Critical Role of Ultrathin Graphene Films with Tunable Thickness in Enabling Highly Stable Sodium Metal Anodes.

Sodium (Na) metal has shown great promise as an anode material for the next-generation energy storage systems because of its high theoretical capacity, low cost, and high earth abundance. However, the extremely high reactivity of Na metal with organic electrolyte leads to the formation of unstable solid electrolyte interphase (SEI) and growth of Na dendrites upon repeated electrochemical stripping/plating, causing poor cycling performance, and serious safety issues. Herein, we present highly stable and dendrite-free Na metal anodes over a wide current range and long-term cycling via directly applying free-standing graphene films with tunable thickness on Na metal surface. We systematically investigate the dependence of Na anode stability on the thickness of the graphene film at different current densities and capacities. Our findings reveal that only a few nanometer (∼2-3 nm) differences in the graphene thickness can have decisive influence on the stability and rate capability of Na anodes. To achieve the optimal performance, the thickness of the graphene film covered on Na surface needs to be meticulously selected based on the applied current density. We demonstrate that with a multilayer graphene film (∼5 nm in thickness) as a protective layer, stable Na cycling behavior was first achieved in carbonate electrolyte without any additives over 100 cycles at a current density as high as 2 mA/cm with a high capacity of 3 mAh/cm. We believe our work could be a viable route toward high-energy Na battery systems, and can provide valuable insights into the lithium batteries as well.