In-Situ STM Study of Sodium Deposition Onto Au(111) in [Mppip][Tfsi]:Initial Stages, Nucleation, Cluster Growth, and Alloy Formation
Authors: Heubach Maren-Kathrin , Abdelrahman Areeg , Schuett Fabian M. , Kibler Ludwig A. , Jacob Timo 
Published: July 2022

The development of battery systems fabricated from abundant materials that meet the highest safety requirements is a major challenge, while the demand for efficient energy storage is continuously increasing.[1] Sodium metal batteries might be an adequate substitute for lithium-ion batteries. In comparison to lithium, sodium is more abundant, is a strong reducing agent, and has a less negative standard electrode potential.[2] Investigations of the deposition and the dissolution behavior of sodium are of utmost importance since dendritic growth is amongst the key factors limiting the lifetime and safety of sodium metal batteries.[3]

In-situ scanning tunneling microscopy (STM) combined with cyclic voltammetry allows observing topographical changes in real-time and to determine potential regions of related electrochemical processes. Single-crystal surfaces are commonly used in fundamental studies of metal deposition due to their (i) well-defined structure, (ii) ease and reproducibility of preparation, and (iii) stability in the electrochemical environment.[4] However, metallic sodium is extremely reactive. In particular, Na reacts violently with water and molecules containing acidic hydrogen atoms, but even with commonly used battery electrolytes as organic carbonates.[5] Fortuitously, ionic liquids (ILs) are promising solvents in sodium metal batteries.[6] Due to their unique properties, they are appropriate for preparing electrolytes in investigations of the deposition and dissolution behaviour of metals. They are known for their relatively wide electrochemical stability windows for both sodium and lithium deposition, as well as for low diffusion rates, which facilitate real-time deposition studies.[7] In this work, the initial stages of sodium underpotential deposition (UPD) onto a Au(111) electrode surface from the ionic liquid N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide ([MPPip][TFSI]) is investigated using in-situ STM and classical electrochemical measurements.
Four subsequent stages in the UPD process of sodium on Au(111) can be discerned. (i) Nucleation of sodium starts around 1.1 V vs. Na/Na+ at the so-called elbows of the reconstructed Au(111) surface. (ii) Small mono-atomically high islands grow around the nuclei at a slightly more negative electrode potential of 1.0 V. (iii) The islands slowly coalesce into smooth layers after lowering the potential to 0.5 V (Figure 1), resulting in several clusters with defined steps. (iv) More islands grow on top of the existing layer and the preferred deposition mode changes from smooth layer formation to 3D-growth, resulting in cauliflower-like structures, especially at the step edges of the clusters. The deposition of several layers of sodium in the UPD regime can be explained by the alloy formation of sodium and sodiophilic gold.[8] It is worth mentioning that this deposition behaviour shows several similarities to that of lithium UPD on Au from ILs, including island formation and multiple layer growth.[7,9] So far, nucleation at the reconstruction elbows has not been reported for lithium, but it has already been observed during deposition of other metals such as nickel, cobalt, and palladium both for electrodeposition[10] and under UHV conditions.[11,12]

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Figure 1