Sodium-ion battery technologies are known to suffer from kinetic problems associated with the solid-state diffusion of Na in intercalation electrodes, which results in suppressed specific capacity and degraded rate performance. Here, a controllable selective etching approach is developed for the synthesis of Prussian blue analogue (PBA) with enhanced sodium storage activity. On the basis of time-dependent experiments, a defect-induced morphological evolution mechanism from nanocube to nanoflower structure is proposed. Through in situ X-ray diffraction measurement and computational analysis, this unique structure is revealed to provide higher Na diffusion dynamics and negligible volume change during the sodiation/desodiation processes. As a sodium ion battery cathode, the PBA exhibits a discharge capacity of 90 mA h g, which is in good agreement with the complete low spin Fe(C) redox reaction. It also demonstrates an outstanding rate capability of 71.0 mA h g at 44.4 C, as well as an unprecedented cycling reversibility over 5000 times.

Soft carbon has attracted tremendous attention as an anode in rocking‐chair batteries owing to its exceptional properties including low‐cost, tunable interlayer distance, and favorable electronic conductivity. However, it fails to exhibit decent performance for sodium‐ion storage owing to difficulties in the formation of sodium intercalation compounds. Here, microporous soft carbon nanosheets are developed via a microwave induced exfoliation strategy from a conventional soft carbon compound obtained by pyrolysis of 3,4,9,10‐perylene tetracarboxylic dianhydride. The micropores and defects at the edges synergistically leads to enhanced kinetics and extra sodium‐ion storage sites, which contribute to the capacity increase from 134 to 232 mAh g−1 and a superior rate capability of 103 mAh g−1 at 1000 mA g−1 for sodium‐ion storage. In addition, the capacitance‐dominated sodium‐ion storage mechanism is identified through the kinetics analysis. The in situ X‐ray diffraction analyses are used to reveal that sodium ions intercalate into graphitic layers for the first time. Furthermore, the as‐prepared nanosheets can also function as an outstanding anode for potassium‐ion storage (reversible capacity of 291 mAh g−1) and dual‐ion full cell (cell‐level capacity of 61 mAh g−1 and average working voltage of 4.2 V). These properties represent the potential of soft carbon for achieving high‐energy, high‐rate, and low‐cost energy storage systems.

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Coordination compounds such as Prussian blue and its analogues are acknowledged as promising candidates for electrochemical sodium storage owing to their tailorable frameworks. However, a key challenge for these electrode materials is the trade-off between energy and power. Here, we demonstrate that Prussian white (Na 3.1 Fe 4 [Fe(CN) 6 ] 3 ) hierarchical nanotubes with fully open framework configurations render extrinsic Na + intercalation pseudocapacitance. The cathode exhibits a capacity up to 83 mAh g -1 at an ultra-high rate of 50 C and an unprecedented cycle life over 10000 times for sodium storage. In situ Raman spectroscopy together with In situ X-ray diffraction analysis reveal that intercalation pseudocapacitance enables full reaction of N-Fe III /Fe II sites in Prussian white with a negligible volume expansion (< 2.1%). The discovery of surface-controlled charge storage occurring inside the entire bulk of intercalation cathodes paves a new way for developing high power, high energy, and long life-span sodium-ion batteries.

Prussian blue analogs (PBAs) featuring large interstitial voids and rigid structures are broadly recognized as promising cathode materials for sodium-ion batteries. Nevertheless, the conventionally prepared PBAs inevitably suffer from inferior crystallinity and lattice defects, leading to low specific capacity, poor rate capability, and unsatisfied long-term stability. As the Na+ migration within PBAs is directly dependent on the periodic lattice arrangement, it is of essential significance to improve the crystallinity of PBAs and hence ensure long-range lattice periodicity. Herein, a chemical inhibition strategy is developed to prepare a highly crystallized Prussian blue (Na2Fe4[Fe­(CN)6]3), which displays an outstanding rate performance (78 mAh g–1 at 100 C) and long life-span properties (62% capacity retention after 2000 cycles) in sodium storage. Experimental results and kinetic analyses demonstrate the efficient electron transfer and smooth ion diffusion within the bulk phase of highly crystallized Prussian blue. Moreover, in situ X-ray diffraction and in situ Raman spectroscopy results demonstrate the robust crystalline framework and reversible phase transformation between cubic and rhombohedral within the charge–discharge process. This research provides an innovative way to optimize PBAs for advanced rechargeable batteries from the perspective of crystallinity.