Li-ion battery separator

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."... 

Costa, C.M., Rodrigues, L., Sencadas, V, Silva, M., Rocha, J.G., Lanceros-Mndez, S., 2012. Effect of degree of porosity on the properties of polyfvinylidene fluoride-trifluorethylene) for Li-ion battery separators. 

It is implicit in the permeability requirement that typically Li-Ion battery separators have a porosity of 40%. Control of porosity is very important for battery separators. Specification of percent porosity is commonly an integral part of separator acceptance criteria. The porosity of separators used in alkaline zinc MnO cells is typically around 80-90%. 

Recently Teijin Techno Products Limited introduced its commercially viable nanofibre based product Dream Weaver Gold as Li-ion battery separator. Teijin s... 

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. 

Once in an operational battery, the separator should be physically and chemically stable to the electrochemical environment inside the cell. The separator should prevent migration of particles between electrodes, so the effective pore size should be less than 1pm. Typically, a Li-ion battery might be used at a C rate, which corresponds to 1-3 mAcm2, depending on electrode area the electrical resistivity of the separator should not limit battery performance under any conditions. 

Layered solids such as graphite are interesting in separation and sorption applications and can be doped to give interesting materials properties as in Li ion batteries. Their intercalation behaviour is best described by the Daumas-Herold model. 

A Li-ion battery usually consists of a composite cathode, a separator and an anode. The outer ends of the batteries are connected to a current collector that channels the electrons produced out to the load circuit. 

The principal function of a separator in a Li-ion battery is to keep the positive and negative electrodes apart. This is needed to prevent electrical short circuits and at the same time allow for rapid transport of ionic charge carriers that are critical to complete the... 

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. 

In other words, the mixtures treated in plants which could accept any battery and accumulator mixes are in fact sorted. For both economic and environmental reasons, the NiCd, NiMH and Li-Ion batteries are usually separated and are no longer treated in these plants. 

Ryou, M.H., Lee, Y.M., Park, I.K., Choi, I.W., 2011. Mussel-inspired polydopamine-treated polyethylene separators for high-power Li-ion batteries. Adv. Mater. 23,3066-3070. 

Zhang, S.S., 2007. A review on the separators of liquid electrolyte Li-ion batteries. J. Power Sources 164, 351-364. 

Figure 1.61 Schematic representation of (a) architecture of Li-ion battery and illustration of charging mechanism involving movement (intercalation/de-intercalation) of Li ions between electrodes and across separator through electrolytic medium [539] and (b) architecture of supercapacitor and depiction of charge storage mechanism. Reprinted from [540] with permission from RSC.

Scheme 7.1 Schematic representation of a li-ion battery. Negative electrode (graphite) and positive electrode (LiCoO ) separated by a non-aqueous liquid electrolyte Panel is reproduced with permission [2]. Copyright 2008, Wiley.

Yang, C.R., Jia, Z.D., Guan, Z.C. et al. 2009. Polyvinylidene fluoride membrane by novel electrospinning system for separator of Li-ion batteries. J. Power Sources 189 716-720. 

The purpose of this chapter is to provide a detailed review of separators used in Li-Ion battery applications and their chemical, mechanical, and electrochemical properties. The separator requirements, properties, and characterization techniques are also described with respect to Li-Ion batteries. Despite the widespread use of separators, a great need still exists for improving the performance, increasing its life, and reducing cost. In the following Sections an attempt is made to discuss key issues in various separators with the hope of bringing into focus present and future directions of research and development in separator technologies. 

Sony s introduction of the rechargeable lithium ion battery in the early 1990s precipitated a need for new separators that provided not only good mechanical and electrical properties but also provided added safety through a thermal shutdown mechanism. Although a variety of separators (e.g., cellulose, nonwoven fabric, etc.) have been used in different types of batteries, various studies on separators for Li-Ion batteries have been pursued in the last few years, as separators for Li-Ion batteries require different characteristics than separators used in conventional batteries. 

A novel microporous separator using polyolefins has been developed and used extensively in Li-Ion batteries since it is difficult for conventional separator materials to satisfy the characteristics required in Li-Ion batteries. In Li-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. 

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