Layered cobalt oxide, P2-NaxCoO2, is a prototypical cathode material for sodium-ion secondary battery. We systematically investigated the rate dependence of the discharge capacity (Q) in three thin films of Na0.68CoO2 with different film thickness (d) and in-plane grain radius (r). With subtracting conventional voltage drop effect on Q, we derived an intrinsic rate dependence of Q. We found a scaling relation between the renormalized discharge rate (γ ≡ r2/DT; D and T are the Na+ diffusion constant and discharge time, respectively) and relative capacity (=Q/Q0; Q0 is the value at a low rate limit). The observed scaling relation is interpreted in terms of the Na+ intercalation process at the electrolyte-NaxCoO2 interface and Na+ diffusion process within NaxCoO2.
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