Operando tracking of ion kinetics and state-of-charge via multiresonant fiber-optic grating sensors in sodium-ion batteries.

Ion transport within the sub-micron diffusion layer at the electrode-electrolyte interface governs battery function, yet probing its rapid, confined dynamics under operating conditions remains a challenge. Here, we introduce a compact, highly sensitive multiresonant fiber-optic grating sensor that monitors these processes operando in sodium-ion batteries without interfering with their operation. Using this approach, we reveal an intermediate stage of ion transport between adsorption and diffusion at the interface. We find that a shorter duration of this intermediate stage correlates with superior fast-charging performance. Furthermore, by integrating the sensor's optical intensity in real time, we achieve state-of-charge quantification with unprecedented accuracy (>98%). This operando measurement platform offers a new capability for battery diagnostics and can inform the design of next-generation batteries with enhanced electrochemical properties.