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 cm 2 s −1 . The average D value (=3.0 × 10 −8 cm 2 s −1 ) is much larger than the reported value (=0.5 × 10 −10 cm 2 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.