Na Metal Anode: “Holy Grail” for Room-Temperature Na-Ion Batteries?

Issues such as fossil fuel depletion, environmental pollution, and global warming have triggered much interest in clean/renewable energy sources and the development of electric vehicles (EVs). To address these issues, advanced energy conversion and storage technologies play a crucial role. With relentless efforts in the past decades, Li-ion batteries (LIBs) have become the primary source to power portable electronic devices. Scientists and engineers now direct their attention to the use of LIBs in EVs and grid scale energy storage due to their high energy density and long lifetime. Despite LIB’s promising performance, lithium sources are relatively limited and unevenly distributed across the globe. Inevitably, LIBs will become unaffordable and large-scale production will falter. Just one row down from lithium on the periodic table and the sixth most abundant element in Earth’s crust, sodium (Na) is a promising alternative to lithium for energy storage technologies. Additionally, seawater is a nearly infinite potential resource for Na, which makes Na-based batteries attractive for grid scale energy storage. In the 1970s, sodium−sulfur (Na−S) batteries with naturally abundant materials (Na metal anodes and S cathodes) achieved reasonable energy densities using β-alumina electrolytes. However, high operating temperature (270−350 °C), expensive electrolytes, and additional safety concerns hampered further development of Na−S batteries. Recently, attention has shifted to room-temperature Na-ion batteries (NIBs), which are analogous to conventional LIBs within a liquid electrolyte system. Over the past few years, significant progress has been made on NIB cathode materials. The challenge of NIBs remains the anode since the commercial anode for LIBs, graphite, is not compatible with NIBs with a poorly understood failure mechanism. To date, only a few anode materials such as hard (nongraphitizable) carbon and metal alloys exhibit reasonable performance. On the other hand, Na metal anodes show great potential due to their high theoretical capacity (up to 1166 mAh/g) and low redox potential, which are important for room-temperature Na−S or Na−O2 batteries. Nonetheless, studies on Na metal anodes are scarce. Researchers may presume that Na metal anodes will fail in room-temperature NIBs since Li metal anodes have previously failed in LIBs due to safety concerns and poor cycling performance. Although Li metal anodes are regarded as the “Holy Grail” in LIBs, significant challenges still remain.