CAREER: Elucidation and Development of Electrolyte and Interface Mechanisms Governing Calcium Redox in Nonaqueous Environments

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Electrochemical energy storage plays a critical role in storing energy from intermittent renewable sources, such as solar and wind energy, and making these sources suitable for electrified transportation. Today’s lithium (Li)-ion and emerging Li metal-based batteries have high energy densities, but they suffer from safety and performance issues along with increasing concerns about scarcity of Li resources. Calcium (Ca) presents a compelling alternative as the basis for next-generation batteries: it is the 5th most abundant element in the earth’s crust, and batteries utilizing Ca metal anodes are projected to have energy storage capabilities comparable to Li counterparts with potential for improved safety. However, development of Ca batteries has been hindered by numerous challenges, including formation of blocking interfaces on Ca metal anodes that impede reversibility, and the need to design electrolytes specifically optimized for divalent Ca ion electrochemistry. This research will build the fundamental understanding of Ca-based electrochemical reactions and the resulting interfaces on Ca metal. To achieve these aims, this research will create tools to conduct novel experimental, quantitative analysis of Ca electrolyte and interface thermochemistry that will guide design of improved Ca batteries. As an additional outcome of the work, middle-school students will engage in active virtual teaching modules that use outcomes of this research while fulfilling a Massachusetts state education standard, bringing exposure to scientists and engineers and motivating pursuit of STEM careers.This research effort will conduct a targeted experimental study of the chemical, thermodynamic and interface parameters that govern Ca ion redox behavior and elucidate Ca solid electrolyte interphase (SEI) chemistry and properties. The central hypothesis guiding this work is that the heightened degrees of freedom in Ca2+ ion solvation present new opportunities to intervene in electrochemical pathways, which can allow reversibility and reaction selectivity to be unlocked if better understood. The tools to be developed to test this hypothesis include a new thermodynamic framework for measuring the solvated Ca state using reaction calorimetry; metal deposition methodologies to prepare high-quality Ca metal interfaces for fundamental study; an operando approach to probe the chemical dynamics of Ca SEI formation; and a cathode conversion reaction involving Ca and CO2 to test and strengthen understanding of how Ca2+ solvation affects reactivity. The results will be integrated to identify the fundamental origins that determine electrochemical parameters: redox potential, Coulombic efficiency, cycle life, and rate capability, and therein identify strategies to improve them. Collectively, this work will yield progress in anode, cathode, and electrolyte aspects of Ca batteries.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.