Water Contributes to Higher Energy Density and Cycling Stability of Prussian Blue Analogue Cathodes for Aqueous Sodium-Ion Batteries

In this work, we performed a comprehensive study of Prussian blue and its analogues (PBAs), one of the most promising cathode materials for aqueous sodium-ion batteries for large-scale energy-storage systems, using first-principles calculations. It is confirmed that dry PBAs generally undergo a phase transition from a rhombohedral Na2PR(CN)6 (where P and R are transition metals) to a tetragonal/cubic PR(CN)6 during Na extraction, in agreement with experimental observations. Using a grand potential phase diagram construction, we show that water and Na co-intercalation result in fundamentally different phase transition behavior and, hence, electrochemical voltage profiles in wet versus dry electrolytes. Lattice water increases the average voltage and reduces the volume change during electrochemical cycling, resulting in both higher energy density and better cycling stability. Finally, we identified four new PBA compositions, Na2CoMn(CN)6, Na2NiMn(CN)6, Na2CuMn(CN)6, and Na2ZnMn(CN)6, that show great promise as cathodes for aqueous rechargeable Na-ion batteries.