High-capacity, lithium and manganese-rich cathodes (HCMR) have aroused great interest in their application in lithium-ion batteries (LIBs) due to their high coulombic capacites and low cost. However, its commercial application has been hindered by various fundamental and practical challenges. In this paper, one of those challenges, the source of the high impedance at low states of charge (SOC) is investigated via electrochemical impedance spectroscopic (EIS) measurements of four different HCMR materials. It is found that the property of both solid state lithium diffusion and charge transfer make significant and comparable contributions to the impedance rise between 3.3 V and 3.7 V; while below 3.3 V, the charge transfer kinetics improve and offset the continuous impedance rise attributed to solid state lithium diffusion. Estimates of the exchange current densities and diffusion coefficients at various SOCs are provided. Methods aimed both at enhancing the solid state lithium diffusion process and the exchange current density should be targeted to promote the commercialization of HCMR. (C) 2016 The Electrochemical Society. All rights reserved.