Page 655 - Handbook of Battery Materials

18.2 Solvent-Free Polymer Electrolytes 629

[6, 8]. Although higher conductivities are preferable, 100-fold or 1000-fold increases
are not essential, as a thin-film electrochemical cell configuration can largely
compensate for these lower values. Less favorable is the tendency for ion association
and low cationic relative mobility (a property shared with aprotic liquids, as opposed
to ceramic or glassy electrolytes) in polyether-based materials. These fundamental
properties can affect cell performance and must influence the design of new
polymeric electrolytes to make them competitive as battery materials.

18.2
Solvent-Free Polymer Electrolytes

18.2.1
Technology

The first report of the performance of small-scale polyether-based batteries came in
◦
1983. The choice of salt necessitated operation at 120 C, which contributed to the
severe decline in capacity and restricted operation to only 50 cycles. Improved cell
performance and working temperature were achieved in the first instance by alter-
ing the salt and the intercalation cathode. Once the principle of a polymer-electrolyte
secondary battery was established, the level of commercial interest remained. The
advent of stringent environmental laws, however, have led to government-backed
R&D efforts, such as in North America with the United States Advanced Battery
Consortium (USABC) and in Japan with the national 10-year research programme
involving the Lithium Battery Energy Storage Technology Research Association
(LIBES). On a more fundamental level, the timing of developments in realistic
alternatives to lithium-metal anodes has also been fortuitous. Because of perceived
limitations in the use of polyether-based solvent-free electrolytes, commercial in-
terest has largely focused on lithium-ion electrodes with gel-type electrolytes, at
least for small (<10 Wh) devices. No alternative lithium source anode materials are
available, however, to replace lithium metal without sacrificing anode capacity, cell
voltage, and consequently energy density. For this reason, a number of commer-
cially based programs are dedicated to the development of ‘dry’ electrolyte–lithium
metal anode technology.
The major investor in the development of ‘dry’ electrolyte technology has been
Hydro-Qu´ ebec, a Canadian electricity utility, in partnership with various groups and
companies since 1980 (Elf-Aquitaine, Yuasa, 3M). A large proportion of the work
being carried out at present by the present consortium is funded by USABC. Much
of the improvement in performance of cells under trial in the 1980s and early 1990s
can be attributed to modifications to the PEO–salt polymer electrolyte. Development
of amorphous modified polyethers and new plasticized anions permits operation
◦
at 60 C and below, higher bulk conductivities, and much improved lithium-ion
transport. New materials, still under development, are hoped to improve rate
capability, operating temperature, and cycle life. Generally, the cycling efficiency
of cells is very good [9]. Cells can be fully discharged for over 500 cycles at