There has been a considerable interest in the battery system beyond lithium-ion. Magnesium-based systems remain the stronger candidate because of their earth abundance, and their ability to use Mg-metal as anode, which has been shown to deposit non-dendritically, a major impediment for realization of Li-metal system. However, the role of the surface passivating film (SEI) in the deposition/stripping of Mg at the anode/electrolyte interface during the reversible charge/discharge cycle has not been fully explored. For example, some borohydride-based electrolytes such as Mg(BH4) suffer from lower oxidative potential (1.7v vs. Mg), and are prone to undergo electrochemical changes in the anode/electrolyte interface during cycling. Combined operando approaches using electrochemical-synchrotron soft X-ray absorption (sXAS) and transmission electron microscopy (TEM) provide perfect platforms to observe such representative electrochemical processes in great details.
Both sXAS and TEM utilize operando electrochemical cell consisting of microfabricated Si3N4 window and Pt electrode contacts. In sXAS, two additional electrode – Mg foil and a platinum wire are inserted, and the electrochemical interface at the thin Pt layer is probed. In TEM, the enclosed liquid cell consisting of Pt electrode electrochemistry chips is supplied with a continuous flow of electrolyte with a gas tight syringe, and cyclic voltammograms are recorded at 2 mV/s. All TEM analysis are performed using a JEOL JEM-2100 operating at 200kV, and a liquid electrochemistry is performed using a Hummingbird Scientific liquid-electrochemistry TEM holder.
Here, we present operando cross-platform sXAS/TEM methods to study the electrochemical studies of Mg-metal in Mg(BH4)-based electrolytes to gain insights into the role of anode/electrolyte interface and interphase during Mg deposition and stripping processes. We use extended X-ray absorption fine structure (EXAFS) to investigate the solvent coordination of the as-prepared Mg(BH4)2: 3LiBH4/DME electrolyte and after one reduction/oxidation cycle. The decrease in the intensity of Mg-O bond after the first cycle suggested the loss in solvent coordination at the interphase. Upon studying the oxidation/reduction processes in liquid TEM, we observe the formation of passivating surface SEI layer and H2 gas beneath the magnesium metal deposit. Further study of B K-edge XAS indicated that [BH4]- anion promoted the reductive formation of H2 gas, and the reduction of Mg2+. These results present a new paradigm into using magnesium borohydride-based electrolytes for rechargeable magnesium batteries.