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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 Na 2/3 Ni 1/3 Mn 2/3 O 2 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-Na 2/3 Ni 1/3 Mn 2/3 O 2 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 (≈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 Na 2/3 Ni 1/3 Mn 2/3 O 2 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.

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Iron/manganese‐based layered transition metal oxides have risen to prominence as prospective cathodes for sodium‐ion batteries (SIBs) owing to their abundant resources and high theoretical specific capacities, yet they still suffer from rapid capacity fading. Herein, a dual‐strategy is developed to boost the Na‐storage performance of the Fe/Mn‐based layered oxide cathode by copper (Cu) doping and nanoengineering. The P2‐Na 0.76 Cu 0.22 Fe 0.30 Mn 0.48 O 2 cathode material synthesized by electrospinning exhibits the pearl necklace‐like hierarchical nanostructures assembled by nanograins with sizes of 50–150 nm. The synergistic effects of Cu doping and nanotechnology enable high Na + coefficients and low ionic migration energy barrier, as well as highly reversible structure evolution and Cu/Fe/Mn valence variation upon repeated sodium insertion/extraction; thus, the P2‐Na 0.76 Cu 0.22 Fe 0.30 Mn 0.48 O 2 nano‐necklaces yield fabulous rate capability (125.4 mA h g −1 at 0.1 C with 56.5 mA h g −1 at 20 C) and excellent cyclic stability (≈79% capacity retention after 300 cycles). Additionally, a promising energy density of 177.4 Wh kg −1 is demonstrated in a prototype soft‐package Na‐ion full battery constructed by the tailored nano‐necklaces cathode and hard carbon anode. This work symbolizes a step forward in the development of Fe/Mn‐based layered oxides as high‐performance cathodes for SIBs.

Nickel-rich layered oxides, as the most promising commercial cathode material for high-energy density lithium-ion batteries, experience significant surface structural instabilities that lead to severe capacity deterioration and poor thermal stability. To address these issues, radially aligned grains and surface Li x Ni y W z O-like heterostructures are designed and obtained with a simple tungsten modification strategy in the LiNi0.91Co0.045Mn0.045O2 cathode. The formation of radially aligned grains, manipulated by the WO3 modifier during synthesis, provides a fast Li+ diffusion channel during the charge/discharge process. Moreover, the tungsten tends to enter into the lattice of the primary particle surface, and the armor-type tungsten-rich heterostructure protects the bulk material from microcracks, structural transformations, and surface side reactions. First-principles calculations indicate that oxygen is more stable in the surface tungsten-rich heterostructure than elsewhere, thus triggering an improved surface structural stability. Consequently, the 2 wt % WO3-modified LiNi0.91Co0.045Mn0.045O2 (NCM@2W) material shows outstanding prolonged cycling performance (capacity retention of 80.85% after 500 cycles) and excellent rate performance (5 C, 188.4 mA h g–1). In addition, its layered-to-rock salt phase transition temperature is increased by 80 °C compared with that of the pristine cathode. This work provides a novel surface modification approach and an in-depth understanding of the overall performance enhancement of nickel-rich layered cathodes.

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