Fast discharge process of layered cobalt oxides due to high Na+ diffusion

Sodium ion secondary battery (SIB) is a low-cost and ubiquitous secondary battery for next-generation large-scale energy storage. The diffusion process of large Na + (ionic radius is 1.12 Å), however, is considered to be slower than that of small Li + (0.76 Å). This would be a serious disadvantage of SIB as compared with the Lithium ion secondary battery (LIB). By means of the electrochemical impedance spectroscopy (EIS), we determined the diffusion constant ( D ) of Na + in thin films of O3- and P2-type NaCoO 2 with layered structures. We found that the D values (~ 0.5–1.5 × 10 −10 cm 2 /s) of Na + are higher than those (< 1 × 10 −11 cm 2 /s) of Li + in layered LiCoO 2 . Especially, the D values of O3-NaCoO 2 are even higher than those of P2-NaCoO 2, probably because O3-NaCoO 2 shows successive structural phase transitions from the O3, O’3, P’3, to P3 phases with Na + deintercalation. We further found that the activation energy ( E D ~ 0.4 eV) for the Na + diffusion is significantly low in these layered cobalt oxides. We found a close relation between the relative capacity and the renormalized discharge rate ( = L 2 / DT , where L and T are the film thickness and discharge time, respectively).