Insertion of Magnesium into Antimony Layers on Gold Electrodes:Kinetic Behaviour

Abstract

Magnesium based secondary batteries are regarded as a viable alternative to the immensely popular Li‐ion systems. One of the largest challenges is the selection of a Mg anode material since the insertion/extraction processes are kinetically slow because of the large ionic radius and high charge density of Mg2+. In an attempt to bridge the gap between insertion measurements in 3D composite electrode materials and that in an idealized pure model system, we studied the insertion and diffusion of Mg in a thin, massive layer of Sb deposited on Au by using PITT, CV, and potential step experiments. Sb has been suggested as an insertion material because magnesium can form intermetallic compound with it. The layered crystal structure of Sb leads should facilitate formation of such an intermetallic phase. Mg insertion from a MACC/tetraglyme electrolyte into Sb starts 300 mV positive of the onset potential of Mg deposition as shown by cyclic voltammetry. The molar ratio of Mg to Sb agrees well with the stoichiometry of Mg3Sb2 alloy (Zintl‐phase). The diffusion coefficient of Mg‐insertion into Sb – layers and the charge transfer rate have been estimated by the above techniques. Such diffusion coefficients, albeit still somewhat “apparent”, are much more closely related to the true diffusion coefficient in the metal or alloy. The solid‐state diffusion coefficient of Mg into the Sb layers is in the range of 4–8×10−14 cm2 s−1. A very high Tafel slope of 370 mV/dec was found in potential step experiments. Mg insertion was further investigated by XPS measurements. Besides Mg and Sb, Al and Cl signals were also detected, particularly at the outer parts of the layer.

Grain‐Boundary‐Rich Triphasic Artificial Hybrid Interphase Toward Practical Magnesium Metal Anodes

“…The Sb 3d core level spectrum (Figure 2f, top) can be fitted into three peaks located at 528.1, 528.8, and 530.1 eV, which are assigned to Sb, SbCl 3 , and SbO x , respectively. [ 14a,18 ] The residual SbCl 3 and trace SbO x species dramatically decrease after 30 s of Ar‐ion bombardment. Meanwhile, a characteristic peak of Mg 3 Sb 2 alloy appears at 526.9 eV, implying that the Mg 3 Sb 2 alloy only exists in the inner region of the artificial hybrid interphase.…”

“…This demonstrates the robust mechanical stability enabled by the triphasic components in the Sb‐Mg/MgCl 2 hybrid artificial interphase, [ 3c,14b ] which can eliminate electrode fluctuations arising from large volume change and irregular Mg deposition during repeated cycling. [ 18 ] Besides the high value of Young's modulus, the Sb‐Mg/MgCl 2 artificial hybrid interphase also possesses high ionic conductivity. From the electrochemical impedance spectroscopy (EIS) results in Figure 3b, [ 14a,17b ] the ionic conductivity of the Sb‐Mg/MgCl 2 @Mg electrode was quantitatively derived to be up to 3.47 × 10 −7 S cm −1 .…”

“…[ 4a,12a ] Moreover, the mechanical robustness of these artificial layers to buffer the non‐negligible interface volume fluctuation during long‐term cycling is questionable. [ 18 ] Therefore, to ensure durable protective effects toward Mg metal anodes under high current densities, artificial SEI layers should integrate compositional and structural advantages to offer both uniform and rapid Mg 2+ diffusion, as well as high mechanical and electrochemical stability.…”

Friction on I‐Modified Au(111) in a Tetraglyme Electrolyte

“…Finally, the electrochemistry in this electrolyte is important because such ether‐based electrolytes have been studied as an alternative electrolyte for the Li‐O 2 battery due to their greater stability during oxygen reduction reaction (ORR) compared with organic carbonate‐based electrolytes (e. g. propylene carbonate (PC)) [49–51] . G4 is also of interest for application as an electrolyte solvent for Mg deposition‐dissolution processes [52–57] …”