Neutron Scattering Experiments and Computer Simulation Studies of a Polymer Electrolyte
Polymer electrolytes are regarded as key components in promising new types of batteries for portable electronic devices and electric cars. Batteries for such applications require polymer electrolytes with improved properties, e.g. a higher ionic conductivity. The route to improved polymer electrolytes is however unclear since there is no satisfactory understanding of the structure, dynamics and ion conduction mechanism on the microscopic length scale. The mechanism is furthermore interesting per se since it must be fundamentally different from the mechanism in low molecular weight electrolytes.
This thesis presents an investigation of the microscopic structure and dynamics of a solvent free polymer electrolyte, poly(propylene oxide) (PPO) complexed with LiClO4, with emphasis on properties of the polymer host. The structure was studied using neutron and x-ray diffraction experiments in combination with reverse Monte Carlo (RMC) and Molecular Dynamics (MD) simulations. We show that even for these complex materials, it is possible to construct three-dimensional models based on experimental data only, using the RMC method. The MD simulations of PPO, which were performed using an ab initio force field, were found to be in quantitative agreement with neutron and x-ray diffraction data. New insights were gained concerning the local chain conformations and inter-chain correlations of the polymer host materials as well as on the effects of the salt solvation on the polymer host structure and ion association. We show that for low salt concentrations, the inter-chain distance is constant, indicating a large excess volume between the chains which can completely host the salt. Furthermore, we find significant polymer-anion and inter-anionic correlations, the latter indicating an inhomogeneous distribution of the salt on intermediate length scales. The dynamics have been studied using high resolution quasi-elastic neutron scattering (QENS) experiments. We find that as the salt is introduced in the polymer matrix, the segmental relaxation behaviour becomes (i) slower, (ii) more stretched in time domain and that (iii) the relaxation times are no longer proportional to the viscosity. Results (i) and (iii) may be explained by that ions form transient inter-chain cross links, which temporarily hinder the relaxation. Analysis of result (ii) yield that the extra stretching of the relaxation behaviour is intrinsic, indicating a complex dynamics of the chain which can not be regarded as a superposition of independent contributions from heterogeneities. The molecular length scale of the experimental method allows for the first time a direct connection to the renewal time in the dynamics disordered hopping model for ion transport in polymer electrolytes.
reverse Monte Carlo simulation