Developments in Polymer Electrolytes for Lithium Batteries
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Developments
in Polymer Electrolytes for Lithium Batteries Peter V. Wright
Abstract Recent developments in polymer electrolyte materials for lithium batteries are reviewed in this article. Four general classifications are recognized: (1) solventcontaining systems in which a liquid electrolyte solution either is fully miscible with a single-phase swollen polymer matrix (gel) or is a two-phase system in which “free” liquid occupies micropores within a swollen polymer network (hybrid), and conductivity (1 mS cm1 at ambient temperature) is essentially independent of the polymer segmental motion (the thermal motion of segments of atoms along the backbone of a flexible polymer chain); (2) solvent-free, ion-coupled systems (typically polyether–Li salt complexes) in which both anions and cations are mobile within an amorphous, rubbery phase (conductivity 0.1 mS cm1 at ambient temperature); (3) “single-ion” systems with anions fixed to the polymer backbone or systems with anion mobilities reduced by incorporation within larger molecules or by associations with the chain (conductivity 105 S cm1 at ambient temperature); and (4) decoupled systems in which ionic mobility through channeled structures involves minimal local segmental displacements (conductivity 0.1–1 mS cm1 at ambient temperature). Keywords: ion conductors, polymers, rechargeable lithium batteries, structure.
Introduction The polymer electrolyte in a rechargeable lithium polymer battery plays a critical role as the electrode separator and, for cells employing composite cathode structures, as a mechanical binder for the composite. The electrolyte must thus allow the passage of ions, while blocking electron conduction between the active components of the battery. In the March 2000 issue of MRS Bulletin, Scrosati and Vincent described the primary requirements of polymer electrolytes for Li rechargeable batteries and provided a general discussion of approaches being pursued, categorizing these into five classes according to electrolyte composition and morphology.1 In this article, we adopt a somewhat different classification scheme based upon four distinct mechanisms for ion transport and provide a review of the most recent research developments in each case.
MRS BULLETIN/AUGUST 2002
In general, polymer electrolyte strategies currently being pursued exploit one of the following mechanisms for ion mobility: (1) the translation of lithium salts through liquid solvents in gels or hybrid materials of various kinds; (2) solventfree, salt–polymer complexed systems in which the ion motion is coupled to the micro-Brownian motion of segments of the polymer chains above the glass- or melting-transition temperature of the system; (3) “single-ion” systems, in which the lithium ion moves by a hopping process between anionic sites fixed to the polymer chain or systems with reduced mobility of anions (solvent-containing or solventfree); and (4) solvent-free, salt–polymer complexed systems in which ion mobility is decoupled from the motions of polymerchain segments.
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