Battery separators characterization

The term leaf separator characterizes the customary stiff version of a starter battery separator that can be inserted individually between the electrodes on automatic stackers, in contrast to pocket separators. This processing requires considerably higher bending stiffness than for pocket separators, calling for thicker backwebs, typically 0.4-0.6 mm (Fig. 18 and 19). 

Battery makers sometimes view separators with disdain the separator is needed but does not actively contribute to battery operation. Consequently, very little work (relative to that on electrode materials and electrolytes) is directed towards characterizing separators. In fact, development efforts are under way to displace microporous membranes as battery separators and instead to use gel electrolytes or polymer electrolytes. Polymer electrolytes, in particular, promise enhanced safety by elimi-... 

Very little work (relative to research of electrode materials and electrolytes) is directed toward characterizing and developing new separators. Similarly, not much attention has been given to separators in publications reviewing batteries.A number of reviews on the on cell fabrication, their performance, and application in real life have appeared in recent years, but none have discussed separators in detail. Recently a few reviews have been published in both English and Japanese which discuss different types of separators for various batteries. A detailed review of lead-acid and lithium-ion (li-ion) battery separators was published by Boehnstedt and Spot-nitz, respectively, in the Handbook of Battery Materials. Earlier Kinoshita et al. had done a survey of different types of membranes/separators used in different electrochemical systems, including batteries."... 

The testing of battery separators and control of their pore characteristics are important requirements for proper functioning of batteries. Mercury porosim-etry has been historically used to characterize the separators in terms of percentage porosity, mean pore size and pore size distribution. In this method, the size and volume of pores in a material are measured by determining the quantity of mercury, which can be forced into the pores at increasing pressure. Mercury does not wet most materials, and a force must be applied to overcome the surface tension forces opposing entry into the pores. 

Another technique, capillary flow porometry has been developed by Porous Materials Inc. ° to characterize battery separators.The instrument can measure a number of characteristics of battery separators such as size of the pore at its most constricted part, the largest pore size, pore size distribution, permeability, and envelope surface... 

The purpose of this chapter is to describe separators used in secondary batteries and characterization of their chemical, mechanical, and electrochemical properties, with particular emphasis on separators for lithium-ion batteries. The separator requirements, properties, and characterization techniques are described with respect to lithium-ion batteries. Despite the widespread use of separators, a need still exists for improving the performance, increasing its life, and extending the operating range. 

M. J. Zientek, and R. J. Bender, Battery Conference on Applications and Advances Characterization of Microglass Laid on Non-wovens Used as Battery Separators, 11th Long Beach Battery Conference, 1996, 273. 

Battery separators are characterized by numerous properties, including material nature, membrane stractural and functional properties. Material nature includes chemical stability, crystalline structure, hydrophilicity, thermal shrinkage, melting point, M and Mv,/M of polyolefin materials. Structural properties include thickness, porosity, pore size, pore shape, pore tortuosity, and pore distribution. Functional properties include mechanical strength, electrical resistivity, air permeability, thermal shutdown, electrolyte wettability and retention. Many of the above properties are affected with each other and may be in a trade-off relationship. For example, the mechanical strength is affected in opposite manner by the thickness, porosity and permeability, as required by the battery performance. 

The Celgard microporous materials made by Polypore Corporation are the best-characterized battery separators. Bierenbam et al. [49] describe the process. 

Liang Y, Ji L, Guo B, Lin Z, Yao Y, Li Y, Alcoutlabi M, Qiu Y, Zhang X (2011) Preparation and electrochemical characterization of iomc-conductmg litfaium lanthanum titanate oxide/polyacrylonitrile submicron composite fiber-based lithium-ion battery separators. J Power Sources 196(1) 436-441. doi 10.1016/j.jpowsour.2010.06.088... 

Liang, Y, Ji, L., Guo, B., Lin, Z., Yao, Y, Li, Y, Alcoutlabi, M., Qiu, Y, and Zhang, X., Preparation and Electrochemical Characterization of Ionic-Conducting Lithium Lanthanum Titanate Oxide/Polyaciylonitrile Submicron Composite Fiber-Based Lithium-Ion Battery Separators , Journal of Power Sources, 1%, 436-441,2011. 

The simulated short-circuit test was developed to characterize the response of the separator to a short circuit without the complications of battery electrodes. The separator was spirally wound between lithium foils and placed in an AA-size can. To avoid lithium dendrite formation, an alternating voltage was applied to the cell. The cell current and can temperature were monitored. Figure 6 shows the behavior of Celgard membranes. 

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