By specific choice of functionalized alkoxysilanes and addition of plasticizers conductivities of up to 1023 S cm21 can be achieved186 while a good mechanical stability is also maintained. These materials are electrochemically stable up to 4.2 V. Prototype battery production based on an up-scaled ORMOCER1 electrolyte separator from Fraunhofer ISC has started at Varta Company.
Recent developments at the Fraunhofer ISC aim at systems which can be applied without adding liquid plasticizers. Such electrolytes will have enhanced dimensional stability. So, very thin electrode foils without further encapsulation measures can be used. Such electrolytes have reached conductivities of about 5 6 1025 S cm21 at room temperature until now. These values are below those for systems containing liquid plasticizers but they are sufficient for only 20 mm thick layers which are achieved in the battery concept. This conductivity in addition to an electrochemical stability of 4.2 V shows the very high potential of this new electrolyte for thin film lithium–polymer batteries (see Fig. 21).
In parallel and in cooperation at LEPMI-INPG, organic– inorganic gels have also been synthesized by interchanging some alkoxy groups of Si(OR)4 precursors with polyethyleneglycol (PEG) chains. The PEG was used to solvate small cations such as lithium, leading to a good ionic conductivity. 181 However the slow hydrolysis of the Si–O–PEG bonds leads to a degradation of such materials. This stability problem can be avoided by using AMINOSILS.181 These compounds were recently synthesized via the hydrolysis and condensation of Si(OR)3R9 precursors (R9 = –(CH2)n–NH2). The nonhydrolyzable alkylamino group can solvate, via the amino group, anions such as CF3SO3 2 rendering free for conduction the associated counter ions (protons). The resulting gels exhibit a rather good protonic conductivity at room temperature (s = 1.4 6 1025 S cm21 for Si(OR)3(CH2)3NH2,(CF3SO3H)0.1 based systems).181,187
Among electrochemical devices, electrochromic displays using transition metal oxides (WO3, TiO2, MnO2, IrO2) as active electrodes can be built by using protonic conductor gels as electrolytes. However, in such acidic conditions and upon electrochromic solicitations the oxide layers are corroded, because the stability of many oxides lies in the 4 to 12 pH range. To overcome this problem, new proton vacancy conducting transparent polymers which work in a higher pH range were developed.183 However when the different components (POE, sulfamide, guanidinium cation) are mixed without covalent bonding between the different phases, the resulting polymer electrolyte is in a metastable amorphous state.
Slow crystallization responsible for a drastic decrease in conductivity occurs in a few month of storage. In order to overcome both acidity and crystallinity problems, new proton vacancy conductors based on anion-grafted ormosils have been synthesized via a sol–gel process. These ionic conductors are based on a three component system: a solvating polymer (a,v-di-(3- ureapropyltriethoxysilane)poly(oxyethylene-co-oxypropylene)), a proton source (3-methanesulfonamidopropyltrimethoxysilane) and a deprotonating agent or proton vacancy inducer (imidazolium cations introduced through 3-(2-imidazolin-1- yl)propyltriethoxysilane, where imidazoline is used as a strong base).183 These materials are obtained by the copolymerization of sulfonamide-containing groups, partially deprotonated, and POE as an internal plasticizer. All organics groups are anchored to trialkoxy silanes which, through hydrolysis and condensation reaction, lead to a silica based backbone. In the presence of a deprotonating agent, conductivity is greatly enhanced, being now solely due to the motion of proton vacancies. The conductivity is 1025 S cm21 at 80 uC.