Identifying the Synergistic Na⁺/Zn²⁺ Co-Intercalation Mechanism for Boosting Electrochemical Performance of Na₄Fe₃(PO₄)₂P₂O₇ in Zn-Ion Batteries

Rechargeable aqueous zinc-ion batteries (ZIBs) have been regarded as a promising battery technology for stationary energy storage applications due to high safety, long-term sustainability, and low cost. The main challenge for ZIBs is the lack of robust structure to accommodate repeated ion insertion/extraction in aqueous solutions. In contrast to transition metal oxides cathodes (MnO 2, V 2 O 5 ), iron-based polyanionic compounds have rigid structure and open framework, hence may serve as compatible cathodes for aqueous batteries. However, only several iron-based polyanionic cathodes such as Na 4 Fe 3 (PO 4 ) 2 P 2 O 7, have received scant attentions to date, not to mention that their underlying complex reaction mechanisms in aqueous solutions have not yet been clearly revealed. In this work, we identify the Na + /Zn 2+ cointercalation mechanism of Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 in zinc-ion batteries by both experimental spectra and DFT calculations. Benefiting from the synergistic chemistry, the Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 demonstrates enhanced structural stability and ion diffusion kinetics upon the Na + /Zn 2+ cointercalation in comparison with single metal ion (only Na + or Zn 2+ ) storage reaction, hence a high-power density (6.73 kW kg –1 ) and long cycle life (54.6% after 5000 cycles) are exhibited. In particular, the prominent reaction kinetics endows the battery with low-temperature (−30 °C) operation capability (capacity retention of 87.6%).