A high-voltage concept with sodium-ion conducting β-alumina for magnesium-sodium dual-ion batteries

Rechargeable magnesium-sodium dual-ion batteries that use dendrite-free magnesium metal as an anode, magnesium-sodium dual-ion electrolyte and sodium-ion cathode are appealing as safe, low-cost systems for large-scale stationary electricity storage. Although substantial advances have been made on magnesium-sodium dual-ion batteries with various sodium-ion cathodes, further development of such batteries is inherently limited by the low electrochemical oxidative stability of known dual-ion electrolytes (2–2.6 V vs. Mg2+/Mg). Here, we examine a magnesium-sodium dual-ion concept, which allows for higher operating voltages of magnesium-sodium dual-ion batteries by using oxidatively stable sodium-ion electrolytes along with a sodium-ion conducting β-alumina membrane on the cathode side. The proposed concept is assessed using a high-voltage Na1.5VPO4.8F0.7 cathode demonstrating a high average discharge voltage of 3.0 V, a cathodic capacity of 110 mAh g−1 and an energy efficiency of 90%.