Solid-Electrolyte Interphase Engineering for Multivalent-Ion Batteries

As Li-ion batteries approach practical performance limits and Li metal batteries grapple with issues such as dendrite formation, batteries based on multivalent (MV) metal anodes such as Ca and Mg are gaining increased research attention. Such systems are attractive because of high gravimetric and volumetric energy densities, high natural abundance, and seemingly low propensity for dendrite formation [1,2]. Implementation of MV metal anodes is challenging due to sluggish transport of Ca²⁺ and Mg²⁺ and thermodynamically favored electrolyte decomposition, often to ionically insulating products. To achieve high (&gt;99.9%) coulombic efficiencies (CE) required for long-term cycling, MV anodes require a robust solid-electrolyte interphase (SEI) that suppresses parasitic reduction of electrolyte components while permitting cation transport at appreciable rates. The study of MV SEIs is in its infancy; little is known about interphase composition and structure and their correlation to anode performance [3].<br/><br/>We recently discovered that a nanometric heterogeneous oxide [4] enables reversible Ca electrodeposition in a model electrolyte, Ca(BH₄)₂ in THF [5]. Cryogenic focused ion beam (cryo-FIB) liftout and transmission electron microscopy (cryo-TEM) techniques enabled analysis of Ca electrodeposits with the liquid electrolyte vitrified in place, thereby preserving the native SEI formed during electrodeposition and open-circuit holds. Electron energy loss spectroscopy mapping, selected-area diffraction, energy-dispersive X-ray spectroscopy, and high-resolution cryo-TEM imaging were used to map the structure and chemistry of nanometric heterogeneous interphases formed on calcium anode surfaces.<br/>In this contribution, we build on our accomplishment by intentionally engineering SEIs for MV metals. We rationally design electrolytes to control interphase chemistry, structure, and heterogeneity, and we then correlate such properties with electrochemical performance enhancements. One case study we discuss is the addition of weakly coordinating calcium salts, such as calcium carba-<i>closo</i>-dodecaborate [Ca(CB₁₁H₁₂)₂], to Ca(BH₄)₂ in THF. This addition modifies the Ca²⁺ coordination in the bulk electrolyte, which enhances interfacial reactivity and, in tandem with CB₁₁H₁₂^- anions, increases oxide interphase heterogeneity. Another case study, aimed at generalizing SEI engineering principles to other MV systems, utilizes an electrolyte of mixed Mg salts in ether solvents. For both cases, we use cryogenic techniques to map the native phases and chemistry of the deposited and cycled structures, correlate morphological and chemical features of engineered SEI layers with the accommodation of newly deposited metal, and highlight potential mechanisms contributing to degradation and failure of MV metal anodes. Our results are a critical step towards the design of ideal SEIs for high-efficiency rechargeable MV metal batteries.<br/>This work was supported by the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy (DOE) and was performed, in part, at the Center for Integrated Nanotechnologies (CINT), an Office of Science User Facility operated for the U.S. DOE Office of Science. 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. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.<br/><u>References:</u><br/>[1] X. Zhang et al. <i>Advanced Functional Materials</i> <b>2020</b>, 30(45), 2004187.<br/>[2] Y. Liang et al. <i>Nature Energy </i><b>2020</b>, 5(9), 646-656.<br/>[3] H. Wang et al. <i>ChemElectroChem</i> <b>2021</b>, 8(16), 3013-3029.<br/>[4] S. A. McClary et al. <i>Submitted</i>.<br/>[5] D. Wang et al. <i>Nature Materials</i> <b>2018</b>, 17(1), 16-20.