Deciphering the Impact of Crystal Water on Prussian Blue Electrochemistry Using a pH-Controlled Synthesis Strategy for Sodium-Ion Batteries

Prussian blue (PB), a promising cathode material for sodium-ion batteries, often suffers from high water content, which compromises its structural and electrochemical properties. This study presents a pH-controlled synthesis strategy to effectively regulate water content, reducing it from 15.67% (PB-6.8) to 9.18% (PB-8.2). We examined how water content influences electrochemical behavior and explained the underlying mechanism using solubility product constants. Results show that higher pH reduces water content and increases sodium content, leading to a specific capacity of 146.68 mAh g^-1 at 100 mA g^-1, with low-spin Fe contributing 45.1% of the total capacity (PB-8.2). However, excessive dehydration in PB-8.2 caused structural collapse and defects, slowing reaction kinetics and resulting in poor cycling stability─only 51.91% capacity retention after 500 cycles at 500 mA g^-1. Thus, water content must be optimized, as crystalline water plays a dual role: it can trigger side reactions but is also essential for structural integrity. This work offers a viable route to tailor water content in PB and clarifies the dual function of crystalline water.