Ball-in-ball hierarchical design of P2-type layered oxide as high performance Na-ion battery cathodes

Hierarchical microstructures assembled with nanostructures can effectively enhance the electrochemical performances of electrode materials for rechargeable Li/Na-ion batteries. Due to the large ionic radius of sodium, this kind of design and synthesis is more important for Na-battery cathodes, which represent a cost-effective alternative for energy storage applications. Here ball-in-ball hierarchical microspheres assembled with micron shaped flakes of the P2-type Na0.7Ni0.18Mn0.64Co0.18O2 have been designed and synthesized via hydrothermal reaction followed by stepwise calcination process. As a cathode material of the Na-ion battery, this novel and uniform hierarchical structure exhibits very high specific capacity and superior rate performance with discharge capacities of 208 mAh g−1 at 0.05 C and 46 mAh g−1 at 10 C, and an excellent cycling stability with about 78% capacity retention at 1 C after 50 cycles. These performances are better than any layered oxide cathode material for reversible Na-ion batteries reported in the literature, prepared via the conventional solid-state method. Combining ex-situ x-ray absorption spectroscopy characterization, x-ray diffraction, field emission scanning electron microscopy, scanning transmission electron microscopy and electrochemical characterization, we demonstrate that the superior performances can be attributed to the unique ball-in-ball hierarchical structure assembled with micron shaped flakes. This configuration could effectively reduce the path of Na ion diffusion, increases the contact area between electrodes and electrolyte and buffers the volume changes during the Na ion insertion/extraction processes.