Li-Mg alloys as anode for Li batteries in liquid electrolytes: Preparation, shaping, characterization and parametric electrochemical study

Lithium-metal batteries offer significant energy density advantages over classical Li-ion technology but their practical implementation is hampered by the poor reversibility of the lithium deposition/stripping processes. To solve this issue, Li-Mg alloys have already been considered as possible alternative and are examined in greater details herein. Homogeneous Li-rich alloys (β-phase) with controlled composition (5–13 at.%Mg) were fabricated and electrochemically reacted with Li to explore the kinetics of the Li-driven β−α phase equilibrium. We showed that the (de)lithiation of the β-phase that occurs at ∼0 Volt vs Li+/Li°, and therefore the most interesting, is strongly limited with the early appearance of Mg-rich α-phase at higher voltages. We demonstrated that this β-domain can be largely extended by using moderate cycling temperature (40 °C), low current density, high initial Li/Mg ratio and rest sequences. Post-cycling observations reveal that repeated (de)alloyings proceed without dendritic growth but rather involve the continuous growth of a porous surface film at the expense of the dense Li-Mg alloy resulting in a sudden drop in capacity when the electrode is becoming completely porous. By limiting the capacity, it is possible to control the progression of this porous layer, improve the efficiency of the Li plating-stripping process, and thus achieve a thousand cycles (0.8 mA.cm-2, 0.8 mAh.cm-2). In full cells, better reversibility is spotted for LiFePO4/Li17Mg compared to LiFePO4/Li cells provided that cycling rate is not pushed over ∼1.2 mAh.cm-2. Overall, we hope these results will provide useful insights for the design of lithium alloy electrodes to improve lithium-metal batteries.