Insights into Electrochemical Sodium Metal Deposition as Probed with in Situ 23Na NMR

Sodium batteries have seen a resurgence of interest from researchers in recent years, owing to numerous favorable properties including cost and abundance. Here we examine the feasibility of studying this battery chemistry with in situ NMR, focusing on Na metal anodes. Quantification of the NMR signal indicates that Na metal deposits with a morphology associated with an extremely high surface area, the deposits continually accumulating, even in the case of galvanostatic cycling. Two regimes for the electrochemical cycling of Na metal are apparent that have implications for the use of Na anodes: at low currents, the Na deposits are partially removed on reversing the current, while at high currents, there is essentially no removal of the deposits in the initial stages. At longer times, high currents show a significantly greater accumulation of deposits during cycling, again indicating a much lower efficiency of removal of these structures when the current is reversed.