High-power Li-ion cells stored or cycled at elevated temperatures showed a significant impedance rise and power loss associated primarily with the LiNi0.8Co0.2O2 cathode (positive electrode). The processes which led to this impedance rise were assessed by investigating the cathode surface electronic conductance, surface structure, and composition at the microscopic level with local probe techniques. Current-sensing atomic force microscopy imaging revealed that the cathode surface electronic conductance diminished significantly in the tested cells, and that the rate of change of the electronic conductance increased with cell test temperature. Raman microscopy measurements provided evidence that surface phase segregation of nickel oxides is at least partially responsible for the observed cathode impedance rise. (C) 2002 The Electrochemical Society.