Short and long range transport effects in salt containing solid-liquid composites (2014)

This work is dedicated to the investigation of long-range and short-range ion transport in composite lithium battery electrolytes. In previous work it was shown that the addition of insulating oxide nanoparticles to liquid electrolytes consisting of lithium salt dissolved in a solvent with low dielectric constant can lead to a significant improvement of the electrical and mechanical properties. Furthermore, it was observed that the lithium transference number can be simultaneously increased. The effects were ascribed to the adsorption of the salt anions on the oxide particle surface coupled with enhanced dissociation of ion pairs resulting in the release of highly mobile lithium ions in the particle space charge zone. As the nanoparticles form networks due to their aggregation tendency the space charge zones eventually stretch throughout the electrolyte leading to an improved overall ionic transport. The conductivity in these systems decreases over time due to unfavorable network restructuring, particularly in the case of rather high particle volume fractions. In this work it is shown that a conductivity increase is found in the course of time when the particle volume fraction in lithium perchlorate / tetrahydrofuran is low. As proven by different measurement techniques as well as by Monte Carlo simulation in combination with numerical FEM calculations, this can be assigned to the network formation process that is recorded in situ by means of ionic conductivity experiments. Furthermore, this system is used to successfully predict for the first time the expected conductivity and lithium transference number increase with help of literature data (such as association constant) based on the parallel switching model originating from solid-solid heterogeneous doping theory. Additionally, the already mentioned Monte Carlo simulation with adjacent numerical FEM calculation proved to be a good tool to investigate the relationship between the given particle characteristics such as its aggregation tendency and the resulting electrical properties of the composite electrolyte. With respect to applied materials research, mesoporous materials with high specific surface areas are investigated as potential soggy-sand fillers. It is observed that such silica particles lead to a conductivity and lithium transference number increase that is comparable to the case when small nanoparticles are chosen for polyethylene glycol-150 system. The observed effects are ascribed to the additional ionic pathways provided by the highly porous materials. It was found that the beneficial electrical properties are here coupled with tunable mechanical properties (gel-like materials) making such materials suitable candidates for the application as lithium battery electrolytes. Filler materials are investigated with respect to their Ion Exchange Capacity allowing for more detailed insights in their capability to adsorb anions. Linked to this, it is shown that the ion adsorption on particles in composite leads to a decrease of the salt concentration in the bulk electrolyte resulting in a conductivity decrease therein. This effect of salt exhaustion especially plays a significant role for low lithium salt concentrations. As ionic liquids have come into focus over the last years as potential battery electrolytes due to their high pristine room temperature ionic conductivity, heterogeneous doping of a model system is also investigated. Here, remarkably, the ionic conductivity can be kept constant in spite of the higher electrolyte viscosity. With regard to measurements of the ionic diffusion in solid polymer electrolytes, as well as in pure liquid electrolytes, it has recently been proposed that also ion pairs contribute to the ionic conduction by means of a vehicular transport mechanism. In this work this effect is investigated for pure liquid electrolyte model system (lithium triflate in polyethylene glycol-150) which now allows for the determination of the contribution of individual ionic species to the overall ionic conductivity. The analysis is realized by the combination of results originating from tracer diffusion, AC and DC polarization experiments.