The crystal structure and electrochemical performance of Li ion cathodes doped and co-doped with Mg and F (Li1.167Mn0.548-xMgxNi0.18Co0.105O2-yFy where x = 0, 0.02 and/or y = 0, 0.02) was investigated. The effects of doping and co-doping were measured with XRD, XPS, and galvanostatic charge/discharge tests. The lattice constant and cell volume decreased when doped with Mg, while they increased when doped with F. The change in local structure was due to changes in the oxidation states of the transition metals. Mg doping decreases the Ni2+/Ni3+ ratio and increases the Mn3+/Mn4+ ratio, while F doping increases the Ni2+/Ni3+ ratio. The altered oxidation states of Ni and Mn also influence the electrochemical performance. Mg-and F-doped cathodes showed increased initial discharge capacity (253 mA h g(-1)) and cycle performance (95%) respectively when compared to the undoped performance (248 mA h g(-1) and 86%). However, Mg and F doping deteriorates cycle performance and lowers discharge capacity respectively. While Mg and F doping had both positive and negative aspects, it was found that co-doping caused synergistic behavior, with the dopants offsetting the changes in local structure. This increased the discharge capacity (259 mA h g-1) and improved cycle performance (90%).