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NaCrO2 crystallizes in the hexagonal P6/mmm space group. The structure is one-dimensional and consists of one NaCrO2 ribbon oriented in the (0, 0, 1) direction. Na1+ is bonded in a linear geometry to two equivalent O2- atoms. Both Na–O bond lengths are 2.13 Å. Cr3+ is bonded in a linear geometry to two equivalent O2- atoms. Both Cr–O bond lengths are 1.72 Å. O2- is bonded in a linear geometry to one Na1+ and one Cr3+ atom.
NaCrO 2 is a promising cathode for Na-ion batteries. However, further studies of the mechanisms controlling its specific capacities and cycle stability are needed for real-world applications in the future. This study reveals, for the first time, that the typical specific capacity of ~110 mAh/g reported by many researchers when the charge/discharge voltage window is set between 2.0 and 3.6 V vs. Na/Na + is actually controlled by the low electronic conductivity at the electrode/electrolyte interface. Through wet solution mixing of NaCrO 2 particles with carbon precursors, uniform carbon coating can be formed on the surface of NaCrO 2 particles, leading to unprecedented specific capacities at 140 mAh/g, which is the highest specific capacity ever reported in the literature with the lower and upper cutoff voltages at the aforementioned values. However, such carbon-coated NaCrO 2 with ultrahigh specific capacity does not improve cycle stability because with the specific capacity at 140 mAh/g the Na deintercalation during charge is more than 50% Na ions per formula unit of NaCrO 2 which leads to irreversible redox reactions. The insights from this study provide a future direction to enhance the long-term cycle stability of NaCrO 2 through integrating carbon coating and doping.