Separators for Lithium-Ion Batteries

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- 

This section reviews the state-of-the-art in battery separator technology for lithium-ion cells, with a focus on separators for spirally wound batteries in particular, button cells are not considered. 

Note that a review of battery separators for lithium-ion cells was recently published [1] in Japanese. 

The requirements for a battery separator can best be understood in the context of how the separator is used. The conventional process (Fig. 1) for making spirally wound cells involves threading the separator (a) through a winding pin (b). 

The electrodes (c and d) are interspaced between the two layers of separator, and the layers are wound as tightly as possible to ensure good interfacial contact. 

The lithium-ion battery market has grown tremendously over the last decade to keep pace with consumer electronics. In the last few years, high-power Hthium-ion batteries have penetrated into the power tool market, and now, large-scale lithium-ion batteries are finding use in stationary and transportation apphca-tions. These new markets have led to new demands on separators for Hthium-ion batteries. 

Traditionally, battery separators have been used as spacers to prevent electronic 

New markets and new requirements are driving many innovations in lithium-ion battery separators. This chapter reviews the present state of separator development for lithium-ion batteries. Note that a good review of battery separators including 

Handbook of Battery Materials, Second Edition. Edited by Claus Daniel and Jurgen 0. Besenhard. 

There are a number of different hthium-ion chemistries now available that involve different positive and negative active materials. Traditionally, the positive active material consisted of hthium cobalt oxide, with some smaU market share going to lithium manganese oxide. Now, cells with positive active materials consisting of lithium nickel cobalt oxide, hthium nickel manganese cobalt oxide, or hthium iron phosphate are available. For the negative, carbons have traditionaUy been used, but 

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The purpose of this paper is to describe the various types of separators based on their applications in batteries and their chemical, mechanical and electrochemical properties, with particular emphasis on separators for lithium-ion batteries. The separator... 

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 added safety through a thermal shutdown mechanism. Although a variety of separators (e.g., cellulose, nonwoven fabric, etc.) have been used in different type of batteries, various studies on separators for lithium-ion batteries have been pursued in past few years as separators for lithium-ion batteries require different characteristics than separators used in conventional batteries. 

ENTER Membranes LLC has developed Teklon— a highly porous, ultrahigh molecular weight polyethylene separator for lithium-ion batteries. At the writing of this publication, the separator is available in small quantities. Pekala et al. characterized Celgard, Setela, and Teklon separators in terms of their physical, mechanical, and electrical properties. ... 

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. 

Cho, T., Sakai, T., Iknase, S., Kimura, K., Kondo, Y, Tarao, T., Tanaka, M., 2007. Electrochemical performances of polyacrylonitrile nanofiber-based nonwoven separator for lithium-ion battery. Electrochem. Solid-State Lett. 10, A159-A162. 

Jeschke, S., Mutke, M., Jiang, Z., Alt, B.,WiemhFer, H.D., 2014. Study of carbamate-modified disiloxane in porous PVDF-HFP membranes newelectrolytes/separators for lithium-ion batteries. Chemphyschem 15,1761-1771. 

Saunier, AUoin, E, Sanchez, J.-Y., Maniguet, L, 2004. Plasticized microporous PVdF separators for lithium ions batteries. Part 111 gel properties and irreversible modifications of PolyfvinyUdene fluoride) membranes under swelling in liquid electrolyte. J. Polym. Sci. Part B 42,2308-2317. 

Shirai, H., Spotnitz, R., Atsushi, A. Characterization of Separators for Lithium Ion Batteries -A Review, Chemical Industry, 48 (1997) 47 (in Japanese)... 

USABC Development of low cost separators for lithium-ion batteries , REPI 2001... 

Polymer electrolytes (e.g., poly(ethylene oxide), poly(propylene oxide)) have attracted considerable attention for batteries in recent years. These polymers form complexes with a variety of alkali metal salts to produce ionic conductors that serve as solid electrolytes. Its use in batteries is still limited due to poor electrode/ electrolyte interface and poor room temperature ionic conductivity. Due to its rigid structure it can also serve as the separator. Polymer electrolytes are discussed briefly in the section Separators for Lithium-Ion Batteries. 

The ceramic fillers (e.g., AI2O3, SiOa, TiOa) can greatly influence the characteristics and properties of polymer electrolyte by enhancing the mechanical stability and the conductivity [135, 175-178]. Prosini et al. [179] in a PVdF-HFP polymer matrix used y-LiAlOa, AI2O3, and MgO as fillers to form self-standing, intrinsically porous separators for lithium-ion batteries. The MgO-based separators showed the best anode and cathode compatibilities. 

Shirai H, Spotnitz R, Atsushi A (1997) Characterization of separators for lithium ion batteries - a review. Chem Ind 48 47 (in Japanese)... 

Jeong HS, Hong SC, Lee SY (2010) Effect of microporous structure on thermal shrinkage and electrochemical performance of A1203/poly(vinylidene lluoride-hexalluoro-propylene) composite separators for lithium-ion batteries. J Membr Sci 364 177-182. doi 10.1016/j.memsci.2010.08.012... 

F. Zhang, X. Ma, C. Cao, J. li, Y. Zhu, Poly(vinlyidiene fluoride)/Si02 composite membranes prepared by electrospinning and their excellent properties for nonwoven separators for lithium ion batteries, J Power Sour 251 (2014) 423—431. 

Venugopal G, Moore J, Howard J, Pendalwar S (1999) Characterization of microporous separators for lithium-ion batteries. J Power Sources 77 34-41... 

Fang J, Kelarakis A, Lin YW, Kang CY, Yang MH, Cheng CL, Wang Y, Giannelis EP, Tsai LD (2011) Nanoparticle-coated separators for lithium-ion batteries with advanced electrochemical performance. Phys Chem Chem Phys 13 14457-14461... 

The desirable properties of separators for lithium-ion batteries include low resistance, low shrinkage, and uniform pore structure. 

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