Source: Dissertation Abstracts International, Volume: 76-07(E), Section: B.

Adviser: Andre D. Taylor.

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The phenomena at material interfaces are some of the most important aspects of study, design and optimization in lithium batteries. These interfaces are the source of electrolyte decomposition and passivation, active material formation, phase changes and even dendrite growth in lithium-based cells. Each reaction plays a crucial role in the cell life cycle but their effects are frequently difficult to disentangle. Developing a better understanding of these reactions and how they are affected by material design will yield a path toward improving the performance of Li-ion and Li-O2 batteries.

Thin film polymer-composites are appealing subjects of study both at a fundamental level by examining the effect of active material-polymer interaction and at a design level by implementing novel methods of deposition. Thin conductive electrodes for electrochemical applications have been sought for their flexibility, transparency, and strength, yet poor control in processing has generally restricted their application. Here we present a fully-automated spin-spray layer-by-layer (SSLbL) technique for the rapid production of high quality, tunable multilayer films. SSLbL is shown to exhibit nano-level control over film growth and permits the efficient formation of percolating conductive networks. To explore the growth of percolating networks in conducting multilayers, a stratified rod network model was developed. This validated model demonstrates the importance of interlayer junction distance on percolation in multilayer composites. Ultrathin and transparent polymer-composite multilayers were also evaluated as Li-ion battery electrodes, emphasizing the practical application of this technique. Carbon nanotube and V2O5 nanowire containing composite films were demonstrated as ultrathin anodes and cathodes. Practical solid polymer electrolyte multilayers with integrated Li salt were also generated. All three cell components exemplify the potential of the SSLbL technique in the development of energy storage systems.

To explore the effect of active material-polymer interaction on electrochemical activity, a core-shell nanocomposite consisting of high aspect ratio V 2O5 nanowires coated with electroactive polyaniline (PANI) was synthesized. The composite nanowire cathode structure was shown to benefit cycle stability as a result of improved electrical conductivity and passivation. Improvements in rate capability were also correlated with increasing thicknesses of the PANI coating layer. This study provides evidence that an electroactive polymer coating can enhance the underlying capacity of a cathode material.

Understanding the interactions between the catalyst surface and electrolyte in Li-O2 or Li-air systems is crucial to improving energy density, efficiency, and cycle life in these cells. The co-functionality of several noble metal catalyst-electrolyte pairs was assessed through an electrochemical study of stability, kinetics, and activity. We present evidence of a synergistic effect between Pt and Pd catalysts and a DMSO-based electrolyte which enhances the kinetics of oxygen reduction and evolution reactions in Li-O2 cells. Direct observation of the cathode interface during cycling is also necessary to improve our understanding of discharge product formation and evolution in practical Li-O2 batteries. To achieve this, a gold electrode surface was monitored by operando surface-enhanced Raman spectroscopy and in situ electrochemical impedance spectroscopy during typical discharge and charge cycling. The precipitation of lithium superoxide (LiO2) and lithium peroxide (Li 2O2) was observed during discharging. Upon charging, a superficial layer of these species (∼1 nm) was preferentially oxidized at low overpotentials, leaving residual products in poor contact with the Au electrode surface. These operando and in situ studies of the oxygen electrode interface, coupled with ex situ characterization, illustrate that the composition of discharge products and their proximity to the catalytic surface are important factors in the reversibility of Li-O2 cells.

The work presented in this dissertation on polymer-composite Li-ion and catalyzed Li-O2 systems illustrates the importance of focusing on interfacial phenomena to improve our fundamental understanding of electrochemical activity in energy storage devices.