Lithium-rich metal oxides Li 1+z MO 2 (M = Ni, Co Mn, etc) are promising positive electrode materials for high-energy lithium-ion batteries, with capacities of 250-300 mAh.g -1 that closely approach theoretical intercalation limits. Unfortunately, these materials suffer severe capacity fade on cycling, amongst other performance issues. Whilst ion substitution can improve the performance of many of these materials, the underlying mechanisms of property modification are not completely understood. In this work we show enhanced performance of the Li 1+z MO 2 electrode, consisting of Li 2 MnO 3 (with C2/m space group) and LiMO 2 (with R3m space group) phases, and establish the effects of cationic and anionic substitution on the phase and structure evolution underpinning performance changes. Whist the undoped material has a high capacity of ~ 270 mAh.g -1 , only 79% of this remains after 200 cycles. Including ~ 2% Cr in the material, likely at the R3m metal (3a) site, improved cycle performance by ~ 13% and including ~ 5% F in the material, likely at the R3m oxygen (6c) site, enhanced capacity by ~ 4-5% at the expense of a ~ 12% decline in cycle performance. Moreover, Cr doping enhances energy density retention by ~ 13% and F doping suppresses this by 17%. We find that these changes arise by different mechanisms. Both anionic and cationic substitution promote faster Li diffusion, by 48% and 20%, respectively, as determined using cyclic voltammetry and leading to better rate performance. Unlike anionic substitution, cationic substitution enhances structural stability at the expense of some capacity, by suppressing lattice distortion during Li insertion and extraction. This work implicates strategic cationic-anionic co-doping for enhanced electrochemical performance in lithium-rich layered metal-oxide phases.