The Role of Sodium Electrolyte Impurities in Governing Interphase Formation and Impacting Calcium Anode Electrochemical Performance

Abstract

Calcium-based rechargeable batteries have received considerable interest due to high theoretical energy densities, low reduction potential of the Ca2+ cation, and high crustal abundance of calcium. The reversible plating and stripping of calcium metal has recently been demonstrated using several anion/solvent combinations, including (BF4)-/ethylene carbonate:propylene carbonate [1,2], (BH4)-/THF [3], (Bhfip)-/THF-glymes [4-6], and (BF4)-/1-ethyl-3-methylimidazolium trifluoromethanesulfonate [7]. These studies are significant advancements towards implementation of Ca-ion batteries, but the coulombic efficiencies (CE) have not yet reached high enough values for practical use. Secondary phases, including CaH2 and CaF2, likely contribute to CE suppression, but other factors, such as impurities and microstructure, have not yet been studied in detail.In this contribution, we describe the critical role that sodium plays in governing the electrochemical performance and morphology of electrodeposited calcium. Trace metal analysis reveals that sodium impurity levels vary across different sources of calcium salts; such variations are correlated with highly variable CE’s and deposition morphologies. By adding controlled levels of sodium (at the hundreds of ppm level) to calcium-based electrolytes, the CE of calcium cycling exceeds 98%, indicating significantly suppressed parasitic reactions. Additionally, sodium addition causes the dominant morphology to transition from loosely packed, ~50 nm calcium platelets to densely packed columnar structures. Detailed studies of electrolyte speciation (dielectric relaxation spectroscopy, Raman spectroscopy), electrochemical response (cyclic voltammetry, chronopotentiometry), deposit microstructure, and interphase composition (SEM, TEM, TOF-SIMS, XRD) are conducted to determine the specific role that sodium plays in directing interphase formation and microstructure evolution. Our results highlight the sensitivity of calcium electrodeposition to small changes in the electrolyte and underscore the broader need for consideration of impurity impact on redox processes in multivalent electrolyte systems.This work was supported by the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.