Cryogenic Analytical Transmission Electron Microscopy for Multivalent Ion Based Energy Storage

Limited knowledge of the chemistry and time dependent formation of Ca or Mg solid electrolyte interphases exists as current tools do not combine surface sensitivity, high spatial resolution, chemical specificity and preservation of the electrolyte environment. In response to this challenge, we have developed and demonstrated cryogenic transmission electron microscopy (cryo-TEM) combining the above attributes to determine the identity and properties of anode interphases that enable long cycle lifetimes in emergent Mg and Ca metal batteries. We find that a nanometric, heterogeneous oxide, not a hydride as previously thought, is the interphase responsible for reversible Ca cycling in a calcium borohydride - tetrahydrofuran electrolyte. Ca²⁺ conductivity in such a ubiquitous material, specifically along oxide, borate, and carbonate phase boundaries, raise the possibility of controlling SEI formation to support working cation conductivity in a broader class of chemistries. We demonstrate a strong dependence of SEI composition and properties on Ca and Mg cation solvation structure, highlighting the possibility of directing reactivity by controlling speciation within the electrolyte. In this presentation, we explore this concept of directing reactivity and correlate outcomes with SEI performance.<br/> <br/>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.