High-rate performances of a lithium-ion battery after storage at elevated temperature are investigated electrochemically by means of three-electrode system. The high-rate capability is decreased significantly after high-temperature storage. A 3C discharge capacities after room-temperature storage and 60°C storage are 650 and 20mAh, respectively. Lithium-ion diffusion in lithium cobalt oxide cathode limits the battery's capacity and the results show that storage temperature changes this diffusion behavior. Transmission electron microscopy (TEM) images show that many defects are directly observed in the cathode after storage compared with the fresh cathode; the structural defects block the diffusion within the particles. Electrochemical impedance and polarization curve indicate that mass-transfer (diffusion) dominates the discharge capacity during high-rate discharge.