To increase the energy density of a vanadium redox flow battery (VRFB), the Mn(II)/Mn(III) system was used as a positive reaction and its effect on the performance and cycle life were investigated. The discharge voltage of the V/Mn system increased due to the higher redox potential of Mn(II)/Mn(III), which led to a 47% increase in initial energy density from 21 Wh L-1 to 31 Wh L-1. However, Mn(III) ions in the positive electrolyte are converted to MnO2 upon charging and remain in the precipitate without being reduced upon discharge, thus decreasing the energy density of the V/Mn system up to the 10th cycle. As cycles progressed further, the number of vanadium ions permeating to the positive electrolyte increased, and the particle size of MnO2 decreased. As a result, MnO2 could participate in the reduction reaction without precipitating, resulting in increased energy density. These results show the possibility of using Mn ions for the positive reaction by appropriately controlling the particle size of MnO2. (c) 2018 The Electrochemical Society.