Electrochemical Property-Structure Correlation for Ni-Based Layered Na-Ion Cathodes.

A class of Ni-based layered Na xNi0.5Mn0.3Co0.2O2 oxide composites is prepared via a solid-state process. Mixed P-, O-, and P-/O-type phases can be obtained by tuning the Na content and annealing temperature, as demonstrated by structural and chemical characterization. Among these materials, the triphase P2/P'3/O'3-type composite exhibits the best overall electrochemical performance. Specifically, this triphase composite delivers a high specific capacity of 126 mA h g-1 in the potential range of 1.5-4.2 V, high rate capability (∼72% of its initial capacity at a rate of 5 C), and good capacity retention after 100 cycles at 0.5 C. The structural transition mechanism for each phase upon electrochemical cycling is investigated, providing insights into the correlation between electrochemical properties and the crystal structure of Ni-rich layered Na xNi0.5Mn0.3Co0.2O2 oxide composites.