Combining operando X-ray experiments and modelling to understand the heterogeneous lithiation of graphite electrodes
Résumé
Heterogeneous potential and lithium distribution inside lithium ion batteries highly affect their performance
and durability. In particular, increased lithium gradients are expected in thick electrodes with high energy
densities or cycling at high currents. To optimise electrodes and cells designs, it is thus crucial to predict
and probe the local lithium concentration across the depth of the electrode, as a function of the
electrode material properties. Here, we follow the lithium distribution across a 80 mm thick porous
graphite electrode using a 1 mm focused synchrotron X-ray beam during a complete delithiation. The
sequential formation of the individual LixC6 phases is extracted from X-ray diffraction patterns, allowing
the quantification of lithium concentration across the electrode thickness and its heterogeneities. We
report a striking pattern, with strong heterogeneities at low stoichiometries, even at C/5, and a much
more homogeneous Li distribution during the stage 1 / 2 transition. A porous electrode model can
actually capture the former but fails at predicting the latter. Revisiting the model, we could match the
experimental tendencies by considering that the kinetics of lithium (de)intercalation is reduced during
the LiC6/LiC12 phase transition compared to the other transitions. By combining modelling and operando
X-rays characterisation, we therefore establish that the main competing forces in graphite are distinctly
balanced depending on the lithiation stage.