Combining operando X-ray experiments and modelling to understand the heterogeneous lithiation of graphite electrodes

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 μm thick porous graphite electrode using a 1 μm 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.