POLYMER ASSISTED ASSEMBLY OF INORGANIC MATERIALS FOR NEXT GENERATION BATTERIES

Abstract

Nanoscale materials have desirable electronic features (e.g. high surface areas, reduced mass and transport paths) that can be harnessed for a variety of technological applications. In most storage devices, there is a particular interest in nanostructured electrodes and solid-state electrolytes. A key challenge is the reproducible fabrication of these nanostructured materials. Polymers are nanoscale materials that could be used for nanoscale fabrication with improved reproducibility. In this thesis I explored two nanostructured systems using novel polymer assisted assembly methods. I fabricate a nano-structured MoS2 electrode and a nano-structured Li7La3Zr2O12 solid-state electrolyte with a garnet-type structure.

A clear redox mechanism for MoS2 is currently being sought. Using our electrode, we propose a mechanism to understand the total or partial decomposition of the electrode and the formation of long soluble polysulfides. We complete a fundamental study to determine the peaks on a cyclic voltammetry curve of nanostructured MoS2. We resolve these peaks by building a novel but simple system of restacked MoS2 with a conformal polyaniline (PANI) coating. We propose that the novel coating functions by absorbing, capturing, and promoting charge transfer (oxidization and reduction) of sulfur atoms remaining at the surface. Our data suggests that PANI acts as redox mediator. Redox mediators can be molecules or solid surfaces that aid in the charge transfer to redox species, traditionally oxide species. Our findings suggest that sulfur behavior dominates the redox chemistry at 0.7 V even earlier than the proposed deep discharge. We propose that longer chain polysulfides are formed through surface mediated interactions with persistent lattice planes of MoS2.

Solid-state electrolytes like cubic garnet type Li7La3Zr2O12 offer safety advantages over flammable liquid electrolytes, which is especially significant to the advancement of high energy density battery devices. Garnet however is unstable in air, suffers from low preparation efficiency and degradation into a two competitive phases, tetragonal type garnet and lithium carbonate phases, which have low conductivity. For two polymers systems, poly(styrene)-block-poly(acrylic acid), PS(0.3)-b-PAA(0.7) and PS(0.8)-b-PAA(0.2), we synthesize cubic Li7La3Zr2O12 garnet. We systematically investigate the effect of growth parameters, temperature and excess lithium content, to find the optimized synthesis conditions of 750 °C for ~5 h with 60 wt.% and 65 wt.% excess lithium salt, for the polymer systems.