A Li/Li1.27Cr0.2Mn0.53O2 cell is found to deliver an excellent discharge capacity of around 260mAhg−1, but exhibits a continuous increase in capacity on extended cycling. To explain this latter behaviour, various electrochemical measurements and ex situ X-ray diffraction (XRD) are performed. Both cyclic voltammetry and ex situ XRD reveal that an accumulation of transition metal atoms (Cr or Mn) in the lithium layer with cycling results in a monoclinic phase and capacity increase. Studies using ac impedance spectroscopy reveal that the structural change mainly occurs during the charging process.