Understanding the behavior of lithium-ion batteries exposed to thermal excursion is of great interest to plug-in hybrid electric vehicle (PHEV) applications, because vehicles often endure wide weather conditions in operation. Here we investigate a composite {LixMn(2)O(4) + LixNi(1/3)Mn(1/3)Co(1/3)O(2)}-based commercial cell design to assess performance and degradation under thermal excursion from 25 degrees C; through -20 degrees C, -5 degrees C, 10 degrees C, 25 degrees C, 40 degrees C, and 60 degrees C; to 25 degrees C. In each isothermal regime, a reference performance test with charge and discharge cycles at C/25, C/5, C/2, 1C, and 2C is conducted to quantify cell capacity, rate capability, and other performance variations. The capacity fade caused by the thermal excursion is attributed to origins including loss of active material, degradation in reaction kinetics, and the ohmic resistance increases. Using electrochemical inference techniques, we found that thermal excursion in the range of -5 degrees C to 40 degrees C is benign to capacity fade. Exposure to -20 degrees C and 60 degrees C respectively leads to irreversible fade. The capacity fade at -20 degrees C induced Li inventory loss and did not cause kinetic degradation, whereas the exposure at 60 degrees C resulted in degradation in reaction kinetics. The evaluation protocols and results are helpful in assisting the study of path dependence of cell degradation in thermal aging. (C) 2012 The Electrochemical Society. [DOI:10.1149/2.063301jes] All rights reserved.