Hierarchical Engineering of Porous P2-Na2/3Ni1/3Mn2/3O2 Nanofibers Assembled by Nanoparticles Enables Superior Sodium-Ion Storage Cathodes

Layered transition metal oxides (TMOs) are appealing cathode candidates for sodium-ion batteries (SIBs) by virtue of their facile 2D Na+ diffusion paths and high theoretical capacities but suffer from poor cycling stability. Herein, taking P2-type Na2/3Ni1/3Mn2/3O2 as an example, it is demonstrated that the hierarchical engineering of porous nanofibers assembled by nanoparticles can effectively boost the reaction kinetics and stabilize the structure. The P2-Na2/3Ni1/3Mn2/3O2 nanofibers exhibit exceptional rate capability (166.7 mA h g(-1) at 0.1 C with 73.4 mA h g(-1) at 20 C) and significantly improved cycle life (approximate to 81% capacity retention after 500 cycles) as cathode materials for SIBs. The highly reversible structure evolution and Ni/Mn valence change during sodium insertion/extraction are verified by in operando X-ray diffraction and ex situ X-ray photoelectron spectroscopy, respectively. The facilitated electrode process kinetics are demonstrated by an additional study using the electrochemical measurements and density functional theory computations. More impressively, the prototype Na-ion full battery built with a Na2/3Ni1/3Mn2/3O2 nanofibers cathode and hard carbon anode delivers a promising energy density of 212.5 Wh kg(-1). The concept of designing a fibrous framework composed of small nanograins offers a new and generally applicable strategy for enhancing the Na-storage performance of layered TMO cathode materials.