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.