(Invited) Alloying Element-Based Functional Interface for Magnesium Metal Negative Electrode
Authors: Mandai Toshihiko 
Published: December 2024

Rechargeable Mg batteries (RMBs), where Mg metal served as an active material of negative electrodes, are a fascinating candidate of next generation batteries owing to the advantageous properties of Mg, such as large natural abundance, cost-effectiveness, high chemical stability, sufficiently low standard electrode potential, and high specific/volumetric capacities. However, despite of the recent dramatic advancement in development of individual battery components in the last decade, there remains serious challenges that impede realization of practical RMBs. One of the significant obstacles for RMB realization is the instability of [Mg | electrolyte] interface. Such interfacial instability would lead to cell failure due to passivation and consequent three-dimensional growth of spherical Mg deposits.
In this study, we investigated the surface engineering approach combined with halide-free electrolytes to achieve robust interface. Simple galvanic replacement reactions were adopted to construct an artificial interface. As the physical structure of metallic electrodes would change successively upon repeated deposition/dissolution cycling, the artificial interface was constructed with the elements those capable to form alloys/intermetallic compounds with Mg, such as Bi, In, Ge, Sn, and Zn. The mechanically polished Mg electrodes were soaked in ethereal solutions of halide or triflate salts incorporating desired alloying elements for pre-determined period, washed with anhydrous dimethoxyethane several times, dried under vacuum, then subjected as working/negative electrodes. Diglyme solutions of Mg[Al(HFIP)4]2, Mg[B(HFIP)4]2, or Mg[TFSA]2 were used as electrolytes and glass fiber filer (GF/A) was served as a separator.
The element-dependent electrochemical performance was clearly observed for the galvanostatic Mg deposition/dissolution cycling. Through the systematic survey for a series of pretreatment solutions, the interface derived from ZnCl2 was found to impart outstanding performance with the weakly-coordinated anion-based electrolytes. The ZnCl2-modified Mg electrodes support exceptionally long-term galvanostatic Mg deposition/dissolution cycling over 1500h and 750h at 6.5% and 32.5% utilization ratio of 40 µm-thick Mg electrodes, respectively, while unmodified ones showed substantially inferior performance at the same experimental condition. The microscopic observation combined with chemical and structural analyses revealed that the Mg2+ conductive nature of somewhat amorphous Mg-Zn-Cl integrated interface can stabilize the interface and allow Mg deposition/dissolution beneath the interface. The above Mg-Zn-Cl integrated interface enables the construction of RMBs with sulfide-based positive electrodes. To achieve further high-energy-density RMBs by adopting high-voltage oxide-based positive electrodes, halide-free artificial interface was also developed.
Acknowledgement
This work was financially supported by the NEXT Center of Innovation Program (COI-NEXT, Grant Number JPMJPF2016) and GteX Program Japan (Grant Number JPMJGX23S1) of the Japan Science and Technology Agency and Grant-in-Aid for Scientific Research (KAKENHI, JP21K05263) of Japan Society for the Promotion of Science.
Reference
Mandai, U. Tanaka, M. Watanabe, “Mg–Zn–Cl-integrated functional interface for enhancing the cycle life of Mg electrodes”, Energy Storage Mater., 2024, 67, 103302.