Voltage Delay Action of Na Metal in Na/MnO2 Non-Aqueous Cells

Abstract

The Li based battery systems, namely Li-ion, Li-O2 and Li-S batteries have occupied top importance in research and development activities at present. However, Na based battery systems are gaining interest for the next generation non-aqueous batteries. This is primarily because of limited and uneven global resources of Li for large scale production of Li-ion batteries for future applications such as electric vehicles. Plentiful resources of Na and its electrochemical properties close to those of Li have instigated research activities on Na-ion batteries similar to Li-ion batteries. Electrode materials suitable for Na-ion batteries are under investigations and they are characterized for their electrochemical properties by assembling experimental cells in combination with Na metal as the counter electrode as well as reference electrode [1]. Primary batteries employing Na metal as the anode similar to Li based primary batteries are also expected to gain momentum [2]. The cells employing Na metal in non-aqueous electrolytes experience voltage delay on initiation of cell discharge. The voltage delay action arises due to the presence of surface film on Na. In the present studies, Na/MnO2 cells are assembled in a non-aqueous electrolyte and the voltage delay properties are investigated. Coin cells of type CR2032 with Na foil and MnO2 coated stainless steel mesh as the negative and positive electrodes, respectively, are assembled. The electrolyte was 1 M NaPF6 in a mixture of ethylene carbonate and propylene carbonate (1:1 by volume). Na foil was sliced from a lump of Na in Ar atmosphere glove box before the cell assembly. Amorphous MnO2 was used as the positive electrode material. As shown in Fig. 1 typically, there is a large voltage drop on commencing the cell discharge by passing current in galvanostatic mode. Following the instantaneous voltage fall from open circuit voltage (V1) to dip voltage (V2), then, cell voltage rises gradually and reaches the operating plateau voltage (V3) of the cell. The surface film on Na exhibits its first appearance and it undergoes dielectric breakdown in the high field, which are reflected in the initial voltage fall. Subsequent to the breakdown of surface film, bare Na surface contacts the non-aqueous electrolyte and undergoes oxidation to Na+ ions under the galvanostatic conditions with a simultaneous building of electric double layer. From the data obtained under several experimental conditions such as current density and cell ageing, the properties of surface film on Na metal are evaluated. Results of these studies will be discussed.