Multi-physical modeling of the graphite electrode within a lithium-ion battery: Study of heterogeneities and aging mechanisms Nicolas Dufour

 SUMMARY

The negative electrode of lithium-ion batteries is commonly made of graphite. Although having an interesting specific capacity, aging, intercalation kinetics and lithium transport both in the active material and the electrode porosities limit its optimal and homogeneous operation. In this thesis work, the mechanisms behind these limits are explained using a multi-physics model of the porous electrode type. A sensitivity study of the model parameters showed the importance of the parameters related to intercalation kinetics and lithium transport in solid and liquid phases. The exploitation of the model, validated experimentally, shows that, during the operation of the electrode, the appearances of lithiation heterogeneity are correlated with the particular shape of the equilibrium potential of graphite with respect to its lithiation rate. The modeling of the particle size distribution greatly amplifies these heterogeneities and significantly degrades the overall performance of the electrode. As a first approach, an operando measurement of the distribution of lithiation states confirms the heterogeneous aspect of the electrode operation. Data on the cycling and calendar performances of graphite-NMC cells have allowed the construction of different electrode aging models. The growth of the passivation layer (SEI) can alone explain the loss of cyclable lithium. The SEI heterogeneities obtained by the model are negligible in the current state. The capacity gains and sudden losses are explained respectively by SEI dissolution and lithium-plating formation mechanisms.

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