A study of copper as a cathode material for an ambient temperature sodium ion battery

The electrochemical behavior of copper was examined as a possible metal/met al(II) chloride cathode for a room temperature sodium battery in a nonaqueo us melt comprised of methanesulfonyl chloride, aluminum chloride, and sodiu m chloride. Electrodes plated with a finite amount of copper showed a decre ase in the amount of stored charge with increasing cycles, while solid copp er electrodes showed a steady amount of oxidation and reduction. To keep th e electrolyte buffered, the electrolyte contained excess NaCl in the form o f a slurry. Results with a working electrode in NaCl slurry showed a Cu/Cu( II) coulombic efficiency (reduction/oxidation) of less than 100%, while a c oulombic efficiency of 55% was observed for the electrode in the buffered s lurry. On the other hand, a coulombic efficiency over 100% was observed for trials where the electrode surface was not submerged in the NaCl slurry re sulting in a lower buffer capacity. This was probably due to local acidity change near the electrode surface caused by the decrease in chloride ions a s the copper is oxidized to copper chloride. This local acidity may cause s ide reactions to take place that chemically participated in the oxidation. This was supported by results using an electrochemical quartz crystal nanob alance that show a greater increase in mass than expected from the amount o f current passed during oxidation. (C) 2001 The Electrochemical Society.