The kinetic behavior of heat-treated electrolytic manganese dioxide (HEMD) was investigated using real-time electrochemical impedance spectroscopy (EIS) under a nonequilibrium state at a discharge current of 0.1 mA cm(-2). The kinetic parameters were obtained under real nonequilibrium conditions, indicating that the charge-transfer step and the diffusion of lithium ions proceeded together with the controlled rate of lithium insertion, except in the reduction range of 0.5 <= x <= 0.55, in which the charge-transfer step is the limiting step for lithium ion insertion during the discharge of HEMD. In addition, a second time constant in the intermediate-frequency region was detected in the EIS experiment, which demonstrates that a closed phase interface has been formed in the two-phase reaction region. Additionally, our observations show that the phase boundary has blocking effects on the diffusion of lithium ions. Based on the analysis of the structural evolution and the variation in the dynamic behavior, we present a detailed core/shell-model mechanism to explain the discharge mechanism of HEMD. (C) 2014 The Electrochemical Society. All rights reserved.