An advanced model is proposed, describing the capacity losses of C 6 /LiFePO 4 batteries under storage and cycling conditions. These capacity losses are attributed to the growth of a Solid Electrolyte Interface (SEI) at the surface of graphite particles in the negative electrode. The model assumes the existence of an inner and outer SEI layer. The rate determining step is considered to be electron tunneling through the inner SEI layer. The inner SEI layer grows much slower than the outer SEI layer. Another contribution to the degradation process is the exfoliation of SEI near the edges of graphite particles during discharging and the formation of new SEI induced by the volumetric changes during the subsequent charging. The model has been validated by storage and cycling experiments. The simulation results show that the capacity losses are dependent on the State-of-Charge (SoC), the storage time, cycle number and graphite particle size. The model can be used to predict both the calendar and cycling life of the Li-ion batteries. Li-ion batteries, based on the C 6 /LiFePO 4 chemistry, attracts nowadays much attention for application in, for example, electric vehicles due to the excellent cycling stability of the LiFePO 4 electrode. The main electrochemical storage reactions of this battery type can be represented byDuring charging electrons and Li + ions are extracted from the LiFePO 4 (0 ≤ x ≤ 1) electrode and flow into the graphite electrode. The reverse reactions take place during discharging. The structure of LiFePO 4 (LFP) is highly robust due to the strong bonding between the P and O atoms. LFP batteries therefore combine a good cycling stability with excellent thermal stability and long lifespan.1-5 The volume decreases only 6.8% 6 when all Li ( x = 1) is extracted and these small volumetric changes do not influence the structure of LiFePO 4 and FePO 4 . Favorably, it almost fully counterbalances the volumetric changes of the graphite electrode during Li (de)insertion.Due to these favorable properties, the capacity losses caused by the LFP cathode are generally assumed to be negligible. Furthermore, Broussely et al. showed that the capacity of the graphite electrode can be fully recovered even after more than 1 year storage under 'floating' conditions at 60• C, demonstrating that the stability of the carbon electrode is also excellent.7 However, capacity losses are still found in LFP batteries. It is generally accepted, that the formation of a Solid Electrolyte Interface (SEI) is one of the main reasons for the capacity losses of graphite-based Li-ion batteries, especially under moderate conditions during the initial stages of aging.The SEI plays a dual-role in the performance of Li-ion batteries. On the one hand, it protects the negative electrode from solvent cointercalation, preventing exfoliation of the graphene layers. On the other hand, it consumes cyclable lithium inside the battery, which is therefore no longer available for the energy storage process and hence * Electrochemical Society Active Member...

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