Nanometre-precision terahertz interferometry for battery electrode metrology

High-precision, non-destructive thickness measurement is essential for lithium-ion battery (LIB) manufacturing. Existing approaches, such as X-rays, acoustic waves, and optical lasers, are limited by speed, resolution, or penetration into conductive materials. Here, we demonstrate nanometre-precision thickness measurements of LIB electrodes using terahertz (THz) Fabry-Pérot (FP) interferometry referenced to a photonic frequency comb. Combining the comb's SI-traceable frequency accuracy with THz radiation's immunity to scattering and absorption, our system directly detects FP modes with sub-10 MHz precision at sweep rates exceeding 12 THz/s, while spectral analysis simultaneously yields the complex refractive index. Electrode thicknesses of 50-150 μm are measured with nanometre precision: 70.1 nm (anode) and 465.5 nm (cathode) at 0.2 s, improving to 7.8 nm and 25.2 nm at 25.6 s, representing a one-to-two orders of magnitude improvement over temporal-analysis methods. The system further supports 3D profiling and dynamic thickness monitoring, enabling a unified, calibration-free platform for next-generation LIB metrology.