Stress evolution of lithium alloying electrodes during cycling

Thesis: Ph. D. in Electronic Materials, Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2016.Cataloged from PDF version of thesis. "June 2016."Includes bibliographical references (pages 163-189).Germanium and silicon are ideal candidate materials for use as anodes in microbatteries since both are well established in microelectronics and reversibly store large amounts of lithium. However, implementation of either material has been limited by poor cyclability due to the large volumetric changes occurring during cycling. To better understand the mechanical stresses associated with these changes, in situ stress curvature measurements on thin film electrodes were conducted. The effect of the initial electrode structure on electrochemistry and mechanics was investigated by post-deposition annealing of hydrogenated silicon thin films. Compared to as-deposited films, annealed electrodes lithiated at lower potentials during the first cycle and could form Li₁₅Si₄ . Moreover, annealed electrodes underwent large structural changes in the first two cycles compared to as-deposited films, with polycrystalline silicon films showing the largest change. The changes in electrochemistry and mechanical behavior were attributed to the loss of hydrogen and densification of the film. In addition to silicon, stress evolution in lithium-germanium films was also investigated and stresses were found to be roughly one-third those of silicon. Rate testing revealed that germanium electrodes exhibit a smaller loss in capacity at high cycling rates than silicon. Cycling below 100mV resulted in crystalline Li₁₅Ge₄ formation which appeared as a tensile peak in the stress-capacity plots. Extended cycling of uncoated electrodes resulted in an irreversible reduction of stress but no loss in capacity, an outcome associated with the thin film fracturing while remaining adherent to the substrate. The reduced plastic flow stresses observed in lithium-germanium and their weak dependence on cycling rate may explain why germanium offers improved cyclability and reduced rate sensitivity compared to silicon. In addition to uncoated electrodes, lithium phosphorous oxynitride (LiPON) coated electrodes were also examined. By coating silicon and germanium thin films with LiPON, mechanical degradation (i.e. crack and island formation) and chemical degradation (i.e. reduction in solid electrolyte interphase formation) processes were strongly suppressed. The improvement in mechanical and electrochemical stability enabled more detailed studies of lithium-silicon and lithium-germanium alloying, including the kinetics of phase formation and dissolution (e.g. of Li₁₅Si₄ and Li₁₅Ge₄).by Ahmed Al-Obeidi.Ph. D. in Electronic Material

Comparative performance of ex situ artificial solid electrolyte interphases for Li metal batteries with liquid electrolytes

The design of artificial solid electrolyte interphases (ASEIs) that overcome the traditional instability of Li metal anodes can accelerate the deployment of high-energy Li metal batteries (LMBs). By building the ASEI ex situ, its structure and composition is finely tuned to obtain a coating layer that regulates Li electrodeposition, while containing morphology and volumetric changes at the electrode. This review analyzes the structure-performance relationship of several organic, inorganic, and hybrid materials used as ASEIs in academic and industrial research. The electrochemical performance of ASEI-coated electrodes in symmetric and full cells was compared to identify the ASEI and cell designs that enabled to approach practical targets for high-energy LMBs. The comparative performance and the examined relation between ASEI thickness and cell-level specific energy emphasize the necessity of employing testing conditions aligned with practical battery systems

Condensation on Superhydrophobic Surfaces: The Role of Local Energy Barriers and Structure Length Scale

Water condensation on surfaces is a ubiquitous phase-change process that plays a crucial role in nature and across a range of industrial applications, including energy production, desalination, and environmental control. Nanotechnology has created opportunities to manipulate this process through the precise control of surface structure and chemistry, thus enabling the biomimicry of natural surfaces, such as the leaves of certain plant species, to realize superhydrophobic condensation. However, this “bottom-up” wetting process is inadequately described using typical global thermodynamic analyses and remains poorly understood. In this work, we elucidate, through imaging experiments on surfaces with structure length scales ranging from 100 nm to 10 μm and wetting physics, how local energy barriers are essential to understand non-equilibrium condensed droplet morphologies and demonstrate that overcoming these barriers via nucleation-mediated droplet–droplet interactions leads to the emergence of wetting states not predicted by scale-invariant global thermodynamic analysis. This mechanistic understanding offers insight into the role of surface-structure length scale, provides a quantitative basis for designing surfaces optimized for condensation in engineered systems, and promises insight into ice formation on surfaces that initiates with the condensation of subcooled water.United States. Dept. of Energy. Office of Basic Energy Sciences (Solid-State Solar-Thermal Energy Conversion Center)National Science Foundation (U.S.) (Award ECS-0335765

Nanohole Structuring for Improved Performance of Hydrogenated Amorphous Silicon Photovoltaics

While low hole mobilities limit the current collection and efficiency of hydrogenated amorphous silicon (a-Si:H) photovoltaic devices, attempts to improve mobility of the material directly have stagnated. Herein, we explore a method of utilizing nanostructuring of a-Si:H devices to allow for improved hole collection in thick absorber layers. This is achieved by etching an array of 150 nm diameter holes into intrinsic a-Si:H and then coating the structured material with p-type a-Si:H and a conformal zinc oxide transparent conducting layer. The inclusion of these nanoholes yields relative power conversion efficiency (PCE) increases of ∼45%, from 7.2 to 10.4% PCE for small area devices. Comparisons of optical properties, time-of-flight mobility measurements, and internal quantum efficiency spectra indicate this efficiency is indeed likely occurring from an improved collection pathway provided by the nanostructuring of the devices. Finally, we estimate that through modest optimizations of the design and fabrication, PCEs of beyond 13% should be obtainable for similar devices

Comparative performance of ex situ artificial solid electrolyte interphases for Li metal batteries with liquid electrolytes

Summary: The design of artificial solid electrolyte interphases (ASEIs) that overcome the traditional instability of Li metal anodes can accelerate the deployment of high-energy Li metal batteries (LMBs). By building the ASEI ex situ, its structure and composition is finely tuned to obtain a coating layer that regulates Li electrodeposition, while containing morphology and volumetric changes at the electrode. This review analyzes the structure-performance relationship of several organic, inorganic, and hybrid materials used as ASEIs in academic and industrial research. The electrochemical performance of ASEI-coated electrodes in symmetric and full cells was compared to identify the ASEI and cell designs that enabled to approach practical targets for high-energy LMBs. The comparative performance and the examined relation between ASEI thickness and cell-level specific energy emphasize the necessity of employing testing conditions aligned with practical battery systems