A Study of the Reactivity of De-Intercalated NaNi_0.5Mn_0.5O₂with Non-Aqueous Solvent and Electrolyte by Accelerating Rate Calorimetry

O3-type layered structure NaNi 0.5 Mn 0.5 O 2 was successfully synthesized by solid state reaction. NaNi 0.5 Mn 0.5 O 2 was tested at room temperature using two-electrode Na/NaNi 0.5 Mn 0.5 O 2 coin cells with NaClO 4 /Propylene Carbonate electrolyte. NaNi 0.5 Mn 0.5 O 2 delivered a reversible capacity of 120 mAh/g during the first charge to 3.8 V, which corresponds to the de-intercalation of about 0.5 mol Na per mole transition metal. The reactivity of de-intercalated NaNi 0.5 Mn 0.5 O 2 (∼Na 0.5 Ni 0.5 Mn 0.5 O 2 ) with ethylene carbonate/diethyl carbonate (EC:DEC) solvent and with NaPF 6 /EC:DEC electrolyte was studied using Accelerating Rate Calorimetry (ARC). X-ray diffraction was used to study the products of the reactions of Na 0.5 Ni 0.5 Mn 0.5 O 2 with solvent or electrolyte after the ARC experiments. Na 0.5 Ni 0.5 Mn 0.5 O 2 shows high reactivity in both solvent and electrolyte. In solvent, Na 0.5 Ni 0.5 Mn 0.5 O 2 decomposes at elevated temperature to form nickel manganese oxide and Ni metal, and the released oxygen reacts with the solvent. In NaPF 6 -based electrolyte, the reaction mechanism changes completely as new reaction products, Na 3 (Ni,Mn)F 6 and Na(Ni,Mn)F 3, are formed in a new low temperature exothermic reaction. These results imply important consequences regarding safety for those researchers selecting electrolytes and electrode materials for Na-ion batteries. In both cases, NaF was found as a product of Na 0.5 Ni 0.5 Mn 0.5 O 2 reacting with PVDF binder.