The impact of magnesium content on lithium-magnesium alloy electrode performance with argyrodite solid electrolyte

Abstract

Solid-state lithium-based batteries offer higher energy density than their Li-ion counterparts. Yet they are limited in terms of negative electrode discharge performance and require high stack pressure during operation. To circumvent these issues, we propose the use of lithium-rich magnesium alloys as suitable negative electrodes in combination with Li6PS5Cl solid-state electrolyte. We synthesise and characterise lithium-rich magnesium alloys, quantifying the changes in mechanical properties, transport, and surface chemistry that impact electrochemical performance. Increases in hardness, stiffness, adhesion, and resistance to creep are quantified by nanoindentation as a function of magnesium content. A decrease in diffusivity is quantified with 6Li pulsed field gradient nuclear magnetic resonance, and only a small increase in interfacial impedance due to the presence of magnesium is identified by electrochemical impedance spectroscopy which is correlated with x-ray photoelectron spectroscopy. The addition of magnesium aids contact retention on discharge, but this must be balanced against a decrease in lithium diffusivity. We demonstrate via electrochemical testing of symmetric cells at 2.5 MPa and 30∘C that 1% magnesium content in the alloy increases the stripping capacity compared to both pure lithium and higher magnesium content alloys by balancing these effects.

Synergistic Evolution of Alloy Nanoparticles and Carbon in Solid-State Lithium Metal Anode Composites at Low Stack Pressure

“…Attempts to control the morphological instabilities of Li metal anodes at the SSE interface have also been made by introducing other interfacial materials, such as Li alloys − or Li-hosting scaffolds. − Alloy materials at the Li-SSE interface can beneficially affect both Li plating and stripping. Materials such as silver and gold form Li alloys and can reduce the nucleation overpotential for Li metal deposition, which enables homogeneous Li growth during deposition. ,, However, in the case of intermetallic alloys, stripping can still cause interfacial disconnection, and films undergo significant structural changes due to volume changes during cycling, which can cause capacity decay by alloy agglomeration and pulverization. ,,, Li stripping from solid solution alloys is slightly different; Krauskopf et al demonstrated that stripping from the β-phase Li–Mg alloy mitigated interfacial contact loss in the absence of stack pressure. ,, Retained contact at the SSE interface during delithiation is beneficial in terms of homogenization of the stripping current distribution . However, the Li diffusion kinetics in the Li–Mg alloy was found to be a bottleneck that limited the stripping capacity. ,, …”

“…19,22,29,30 Li stripping from solid solution alloys is slightly different; Krauskopf et al demonstrated that stripping from the β-phase Li−Mg alloy mitigated interfacial contact loss in the absence of stack pressure. 20,21,31 Retained contact at the SSE interface during delithiation is beneficial in terms of homogenization of the stripping current distribution. 22 However, the Li diffusion kinetics in the Li−Mg alloy was found to be a bottleneck that limited the stripping capacity.…”

“…22 However, the Li diffusion kinetics in the Li−Mg alloy was found to be a bottleneck that limited the stripping capacity. 20,21,31 Unlike alloy interlayers, carbon scaffolds can host plated Li within their porous structures. Chen et al demonstrated reversible Li deposition and stripping in a confined carbon nanotube scaffold.…”