In situ optical microscopy investigations of lithium and sodium film formation in buffered room temperature molten salts

Previous work performed in both sodium and lithium buffered chloroaluminate molten salts have shown that the addition of small amounts of SOCl2 promotes the reversible stripping behavior of lithium and sodium metal with cycling efficiencies between 80 and 90%. We have performed a series of optical studies in conjunction with electrochemical experiments at varying SOCl2 concentrations in both lithium and sodium chloride buffered melts. On investigation, the lithium deposit is dendritic in nature and does not form a uniform film on the tungsten electrode. After discharging at moderate current densities, disconnected lithium metal is observed at the electrode surface. In contrast, the sodium deposits as a uniform, flat film on the tungsten electrode with little or no dendritic growth. The sodium electrodeposits undergo complete stripping from the tungsten electrode without dendritic or disconnected sodium metal left on the electrode surface.