The investigation of the aging and degradation mechanism of lithium ion batteries in automotive and energy storage applications is of particular importance for the acceptance of the battery technology. However, several factors interact to generate complicated battery aging phenomena, thus leading to limited accuracy of the established models when applied to degradation prediction under complex real running conditions. Here, a coupled electrochemical-thermal-mechanical model is presented for the degradation investigation of lithium-ion batteries. The model includes both side reactions on anode and the loss of active material of cathode and is employed to study the aging behavior of the battery applying different C-rates and ambient temperatures. Simulation results indicate that the aging of the battery is dominated by various aging factors under different operating conditions. Higher ambient temperature can accelerate SEI formation reaction, while low temperature can cause severe lithium-plating. Active material loss is affected by cycling current significantly and becomes the dominant aging factor under extremely high C-rate. The model is fitted under two accelerated aging cycles and agrees well with the experimental results.