Insights into the Structure Stability of Prussian Blue for Aqueous Zinc Ion Batteries

The reversible storage of Zn2+ ions in Prussian blue analogues with typical aqueous solution was challenged by fast degradation and poor coulombic efficiency, while the mechanism is yet to be uncovered. This study correlates the performance of the nickel hexacyanoferrate to the dynamics of H2O in the electrolyte and the associated phase stability of the electrode. It demonstrates severe Ni dissolution in conventional diluted aqueous electrolyte (1 m ZnSO4 or 1 m Zn(TFSI)(2)), leading to structure collapse with the formation of an electrochemical inert phase. This is regarded as the descriptor for the fast decay of nickel hexacyanoferrate in diluted aqueous electrolyte. However, a well-preserved open framework for zinc storage was obtained in concentrated aqueous electrolyte (1 m Zn(TFSI)(2) + 21 m LiTFSI)-the H2O activity is highly suppressed by extensive coordination-thus, reversible capacity of 60.2 mAh g(-1) over 1600 cycles could be delivered.

Automated summary

The study shows that nickel hexacyanoferrate easily degrades in conventional diluted aqueous electrolyte due to structure collapse, but a well-preserved framework for zinc storage can be obtained in concentrated aqueous electrolyte by suppressing H2O activity.