Polypropylene microporous separators

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... 

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. 

H. Lee, M. Alcoutlabi, J.V. Watson, X. Zhang, Polyvinylidene fluoride-co-chlorotriflu-oroethylene and polyvinylidene fluoride-co-hexafluoropropylene nanoflber-coated polypropylene microporous battery separator membranes. Journal of Polymer Science, Part B Polymer Physics 51 (5) (2013) 349-357. 

Yu, H.-Y., Xu, Z.-K., Lei, H., Hu, M.-X., Yang, Q. (2007). Photoinduced graft polymerization of acrylamide on polypropylene microporous membranes for the improvement of antifouling characteristics in a submerged membrane-bioreactor. Separation and Purification Technology, 53, 119—125. 

By laminating conventional polyethylene and polypropylene microporous membranes together, it is possible to obtain a separator with the desired shutdown function together with protection from rupture. Microporous membrane of liquid crystalline polyester, polyphenylene ether, aromatic polyamide, polyimide, polyamide imide resin, acrylic resin, and cross-linked polymer are now being studied as candidates for lamination with polyethylene in order to gain even greater heat resistance. 

Polypropylene (PP) and polyethylene (PE) microporous separators (e.g. with 20 jxm thickness and 50% porosity) are used for electrically separating the positive electrode and negative electrode. SEM of microporous separator is shown in Figure 12.1.4. As organic solvents are wettable to PP and PE, the solvents can penetrate into such micropores. The pore size of the separator is normally less than 0.5 (tm, in order to ensure that fine active ceramic particles of electrodes do not pass through the separator. A PP/PE/PP layered separator is often used for practical Li-ion batteries because of a shut down effect. When battery temperature approaches tbe melting point of PE (130°C), micropores of only PE are suddenly closed, and the battery reaction coming from Li-ion transportation is stopped by tbe separator. 

In contrast, hthium cells require thin micropo-rous separators based on the low conductivity of organic electrolytes. These microporous separators more closely resemble membranes with a thickness of 25 pm or less with pore diameters in the hundredths of microns. They most often are polyolefins consisting of polyethylene or polypropylene. Despite their thinness, up to three layers of separately extruded films may be used. 

The basic cell structure of this system consists of a lithium anode, a microporous polypropylene film separator, and a cathode that is usually composed of 90% V2O5 and 10% graphite, on a weight basis. When it is used in a reserve battery, the prevalent electrolyte is 2M LiAsFg + 0.4M LiBp4 in methyl formate (MF) because of its excellent stability during long-term... 

Very different microporous separators for alkaline batteries are included in Table 11.16. The very thin ( 25 xm) films of stretched polypropylene ( Celgard ) are generally employed in combination with fleeces, while separators of sintered PVC or filled UHMW PE find use also in single separation of alkaline industrial batteries. Their production process corresponds to the analogous version for lead-acid batteries and is described in detail in Sections 11.2.2.1 and 11.2.2.2 respectively. 

The separators with pore diameter in the 5-lOnm range are called microporous separators. These separators, in principle, can be organic or inorganic, but the majority of them are derived from organic polymers such as polyethylene, polypropylene, poly(tetrafluoroethylene), and polyfvinyl chloride). Naturally occurring rubber and wood has been used as battery separators. Nonwoven fibers of nylon, cotton, polyesters, and glass can also be employed in the fabrication of microporous separators. 

Lee H, Alcoutlabi M, Watson JV, Zhang X (2013) Polyvinylidene fluoride-co-chlorotrifluoroethylene and polyvinylidene fluoride-co-hexafluoropropylene nanofiber-coated polypropylene microporous battery separator membranes. J Polym Sd Part B-Polym Phys 51(5) 349-357. doi 10.1002/polb.23216... 

For the separation of D,L-leucine, Ding et al. [62] used poly(vinyl alcohol) gel-coated microporous polypropylene hollow fibers (Fig. 5-11). An octanol phase containing the chiral selector (A-n-dodecyl-L-hydroxyproline) is flowing countercur-rently with an aqueous phase. The gel in the pores of the membrane permits diffusion of the leucine molecules, but prevents convection of the aqueous and octanol phase. At a proper selection of the flow ratios it is possible to achieve almost complete resolution of the D,L-leucine (Fig. 5-12). 

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.)... 

The ordn uses for polypropylene are varied. It is used in the fabrication of personnel body armor (Refs 6 7) in slurry-type expls for the demolition of concrete structures (Ref 11) as a microporous hydrazine-air (cathode) separator in fuel cells (Ref 9) as a propint binder matl, particularly in caseless ammo, (Refs 5 8) and as a candidate to act as a proplnt aging inhibitor for the 155mm RAP round (Ref 10) Refs 1) Beil 1, 196, (82), [167], 677 and (725) 2) A.V. Topchiev V.A. Krentsel,... 

Fignre 27.3 shows a typical spectroelectrochemical cell for in sitn XRD on battery electrode materials. The interior of the cell has a construction similar to a coin cell. It consists of a thin Al203-coated LiCo02 cathode on an aluminum foil current collector, a lithium foil anode, a microporous polypropylene separator, and a nonaqueous electrolyte (IMLiPFg in a 1 1 ethylene carbonate/dimethylcarbonate solvent). The cell had Mylar windows, an aluminum housing, and was hermetically sealed in a glove box. 

As the separator, microporous polypropylene film (PORP by NPO UFIM, Russia) with the thickness of 25 pm was used. 

In lithium-based cells, the essential function of battery separator is to prevent electronic contact, while enabling ionic transport between the positive and negative electrodes. It should be usable on highspeed winding machines and possess good shutdown properties. The most commonly used separators for primary lithium batteries are microporous polypropylene membranes. Microporous polyethylene and laminates of polypropylene and polyethylene are widely used in lithium-ion batteries. These materials are chemically and electrochemically stable in secondary lithium batteries. 

Lithium CFj. The Li/CF rbattery consists of a lithium anode, polycarbon monofluoride cathode, and microporous polypropylene separator saturated with organic electrolyte. These batteries are used as power sources for watches, portable calculators, memory applications, and so on. 

Lithium SO2. The lithium SO2 systems are mainly used in military and some industrial and space applications. This system is particularly known for its capability to handle high current and high power requirements, for its excellent low-temperature performance, and for its long shelf life. They are typically fabricated in cylindrical structure by spirally winding rectangular strips of lithium foil, a microporous polypropylene separator, the cathode electrode, and a second separator layer. 

Fig. 5.13 Motive power lead-acid cell with tubular positive plates in which the active material is contained in pre-formed terylene tubes, and negative pasted grid plates surrounded by microporous polyvinyl chloride separator envelopes. The case and lid are formed of heat-sealed polypropylene. (By courtesy of Chloride Industrial Batteries.)...

Figure 3.15 Polypropylene structures, (a) Type I open cell structure formed at low cooling rates, (b) Type II fine structure formed at high cooling rates [37]. Reprinted with permission from W.C. Hiatt, G.H. Vitzthum, K.B. Wagener, K. Gerlach and C. Josefiak, Microporous Membranes via Upper Critical Temperature Phase Separation, in Materials Science of Synthetic Membranes, D.R. Lloyd (ed.), ACS Symposium Series Number 269, Washington, DC. Copyright 1985, American Chemical Society and American Pharmaceutical Association...

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