The quest for higher energy density batteries suggests the use of solid electrolytes that can harness the electro-plating and dissolution of reactive metals (Li°, Na°, K°, Mg°, Ca°) as they correspond to the highest capacity possible for the negative electrode. In liquid electrolytes, the reactivity of the organic solvent and the inevitable formation of dendrites have thwarted any effort to operate with these agressive metals. Solid electrolytes offer a safer approach to this problem. Ceramic electrolytes with high conductivity are now known for Li (Argyrodite sulfides, LAGP…) and Na (beta alumina, Nasicon…) but the building of all solid-state batteries stumbles on the loss of contact during operation and the subsequent volume change of the electrodes. Besides, the making of large thin films of the electrolyte is challenging.

Conversely, polymer electrolytes are able to be processed easily in thin films, and with their malleability and adhesiveness, keep a good contact despite the volume changes of the electrodes during operation. Most polymer electrolytes are obtained by dissolution of a low lattice energy salt into a solvating matrix, the most studied being poly(ethylene oxide) — PEO. Other solvating backbones are also known now, in the poly(ester) family with the advantage of being able to operate in contact with high voltage cathodes. As so, with a discrete salt complexed by the polymer, both anions and cations are mobile, which is a handicap, as only cations (Li⁺, Na⁺, K⁺, Mg⁺⁺, Ca⁺⁺) are exchanged at the electrode, resulting in concentration polarization. The most recent tendency is thus to tether the anions to the solvating polymer, or make an alloy of a poly(salt) with the solvating host (PEO for instance). The challenge is to design negatively charged moieties with a “handle” to link to the polymer keeping the high delocalization of the charge needed for conductivity. These so-called “single ion conductors” can operate in batteries with the reactive metals with minimal growth of deleterious dendrites. 

Polymer electrolytes are presently used in the only commercial solid-state batteries, produced by Blue Solutions^® in France and powering busses and cars.

A thorough discussion will be provided on these materials and their inherent electrochemistry.