The adsorption and decomposition mechanism of electrolyte on the cathode surface is one of the governing factors that control the stability, capacity, and cyclic life of Lithium-ion batteries. In this work, first principles calculations are used to study the adsorption and decomposition reaction mechanisms of ethylene carbonate (EC) on the (100) surface of charged (Li0.6Mn2O4), and the fully discharged LiMn2O4. The adsorption strength and electronic properties of various configurations are discussed by using density of states (DOS), projected density of states (PDOS) and electron density difference (EDD). Moreover, the initial decomposition mechanisms of EC on the surfaces is investigated by examining the minimum energy path between two minima using the climbing image nudged elastic band reaction-pathway sampling scheme. Even though adsorption and decomposition reaction can occur on the surfaces, for all configurations studied, the results show that the decomposition of EC is more likely on the charged surface despite having higher adsorption energy on the fully discharged surface. In general, this work aims to give an insight into the initial stages of surface catalyzed electrolyte decomposition reactions on spinel cathode structure.