Polyurethane-Based Gel Electrolyte for Application in Flexible Electrochromic Devices

Abstract

For the application in flexible electrochromic devices (ECDs) on plastic substrates, a new polyurethane-based gel electrolyte was manufactured. In this context, the curing behavior and the influence of the proportion of solvent and salt on the ion conductivity as well as the optical and mechanical properties were investigated. Furthermore, the stoichiometric ratio of the polyurethane matrix was varied to influence the ion conductivity. As an isocyanate component, the aliphatic difunctional polyisocyanate prepolymer, synthesized by Hexamethylen-1,6-diisocyanat (HDI), was chosen since the resulting polyurethane is considered to be particularly lightfast, color-stable and temperature-resistant and therefore frequently used for paints and coatings. As polyol a trifunctional polyetherpolyol was selected to form a wide-meshed crosslinked matrix to achieve a mechanically stable but flexible electrolyte, that enables the processing and bending of film-based ECDs. The additives amount and the matrix stoichiometric ratio affected the curing behavior and curability. The salt content had almost no influence on the measured properties in the chosen experimental space. Solvent content had a great influence on ion conductivity and mechanical properties. An understoichiometric ratio of the polyurethane matrix (0.85) increases the ion conductivity and the mechanical flexibility, but also the optical properties in a negative manner. The best specific ion conductivity with 10−5 S/cm was reached with an understoichiometric ratio of 0.85 and a high solvent content (30 wt%). Concluding, due to its high flexibility and transmittance, color neutrality and sufficient ion conductivity, the application of the researched electroyte in ECDs might be suitable. A demonstrator ECD was successfully manufactured and conducted.

Weathering of a Polyurethane-Based Gel Electrolyte

“…[ 12 ] Thermal decomposition without oxygen takes place at over 250 °C in the first stage due to thermolysis of urethane linkages and in the second stage due to decomposition of the polyol component [ 9 ]. This fits well with the previous result of a thermogravimetric analysis of the subjected polyurethane-based electrolyte [ 6 ]. Sui et al reported that during prolonged temperature storage, degradation of the hard (isocyanate dominated) segment took place first at approximately 175 °C through scission of the urethane bond to form urea, alcohols and CO 2 [ 13 ].…”

“…It is noticeable that, despite weathering, formulation f (0.85_5_10) retains a conductivity at a significantly higher level compared to the other formulations whose values drop comparably to the order of 10 −8 S/cm. The sub-stoichiometric ratio of the polyurethane matrix of formula f (0.85_5_10) thus seems to have a strong positive effect not only on the conductivity as prepared [ 6 ] but also on the aging resistance with respect to conductivity. Interestingly, the samples weathered in the dry nitrogen atmosphere almost retain their conductivity, with a certain variation ( Figure 1 b).…”

“…Lithium trifluoromethanesulfonate (LiTFSi) as a conducting salt and propylene carbonate (both Sigma Aldrich, St. Louis, MO, USA) as a solvent and dibutyl tin dilaurate (DBTL) (Merck KGaA, Darmstadt, Germany) as a curing catalyst were added to the matrix during manufacture. The preparation process and sample types are described in detail in [ 6 ], and the electrolyte formulations are listed in Table 1 .…”

“…Polyurethane, as a matrix in polymer electrolytes, has been studied in particular because of its hard–soft structure, which contains hard isocyanate segments and soft polyol segments [ 1 , 2 , 3 , 4 , 5 ]. Recently, a new cold-curing, polyurethane-based gel electrolyte was characterized and its application in an electrochromic (EC) multilayer film was demonstrated [ 6 ]. In addition to its high light transmittance and colorlessness, the electrolyte’s mechanical strength and flexibility allow for the deformation of EC films.…”

Relationship between the intermolecular interactions of carbonyl (PC) with nitrile (HNBR) functional groups and the flash point of a gel polymer electrolyte

“…The presence of solvent also contributes to the observed ionic conductivities by increasing polymer chain mobility and free volume. 33 Furthermore, the mechanism of ion transport differs between the gel polymer electrolytes and the dry polymer electrolytes. HNBR is mostly amorphous with a small degree of critallinity.…”