Studies of the Electrode/Electrolyte Interface in Magnesium Batteries Using Ambient Pressure X-Ray Photoelectron Spectroscopy

Abstract

Introduction With the high volumetric energy density, high negative reduction potential, and relatively good safety features, magnesium metal has been considered as an attractive battery anode.1 As the reversible charge and discharge is one of the most important electrochemical properties of a battery, understanding the processes of plating and stripping of the metal across the electrode/electrolyte interface is crucial in the advancement of battery performance.2 However, the current knowledge of the chemical and electrochemical interactions/reactions at the interface is limited because of the paucity of suitable in situ and operando techniques to probe these issues. In this talk, we present how we combine an ambient pressure X-ray photoelectron spectroscopy (AP-XPS) technique, a “dip & pull” approach3, with the “tender” X-ray synchrotron source to access the solid/liquid interface and to obtain in situ chemical specific information in Mg batteries. The ability to access the solid/liquid interface stems from that the photoelectron probing depth reaches 10 nm with a ‘tender’ X-ray photon source. The sensitivity to the thin interface region is also maintained in this photon energy range.3 Results In this study, magnesium(II) bis(trifluoromethane sulfonyl) imide [Mg(TFSI)2] dissolved in diglyme solvent is used as the electrolyte solution. The operating pressure stabilizes at 1-2 Torr with diglyme vapor when the electrolyte solution is introduced into the AP-XPS analysis chamber at beamline 9.3.1 at the Advanced Light Source, Lawrence Berkeley National Laboratory. A three-electrode electrochemistry sample holder is used to facilitate electrochemistry measurements and to create a stable nanometers-thick liquid electrolyte film on the Mg working electrode (WE) surface for investigation. Electrochemical measurements are conducted by using a potentiostat with potentials referenced to the Mg reference electrode. Figure 1 shows an actual photograph of the three Mg electrodes immersed in the electrolyte while obtaining XPS spectra. A representative Mg 1s spectra at the electrode/electrolyte interface in presence of 0.8 M Mg(TFSI)2 electrolyte solution is shown in Figure 2. Initially, a thin oxide/hydroxide layer with small presence of carbonate covers the Mg metal. After a thin film of electrolyte is created and stabilized on the electrode by the “dip & pull” method, distinct photoemission signal from the Mg(TFSI)2 electrolyte solution is observed at the binding energy of 1303.5 eV. In this talk, we will provide details about how this spectroscopy technique is used in probing the solid/liquid interface and provides in situ chemical information. We will also address the behavior of Mg(TFSI)2 electrolyte at this interface and its response to different electrochemical conditions.