Electrolytes and Separators for Lithium Batteries

With these introductory remarks, one can provide a concise description of the major issues involved in the choice and use of various classes of electrolytes in lithium batteries. It should be immediately stated that most of the work in the literature pertains to the dominant class of these batteries, namely, lithium-ion batteries. These are the intercalation batteries in which both the anode and the cathode are hosts for the lithium ions and no metallic lithium or its alloys are involved. 

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Aprotic electrolytes of an adequate high conductivity are necessary for lithium batteries and super capacitors. Therefore, recently, much industrial research has been done in this area and highly sophisticated electrolyte systems have been developed (e.g. [64]). The supporting electrolytes for aprotic solvents generally are more or less expensive and toxic. After the reaction, their separation and recycling is inevitable and frequently needs considerable efforts. 

At this time the only commercially available all-solid-state cell is the lithium battery containing Lil as the electrolyte. Many types of solid lithium ion conductors including inorganic crystalline and glassy materials as well as polymer electrolytes have been proposed as separators in lithium batteries. These are described in the previous chapters. A suitable solid electrolyte for lithium batteries should have the properties... 

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."... 

To overcome the poor mechanical properties of polymer and gel polymer type electrolytes, microporous membranes impregnated with gel polymer electrolytes, such as PVdF. PVdF—HFP. and other gelling agents, have been developed as an electrolyte material for lithium batteries.Gel coated and/ or gel-filled separators have some characteristics that may be harder to achieve in the separator-free gel electrolytes. For example, they can offer much better protection against internal shorts when compared to gel electrolytes and can therefore help in reducing the overall thickness of the electrolyte layer. In addition the ability of some separators to shutdown... 

A battery cell where both the electrodes consist of dopable polymer is shown in Figure 5.23. The electrolyte in this case consists of Li+ClO 4 dissolved in an inert organic solvent, usually tetrahydro-furan or propylene carbonate. When two sheets of polyacetylene or PPP are separated by an insulating film of polycarbonate saturated in an electrolyte (lithium perchlorate), and completely encapsulated in a plastic casing, a plastic battery can be made. The two sheets of polyacetylene or PPP act as both anode and cathode for the battery. A schematic is shown in Figure 5.24. Although doped polyacetylene and polyaniline electrodes have been developed, polypyrrole-salt films are the most promising for practical appKcation. 

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. 

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. 

Polymer Electrolytes. An alternative to the liquid electrolytes is a solid polymer electrolyte (SPE) formed by incorporating lithium salts into polymer matrices and casting into thin films. These can be used as both the electrolyte and separator. These electrolytes have lower ionic conductivities and low lithium-ion transport numbers compared to the liquid electrolytes, but they are less reactive with lithium, which should enhance the safety of the battery. The use of thin polymer films or operation at higher temperatures (60-100°C) compensate in part for the lower conductivity of the polymer film. The solid polymers also offer the advantages of a nonliquid battery and the flexibility of designing thin batteries in a variety of configurations. 

The polymer electrolyte lithium batteries contain aU solid-state components lithium as the anode material, a thin polymer film as a solid electrolyte and separator, and a transition metal chalcogenide or oxide, or a sulfur-based polymer as tbe cathode material. These features offer the potential for improved safety because of tbe reduced activity of lithium with the solid electrolyte, flexibility in design as tbe cell can be fabricated in various sizes and shapes, and high energy density. 

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