PMMA based gel electrolyte for EC smart windows S.A. Agnihotry a, *, Pradeep a , S.S. Sekhon b a National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110 012, India b G.N.D. University, Amritsar 143 005, India Received 9 September 1998; received in revised form 27 October 1998 Abstract Preliminary studies of gel electrolytes based on PMMA and solutions of the lithium salt LiN (CF 3 SO 2 ) 2 in propylene carbonate (PC) and the mixed solvent propylene carbonate+ethylene carbonate (PC+EC) are presented. The in¯uence of salt concentration on conductivity with dierent proportions of PMMA and dierent volume ratios of EC and PC has been investigated. Comparison of conductivity studies of the liquid electrolytes with the salts LiCF 3 SO 3 and LiN(CF 3 SO 2 ) 2 has also been made. The addition of PMMA to the liquid electrolytes increases the viscosity considerably without aecting their stable potential window resulting in thermally stable gels. The high conductivity attained by the gels (3 Â 10 À3 S cm À1 ) at room temperature and their moderate variation with temperature made them very promising for electrochromic window applications # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Gel electrolyte; PMMA; LiN(CF 3 SO 2 ) 2 ; Smart windows; Mixed electrolytes 1. Introduction Electrochromic devices (ECDs) based on solid poly- meric electrolytes (SPEs) are desirable for development of energy ecient windows, so-called `Smart Windows'. A search has been going for elastomeric and adhesive electrolytes with high conductivity at room temperature and with electrochemical and ther- mal stability. In order to have an ecient and reprodu- cible performance of an ECD the ohmic drop across the device needs to be minimized. This can be achieved with an electrolyte having conductivity greater than 10 À4 S cm À1 at room temperature. Several approaches have been followed to prepare SPEs with such proper- ties. Conventional polymeric electrolytes such as PEO- Li (X) where PEO is polyethylene oxide and LiX is a Li salt, with large anions such as AsF 6 À , BF 4 À or CF 3 SO 3 À . Materials of such compositions can be iso- lated as mechanically strong, free standing ®lms but their conductivities are too low (10 À9 ±10 À5 S cm À1 at 258C) for use in ECDs. The conductivity of SPEs can be enhanced by using amorphous polymer hosts hav- ing a low glass transition temperature (Tg). However, it is generally observed that high conductivity is achieved at the expense of good dimensional stability. There have also been attempts [1] to increase the amor- phous domains of electrolytes containing crystalline polymers like PEO, by using a salt such as LiN(CF 3 SO 2 ) 2 . Such a salt with extensive dispersion of the negative charge on the anion has a plasticizing eect and easily forms complexes with PEO with excel- lent ionic conductivities in the range 5 Â 10 À5 S cm À1 . These electrolytes have some tendency to slowly crys- tallize. High conductivities, greater than 10 À4 S cm À1 at room temperature, can be attained by an alternative approach of gel electrolytes. In such polymer±salt±sol- Electrochimica Acta 44 (1999) 3121±3126 0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0013-4686(99)00029-8 * Corresponding author.