Polymer Electrolytes: Ionic Transport Mechanisms and Relaxation Coupling
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Mark A. Ratner, Patrik Johansson, and Duward F. Shriver Introduction Polymer electrolytes have been a focus of the scientific community for a quarter century. They extend the realm of traditional solid-state ionics from hard materials to include soft materials and, in doing so, pose issues both fundamental (e.g., how are structure and transport defined in a concentrated electrolyte with an immobile solvent?) and technological (e.g., how can polymer ionics be used to construct electrochemical devices such as sensors, batteries, and fuel cells?). Several overviews of the polymer electrolyte area have been given,1–3 including a brief yet very good discussion of their applications in lithium metal batteries (see the article by Scrosati and Vincent in this issue.) An earlier MRS Bulletin issue2 focused largely on polymer/salt complex structures. This article focuses on mechanistic issues involving ion transport; the aim is to provide an introduction to these issues as a complement to Scrosati and Vincent’s article on applications. Historically, solid ionics was first discussed carefully by Faraday,4 who concentrated on such salts as silver sulfide and lead fluoride. Important subsequent work included careful measurements of silver salt conductors early in the 20th century,5 work at mid-century in developing hardoxide ionics,6 and the introduction of polymer ionics in the 1970s.7,8 Although polymer ionics has developed into ramified sub-areas during the past quarter century, the mechanistic issues can best be understood with reference to Class 1 of Scrosati and Vincent— that is, neat (not diluted) polymer/salt complexes and their generalization to MRS BULLETIN/MARCH 2000
polyelectrolytes, in which the counterion is covalently bound to the backbone. This overview will focus on that class of materials, whose formation can be exemplified in a simplified fashion as PPOn LiCF3SO3 l PPOn · LiCF3SO3. (1) Here n indicates the number of repeat units of the poly(propylene oxide) (PPO) (CH2CH(CH3)O)n , and the dot on the right-hand side indicates a complexation/ association process. Upon forming the complex (usually by casting a film in a cosolvent such as methanol, which is subsequently evaporated), the glass-transition temperature of the polymer normally increases by 50–100 K, and the material hardens, stiffens, and increases in conductivity by at least 105.
characterizes the four general classes of solid ionic materials. The polymer and glassy materials are amorphous, exhibiting no long-range structural order; both polymer electrolytes and glassy electrolytes generally exhibit a glass-transition temperature. Structure studies show that ceramic and salt crystalline ionics conduct because motion pathways are available to the ions—often, but not always, by means of ions moving through vacancies. In the beta-alumina family, the conduction pathways are between favored sites in the twodimensional plane containing the mobile alkali ion (see the article by Sudworth et al. in this issue). In silver halides, the fully disordered (high
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