An electrochemical characterization was performed in electrodes with different weight percentages of LiMnPO4 and multi-walled carbon nanotubes (MWCNTs) in aqueous solution. The redox potential of LiMnPO4 cathode is close to the electrolyte decomposition, which provides an ideal scenario to study multiple reactions on a single electrode surface involving parallel steps and species transformation in both solid and liquid state. Different processes were deconvoluted using cyclic voltammetry and electrochemical impedance spectroscopy. In addition, a surface coverage model was employed to theoretically quantify the limiting step of the electrochemical process. The results show the addition of MWCNTs increased the electrical conductivity of the cathode and improved the intercalation process in LiMnPO4. The optimal concentrations of MWCNTs, which enhanced the electrical properties and decreased the water oxidation effect, were 20 and 40 wt.%.