Layered Na0.71CoO2: a powerful candidate for viable and high performance Na-batteries

The present study reports on the synthesis and the electrochemical behavior of Na0.71CoO2, a promising candidate as cathode for Na-based batteries. The material was obtained in two different morphologies by a double-step route, which is cheap and easy to scale up: the hydrothermal synthesis to produce Co3O4 with tailored and nanometric morphology, followed by the solid-state reaction with NaOH, or alternatively with Na2CO3, to promote Na intercalation. Both products are highly crystalline and have the P2-Na0.71CoO2 crystal phase, but differ in the respective morphologies. The material obtained from Na2CO3 have a narrow particle length (edge to edge) distribution and 2D platelet morphology, while those from NaOH exhibit large microcrystals, irregular in shape, with broad particle length distribution and undefined exposed surfaces. Electrochemical analysis shows the good performances of these materials as a positive electrode for Na-ion half cells. In particular, Na0.71CoO2 thin microplatelets exhibit the best behavior with stable discharge specific capacities of 120 and 80 mAh g-1 at 5 and 40 mA g-1, respectively, in the range 2.0–3.9 V vs. Na+/Na. These outstanding properties make this material a promising candidate to construct viable and high-performance Na-based batteries.