Protected Calcium Anodes: Lessons from Native Interphases

Abstract

Reversible ambient temperature Ca electrodeposition has thus far been demonstrated in only a few electrolytes. Additional knowledge is required to translate these early successes to electrolytes capable of high coulombic efficiency and stability toward self discharge required for a practical Ca metal ion battery. Calcium borohydride in tetrahydrofuran is promising in both regards, representing an opportunity to develop electrolyte design principles to support a Ca metal anode, despite a low oxidation threshold and low ionicity. Where efficiency and stability are attributed to calcium hydride shown to form during deposition, no direct evidence for the presence of hydride within the interphase has been reported. In an effort to determine how the interphase functions for Ca cation transport, we have conducted compositional and structural mapping of this interphase using analytical transmission electron microscopy, including cryogenic methods. We demonstrate the interphase that forms on the dense Ca deposit is a discontinuous, only partially crystalline oxide that contains B species, not unlike the interphase reported to form in carbonate electrolytes containing the tetrafluoroborate anion at elevated temperature. In this presentation, we discuss how the interphase attributes vary as a function of Ca deposition conditions, including tuned bulk and interfacial speciation and explore ideas of enhancing Ca anode function.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.