Polymer Electrolytes

During the 20th century most synthetic polymers have been used as structural materials or as electric insulators. But in the past 20 years, they have been tailored as electron or ion conductors; when combined with appropriate salts their ionic conductivity can be put to use as an electrolyte. Peter V. Wright, a polymer chemist from Sheffield, first showed in 1975 that poly-ethylene oxide (PEO) can act as a host for sodium and potassium salts, thus producing a solid electrical conductor polymer/salt complex (P.V. Wright, British Polymer Journal, 7 (1975), p. 319). Michel Armand, who had suggested the use of graphite intercalation compounds for electrodes, immediately realized that lithium/PEO complexes could be deployed as solid electrolytes matching perfectly intercalation electrodes. A lithium salt could be dissolved in a solvating polymer matrix through direct interaction of the cation and electron pairs. The complex formed (as result of the favorable competition between the solvation energy and the lattice energy of the salt) becomes a good conductor at 60-80°C. Armand's suggestion met with considerable interest at the Second International Meeting on Solid Electrolytes held at St Andrews in Scotland (M.B. Armand, J.M. Chabagno and M. Duclot, in Second International Meeting on Solid Electrolytes, St Andrews, Scotland, 20-22 Sept., 1978, Extended Abstract; M.B. Armand, J.M. Chabagno and M. Duclot, “Poly-ethers as solid electrolytes”, in P. Vashitshta, J.N. Mundy, G.K. Shenoy, Fast ion Transport in Solids. Electrodes and Electrolytes, North Holland Publishers, Amsterdam, 1979). Armand's short paper opened up new perspectives in the international solid-state ionics community.

However, understanding the formation of a salt complex and the nature of the charge transport mechanism proved no simple matter. They required bridging the two communities of polymer and solid-state chemistry, traditionally concerned with inorganic compounds and crystal structures. The standard model with roots in the study of ceramic-type electrolytes (e.g. beta-alumina) linking ionic conductivity to defects in a crystal structure proved misleading for polymers. Experiments and detailed mechanistic studies clearly established in 1983 that ionic motion in salt-polymer complexes is not due to charges hopping from site to site. Rather it is a continuous motion occurring in the amorphous region of the polymeric material (C. Berthier, W. Gorecki, M. Minier, M.B. Armand, J.M. Chabagno, P. Rigaud, Solid State Ionics 11 (1983) p. 91; M. Minier, C. Berthier, and W. Gorecki, Journal de Physique 45 (1984) p. 739). The intrinsic phenomenon of a solid material exhibiting liquid-like conductivity without motion of the solvent itself was fascinating from a theoretical point of view and the applications to electrochemical devices sounded very promising in a time of emerging concerns with energy and pollution. A polymer electrolyte can be easily manufactured into shapes not available to liquid containing systems, and it is safer than liquid electrolytes.

Like beta-alumina in the 1970s, the PEO served as a prototype material in the 1980s for investigating alternative models of ion transport, and for developing the concept of the film cell for solid batteries (see figure 1 Schematic of a polymer electrolyte battery (D.F. Shriver and G.C. Farrington, Chemical and Engineering News, 6, May 20, 1985, p. 42).

However, from a practical standpoint the PEO was not itself an ideal electrolyte. Several manipulations were needed to prevent its crystallization and to extend the domain of existence of the elastomeric phase favorable to high ionic conductivity: plasticizing the matrix by addition of a low molecular weight polar molecule or forming block or comb copolymers. Meanwhile other structurally similar polymers were extensively studied, such as poly-propylene oxide (PPO); and other classes of conductive polymers have been prepared in the 1980s. As electronically conductive polymers captured the attention of many scientists, the interest of the solid-state ionics community shifted toward mixed conductors (backbone polymers or redox polymers) which, in turn, raised new theoretical challenges.

Research and application of polymer electrolytes forked into two directions: on the one hand it was demonstrated that modified PEO electrolytes, operating at 40 to 80°C, allowed the harnessing of the electrochemical with maximum energy per unit weight: lithium metal and an intercalation compound. The main application is in the automotive industry: electric and hybrid cars where it is easy to regulate the temperature above ambient. This is indeed considered as a part of the safety, a topic of great concern when huge amounts of energy (>20kWh) are stored in a confined volume (car) and destined for the public. Other potential applications may be found in load leveling, and in the emergency energy supply of remote telecommunication relays where the density of the electronics components maintains a temperature above 40°C, a temperature deleterious to the life span of ordinary electrochemical systems.

On the other hand, it became clear from an understanding of the polymer electrolyte conduction mechanism, that they would not lead to materials with a conductivity sufficiently high for the portable electronics industry, where operation at –20° at least is required. The extraordinary development of “lithium-ion” batteries (using two intercalation electrodes) that started in the early 1990's and that now amounts to an $8 billion/year market, was mainly achieved using liquid electrolytes. The danger presented by these flammable liquids is considered as acceptable for such small devices. However, the advantages offered by solid electrolytes have led to a compromise: heavily plasticized polymers are used as a gel-type, associating acceptable mechanical properties with high conductivities at all common temperatures. These “polymer lithium-ion batteries”, keeping the concept of thin film configuration and a plastic electrolyte are spreading rapidly in the electronics market, and might one day replace all existing battery systems based on liquid electrolytes alone.

This page was written by Bernadette Bensaude-Vincent and Michel Armand. It was last updated on 24 April by Arne Hessenbruch.