A new mechanistically derived 0D model of a phase separating active cathode particle was designed for use in a multi particle porous electrode theory based model. The proposed 0D model was obtained by integration of dynamic equations of the spatially resolved single particle model, based on the regular solution theory. The described analytic procedure yields a thermodynamically consistent 0D model that preserves physicochemical relevance of the spatially resolved model and reduces computational times by up to six orders of magnitude. Besides its computational efficiency, the 0D model features high levels of consistency with the spatially resolved model in the low and high overpotential limit, which are most relevant for cell modelling. A transparent relation to the spatially resolved model provides validation of the suitability of previously applied intuitive approaches assigning spinodal average chemical potential to the entire particle. It also offers additional insights into the physicochemical phenomena inside a multiparticle electrode. The proposed 0D approach provides the basis for modelling advanced experimental observations covering: charge/discharge hysteresis, varying active particle population, solid solution vs. phase separating state of particles within active population, and memory effect when implemented in the multi particle phase separating porous electrode theory based model. (C) 2019 The Electrochemical Society.