The proton diffusion coefficients in an electrolytic manganese dioxide (EMD) electrode at various stages of the constant current discharge have been determined by means of AC impedance technique. The finite diffusion model was applied in the numerical fitting. The crystallite size of EMD was estimated from (110), (121) and (021) diffraction peaks with both Scherrer equation and Warren–Averbach theory. X-ray diffraction (XRD) peak broadening caused by instrument, crystallite size and microstrain was separated. The crystallite size of ramsdellite was assumed to be the proton diffusion length.