Polyolefin separator

Typical of the temporary or manufacturing aid coating systems is the RISTON dry film photoresist for printed circuit (PC) board fabrication. This was the first of these systems developed. The RISTON product stmcture and the basic steps in its use are shown in Figure 2. It consists of a photopolymer sheet laminated between a Mylar cover sheet and a polyolefin separation sheet. It is manufactured as a continuous web (see Coating PROCESSES, survey), and is suppHed in roUs of varying width and photopolymer composition. 

Fig. 2. Schematic of the RISTON dry film photoresist process, (a) Removal of polyolefin separator sheet and laminate resist to clean surface, using special laminator (b) exposure to uv source using positive or negative phototool (positive to plate negative for print-and-etch) (c) removal of the protective Mylar, which is readily removed by hand and (d) development using a special processor (3).

A great variety of polyolefin separator types are now used in Li ion batteries. They must be stable in the organic electrolytes. Typically they may not be properly wetted by the electrolytes of the optimized composition, e. g., mixtures with PC, PE, and others. Therefore some proprietary treatments are needed to provide hydrophilic behavior. Generally, a micro-porous nonwoven morphology with a large surface gives a good wettability. 

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. 

Figure 2. Polyolefin separators used in lithium-ion batteries.

Nonwoven materials such as cellulosic fibers have never been successfully used in lithium batteries. This lack of interest is related to the hygroscopic nature of cellulosic papers and films, their tendency to degrade in contact with lithium metal, and their susceptibility to pinhole formation at thickness of less than 100 fjim. For future applications, such as electric vehicles and load leveling systems at electric power plants, cellulosic separators may find a place because of their stability at higher temperatures when compared to polyolefins. They may be laminated with polyolefin separators to provide high-temperature melt integrity. 

Prior work related with shutdown separators also involved application of waxes on membranes." " In these cases, the wax or low melting polymers were coated on the polyolefin separator. The disadvantage of this technique is that the coating can block the pores of the separator and thus can affect the performance by increasing separator resistance. Moreover, the coating level has to be very high to get complete shutdown. 

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 list of examples of successful Th-FFF separations of lipophilic polymers is extensive and includes polystyrene [29,34,76,118,144,164,165,168,196,200,345-350], polyisoprene [55,110,144,196,349,350], polytetrahydrofuran [144,196,349,350] and poly(methyl methacrylate) [55,110,144,196,349,350], polybutadiene [349], poly(ethyl methacrylate), poly(n-butyl methacrylate), polyfoctadecyl methacrylate), poly(a-methylstyrene), poly(dimethylsiloxane), poly (vinyl acetate), po-ly(vinyl chloride) and poly(vinyl carbazole) [144],polyethylene [351] and other polyolefins [221]. The polyolefin separations were achieved in a special high temperature channel [15,351]. Asphaltenes have also been separated with Th-FFF [352]. 

Both the anode and the cathode are composed of a coating of the electrochemically active material onto a current collector (copper or aluminum). Another key component of the battery is the separator that physically separates the two electrodes and prevents contact between them. In the case of a liquid technology battery, a polyolefin separator is typically used and a liquid electrolyte is used to transport the Li ions from one side of the porous separator to the other. In the case of a polymer Li ion battery, a polymer, such as PVDF, is used to form a porous structure, which is then swollen with a Li" " conducting liquid electro-lyte. " This results in a gel-type electrolyte, which plays the dual role of electrolyte and separator, with no free liquid present. 

Fig. 20.3 (a) Polyolefin separators used in Li-Ion batteries, (b) A simplified flowchart for separator manufacturing process. Each step of the separator manufacturing process has online detection systems to monitor the quality of the separator. Reprinted with permission from Chem. Rev. 104 (2004) 4419-4462, copyright (2(X)4), American Chemiceil Society... 

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. 

The separator used in LIBs might be degraded by physical contact with high-voltage positive electrode materials. The polyolefin separator is oxidized and gaseous products are produced, which causes swelling of the so-called pouch cells. 

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