Our work presents a convenient way to capture inhomogeneous lithium distribution along the in-plane dimension of graphite electrodes. The methodology presented determines post-mortem the different grades of lithiation (or state-of-charge, SOC) spatially via diffusive reflectance spectroscopy (DRS).
To create a DRS-spectra to SOC map in a first step, graphite active material is harvested from a NCR18650B cylindrical cell and used to build coin cells from it. After initial test, the coin cells are charged to specific SOCs and disassembled to access the electrode coins. The DRS spectra of the different lithiated graphite coins are captured and further used to build a model which uses the spectra to estimate SOC.
In a second step a fully charged cylindrical cell is discharged and subsequently disassembled to harvest the electrode sheets. The sheets DRS spectra is captured in a grid with a resolution of 1 cm x 1 cm and the spatial SOC determined through the model. Finally, spatial effects like the anode overhang effects are clearly visible. This highlights the potential of diffuse reflectance spectroscopy for the determination of spatial inhomogeneity in lithium concentration of active material.