Catalytic and Stability Performance of the Electrode and Electrolyte in Na-O2 Batteries: Electrochemical Insight

Abstract

Using a metal anode in a metal air battery should maximise the cell voltage and energy density compared to the counterpart metal-ion technology but currently is challenging [1]. In sodium and lithium air batteries sodium or lithium dendrite formation at the anode is problem both in terms of performance and safety [2]. Oxygen (O2) may also diffuse to the anode and partly passivate the metallic surface and increase the cell impedance [2]. Side reactions of the metallic anode with the liquid electrolyte, O2 can also take place at the anode surface [3].Here, we report how the Na anode, the cathode and electrolyte limit the performance of Na-O2 batteries. The capacity limitation of the Na-O2 batteries not only relates to air blockage of the cathode by the discharge products but also to the metallic anode due to its side reactions with electrolyte and O2. A pX logged cyclic voltammetry studies showed that metallic Na is passivated significantly in aerated electrolyte as indicated by a shift in the Na/electrolyte interface's oxidation and reduction reaction potential to both positive and negative directions. Consequently, the overall cell potential decreases while the charging over potential increases (Fig. 1). The isolation by a solid-state electrolyte of the metallic anode from the direct contact of liquid electrolyte and O2 resulted in a long operating time with cell potential close to the theoretical value. A high stability was achieved at slower charge-discharge rates due to lower ionic conductivity of the solid-state electrolyte at room temperature. We will examine a number of solid-sate electrolytes for high performance Na-O2 batteries. This study will also analyse ex situ characterisation of the products formed at the metallic Na/electrolyte and non-precious metal oxide air-cathode/electrolyte interfaces that correlates the catalytic and stable performances of Na-O2 batteries.