Lithium‐Ion Transport in Carbon Fibers for Structural Batteries

Structural batteries that unite mechanical integrity with electrochemical function hinge on carbon fiber anodes capable of sustaining efficient lithium transport. Carbon fibers possess unique microstructures and multifunctional demands, yet their lithium transport kinetics remain largely unexplored in the context of structural batteries. Here, diffusion processes and interfacial characteristics are quantified in two intermediate‐modulus polyacrylonitrile‐based fibers (T800S and T800H), which share identical core microstructures but differ in polymer sizing and electrode architecture. T800S outperforms T800H in liquid electrolyte, delivering higher lithiation capacity (≈295 vs. ≈283 mAh g −1 ) and lower irreversible loss (31% vs. 36%), consistent with more efficient solid electrolyte interphase (SEI) formation and faster charge‐transfer dynamics. Under structural battery electrolyte conditions, both fiber types exhibit suppressed capacity, with diffusion coefficients reduced by up to two orders of magnitude (≈10–13 to ≈10–15 cm 2 s −1 ), as revealed by galvanostatic intermittent titration and impedance spectroscopy. Elevated charge‐transfer resistance and diminished interfacial capacitance further highlight the transport limitations imposed by the biphasic structural electrolyte matrix. The results demonstrate that fiber microstructure governs performance in liquid electrolytes, whereas interfacial chemistry and electrode architecture dominate under structural battery electrolyte operation. This mechanistic framework identifies interface engineering and mesoscale design as key strategies for advancing multifunctional structural energy storage.