Functional Design and Investigation of Mg-Ion Conductors for Solid-State Mg Batteries

Secondary batteries using multivalent cations as charge carrier have attracted increasing attention in recent years due to the high theoretical energy density given by multi-electron redox reactions. Nevertheless, the large charge density of these cations inevitably causes sluggish kinetics of the ion migration under room temperature, which is challenging for the development of multivalent-ion solid-state batteries.^[1] Taking Mg²⁺ as a case study, we investigated a series of functional materials to figure out the design principle for Mg-ion solid-state conductors. First of all, an ionogel electrolyte using a MOF (metal-organic framework) as skeleton was prepared.^[2] An ionic liquid was incorporated into UiO-66, which has a large inner surface area as well as a rich porous structure. Comparably high ionic conductivities of 5.7 × 10^–5 S cm^–1 and 2.4 × 10^–4 S cm^–1 could be achieved at room temperature and at mildly elevated temperature (60 °C), respectively. Apart from the good ionic conductivity, the prepared electrolyte also exhibits good chemical and electrochemical stability against magnesium metal. By pairing the magnesium metal anode with an aromatic organic material as cathode material, a proof-of-concept quasi-solid-state Mg battery can work reversibly at 60 °C. Secondly, we replaced the ion-insulating MOF framework by a new Mg-ion conducting NASICON-structured material, Mg_0.5Sn₂(PO₄)₃. By combining it with a small amount of Mg ionic liquid to improve the Mg²⁺ migration at grain boundary, the prepared Mg-ion conducting hybrid solid electrolyte shows superior room-temperature ionic conductivity of 1.1 × 10^–4 S cm^–1 and an activation energy of 0.36 eV. Through in situ scanning electron microscopy, for the first time we observed the room temperature Mg growth inside a solid-state cell by the evidence of clear-cut metal particle formation and electrolyte particle cracking. The results shown here can act as a good starting point for the understanding of Mg transport behavior in solid-state batteries.<br/><br/><br/><b>References</b><br/><br/>[1] Y. Liang, H. Dong, D. Aurbach, Y. Yao, Nature Energy <b>5</b>, 646–656 (2020)<br/>[2] Z. Wei, R. Maile, L. M. Riegger, M. Rohnke, K. Mueller-Buschbaum, J. Janek, Batteries & Supercaps <b>5</b>, e202200318 (2022)