Lithium-ion batteries (LIBs) are increasingly popular for electric vehicle and grid storage applications, but degradation mechanisms remain poorly understood and are difficult to characterize accurately. Here a new method for in situ monitoring of internal degradation in LIBs using an electrochemical model (EM) is introduced. The main contributions of this work can be summarized as follows: (1) An EM is developed based on a single particle model, the parameters of which are reorganized from the original physical property parameters for convenience of identification, with each new parameter assigned a specific physical meaning. (2) Identification methods for all parameters are determined through activation-and-response analysis, and a combined load profile for parameter identification is developed. (3) Cyclic aging experiments are carried out and the aforementioned model and parameter identification methods are applied at appropriate intervals. The predicted trends of 7 (internal resistance included) of the 10 model parameters show strong correlations with the experimentally observed degradation.