructions. These require very thin electrodes due to the overproportionally increasing polarization effects at higher current densities and consequently also low distances most modern versions have separators only 0.6 mm thick. Such narrow spacings enforce microporous separation ... 

A stiff, microporous separator is formed with a very narrow pore size distribution with an average of 0.5 jum — about 90 percent of all pores being between 0.3 and 0.7 fjm in diameter ... 

Batteries with gelled electrolyte have been shown to require a separator in the conventional sense, to secure spacing of the electrodes as well as to prevent any electronic shorts the latter is achieved by microporous separators. An additional important criterion is minimal acid displacement, since these batteries — in comparison with batteries with liquid electrolyte — lack the electrolyte volume share taken up by gelling and by the cracks. 

Very different microporous separators for alkaline batteries are included in Table 16. The very thin (-25 ftm) films of stretched polypropylene ( Celgard ) are generally employed in combination with... 

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. 

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. 

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. 

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. 

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 two basic kinds of nickel—zinc main separators are the membranes and the microporous separators. Membrane separators are those in which ionic transport occurs through the interaction of the hydrophilic groups attached to the polymer with the ionic groups in the electrolyte. Ionic transport through mi-... 

Microporous separators have the advantage of being relatively low-cost and adequately stable in the electrolyte, but unfortunately they contribute to energy-efficiency losses in the battery. Rapid trans-... 

Almost all modern practical aqueous primaries are referred to as dry cells . This designation should not be confused with the rather specialized solid state cells which make use of the recently discovered true solid electrolytes. Rather, the term implies that the aqueous electrolyte phase has been immobilized by the use of gelling agents or by incorporation into microporous separators. Such procedures permit the cells to operate in any orientation and reduce the effects of leakage should the container become punctured. 

The cross-sections of two typical cylindrical cells are shown in Fig. 3.9. In Fig. 3.9(a) a D-size unit used for flashlights and similar applications is shown a large capacity alarm cell is shown in Fig. 3.9(b). The electrolyte or paste separator in Fig. 3.9(a) is a relatively thin layer of electrolyte solution immobilized in a gel or microporous separator. Different manufacturers favour different forms of separator. These range from gelled... 

Fig. 9.21 Schematic diagrams of zinc-bromine battery systems (a) cell with cation selective membrane (b) cell with reservoir for poly bromide and microporous separator...

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. 

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