Interfaces and Interphases in Ca and Mg Batteries

The development of high energy density battery technologies based on divalent metals as the negative electrode is very appealing. Ca and Mg are especially interesting choices due to their combination of low standard reduction potential and natural abundance. One particular problem stalling the technological development of these batteries is the low efficiency of plating/stripping at the negative electrode, which relates to several factors that have not yet been looked at systematically; the nature/concentration of the electrolyte, which determines the mass transport of electro-active species (cation complexes) toward the electrode; the possible presence of passivation layers, which may hinder ionic transport and hence limit electrodeposition; and the mechanisms behind the charge transfer leading to nucleation/growth of the metal. Different electrolytes are investigated for Mg and Ca, with the presence/absence of chlorides in the formulation playing a crucial role in the cation desolvation. From a R&D point-of-view, proper characterization alongside modeling is crucial to understand the phenomena determining the mechanisms of the plating/stripping processes. The state-of-the-art is here presented together with a short perspective on the influence of the cation solvation also on the positive electrode and finally an attempt to define guidelines for future research in the field.J.D.F.-S. and D.S.T. contributed equally to this work. Funding from the European Union’s Horizon 2020 research and innovation programme ERC-2016-STG (CAMBAT Grant Agreement No. 715087) and H2020 FETOPEN-1-2016-2017 (CARBAT, Grant Agreement No. 766617) is gratefully acknowledged. ICMAB-CSIC members thank the Spanish Agencia Estatal de Investigación Severo Ochoa Programme for Centres of Excellence in R&D (CEX2019-000917-S). J.D.F.-S. is grateful to 2020-MSCA-COFUND-2016 (DOC-FAM, Grant Agreement No. 754397). J.D.F.-S. contributed to this work in the frame of the Doctoral Degree Program in Materials Science by the Universitat Autònoma de Barcelona.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe