Electrochemical investigation of lithium and tin reduction at a graphite cathode in molten chlorides

The electrochemical reduction behaviour of lithium and tin at a graphite electrode in molten lithium chloride and a molten mixture of lithium chloride and tin(II) chloride (4.5 wt% of tin(II) chloride) was investigated at 625 °C by means of cyclic voltammetry, chronoamperometry and chronopotentiometry. It was found that, during cathodic polarization, deposition of metallic tin from the chloride mixture onto the graphite surface occurred, while no intercalation of tin into the graphite bulk took place. In contrast, and in agreement with previous findings, lithium intercalation into graphite occurred before deposition of metallic lithium started. The extent of erosion at the graphite cathode caused by tin deposition was much less than that resulting from lithium intercalation. This finding may explain why the reduction of tin at a graphite cathode, unlike the reduction of lithium, does not lead to the formation of nano-sized carbon materials. It was further deduced that the difference in the metal/graphite interaction has to be attributed to electronic rather than geometric factors. From chronoamperometric and chronopotentiometric measurements, the diffusion coefficient of the Sn2+ ion in the lithium chloride/tin(II) chloride mixture at 625 °C was estimated to be in the order of 3 × 10−4 cm2 s−1.