Enhanced conductivity and stability of Prussian blue cathodes in sodium-ion batteries by surface vapor-phase molecular self-assembly
Tao Yuan, Xiaopan Fu, Yuan Wang, Mingjie Li, Shuixin Xia, Yuepeng Pang, Shiyou Zheng
Abstract
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 Fe 2+ 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.