The effect of treating the lithium-rich layered Li1.2Ni0.2Mn0.6O2 with (NH4)(2)HPO4 solution followed by annealing at 400 - 700 degrees C on the structure and electrochemical properties has been investigated. It is found that the treatment with (NH4)(2)HPO4 solution (pH = 8.0) leach out lithium ions from layered Li1.2Ni0.2Mn0.6O2 through Li+/H+ ion-exchange process, which becomes enhanced at elevated temperatures (60 degrees C). Annealing the 60 degrees C (NH4)(2)HPO4 treated sample causes a lithium-deficiency-driven transformation to a 4.7 V LixNi0.5Mn1.5O4 spinel phase, while the sample treated with (NH4)(2)HPO4 at room temperature remains unaltered after such high-temperature annealing. Cathodes containing these annealed samples were prepared and evaluated in half cells by galvanostatic cycling. It is found out that the formed spinel-like phase become stable as the annealing temperature increases, whereas the layered phase in the annealed samples undergoes an irreversible transformation to a new 3 V spinel-like phase. Due to the excellent stability of both the 4.7 V LixNi0.5Mn1.5O4 spinel phase and the newly evolved 3 V spinal-like phase, the 600 and 700 degrees C annealed samples exhibit enhanced energy density, reaching more than 800 mWh g(-1) with good cycle life between 2.0 and 5.0 V. (C) 2013 The Electrochemical Society. All rights reserved.