Sodium trapping in aluminium current collectors

Abstract: The aim of this master thesis was to establish if sodium is trapped in aluminium current collectors, which in turn could affect the capacity fade in sodium-ion battery systems. In the case of lithium-ion batteries, previous studies have shown that a trapping mechanism, where lithium diffuses through the active material and current collectors, can explain the capacity fade observed for several systems. However, no such reports have been published in the sodium case, motivating this pioneering investigation. Contact samples of sodium and aluminium current collector material confirmed the uptake of sodium as shown by ICP-AES analyses. The uptake of sodium in the aluminium was equivalent to a charge of 0.4 µAh after 70 days of contact at 55°C. The main characterisation method was galvanostatic plating and stripping of sodium on aluminium in a pouch-cell configuration. When using a bare aluminium working electrode with a metallic sodium counter electrode in a 1 M NaPF6/diglyme electrolyte, the galvanostatic cycling showed coulombic efficiency instabilities. It was concluded that a more stable, high efficient plating-stripping would be needed to quantify the effects of sodium trapping with the employed electrochemical methods. Coulombic efficiency values that exceeded 100 % were attributed to the oxidation of disconnected (detached) sodium from previous plating cycles. On consecutive cycles some of the disconnected sodium got reconnected, resulting in coulombic efficiency values well over 100 %.