Na⁺ diffusion in Na _x Co[Fe(CN)₆]_0.90 film as investigated by transmission image

Abstract Diffusion of alkali metal ions plays an important role in the rate property of the battery. Here, we investigated macroscopic Na+ diffusion in a thin film of Na x Co[Fe(CN)6]0.90 (NCF90) by means of transmitted light intensity image at 540 nm. The distribution of the Na+ concentration (x) was determined against the elapsed time (t). We found that the diffusion constant (D) for Na+ shows x-dependence as D = 6.0 × 10−8 − (1.6 − x) 6.0 × 10−8 cm2 s−1. The average D value (=3.0 × 10−8 cm2 s−1) is much larger than the reported value (=0.5 × 10−10 cm2 s−1 M. Takachi, Y. Fukuzumi, and Y. Moritomo, Dalton Trans. 45, 458 (2016)) of Na x Co[Fe(CN)6]0.88 (x = 0.8) determined by the electrochemical impedance spectroscopy. The large D value evaluated in the present investigation is ascribed to the fast intergrain Na+ diffusion process.