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).