First Principles Study of Magnesium/Oxygen Batteries and Glassy Solid Electrolytes

Abstract

A transition to electrified transportation would benefit from the development of batteries with energy densities beyond that of Li-ion batteries. New battery chemistries and materials are needed to realize this transition. Metal-air and solid-state batteries have attracted attention due to their potential to meet this demand. This dissertation employs first-principles calculations to characterize materials of importance for two ‘beyond Li-ion’ chemistries: 1) the magnesium-air battery, and 2) solid state Li batteries employing a lithium thiophosphate Li3PS4 glassy electrolyte. Mg-air batteries have high theoretical energy densities and rely on earth abundant materials. Nevertheless, metal-air batteries based on alkaline earth anodes have received limited attention and generally exhibit modest performance. In addition, many fundamental aspects of this system remain poorly understood, such as the reaction mechanisms associated with discharge and charging and the charge transport mechanisms within the discharge products. First principles calculations are employed to study the electrochemical and transport properties of the likely discharge products, MgO and MgO2. Thermodynamic overpotentials for discharge and charge are calculated for several scenarios, including variations in surface stoichiometry and the presence/absence of intermediates in the reaction pathway. The calculations indicate that reaction pathways involving oxygen intermediates such as superoxides or peroxides are preferred. In agreement with recent experiments, our calculations predict that cells that discharge to MgO will exhibit low round-trip efficiencies. In contrast, MgO2-based cells are predicted to approach round- trip efficiencies of 90%, suggesting that performance can be improved by ‘steering’ discharge towards formation of MgO2. Secondly, the transport properties of MgO and MgO2 discharge products were investigated. The transport mechanisms in these compounds either are incompletely understood (in MgO2) or remain a matter of debate (in MgO). For MgO, negative Mg vacancies and hole polarons were identified as the dominant charge carriers. However, their large formation energies suggest low equilibrium concentrations. A large asymmetry in the carrier mobility is predicted: hole polarons are mobile at room temperature, while Mg vacancies are immobile. Accounting for nonequilibrium effects such as frozen-in defects, the calculated conductivity data for MgO is shown to be in remarkable agreement with the three “Arrhenius branches” observed in experiments. In the case of MgO2, electronic carriers are the most prevalent. Similar to MgO, equilibrium concentrations in MgO2 are low, and moderate mobility further limits conductivity. If equilibrium behavior is realized, then (i) sluggish charge transport in MgO or MgO2 will limit battery performance when these compounds cover the cathode support and (ii) what little conductivity exists in these phases is primarily electronic in nature (i.e., polaron hopping). Artificially increasing the carrier concentration via monovalent substitutions, or circumventing solid-state transport altogether via liquid-phase redox-mediators, are suggested as strategies for overcoming transport limitations. Regarding solid electrolytes, the lithium thiophosphate family of glasses has reemerged as a promising candidate electrolyte due to its high conductivity and formability. However, due to their amorphous structure, the ion migration mechanisms that underlie their high Li-ion conductivity have been difficult to characterize. Our calculations reveal that: (i) cation migration events involve the nearly simultaneous migration of multiple adjacent cations, and (ii) cations are dynamically coupled to the reorientation and thermal vibrations of the PS4 anions. This dynamic coupling is expected to enhance the transport of cations through the anion sub-lattice and could be used as a guide in the design of fast ion conductors.