A P2/P3 composite layered cathode for high-performance Na-ion full batteries

Na-ion batteries (NIBs) have been suggested as one of the most promising technologies for low-cost rechargeable batteries due to the earth-abundance resource of sodium. However, the delivery of high-performance cathode is still facing a great challenge especially for the development of Na-ion full batteries towards practical application. Herein, we present a layered biphasic cathode with P2 and P3 integrated structures, which is clearly elucidated by X-ray diffraction refinement, high resolution transmission electron microscopy and selected area electron diffraction. Combining the respective characteristics of P2 and P3 phases and highly reversible P2/P3–P2/OP4 structural evolution, the layered composite delivers a high reversible capacity of 119 mA h g−1 with a superior initial Coulombic efficiency of 94.8%, a high operating voltage of 3.53 V vs Na+/Na based on Ni2+/Ni4+ redox couple, and an outstanding rate performance (capacity retention of 85.5% at 5C compared with 0.2C). Furthermore, the assembled Na-ion full battery of P2/P3-Na0.7Li0.06Mg0.06Ni0.22Mn0.67O2//hard carbon exhibits a high average voltage of 3.36 V with a calculated energy density up to 218 W h kg−1. Our contribution paves a smart way for designing sodium-ion full batteries with both high energy density and good cycling stability toward practical application.