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Enhanced conductivity and stability of Prussian blue cathodes in sodium-ion batteries by surface vapor-phase molecular self-assembly
Nano Research 2024, 17 (5): 4221-4230
Published: 24 January 2024
Downloads:38

With many merits such as facile synthesis, economy, and relatively high theoretical capacity, Prussian blue analogs (PBAs) are considered promising cathode materials for sodium-ion batteries (SIBs). However, their practical applications still suffer from a low actual specific capacity and inferior stability owing to the imperfect crystallinity, irreversible phase transition, and low intrinsic conductivity. Herein, a surface-modification technique for vapor-phase molecular self-assembly was developed to prepare Fe-based PBAs, specifically sodium iron hexacyanoferrate (NaFeHCF), with a uniform conductive polymer protective layer of polypyrrole (PPy) on the surface, resulting in NaFeHCF@PPy. The incorporation of a PPy protective layer not only improves the electronic conductivity of NaFeHCF@PPy, but also effectively mitigates the dissolution of Fe-ions during cycling. Specifically, this advanced vapor-phase technique avoids Fe2+ oxidation and Na+ loss during liquid-phase surface modification. The NaFeHCF@PPy exhibited a remarkably enhanced cycling performance, with capacity retentions of 85.6% and 69.1% over 500 and 1000 cycles, respectively, at 200 mA/g, along with a superior rate performance up to 5 A/g (fast kinetics). Additionally, by adopting this strategy for Mn-based PBAs (NaMnHCF@PPy), we further demonstrated the universality of this method for PBA cathodes in SIBs.

Research Article Issue
Moisture stable and ultrahigh-rate Ni/Mn-based sodium-ion battery cathodes via K+ decoration
Nano Research 2023, 16 (5): 6890-6902
Published: 28 February 2023
Downloads:154

As one of the most promising cathodes for sodium-ion batteries (SIBs), the layered transition metal oxides have attracted great attentions due to their high specific capacities and facile synthesis. However, their applications are still hindered by the problems of poor moisture stability and sluggish Na+ diffusion caused by intrinsic structural Jahn–Teller distortion. Herein, we demonstrate a new approach to settle the above issues through introducing K+ into the structures of Ni/Mn-based materials. The physicochemical characterizations reveal that K+ induces atomic surface reorganization to form the birnessite-type K2Mn4O8. Combining with the phosphate, the mixed coating layer protects the cathodes from moisture and hinders metal dissolution into the electrolyte effectively. Simultaneously, K+ substitution at Na site in the bulk structure can not only widen the lattice-spacing for favoring Na+ diffusion, but also work as the rivet to restrain the grain crack upon cycling. The as achieved K+-decorated P2-Na0.67Mn0.75Ni0.25O2 (NKMNO@KM/KP) cathodes are tested in both coin cell and pouch cell configurations using Na metal or hard carbon (HC) as anodes. Impressively, the NKMNO@KM/KP||Na half-cell demonstrates a high rate performance of 50 C and outstanding cycling performance of 90.1% capacity retention after 100 cycles at 5 C. Furthermore, the NKMNO@KM/KP||HC full-cell performed a promising energy density of 213.9 Wh·kg−1. This performance significantly outperforms most reported state-of-the-art values. Additionally, by adopting this strategy on O3-NaMn0.5Ni0.5O2, we further proved the universality of this method on layered cathodes for SIBs.

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