Galvanostatic intermittent titration technique (GITT) at room temperature (25°C) and at 50°C and electrochemical impedance spectroscopy (EIS) at 25°C as a function of cell voltage and cycle number were performed on the layered cathode, Li(Ni1/2Mn1/2)O2 and the results were correlated with its known electrochemical performance. Comparison of the voltage versus capacity profiles and the cycling performance reveals that a higher extraction and reversible capacity can be obtained by means of increasing the upper cut-off voltage or by changing the cycling mode (from constant-current (CC) to constant-current–constant-voltage (CCCV)) at 25°C or by increasing the temperature to 50°C for V >4.2V versus Li metal, but this leads to capacity-fading over 50 cycles. A minimum in the Li-ion diffusion coefficient (D Li) versus voltage curve was observed at ∼3.8V coinciding with the plateau region in the charge–discharge profiles. The D Li is almost unchanged with a value of 3.0(±0.5)×10−10 cm2/s in the voltage range, 3.8–4.3V versus Li. EIS studies as a function of cycle number after charging the cells to ∼3.8 and 4.4V, after they were cycled in the CC and CCCV modes show that while the surface film and the bulk contribution of the cell resistances remain small and unchanged, a significant increase in the charge transfer resistance (R ct) occurs, revealing a pronounced electrode polarization. This polarization may be the reason for the observed capacity-fading in Li(Ni1/2Mn1/2)O2 when charged to voltages >4.2V.