Synthesis and thermodynamics of transition metal oxide based sodium ion cathode materials

While finding sustainable renewable energy sources is of high priority, it is equally important to develop solutions for energy storage. Advances in lithium ion batteries (LIB) indeed represent a great step in this direction. However, lithium is scarce and expensive, Batteries based on more earth-abundant elements such as sodium are attractive replacements. Layered transition metal oxides with sodium in the interlayer are promising battery cathode materials. Here we report the synthesis of a series of NaMO₂ (M = Mn, Fe, Co and Ni) materials and analysis of their thermodynamic stability using high temperature oxide melt solution calorimetry. Their formation enthalpies from corresponding binary oxides are highly exothermic. The NaMO₂ materials are thermodynamically more stable than their LiMO₂ counterparts owing to higher electropositivity of Na⁺ over Li⁺. The formation enthalpies become less exothermic in the sequence Mn, Fe, Co, Ni, a trend seen in many other ternary transition metal oxides.