Electrochemical Behavior Of Magnesium-Lithium Based Alloys In Sodium Chloride Solution

Magnesium has more negative standard electroreduction potential, high discharge activity and high energy density, thereby, shows excellent performance as anode of various chemical power sources, such as, primary and secondary battery, seawater activated battery, magnesium air battery, magnesium-hydrogen peroxide and magnesium-oxygen dissolved in seawater semi fuel cell. These power sources, using seawater as electrolyte in general, have the advantages of high energy density, stable discharging ability, safety and reliability, environmentally acceptability, and low cost. They are attractive undersea power sources. As anode material, magnesium suffers the drawbacks of low current efficiency due to its severe parasitic corrosion reaction resulting in the evolution of hydrogen and the"negative difference effect". Besides, the passive film formed on magnesium surface leads to the occurrence of a cell voltage delay. Doping magnesium with other alloy elements and modifying the electrolyte by including additives are effective ways for the improvement of magnesium anode performance. Lithium is the most active metal anode and has more negative electrode potential and larger energy density than magnesium. Therefore, by alloying magnesium with lithium, the discharge activity and energy density of magnesium anode can be enhanced and the voltage delay effect can be depressed. The deformation processing ability can also be improved. Therefore, it is useful to investigate the discharge performance of Mg-Li based alloys.In this study, the discharge performance of binary Mg-Li alloys and multinary alloys formed by adding Al, Ce, Sn, Zn, and Mn to Mg-Li alloy are investigated by means of potentiodynamic polarization, potentiostatic oxidation and electrochemical impedance technique. The influence of alloying elements on the discharge activity and current efficiency of Mg-Li alloys were analyzed. The surface morphology after discharge was examined using scanning electron microscopy. The effect of thermal treatment and rolling process on alloy discharge performance was studied. The performance of Mg-H₂O₂ semi-fuel cell with Mg-Li based alloy anode was evaluated. The obtained results provide useful information for the development of high performance Mg-Li based alloy anodes.Mg-14Li binary alloy with singleβ-Li phase exhibits more negative open circuit potential, higher discharge activity, but lower current efficiency than Mg-8.5Li alloy withα-Mg+β-Li two phase structure. Both alloys show the"negative difference effect", and their current efficiencies undergoing intermittent discharge are smaller than that after continuous discharge. The oxidation products of Mg-14Li forms sphere-like particles, which loosely attached on electrode surface and peel off easily. While the discharge products of Mg-8.5Li are in the form of dense and thick micro-clumps.Mg-8Li-3Al and Mg-8Li-3Al-1Ce alloy electrodes formed by adding Al or/and Ce to Mg-Li alloy displayed higher discharge current efficiency than Mg-8.5Li binary alloy electrode, indicating that Al and Ce functionalized as corrosion inhibitor. Mg-8Li-3Al-1Ce alloy electrode has larger discharge current density and shorter transition time than Mg-8Li-3Al alloy electrode, demonstrating that Ce improved the discharge activity of Mg-Li-Al alloy as an activation agent, and reduced its voltage delay effect. Mg-8Li-3Al-1Ce alloy electrode surface discharges uniformly, and the discharge product loosely packed on the electrode surface.The addition of Ga₂O₃ to electrolyte solution improved the discharge current density and utilization efficiency of Mg-8Li-3Al-1Ce alloy electrode, implying that Ga₂O₃ plays a dual-role of activation and corrosion inhibition. The utilization efficiency of Mg-8Li-3Al-1Ce discharged at -1.0V in the presence of Ga₂O₃ reached as high as around 87%. Clearly, Ga₂O₃ is an effective electrolyte additive for reducing mass loss of magnesium alloys caused by corrosion and other parasitic reactions. The hot-rolled Mg-Li-Al-Ce alloy electrodes show higher discharge current density and higher current efficiency than the as-cast alloy electrodes, indicating hot-rolling enhanced alloy discharge performance.Sn activated Mg-Li-Al alloy and obviously improved its discharge activity,but reduced its current efficiency and shows no corrosion inhibition effect. The current density of Mg-8.5Li-3Al-1Sn electrode discharged at -1.0V reached 50 mA cm^-2. Mg-8.5Li-3Al-1Sn exhibits higher discharge current density than Mg-8.5Li-3Al-1Ce-1Sn, but lower current efficiency, indicating that Ce functionalized as a corrosion inhibitor, but not an activation agent. Both alloys show the"negative difference effect", and their current efficiencies undergoing intermittent discharge are smaller than that undergoing continuous discharge. For both alloys, when the potential or current abruptly changes during discharge, the corresponding current or potential instantly changed to their new steady state values, showing that both alloys have quick response to load changes.Discharge activity and current efficiency of Mg-5.5Li-3Al-1Ce-1Zn-1Mn are higher than that of Mg-5.5Li-3Al-1Ce-1Zn alloy, demonstrating that Mn is an effective activation agent and corrosion inhibitor for Mg-5.5Li-3Al-1Ce-1Zn alloy electrode. Mg-5.5Li-3Al-1Ce-1Zn-1Mn and Mg-5.5Li-3Al-1Ce-1Zn alloy displayed no obvious"negative difference effect". The discharge performance of Mg-Li-Al-Ce alloy electrode is not improved by decreasing Li content and adding Zn. But by decreasing Li content and adding both Zn and Mn, its discharge activity increased and current efficiency remains unchanged. The current density of Mg-5.5Li-3Al-1Ce-1Zn-1Mn electrode discharged at -1.0 V reached 49 mA cm^-2, and its current efficient is around 80%, showing that Mg-5.5Li-3Al-1Ce-1Zn-1Mn has superior overall discharge performance. The Mg-H₂O₂ semi-fuel cell with Mg-5.5Li-3Al-1Ce-1Zn-1Mn anode presented a maximum power density of 110 mW cm?2 at 40°C operation temperature running on 0.6 mol L-1 H₂O₂ in the catholyte at a flow rate of 150 mL min-1. The cell performance increased with the increase of H₂O₂ concentration, catholyte flow rate and operation temperature.