P2-type layered structure NaxCoO2 (x similar to 0.65) was successfully synthesized by solid state reaction. The electrochemical performance of Na0.65CoO2 as a positive electrode material for sodium-ion batteries was tested at room temperature using two-electrode coin cells with NaClO4/PC electrolyte. Na0.65CoO2 delivered a reversible capacity of 70 mAh/g during the first charge to 4.3 V, which is related to the de-intercalation of about 0.3 mol Na per mole Co. The reactivity of de-intercalated NaxCoO2 (similar to Na0.35CoO2) with ethylene carbonate/diethyl carbonate (EC:DEC) solvent and with NaPF6/EC:DEC electrolyte was studied using Accelerating Rate Calorimetry (ARC). X-ray diffraction was used to study the products of the reactions of Na0.35CoO2 with solvent or electrolyte after the ARC experiments. De-intercalated NaxCoO2 shows high reactivity in both solvent and electrolyte. Na0.35CoO2 reacts with solvent at elevated temperature in a manner which is similar to de-intercalated LixCoO2, leading to the products Na0.7CoO2 and Co3O4. Then Na0.7CoO2 reacts to form Co3O4 and NaHCO3. Then Co3O4 reacts to form CoO or Co metal. In NaPF6-based electrolyte, the reaction mechanism changes completely as the major reaction products are NaCoF3 and CoO or Co. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.102205jes] All rights reserved.