Low-Cost Carbonaceous Metal-Free Wetting Layer as Sodiophilic Treatment for Solid and Molten Na Batteries

Sodium-metal batteries are a promising alternative to Li-metal batteries due to the natural abundance of Na, especially for grid scale energy storage where the cost premium of Li puts strain on the economic feasibility of scalable renewables-plus-storage. Low temperature Na-metal batteries, like their Li-based counterparts, suffer from Na-dendrite formation and dangerous cell failure upon extended cycling or plating at high current densities. Also like their Li-metal counterparts, Na-metal solid-state batteries (SSB) are unable to prevent dendrite intrusion without careful engineering of the Na-metal/solid-electrolyte interface. An alternative approach is to use elevated temperature batteries with a molten anode, which can enable categorically higher current densities with no dendrite issues. Where classically molten Na-batteries such as high temperature Na-S or 'ZEBRA' batteries operate well above 200 °C, it is desirable to reduce operating temperatures to unlock lower cost materials of construction and increase energy efficiency by reducing thermal losses. Unfortunately, it is well known that molten Na has poor wettability in contact with common solid electrolytes like sodium beta-alumina solid-electrolytes (Na-ß"-Al₂O₃, "BASE") or NaSICON (Na-Superionic Conductor) ceramic electrolytes.<br/><br/>Various wetting agents and interface modification strategies have been employed to improve wettability of molten Na and reduce interface resistance in Na-SSBs. Some rely on sputter deposition of a suitable metal such as Sn or deposition of metal precursors from solution followed by a heat treatment to form pure metals such as Pb, Sn, or Bi. Some strategies utilizing exotic phases of carbon such as graphene or carbon nanotubes have also been employed. Still others rely on high temperature annealing to drive off surface contaminants under vacuum or inert atmosphere. All of these strategies lack one or more crucial components such as low-cost, non-toxicity, simplicity, or scalability. This has motivated the development of a 'metal-free wetting layer' (MFWL) based purely on low-cost carbon sources, that can be applied via scalable processes such as spin-coating, spray-coating, or even drop-casting from solution. Our MFWL shows excellent uniformity when applied to a suitable solid-electrolyte such as BASE or NaSICON, and facilitates excellent wetting of molten Na as low as the melting point of Na. The meso/microstructure of MFWLs and mechanism of Na-wettability will be discussed in detail. Symmetric Na-Na cell data confirm that cells with low specific resistance and excellent cycling stability can be obtained at elevated temperature (T &gt; 100 °C), forming extremely intimate contact between Na and the underlying solid-electrolyte. Further, the utility of this MFWL can be extended to solid-state symmetric Na-Na cells, showing that even without applied pressure, initial experiments with BASE electrolytes treated with MFWL demonstrate a critical current density of ~ 0.5 mA cm^-2 at 30 °C. This simple, inexpensive, and scalable interfacial modification strategy shows great promise for enabling pure Na-anodes across solid-state and elevated temperature batteries.