Understanding Ion Conduction in Polymer-Ceramic Composite Electrolytes for Lithium Ion Batteries

Abstract

In the transition to safer, more energy-dense solid state batteries, polymer-ceramic composite electrolytes may offer a potential route to achieve simultaneously high lithium ion conductivity and enhanced mechanical stability. Despite numerous studies on the polymer-ceramic composite electrolytes, disagreements persist on whether the polymer or the ceramic are the primary conduction pathway and how each phase is impacted by the proximity of the other. This lack of understanding limits the design of effective composite solid electrolytes. Therefore, the goal of this thesis is to establish the role that the polymer plays in the ionic conduction and what changes in this phase to allow for enhanced conduction. I present a collection of well-controlled experiments using model systems and minimizing the parameter space, as well as utilizing advanced characterization techniques. In doing so, we probe the underlying mechanisms of how lithium ion conductivity is affected in polymer-ceramic composites. In particular, the use of positron annihilation spectroscopy has been instrumental in revealing the primary mechanisms that alter lithium ion conductivity in the polymer matrix. First, we present a comprehensive and in-depth review of the literature in Chapter 2. This chapter statistically analyzes the trends in ionic conductivities achieved in the field and present the various arguments for ion conduction pathways and interfacial effects made throughout the last three decades. As a result of this field-wide analysis, we hypothesize that the major component whose ionic conductivity is affected in the composite is the polymer. Thus the following experiments and results focus on how the polymer is altered structurally and/or chemically. Second, in Chapter 3 we present a study utilizing thin polymer films deposited on substrates of varying acidity, as a model system. Comparison of the polymer film conductivities, chemistry and structures allow us to deduce that the modification of polymer structure near a ceramic interface is a major factor, while the ceramic-polymer interface chemistry has a negligible effect on the conductivity. In particular, the crystallinity and free volume of the polymer change appreciably to result in a higher ionic conductivity in thin films as compared to bulk materials. In Chapter 4, we assess well-controlled composite electrolytes with inert, non-lithium conducting ceramic fillers, and active, lithium conducting ceramic fillers in the polymer, in particular by consistently controlling the particle size and filler volume fraction. We find that the increase in the free volume of the polymer phase plays a crucial role in the conductivity of both types of composites. The addition of active fillers only minorly improves conductivity over inert fillers, solely due to the conduction path introduced by the active ceramic particles. In Chapter 5, we assess composites containing inert nanometer-site particles to those with inert micrometer-size particles. We find that the ionic conductivity improves with nanoparticles but not with the micrometer-size particles, and again we are able to correlate this improvement with the free volume present in the polymer matrix. Lastly, in Chapter 6, we will summarize several experiments that, though less pertinent to the main aims of this thesis, are valuable to the field in understanding the techniques for characterizing these materials.

Through this thesis, we provide a strong argument for the importance of the polymer’s structure in the composite electrolytes, and particularly an increase in the free volume present in the polymer phase. Such analysis of free volume has been limited in the field thus far. Thus, this thesis fills an important knowledge gap that has been limiting the understanding and control of how ceramic fillers increase lithium ion conductivity in polymer-ceramic composite electrolytes.