45985-AC7
Tailoring Ion-Containing Polymers for Energy Storage Devices
The development of safer and high conductivity polymer-based electrolytes for Li ion batteries is a critical challenge for a growing number of electronic devices. Replacing the current generation of ‘gel’ polymer membranes (which are composed of a porous polymer, high dielectric constant liquids, and a Li salt) with solid polymer electrolytes (SPE), which contain only polymer and ions, would result in important advantages, provided that SPE conductivities can approach those of the gels. As yet, the chemistries available for SPEs do not provide sufficiently high conductivity, and as an intermediate step, plasticized polymer electrolytes (PPEs) with modest plasticizer fractions are of interest. The overarching goal of this program is to develop an understanding of the three-fold interactions occurring in model PPEs – plasticizer-polymer, polymer-salt, and plasticizer-salt – so that the design parameters for a mechanically durable, high-conductivity PPE can be established.
Remarkably, only in a very few cases has the relationship between plasticizer interaction parameters and ionic conductivity been investigated. To facilitate the development of a more extensive understanding, we have chosen to focus our work on one Li+ conducting polymer and explore the role of plasticizers over a wide concentration and temperature range. The plasticizers were chosen to be miscible with the ion-conducting polymer and to span a wide range of dielectric constants, donor numbers, viscosities, and glass transition temperatures (Tg). The ion-conducting polymer under consideration is a single ion conductor (i.e., anions are covalently bound to the chains) consisting of a repeating structure of 13 consecutive units of ethylene oxide (molecular weight of = 600) separated by a 5-sulfoisophthalate unit. Li+ is the counter-ion and no phase separation (ion clustering) is observed for this material at room temperature in small-angle X-ray scattering experiments.
Initial experiments have focused on contrasting the polymer dynamics and ion conductivity of the neat poly(ethylene oxide-based) ionomer with plasticized forms containing 6 wt % of six miscible diluents: propylene carbonate, dioctyl phthalate, dimethyl sulfoxide, N,N-dimethylformamide, ethylene glycol, and triethylamine. Precise measurements of the conductivity and polymer dynamics of all materials over a wide temperature range were conducted using broadband dielectric (impedance) spectroscopy, covering a frequency range from 10 MHz to 10 mHz.
For the neat ionomer, ion motion was found, as expected, to be intimately dependent on the segmental dynamics and the glass transition (Tg) of the polymer. This dependence is also strongly evidenced by the curved form of conduction relaxation time vs 1/T, which indicates Vogel-Fulcher-Tammann -like behavior.
The temperature dependence of conductivity of the plasticized ionomers remains curved in all cases, signifying that ion motion is governed by segmental motion even with the addition of the dipolar small molecules, at least at the low plasticizer concentrations used in our initial study. The addition of the small molecules considerably speeds up the segmental and conduction processes simultaneously.
The observation that the so-called decoupling index decreases on addition of the plasticizers, and usually decreases significantly for samples with higher conductivities, suggests that the observed increases in conductivity in the presence of the plasticizers do not arise from decoupling segmental motion from ion motion. Rather, segmental motion and conductivity are strongly coupled in the PPEs under consideration, and therefore the increase in conductivity follows the decrease in Tg.
A simple model was developed to relate measured conductivity to the four plasticizer parameters: dielectric constant, donor number, viscosity and Tg. The PPEs under consideration here were found to have a slight but statistically insignificant dependence on dielectric constant and donor number, no dependence on viscosity, and an overwhelmingly strong dependence on Tg of the mixtures.
Using these initial findings as a guide, we are now exploring conductivity and its relationship to polymer dynamics for the same plasticizers noted above, but over a wider concentration range. As we progress to mixtures with many more plasticizer molecules, it is anticipated that plasticizer dielectric constant and perhaps other parameters will play a more prominent role in ultimately controlling ion motion and conductivity.
