Synthesis and Stoichiometry of Different Layered Sodium Cobalt Oxides

Layered Na-metal oxides can form in different crystal structures, each with different electrochemical behavior. As a prototype system to better understand how each phase can be formed, we present the conditions under which different layered phases of NaxCoO2 can be stabilized in solid-state synthesis. Using a novel combination of ex situ XRD on as-synthesized samples, with in situ XRD to monitor the relation between Na content and lattice parameters, we are able to construct a phase diagram of NaxCoO2 between 450 to 750 degrees C in air and for Na:Co sample ratios ranging from 0.60 to 1.05. Four single phase domains of O3, O'3, P'3, and P2 are revealed based on the XRD analysis. In contrast to previous reports it is found that pure O3, O'3 and P'3 phase can only form at a fixed stoichiometry of x = 1.00, 0.83, and 0.67, respectively, while the P2 phase forms in a slightly larger composition range from 0.68 to 0.76. Galvanostatic charging of O3-Na1.00CoO2 shows several flat and sloping regions on the voltage profile, which follows the sequence of O3-O'3-P'3-P3-P'3, with increasing interslab distances. Our results indicate that the electrochemically important P2 structure is likely stabilized by entropy.