The interphase that enables reversible, room-temperature calcium electrodeposition from a calcium borohydride – tetrahydrofuran [Ca(BH4)2:THF] electrolyte has been characterized using cryogenic transmission electron microscopy and electron energy loss spectroscopy. Understanding why this interphase supports the highest coulombic efficiency and greatest stability toward self-discharge of the few demonstrated calcium electrolytes is critical for materials discovery in support of multivalent metal battery development. Calcium hydride was originally proposed as the functional constituent of the Ca(BH4)2:THF interphase, but our hydride-sensitive chemical mapping shows that it forms in the bulk of the calcium deposit and is absent from the surface layer. The electronically insulating interphase is instead identified as a heterogeneous nanocrystalline calcium oxide with calcium borate and carbonaceous phases occupying intergranular regions. A plausible reaction pathway, supported by DFT computation, is provided to explain how calcium hydride and calcium oxide form from interfacial decomposition of THF. We propose that the Ca2+ transport function required of a solid-electrolyte interphase occurs along the phase boundaries between the oxide and impurity species. More broadly, we show that the solvent plays a key role in tailoring interphase heterogeneity and anode performance for Ca, Mg, and Zn anodes.

Identifying multivalent metal anode interphases using cryogenic electron microscopy

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ACS Fall 2022

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