NaCrO2 is a Fundamentally Safe Positive Electrode Material for Sodium-Ion Batteries with Liquid Electrolytes

Layered structure NaCrO 2 (R-3m) was successfully synthesized by solid state reaction. The electrochemical performance of NaCrO 2 as a positive electrode material for sodium-ion batteries was tested at room temperature using two-electrode coin cells with NaClO 4 /PC electrolyte. NaCrO 2 delivered a reversible capacity of 110 mAh/g during the first charge and had good cyclability. The reaction between deintercalated Na x CrO 2 (∼Na 0.5 CrO 2 ) and ethylene carbonate/diethyl carbonate solvent was studied using Accelerating Rate Calorimetry (ARC). The results were compared to analogous results for Li 0.5 CoO 2 and Li 0 FePO 4 under the same conditions. Na 0.5 CrO 2 showed an unexpected excellent thermal stability in solvent with no heat release in the ARC test temperature range. An X-ray diffraction study showed that Na 0.5 CrO 2 decomposes to NaCrO 2 and P3-type layered structure CrO 2-δ with minimal oxygen release during the ARC tests, which explains the lack of exothermic behavior.