Modulating the Electrochemistry of Calcium Metal Anodes

Abstract

Rechargeable lithium ion (Li-ion) batteries have been a foundational energy storage technology. However, there are significant technoeconomic, geopolitical, and sustainability concerns regarding the procurement of Li-ion battery components, from transition metals within the cathode to lithium itself. As such, the development of battery chemistries beyond Li is vital to the long-term viability of electrochemical energy storage. Batteries based on calcium (Ca) metal anodes offer a compelling alternative; Ca is the fifth-most abundant element in the earth’s crust at 41,500 ppm (vs. 200 ppm for Li), offering potential improvements in scalability and sustainability. Ca metal also offers attractive electrochemical metrics, with a redox potential 0.17 V more positive than Li and a theoretical volumetric capacity of 2073 mAh/cm³ (vs. 850 mAh/cm³ for graphite and 2062 mAh/cm³ for Li metal). The field of Ca metal batteries is currently in its early stages, however, due to a limited number of electrolytes that can reversibly plate and strip Ca — a requirement for rechargeability. Two important challenges to overcome are (1) the formation of a passivating solid electrolyte interphase (SEI) between Ca and the electrolyte that inhibits Ca²⁺ transport to the anode, and (2) attractive Ca²⁺--anion interactions in the electrolyte that suppress ionic conductivity and hinder Ca electrochemistry. These limitations rendered Ca plating/stripping unattainable until a groundbreaking first demonstration in 2015. In the decade since, only a handful of reversible electrolytes have been reported, reflecting a severely constrained electrolyte design space. This thesis expands upon this design space through interfacial and electrolyte engineering, offering novel techniques to modulate Ca electrochemistry that provide new degrees of freedom for the development of Ca-based batteries.

To begin, the practical assembly and cycling behavior of Ca foil electrodes are examined in a reversible electrolyte system for the first time. In contrast to historical work examining Ca foil in other common battery electrolytes, Ca foils are demonstrated to be electrochemically accessible for both plating and stripping in Ca(BH₄)₂ in tetrahydrofuran (THF). However, the first cyclic voltammetry (CV) cycle reflects persistent, history-dependent behavior from prior handling, which manifests as characteristic interface-derived features. Three exemplar SEI exhibit this interface-dominated behavior during initial CV cycles, though the interfacial features diminish with continued cycling. These results reveal that long-term cycling behavior is, to a greater extent, governed by the electrolyte, informing ensuing research into electrolyte composition and speciation.

Competitive interactions between Ca²⁺, anions, and solvent molecules are next harnessed to modify the Ca²⁺ coordination environment in this baseline electrolyte. An exemplar dual-salt electrolyte with differing Ca²⁺--anion interaction strengths, Ca(BH₄)₂ + Ca(TFSI)₂ in THF, is systematically altered. Introduction of a more-dissociating source of Ca²⁺ via Ca(TFSI)₂ drives re-speciation of strongly ion-paired Ca(BH₄)₂, generating larger populations of charged species and enhancing Ca plating currents. A critical parameter is proposed to govern electroactivity, the BH4 /Ca²⁺ ratio. Parasitic TFSI- decomposition prevents Ca plating when the BH4-/Ca²⁺ ratio is less than one. However, Ca plating in a TFSI--containing electrolyte is demonstrated for the first time when the BH4-/Ca²⁺ ratio is greater than 1, as BH₄⁻ displaces strongly coordinating TFSI⁻ from the Ca²⁺ coordination environment. These results directly evidence the impact of coordination-shell chemistry on plating activity and indicate that Ca²⁺--BH₄⁻ interactions can unlock electroactivity in the presence of other Ca salts, significantly increasing the Ca electrolyte design space.

Ca²⁺--solvent interactions are next examined as a subtler tool for electrochemical manipulation. The systematic introduction of glymes into the baseline electrolyte is shown to induce differential changes in Ca²⁺ coordination, as stronger glyme coordination displaces THF from the Ca²⁺ coordination environment, weakens Ca²⁺--BH₄⁻ interactions, and prompts BH₄⁻ redistribution. Examination of electrochemically-formed SEI indicates that BH₄⁻-facilitated solvent decomposition governs Ca electrochemistry in these systems, as coordinated THF promotes beneficial borate formation in the SEI but coordinated glymes instead favor the formation of Ca²⁺ blocking phases. The link between Ca²⁺ coordination strength and solvent decomposition is corroborated through the quantification of gaseous products. Altogether, these strategies for the modulation of Ca electrochemistry reveal new avenues for electrolyte engineering that will promote further development of Ca-based batteries.