Understanding and controlling crystal water in Prussian blue analogues.

Prussian blue analogues (PBAs) are promising cathodes for sodium-ion batteries (SIBs) owing to their open ion-diffusion channels, scalable synthesis, and competitive energy density. However, crystal water in their frameworks significantly limits capacity, rate performance, safety, and cycling stability, thereby hindering their practical application. This review systematically analyzes the roles of crystal water, focusing on its migration dynamics and multiscale effects on structure and electrochemical behavior. A structure-water-performance relationship is proposed, linking crystal phases, electronic structures, ion transport, and interfacial reactions. Strategies for regulating crystal water are summarized in three categories: controlled synthesis, post-treatment, and electrolyte/interface engineering. Advanced characterization techniques and computational simulations for investigating crystalline water in the PBA framework have been introduced. Industrial challenges are also discussed, with emphasis on the cost-benefit balance of different water-regulation strategies. Finally, future directions are outlined, such as in situ tracking of water during cycling and heating, and predictive models linking water activity to long-term stability. Overall, this review aims to guide the rational design and scalable production of PBAs with controlled crystal water content, thereby enabling safe and durable SIBs for grid-scale storage and portable electronics.