Accurate online state-of-charge (SoC) estimation is a basic need and also a fundamental challenge for battery applications. In order to achieve accurate SoC estimation for the lithium-ion batteries, we employ a coupled thermal-electrochemical model. This coupled system of an ordinary differential equation (ODE) and a partial differential equation (PDE) is simpler than the Doyle-Fuller-Newman (DFN) model, and is more accurate than the single particle model (SPM) alone. Thus, it could serve as a better fit of model for a full state observer design and accurate SoC estimation. PDE backstepping approach is utilized to develop a Luenberger observer for the electrode concentration, and estimation effectiveness of the proposed method is verified by simulation results.