A new model, describing the capacity loss of C6/LiFePO4 batteries under open-circuit storage conditions has been developed. De-gradation is attributed to the formation of a Solid Electrolyte Interface (SEI) at the surface of the graphite particles in the negative electrode. The model takes into account that the Solid Electrolyte Interface consists of an inner and outer SEI layer. The rate determining step of the SEI formation process is proposed to be electron tunneling through the inner SEI layer. The simulation results demonstrate that the capacity loss is dependent on the State-of-Charge (SoC), the electrode potential and storage time. The inner SEI layer was found to grow much slower than the outer SEI layer.