Experiments, numerical simulations, and analytical calculations are used to calibrate a diffuse-interface Cahn-Hilliard model of Li-Sn thin film electrodes that experience a transformation from Sn to Li2Sn5. It is shown that a concentration-gradient dependent mobility can be used in the Cahn-Hilliard equation to give the interface a finite mobility and capture its nonequilibrium behavior. Comparing experiments and simulations, the free-energy of Li-Sn, diffusivity of Li in Sn and Li2Sn5, the exchange current density for the surface reaction, and the mobility of the Sn/Li2Sn5 interface are extracted. The implications of finite interface mobility for practical battery electrodes are discussed. (C) 2016 The Electrochemical Society. All rights reserved.