Insights into the Facet and Morphological Domination on the Electrochemical Performance of Layered NaNi1/3Fe1/3Mn1/3O2 for Sodium-Ion Batteries

While multitransition-metal layered oxides in the form of micrometer-sized single crystals and polycrystalline aggregates show great promise as affordable cathodes for sodium-ion batteries, the relationship between a typical preferential facet as well as morphological domination and electrochemical performance in both forms remains unclear. In this regard, herein, two models of layered O3-type NaNi_1/3Fe_1/3Mn_1/3O₂ (NFM) materials with single-crystalline and polycrystalline characteristics were prepared. Based on in situ X-ray diffraction (XRD) characterization and electrochemical investigations, the common dominated (003) plane in single-crystalline NFM is proved to be an unfavorable ionic accessing plane but can alleviate the proportion of exposed edge planes and strengthen the domains to further improve the cycle life and humid air stability, while the polycrystalline NFM cathodes deliver a superior ionic transportation dynamics. Consequently, the polycrystalline NFM can achieve a high rate capability, with a discharge capacity of 110 mAh g^-1 at 10C at room temperature, and a discharge capacity of 89 mAh g^-1 at 1C even at −20 °C. Last but not the least, good cyclability and processability can be observed in the single-crystalline NFM by realizing a capacity retention of 85.7% after 350 cycles and a capacity retention of 91% after 100 cycles at 1C, even after long-term humid exposure.