Rationalizing Calcium Electrodeposition Behavior by Quantifying Ethereal Solvation Effects on Ca²⁺ Coordination in Well-Dissociated Electrolytes

Abstract

Ca-ion electrochemical systems have been pushed to the forefront of recent multivalent energy storage advances due to their use of earth-abundant redox materials and their high theoretical specific capacities in relation to monovalent or even other more widely explored multivalentcharge carriers. However, significant pitfalls in metal plating and stripping arise from electrolyte decomposition and can be related to the coordination environment around Ca 2+ with both the negatively charged anion and the organic-aprotic solvent. In this study we apply multiple spectroscopic techniques in conjunction with density functional theory to evaluate the coordination environment of Ca 2+ across a class of ethereal solvents. Through the combination of X-ray absorption fine structure and time-dependent density functional theory, descriptive measures of the local geometry, coordination, and electronic structure of Ca-ethereal complexes provide distinct structural trends depending on the extent of the Ca 2+ -solvent interaction. Finally, we correlate these findings with electrochemical measurements of CaBHFIP2 salts dissolved within

Solvation, Rational Design, and Interfaces: Development of Divalent Electrolytes

“…Proposing theories to electrolyte incompatibility at the cathode interface needs the development of methods to tease out problems with the salt, solvent, and cathode. Coordinating efforts from different fields has resulted in new ways to study and develop pioneering MV ion electrolytes (Hahn et al, 2018;Lau et al, 2019;Driscoll et al, 2020), which has enabled the development of new cathode systems (Kwon et al, 2020a;Kwon et al, 2020b). Effectively utilizing these three themes in a circulating fashion holds promise as a robust framework to bridge the gap for practical next-generation batteries.…”

“…Higher chelating ethers have even been shown to block plating and stripping activity (Hahn et al, 2020). This phenomena was further studied in the context of a Ca [B (hfip) 4 ] 2 electrolyte that found cyclic ethers provide a less rigid solvation structure to Ca 2+ as opposed to glyme solvents (Driscoll et al, 2020). Solvation studies have provided a strong basis for understanding how current anion systems may translate in different cation-solvent environments.…”

Solid‐State Post Li Metal Ion Batteries: A Sustainable Forthcoming Reality?

“…The salt is also very important, since in early studies, some incompatibility was shown with classical salts due to the formation of surface passivation films on Ca metal, which prevents ion transport. Recently anion oxidation stability of fluorinated alkoxyborate has been predicted from theoretical calculations, then the calcium tetrakis(hexafluoroisopropyloxy)borate, or Ca[B(hfip) 4 ] 2 salt was synthesized for the first time [ 74 ] and tested in 1,2‐dimethoxyethane (DME), G2 and triethylene glycol trimethyl ether (G3) and tetrahydrofuran (THF) [ 75 and reference therein ]. This salt in ethereal solvents achieved a high oxidative stability up to 4.5 V and a high ionic conductivity thus showing some promise for the realization of room‐temperature Ca batteries.…”

“…Elucidations were also given on the relationship between the solvation structure of Ca 2+ ‐ethereal complexes and the electrochemical behavior, evidencing that weaker coordinating solvents enable efficient and reversible Ca plating/stripping. [ 75 ] Compared to other MV cations, especially Mg that undergoes chemical disproportionation prior the electron transfer, Zn follows a simple two electron reduction mechanism [ 76 ] so that there is no formation of highly reactive intermediates, and sufficiently good ambient temperature kinetics, which are considered as great advantages of Zn‐based battery technologies over other post‐Li storage systems. Although aqueous cells are inexpensive and safe, and less harmful to the environment, organic electrolytes offering higher oxidative stabilitiescould enable high voltage batteries.…”

Why Is Tetrahydrofuran a Good Solvent for Calcium Batteries? Insights From Ab Initio Molecular Dynamics Simulations

“…It is well known that local Ca cation coordination in the solvent plays an important role in reversible plating and stripping of Ca metal. [23][24] Our AIMD simulations allow the visualization of solvated structures in the electrolyte. Figure 5b) shows the number of salt anion and THF atoms coordinating with the salt Ca cation in the electrolyte.…”