Rate capability of Li-excess Mn-based layered cathode materials is improved by treating with (NH4)2SO4. After treatment, discharge capacity of as high as 230 mAh/g can be obtained at a rate of 300 mA/g (∼1.2C). The improvement is attributed to the modification of the surface of the layered material into a spinel-like structure with the treatment, as suggested by Raman spectroscopy and electrochemical charge-discharge. X-ray diffraction results show no change in bulk lattice parameters, indicating that the structural modification is only on the surface of the active material. Chemical analyses show that both lithium and oxygen are extracted from the active material, supporting the formation of a surface spinel layer with the treatment. © 2010 The Electrochemical Society.