(318e) Effect of Polymer Mobility on Conductivity of Single-Ion Conductors

Scientists are turning to the use of polymers as substitutes for liquid electrolytes in lithium ion batteries, because of their mechanical flexibility and non-toxic properties. Physical mixtures of lithium salt and poly(ethylene oxide) (PEO + LiClO4) are commonly chosen because they have potential for high ionic conductivities. However, high mobility of ions in these mixtures results in electrode polarization, which affects battery performance. To isolate the effect of the cation and to reduce the obstacle of concentration polarization, the anion is chemically incorporated into the backbone of the polymer, thereby rendering it immobile. These single-ion conductors are called ionomers.

It is a widely accepted theory that in polymer electrolytes, ion conduction is aided by the segmental mobility of the polymer. The cation electrostatically coordinates with 5-7 ether oxygen atoms and these cations ?hop? from one polymer segment to another by making and breaking temporary ?cross-links?. This study determines how the mobility of the ionomer affects the ion conductivity and how the ion content affects segmental mobility in return.

Neutron scattering experiments have been conducted on ionomers with varying degrees of ionization. We observed that there is an optimum ion content that has the maximum ionomer mobility and the highest conductivity. This can be explained by the fact that when ion content increases, there are more ions present to contribute to conductivity, but electrostatic coordination of cations with ether oxygen atoms reduces the polymer mobility.

Neutron scattering experiments further show that polymers with ions have a second process, which is absent in polymers with no ion concentration at all. This second process is slower that than the segmental motion of the polymer and it constitutes of making and breaking of electrostatic cross-links. This feature becomes more prominent with increasing ion content.

On comparison of the timescales for segmental mobility and the second process we noted that for PEO+LiClO4 mixtures, this second process is significant in improving conductivity. However, for ionomers, this process does not play a major role in the mechanism of conduction. This comparison in terms of mobility as a function of ion concentration gives a better insight for understanding the fundamental mechanism of ion conduction in ionomers. This will enable us to choose an ion loading that gives optimum performance.

We measured ionomer mobility using the High-Flux Backscattering Spectrometer [HFBS] and the Disk Chopper Time-of-Flight Spectrometer [DCS] at the NIST Center for Neutron Research in Gaithersburg, Maryland. DCS primarily detects segmental motion of the polymer, which lies in a timescale between 0.1 ps and 40 ps. HFBS detects motion on a timescale between 240 ps and 2 ns, and this primarily constitutes of the second process. Mobility is measured for ionomers with from 0%, 5%, 10%, 25%, 50% and 100% ionization, at temperatures of 298K, 325K and 350K.