Identifying a Stable Counter/Reference Electrode for the Study of Aprotic Na–O2Batteries

The aprotic Na–O 2 battery has attracted significant interest over the past few years because of its high theoretical specific energy and uncompromised round-trip efficiency. So far, most of the Na–O 2 studies, using a cell configuration of Na metal anode, aprotic Na + electrolyte and porous O 2 cathode, focus on the O 2 reactions in the cathode, which, however, could be complicated by the use of Na metal anode. This is because both the electrolyte and O 2 (from cathode) can react with the Na metal anode and some of the side reaction products could cross over to the cathode and interfere with the O 2 reactions occurring therein. Toward a better understanding of the O 2 electrochemistry in aprotic Na–O 2 batteries, it is essential to set up a clean electrochemical system, i.e., using counter/reference electrodes that react with neither electrolyte nor O 2 . In this report, Na 3 V 2 (PO 4 ) 3 /NaV 2 (PO 4 ) 3 couple having a formal potential of 3.35 V vs . Na/Na + , reacts with neither electrolyte nor O 2 during its multiple cycles of intercalation/deintercalation process, and therefore has been identified to be a stable counter/reference electrode for the study of O 2 electrochemistry in aprotic Na–O 2 batteries.