Electrolyte Confinement Enables a Continuous Transition From Battery‐Like to Capacitor‐Like Na ⁺ Storage via Solvent Co‐Intercalation

Understanding how electrolyte solvation and nanoconfinement govern sodium‐ion intercalation is essential for designing high‐power sodium storage electrodes. Here, we use a model series of hydrogen titanates with systematically expanded interlayer spacings to investigate how the degree of electrolyte confinement governs the transition from battery‐like toward capacitor‐like Na + storage signatures. In carbonate‐based electrolyte, all materials form resistive interphases, exhibit pronounced charge transfer resistance, and show conventional sluggish kinetics. In contrast, diglyme‐based electrolyte enables solvent co‐intercalation of Na + ‐diglyme complexes, limits interphase formation, and produces rectangular/capacitor‐like CV signatures with strongly reduced interfacial impedance and excellent high‐rate performance. Ex situ TEM analysis and operando dilatometry confirm reduced interphase growth and reversible co‐intercalation‐induced lattice expansion in diglyme. These findings provide direct evidence for a confinement‐controlled continuum between Faradaic and capacitive Na + storage and highlight electrolyte solvation and electrode nanoconfinement co‐design as a powerful strategy toward high‐power sodium storage.