A Parameter-Driven Approach to Modulating Chemical Composition in Prussian Blue Analogues Cathodes for Sodium-Ion Batteries

This study explores how various experimental factors, such as temperature, viscosity, and stirring speed, affect Prussian blue analogues (PBAs) materials‘ structural properties and electroneutrality. These factors influence key attributes like sodium ions, vacancies, and water content, which is governed by electroneutrality. Higher temperatures, faster stirring, low viscosity, and high Na⁺ concentration enhance Na⁺ incorporation because of the sufficient Na⁺ supplement, leading to a densified monoclinic structure with fewer vacancies and lower water content. In contrast, lower temperatures, slow stirring, high viscosity, and low Na⁺ concentration lead to less monoclinic or cubic structures with more vacancies and higher water content. Adding carboxymethyl cellulose (CMC) increases viscosity, enables lower Na⁺ diffusion, and renders less Na⁺ incorporation. As a result, an intermediate structure forms with balanced Na⁺ and water content, bridging the cubic and monoclinic forms. The electrochemical performance of cubic structures (PW-Cub) is superior, demonstrating better C-rate performance with distinct charge-discharge plateaus than monoclinic structures acquired at high temperatures (PW-Mc-HT). The PW-Mc-HT shows the mixed redox reactions between Fe²⁺/Fe³⁺ and Mn²⁺/Mn³⁺. This study reveals the root cause for the variation in PBAs′ chemical composition and provides a guideline for synthesizing high-quality PBAs for sodium-ion batteries.