Structural-Defect-Controlled Electrochemical Performance of Sodium Ion Batteries with NaCrO2 Cathodes

In this study, we report the effects of structural defects on the electrochemical cycle stability of Na-ion batteries made of NaCrO2 cathodes. Three processing conditions are used to create NaCrO2 with different structural defects. One condition entails high-energy ball milling of reactants before the solidstate reaction at 900 degrees C, the second one has no high-energy ball milling but simple mixing of reactants before the solid-state reaction, and the last one has high-energy ball milling after the solid-state reaction. The NaCrO2 crystals derived from the process with high-energy ball milling of reactants before the solid-state reaction exhibit the best bulk conductivity and highest specific capacity with little capacity decay over 46 charge/discharge cycles. In contrast, the NaCrO2 crystals derived from the process without high-energy ball milling have significant capacity decay, whereas the NaCrO2 crystals obtained from the process with high-energy ball milling after the solid-state reaction exhibits the lowest bulk conductivity and very low specific capacities. The different electrochemical properties exhibited by these NaCrO2 crystals have been ascribed to different extents of structural defects present in these crystals. This study unambiguously demonstrates that structural defects in NaCrO2 crystals are critical in achieving NaCrO2 electrodes with high specific capacity and excellent capacity retention.