The Catalytic and Mechanical Properties of Lithium Battery Electrodes (2017)

The mass adoption of electric vehicles warrants higher energy densities at lower costs. Novel chemistries such as Li-S or Li-air, high energy density anodes such as lithium (Li) metal are some of the ways to address the aforementioned issue. However, many scientific challenges must be overcome in order to achieve the successful commercialization of these batteries. For Li-air, poor cyclability and low coulumbic efficiency are key obstacles. The search for cathode materials that exhibit high capacity, low discharge/charge overpotential and chemical stability over many cycles is a major area of interest in the field. On the anode side, the application of Li metal is stumped by uncontrollable dendrite growth during the charging, and existing methods such as pulsed charging, physical suppression, and additives in the electrolyte have only had alleviating effects. The first part of this thesis investigates the suitability of various materials as Li-air cathodes. We fabricated 3-dimensional architected electrodes using a variety of materials including Au, Ni, Ti, LaCoO3 (LCO), LaNiO3 (LNO), and LaNi0.5Co0.5O3 (LNCO). Their performances in capacity, overpotential, and cyclability were assessed using galvanostatic battery testing methods. The reaction products were investigated using spectroscopic techniques such as FTIR and Raman. Our experiments corroborated recent findings that even trace moisture contamination can dramatically influence discharge product composition and morphology. Furthermore, Ni nanoparticles may serve as a carbon substitute in investigating the properties of non-conductive catalysts under specific potential windows. By incorporation the perovskites into a Ni based conductive mesh, we found the oxygen reduction reaction capability of the three materials to be ranked as LCO>LNCO>LNO, and the chemical stability ranked as LCO>LNO>LNCO. The instability of DMSO due to chemical reactions with discharge products is observed and discussed in the context of the solution-mediated mechanism of Li2O2 growth. The second part of the thesis investigates the nano-mechanical properties of Li (bcc), as a function of size, temperature, and crystal grain orientation. At room temperature the power law exponent of the strength vs. size log-log plot is -0.68, while at 90°C this value is increased to -1.00. A factor of 3 decrease in the yield strength at 90°C is observed, and the morphology of deformation was found to transition from localized slip planes to homogeneous barreling. Our collaborators at Carnegie Mellon University calculated the elastic constants of Li from 78 K to 440 K (melting temperature of Li is 453 K), and is found to be within reasonable agreement with existing experimental data where applicable (78 -300 K). We proceeded to calculate the elastic and shear moduli of single crystal Li as a function of temperature and orientation. We found that due to the extreme anisotropy of Li, there is a factor of ~4 difference between the strongest and weakest orientation of both the elastic and shear moduli. Our findings are discussed in the context of Li anodes, where we highlight the importance of taking into consideration the size-effect and anisotropy when designing solid electrolytes, or modeling dendrite growth behavior.