The increasing demand for portable electronic devices is driving the development of compact lightweight batteries of high energy density(1). Lithium-ion batteries tend to be the systems of choice, as they offer higher energy densities and longer operational lifetimes than other rechargeable battery systems(1,2). But commercially available lithium-ion batteries make use of layered LiCoO2 cathodes(3,4), and the high cost and toxicity of cobalt therefore motivate the development of cheaper and environmentally benign cathode materials. In this regard, manganese oxides are attractive alternatives, and the spinel LiMn2O4 has been investigated intensively as a cathode(5,6); however, the fading on cycling of its energy-storage capacity poses problems, More recently, attention has been focused on the synthesis of layered LiMnO2 as a cathode material, but its cycling characteristics remain to be established(7-9). Here we report the synthesis and electrochemical performance of a new manganese oxide cathode, the oxyiodide Li1.5Na0.5MnO2.85I0.12. Our material exhibits a high reversible capacity of 260 mA h g(-1) in the range 1.5-4.3 V with excellent cycling characteristics. Furthermore, the amorphous nature of the material (as determined by X-ray diffraction) and smooth discharge behaviour may help to overcome the problems associated with lattice distortions that have plagued manganese oxides with more crystalline structures(5-9).