Li-rich layered transition metal oxides with the nominal composition of Li1+x [Li0.2Mn α Co β Ni γ ]O2 with 0.0 ≤ x ≤ 0.7 are synthesized using a high-pressure technique. The chemical compositions are determined using inductively coupled plasma (ICP) spectroscopy and the structure is identified from X-ray diffraction measurements as a mixture of rhombohedral R-3m and cubic Fm-3m phases. Rietveld refinement analysis reveals that the layered to cubic phase ratio increases as x tends towards 0.7, with an almost complete cubic phase present for x ≥ 0.6. Electrochemical charge–discharge experiments performed using coin cells and show capacities of around 200 mAh g−1 for 0.0 ≤ x ≤ 0.4 samples, although the capacity quickly decreases for x ≥ 0.5, which is consistent with the trend towards a predominantly cubic phase. The plateau for oxygen generation, which appears at the 4.4 V threshold, is reduced as x increases.