The temperature coefficient (alpha) of redox potential (V) is a significant material parameter that coverts thermal energy into electric energy. In this paper, we determined alpha of P2-type NaxCoO2 against the Na+ concentration (x). The solid component (alpha(solid)) of alpha scatters from -0.18 to 0.64 mV K-1. The phase separation model cannot reproduce the x-dependence even qualitatively. We interpreted the unexpected disagreement in terms of the residual configuration entropy in the pseudo-disordered region between the Na+ ordered phases. (C) 2019 The Japan Society of Applied Physics