Crystal Water Engineering in Electrode Materials: Decoding Structure‐Property Dynamics for Advanced Energy Storage Devices

ABSTRACT Crystal water, traditionally viewed as a passive or even detrimental component, has emerged as an integral and dynamic structural unit in advanced energy storage materials. Despite its critical role in modulating structural stability and ion transport kinetics, a comprehensive understanding of the distinct structure‐property dynamics of crystal water across diverse energy storage systems remains elusive. Herein, this review systematically decodes the fundamental mechanisms and multifaceted applications of crystal water engineering. We comprehensively examine its tailored functionalities in monovalent‐ion (e.g., Li + , Na + , K + ) and multivalent‐ion (e.g., Mg 2+ , Zn 2+ , Ca 2+ ) batteries, highlighting distinct mechanistic disparities ranging from mitigating steric hindrance via lattice expansion to shielding electrostatic interactions through solid‐state solvation. The discussion also extends to emerging systems, such as supercapacitors and solid‐state or quasi‐solid‐state electrolytes, where structurally confined water uniquely acts as an ionic lubricant and a rapid ion‐hopping promoter. Finally, critical open questions and emerging opportunities are analyzed, highlighting transformative potential of crystal water in guiding the rational design of next‐generation energy storage materials.