Facile Synthesis and Electrochemical Investigation of Na0.67Mn0.65Fe0.20Ni0.15O2: A Positive Electrode Material for Na-Ion Batteries

Abstract

Research activities on Na-ion batteries have become intensive in recent years, as they are considered as economical alternatives to Li-ion batteries.1,2 Although, the use of small size Li-ion batteries is now wide spread, their range of applications is expected to increase in larger sizes for applications such as electric vehicles in future. One of the major concerns for expansion of Li-ion batteries in large sizes is the limited and unevenly distributed global resources of Li and hence its high cost. As the sources of Na are distributed over the globe in plentiful and being significantly cheaper than Li, research and development activities are now diverted towards Na-ion batteries. Among several cathode materials investigated for rechargeable Na-ion batteries, Na0.67Mn0.65Fe0.20Ni0.15O2 is reported to be a promising material.3 Convenient method of synthesis is one of the key issues in deciding potential use of an electrode material in general. In the present study, a low temperature self-combustion method is employed to prepare Na0.67Mn0.65Fe0.20Ni0.15O2. An aqueous solution consisting of stoichiometric quantities of NaNO3, Mn(NO3)2.4H2O, Fe(NO3)2.9H2O and Ni(NO3)2.6H2O is prepared and sucrose is dissolved as the fuel. The mixed solution is slowly heated to 130 °C, where the fuel ignites and produces a black residue within a few minutes. The residue is heated at 800 °C for 6 h to produce well crystalline phase of Na0.67Mn0.65Fe0.20Ni0.15O2. The material is characterized by physicochemical techniques. For the electrochemical studies, CR2032 coin cells are assembled using Na foil as the counter cum reference electrode in an electrolyte of 1 M NaPF6 dissolved in ethylene carbonate and propylene carbonate (1:1 by volume). When the cells are cycled at a current density of 35 mA g-1, a specific discharge capacity of 178 mAh g-1 is obtained with coulombic efficiency close to 100 %, initially. The capacity decreases to 127 mAh g-1 after 50 cycles which indicates capacity retention of 72% over 50 cycling. The decay in the capacity is studied with the help of electrochemical impedance spectroscopy. Cyclic voltammogram and rate capability experiments are also carried out for detailed study of the material. These results will be presented.