Corrosion of PCC materials in Na-based liquid metal batteries

Liquid metal batteries (LMBs) are attractive candidates for large-scale stationary energy storage. Due to the liquid nature of the electrodes and electrolyte, LMBs show fast reaction kinetics at the interfaces and do not suffer from structural changes upon cycling. However, the elevated working temperature increases the susceptibility to corrosion of the solid cell components, in particular the positive current collector (PCC). Corrosion of the PCC not only limits the lifetime of an LMB but may also deteriorate its electrochemical performance if corrosion products pollute the positive electrode. In this study, the corrosion of Na-based LMB cells is investigated. Na is selected as negative electrode metal due to its relatively low melting point and its economic and environmental advantages over Li. As positive electrode, the heavy liquid metal alloys Sb-Sn and Sb-Bi are employed. The corrosion behavior of various PCC candidate materials (different steels, MAX-phases, Mo metal) in these alloys is investigated both in static exposure tests and under cycling conditions at around 450 °C, the working temperature of the respective Na-LMB cells. ... mehrIn Sb3Sn7 alloy, all investigated steels show severe dissolution corrosion, penetration of Sb and Sn, and formation of intermetallic Sn and Sb compounds. The corrosion depth ranges from 200 μm to 1 mm after 750 h. The same steels exhibit much milder corrosion (20-30 μm deep) and formation of Sb compounds when exposed to Sb-Bi alloys. Under cycling conditions in a Na//SbBi9 cell, 316-type stainless steel experiences similar corrosion rates and forms similar corrosion products as in static exposures to SbBi9, with some additional local corrosion enhancement and a higher penetration of Bi into the corrosion layer. These might be attributed to the influence of cell operation. In contrast to the investigated steels, MAX phase coatings and Mo metal show very good corrosion resistance in Sb-Sn and Sb-Bi alloy in static exposures. The excellent behavior of Mo metal is confirmed under cycling conditions. When employed as PCC in a Na//SbBi9 LMB cell, Mo shows merely some minor dissolution after two months continuous operation, which does not deteriorate the cell performance.