Toward “rocking-chair type” Mg–Li dual-salt batteries

Abstract

High energy-density rechargeable batteries are strongly demanded from the viewpoint of energy and environmental concern. This work is devoted to fundamental electrochemistry on a novel concept of rechargeable batteries, “rocking-chair type” Mg–Li dual-salt batteries (DSBs), where both Mg and Li cations are carrier ions. In this system, dangerous dendritic growth is drastically suppressed by co-electrodeposition of Mg and Li, and Mg–Li alloys can be used as anode materials with high electrical capacities. As a DSB cathode material that can accommodate both Mg and Li cations, we use a spinel oxide MgCo₂O₄, in which an eccentric insertion mechanism, the “intercalation & push-out” process, occurs. Mg insertion occurs at 2.9 V vs. Mg²⁺/Mg and Li insertion occurs at 3.1 V vs. Li⁺/Li, being consistent with ab initio calculations, and its capacity approximately amounts to 150–200 mA h g⁻¹. In the combination of MgCo₂O₄ and Mg₅₀Li₅₀ alloys, the cell voltage during discharge is as high as about 2–3 V. The concept of rocking-chair type DSB systems provides a new strategy for future safe rechargeable batteries combining high energy/power densities.