Microscopic Dynamics and Structure of some Polymer-based Electrolytes

Doctoral thesis, 2004

The electrolyte is a key component in high-capacity lithium batteries and there is a large demand for improved materials to increase the performance. Polymer-based electrolytes offer advantages in the design of safer and lighter batteries. The solvent-free polymer electrolytes lack problems specific to liquid electrolytes, but have a lower ionic conductivity. In polymer gel electrolytes, on the other hand, the ion conduction is coupled to the solvent dynamics and higher, but stability and solvent retention is more critical. A better understanding of the microscopic properties favorable to the ionic conduction and elastic properties is essential for development of new materials with improved performance. Here, investigations of the microscopic structure and structural dynamics of a model polymer electrolyte, poly(propylene oxide)(PPO) complexed with LiClO4, and a polymer gel electrolyte are presented. The segmental relaxation, was investigated using various quasi-elastic neutron scattering (QENS) techniques. For the polymer electrolyte the relaxation is slower and more non-exponential (stretched) compared with the polymer; proportionality between relaxation time and viscosity does not hold, and the Q-dependence of the relaxation time is stronger than for the pure polymer. These effects may be explained by chain segments in the electrolyte being coordinated to lithium ions, which restrains the relaxation. In both the polymer and the polymer electrolyte, the stretching has no significant temperature dependence, which indicates that the segmental relaxation is homogeneous. In pure PPO, a cross-over from Gaussian dynamics at Q=0.4 Å-1 was detected by an extension of previous QENS data to lower Q. Analysis of diffraction data of the polymer electrolyte using RMC modeling, reveal marked correlations between chains and anions and between different anions, both with a possible origin in structural arrangements of chain segments and anions around the lithium cations. At a lower salt concentration, the model structure support a locally heterogeneous ion distribution in the polymer electrolyte. The diffusive dynamics of the polymer gel electrolyte was studied on a length scale where influence from the polymer matrix can be expected. A length scale for the confinement effect of the polymer on the solvent was determined. Within the confined regions liquid-like diffusion was observed.