Lawrence Berkeley National Laboratory

High-power Li-ion cells that were tested at elevated temperatures showed a significant impedance rise, which was associated primarily with the LiNi0.8Co0.15Al0.05O2 cathode. By systematically collecting thousands of Raman spectra from 50 x 80 mm areas at 0.9 mm spatial resolution, and integrating the respective bands of the cathode components for each spectrum, we were able to produce color-coded, semi-quantitative composition maps of cathode surfaces. Raman microscopy images of cathodes from tested cells revealed that cell cycling or storage at elevated temperatures led to significant changes in the LiNi0.8Co0.15Al0.05O2/elemental-carbon surface concentration ratio. The loss of conductive carbon correlated with the power and capacity fade of the tested cathodes. The cathode surface state of charge (SOC) varied between individual grains of active material, and at some locations the spectra indicated the presence of fully charged material, despite the deep cell discharge at the end of testing.