Batteries highly conductive polymer electrolyte

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The future remains bright for the use of carbon materials in batteries. In the past several years, several new carbon materials have appeared mesophase pitch fibers, expanded graphite and carbon nanotubes. New electrolyte additives for Li-Ion permit the use of low cost PC based electrolytes with natural graphite anodes. Carbon nanotubes are attractive new materials and it appears that they will be available in quantity in the near future. They have a high ratio of the base plane to edge plain found in HOPG. The ultracapacitor application to deposit an electronically conductive polymer on the surface of a carbon nanotube may be the wave of the future. 

Lithium polymer electrolytes formed by dissolving a lithium salt LiX (where X is preferably a large soft anion) in poly(ethylene oxide) PEO can find useful application as separators in lithium rechargeable polymer batteries.Thin films must be used due to the relatively high ionic resistivity of these polymers. For example, the lithium-ion conductivity of PEO—Li salt complexes at 100 °C is still only about Viooth the conductivity of a typical aqueous solution. 

For using lithium batteries (which generally have high energy densities) under extreme conditions, more durable and better conducting electrolytes are necessary. Salt-in-polymer electrolytes discovered by Angell et al. (1993) seem to provide the answer. Polypropylene oxide or polyethylene oxide is dissolved in low melting point mixtures of lithium salts to obtain rubbery materials which are excellent lithium-ion conductors at ambient temperatures. 

Electrical Conductivity. The electrical conductivity of carbon blacks is inferior to that of graphite, and is dependent on the type of production process, as well as on the specific surface area and structure. Since the limiting factor in electrical conductivity is generally the transition resistance between neighboring particles, compression or concentration of pure or dispersed carbon black, respectively, plays an important role. Special grades of carbon black are used to donate to polymers antistatic or electrically conductive properties. Carbon blacks with a high conductivity and high adsorption capacity for electrolyte solutions are used in dry-cell batteries. 

The lithium polymer battery (LPB), shown schematically in Fig. 7.21, is an all-solid-state system which in its most common form combines a lithium ion conducting polymer separator with two lithium-reversible electrodes. The key component of these LPBs is the polymer electrolyte and extensive work has been devoted to its development. A polymer electrolyte should have (1) a high ionic conductivity (2) a lithium ion transport number approaching unity (to avoid concentration polarization) (3) negligible electronic conductivity (4) high chemical and electrochemical stability with respect to the electrode materials (5) good mechanical stability (6) low cost and (7) a benign chemical composition. 

Ion conducting polymers may be preferable in these devices electrolytes because of their flexibility, moldability, easy fabrication and chemical stability (for the same reasons that they have been applied to lithium secondary batteries [19,48,49]). The gel electrolyte systems, which consist of a polymeric matrix, organic solvent (plasticizer) and supporting electrolyte, show high ionic conductivity about 10 5 S cnr1 at ambient temperature and have sufficient mechanical strength [5,7,50,51], Therefore, the gel electrolyte systems are superior to solid polymer electrolytes and organic solvent-based electrolytes as batteries and capacitor materials for ambient temperature operation. 

Solvent-free polymer-electrolyte-based batteries are still developmental products. A great deal has been learned about the mechanisms of ion conductivity in polymers since the discovery of the phenomenon by Feuillade et al. in 1973 [41], and numerous books have been written on the subject. In most cases, mobility of the polymer backbone is required to facilitate cation transport. The polymer, acting as the solvent, is locally free to undergo thermal vibrational and translational motion. Associated cations are dependent on these backbone fluctuations to permit their diffusion down concentration and electrochemical gradients. The necessity of polymer backbone mobility implies that noncrystalline, i.e., amorphous, polymers will afford the most highly conductive media. Crystalline polymers studied to date cannot support ion fluxes adequate for commercial applications. Unfortunately, even the fluxes sustainable by amorphous polymers discovered to date are of marginal value at room temperature. Neat polymer electrolytes, such as those based on poly(ethyleneoxide) (PEO), are only capable of providing viable current densities at elevated temperatures, e.g., >60°C. 

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