Sodium metal anodes: Deposition and dissolution behaviour and SEI formation

Abstract

The electrochemical behaviour of sodium metal electrodes is explored in symmetrical Na cells with 1 M NaPF6 EC:DMC (1:1) electrolyte. Like Li metal, Na metal shows a characteristic voltage behaviour during galvanostatic cycling, which correlates with changes of the electrode surface area and the morphology. The deposition and dissolution behaviour of both alkali metals is largely similar: during dissolution pits form, the metal deposition takes place inhomogeneously, resulting in dendrites. In contrast to Li metal, the electrode surface of Na metal undergoes pronounced morphological and chemical modifications within the first cycles. Since the alkali metals react with relevant organic solvents or with electrolyte anions, a passivating surface film, the solid electrolyte interphase (SEI), is formed. This means the evolution of the Na electrode morphology is governed by two competing processes: (i) metal deposition and dissolution, and (ii) electrode morphology modification during SEI growth. Therefore, Na metal electrodes need at least four formation cycles until their behaviour is stable. Using light microscopy, we observed during Na electrodeposition, that deposits detach from the electrode, indicative for a lack of SEI passivation. We applied the distribution of relaxation times analysis to evaluate the impedance spectra and identified the different contributions to polarization losses.