Feasibility of Metal-Anode Rechargeable Battery: 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. The energy density is determined by the specific capacities and redox potentials of electrode materials. In the case of lithium ion batteries (LIBs), cathode materials are frequently restricted to transition-metal complex oxide materials, such as LiCoO2 etc, and it is hard to enhance their capacities significantly. At the negative electrodes of LIBs, carbonaceous materials (372 mAh g-1) are usually employed instead of lithium metal electrodes (3862 mAh g-1) to avoid the dangerous dendritic growth of Li on charge. Due to the low capacity of carbonaceous materials, the development of high capacity anode materials has been expected for a breakthrough in the enhancement of energy density of LIBs. In order to achieve this goal, various candidate materials have been proposed and/or design improvements have been investigated in recent years, but there is still not a reliable and practicable way to enhance energy density sufficiently to meet the current demands. On the other hand, as a candidate of post LIBs, magnesium rechargeable batteries (MRBs) have attracted growing attention because Mg metal (2205 mAh g-1) itself can be adopted as an anode material owing to its non-dendritic growth while charging. In this study, Mg2+ was introduced into the electrodeposition process of Li+, and effects of co-electrodeposition of Mg and Li on the morphology were investigated. Since Mg is readily to be passivated in conventional carbonate electrolytes containing anion salt[1], such as (BF4)- or (ClO4)-, which are commonly used as electrolytes of LIBs, here we adopted the following two types of Mg-Li dual-salt electrolytes which were prepared by adding appropriate Li salts into electrolytes that can support reversible Mg deposition-dissolution: (i) conventional Mg/Li salts electrolytes, Mg(TFSA)2/G3[2], with an addition of Li(TFSA), (ii) magnesium organohaloaluminate electrolytes, Mg(HMDS)2-AlCl3/G3[3], with an addition of Li(HMDS). To investigate the effects of co-electrodeposition, a series of electrodeposition experiments was carried out using three-electrode beaker cells with different concentration ratios of Mg and Li salts. We have found that drastically suppression of dendritic growth was observed despite electrodepositing at a sufficiently low potential (-0.5 V vs. Li+/Li) and non-dendritic electrodeposit was obtained even in the electrolytes with a very low concentration of Mg2+. Based on this result, Mg-Li dual-salt batteries (DSBs)[4], which employ both Mg2+ and Li+as carrier ions, is appeared to be an effective way to obtain high energy density rechargeable batteries because high capacity Mg-Li alloys can be used as anode materials. However, the Daniell type dual-salt battery inevitably requires huge amount of electrolyte, which results in a significant reduction of the energy density. Hence, “rocking-chair” type DSBs should be developed for the practical dual-salt systems[5]. Namely, in the ideal charge/discharge processes in “rocking-chair” type Mg-Li DSBs, Mg2+ and Li+ should be accommodated in cathodes at the discharge state and released simultaneously during charge in order to realize the co-electrodeposition. Since cathode materials of MRBs like MgCo2O4[4] or Mo6S8[6] can accommodate Li+ as well as Mg2+, these were employed as the cathode material in the model case and both of them were confirmed to be able to accommodate Li+ and Mg2+ simultaneously while discharging in Mg-Li dual-salt electrolytes. Furthermore, in order to examine the feasibility of “rocking-chair” type Mg-Li DSBs, two-electrode coin cells were prepared with a set of a fully discharged cathode accommodating both cations of Mg2+ and Li+, a Cu current-collector anode, and a separator soaked with a trace quantity of electrolyte that only contains Li+. After charging such two-electrode cells, the Cu current-collectors with electrodeposits were taken out to observe the surface morphology of the electrode, by which we have confirmed that the electrodeposits consisting of Mg-Li alloy show smooth morphology. These results indicate the feasibility of “rocking-chair” type Mg-Li DSBs, and this novel concept of rechargeable battery system would provide a new strategy for future safe “metal-anode” rechargeable batteries combining high energy densities. Reference: [1] Z. Lu et al., J. Electroanal. Chem., 466, 203–217 (1999). [2] T. Fukutsuka et al., Chem. Lett., 43, 1788–1790 (2014). [3] Z. Zhao-Karger et al., RSC Adv., 3, 16330 (2013). [4] S. Yagi et al., J. Mater. Chem. A, 2, 1144 (2014). [5] T. Ichitsubo et al., J. Mater. Chem. A, 3, 10188–10194 (2015). [6] D. Aurbach et al., Nature, 407, 724–7 (2000).