Charge and discharge properties of spinel LiNi0.5Mn1.5O4 positive electrodes were investigated using concentrated LiBF4/propylene carbonate (PC) electrolyte solutions in the range of 0.833 (ca. 1 mol dm(-3)) to 7.25 (nearly saturated) mol kg(-1). The irreversible decomposition of electrolyte solution was effectively suppressed with an increase in concentration. In addition, the polarization in charge/discharge reactions remained small in the nearly saturated 7.25 mol kg(-1) LiBF4/PC even though the viscosity was very high and the ionic conductivity was low. The rate capability was higher than that obtained with nearly saturated 4.3 mol kg(-1) LiPF6/PC. The structure, viscosity, and ionic conductivity of the concentrated LiBF4/PC were investigated. Based on the experimental results, the rapid charge/discharge reactions at LiNi0.5Mn1.5O4 electrodes were discussed. (C) 2016 The Electrochemical Society. All rights reserved.