title: Fast discharge process of layered cobalt oxides due to high Na⁺ diffusion.
abstract: 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 NaCoO2 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 LiCoO2. Especially, the D values of O3-NaCoO2 are even higher than those of P2-NaCoO2, probably because O3-NaCoO2 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 (ED ~ 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).
authors.affiliation: Fucalty of Pure and Applied Science, Univ. of Tsukuba, Tsukuba 305-8571, Japan.
authors.forename: Takayuki
authors.initials: T
authors.lastname: Shibata
authors.affiliation: College of Physics, Univ. of Tsukuba, Tsukuba 305-8571, Japan.
authors.forename: Yuya
authors.initials: Y
authors.lastname: Fukuzumi
authors.affiliation: 1] Fucalty of Pure and Applied Science, Univ. of Tsukuba, Tsukuba 305-8571, Japan [2] College of Physics, Univ. of Tsukuba, Tsukuba 305-8571, Japan.
authors.forename: Wataru
authors.initials: W
authors.lastname: Kobayashi
authors.affiliation: 1] Fucalty of Pure and Applied Science, Univ. of Tsukuba, Tsukuba 305-8571, Japan [2] College of Physics, Univ. of Tsukuba, Tsukuba 305-8571, Japan [3] Center for Integrated Research in Fundamental Science and Engineering (CiRfSE), Univ. of Tsukuba, Tsukuba 305-8571, Japan.
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Moritomo