Poly /lithium salt systems ionic conductivity

A second class of important electrolytes for rechargeable lithium batteries are soHd electrolytes. Of particular importance is the class known as soHd polymer electrolytes (SPEs). SPEs are polymers capable of forming complexes with lithium salts to yield ionic conductivity. The best known of the SPEs are the lithium salt complexes of poly(ethylene oxide) [25322-68-3] (PEO), —(CH2CH20) —, and poly(propylene oxide) [25322-69-4] (PPO) (11—13). Whereas a number of experimental battery systems have been constmcted using PEO and PPO electrolytes, these systems have not exhibited suitable conductivities at or near room temperature. Advances in the 1980s included a new class of SPE based on polyphosphazene complexes suggesting that room temperature SPE batteries may be achievable (14,15). 

The concept of SPE dates back to 70s, when Armand firstly proposed a new ion conductor based on a lithium salt properly complexed by a polar and aprotic polymer matrix without the use of any liquid component (additives or liquid electrolytes) [65]. At the beginnings, the research on SPEs was exclusively focused on poly(ethyleneoxide) (PEO) as the complexing polymer [66]. Ever since, a lot of polymer/salt systems were deeply investigated, such as those based on PMMA, PAN, PVDF [66-69]. In principle, SPEs must satisfy some basic requirements (i) ionic conductivity higher than 10 " S/cm at room temperature, (ii) good thermal, chemical and mechanical stability, (iii) lithium transport number close to the unity, and (iv) compatibility with the electrodes and consequently wide electrochemical windows [67]. 

Ionic conductivity of alkali metal salt complexes of poly(ethylene oxide) was noted in studies three decades ago by Wright [217, 218]. The interest in this observation has surged, due to the commercial success of lithium ion batteries. Poly(ethylene oxide) plus lithium salts have been the preferred system for solid polyelectrolyte layers, although the commercial systems... 

Quasi-solid-state electrolytes include gel polymer electrolytes, ionic liquids, and plastic crystal systems. It is important to distinguish polymer electrolytes and gel polymer electrolytes. In polymer electrolytes, charged cationic or anionic groups are chemically bonded to a polymer chain, while gel polymer electrolytes are solvated by a high dielectric constant solvent and are free to move. In a classical gel electrolyte, polymer and salts are mixed with a solvent, usually having a concentration above 50 wt%, and the role of the polymer is to act as a stiffener for the solvent, creating a three-dimensional network, where cations and anions move freely in the liquid phase [88]. The solid polymer electrolyte includes poly(ethylene oxide) (PEO)-based lithium ion conductors that typically show conductivities of 10 S cm while the gel polymer electrolytes have semisolid character with much higher ionic conductivities of the order 10 —10 S cm . ... 

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