Diagnostic analysis of electrodes from high-power lithium-ion cells cycled under different conditions

Pouch-type lithium-ion cells, with graphite and LiNi_0.8 Co_0.15 Al_0.05 O₂, were cycled over different ranges of depth of discharge (DOD) and at different temperatures. A combination of electrochemical, physical, and chemical diagnostic evaluations, including Raman, nuclear magnetic resonance (NMR), and Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), energy-dispersive X-ray, and X-ray diffraction (XRD), were carried out on the components removed from the cells to form a clear picture of the mechanism for capacity and power fade in this important cell chemistry. Both capacity fade and impedance rise were found to increase with cycling temperature and the span of DOD. The loss of cycleable Li, most likely due to solvent reduction on the anode, was linear in cell test time, and the room-temperature cells showed a solid electrolyte interface composition and degree of graphite disorder that roughly correlated with the extent of Li consumption. However, electrochemical analysis showed that the cathode was controlling performance loss in the cell. TEM and NMR showed evidence of crystalline defects and degradation of the Li-Ni environment, respectively, though no major new phases were identified, in agreement with the XRD results. FTIR analysis of the cathode revealed no evidence of polymeric deposits on the cathode particles, although both Raman and TEM showed evidence of P-containing deposits. Raman mapping showed a noticeable change of the active material/carbon surface concentration ratio for the cathode cycled 1000 times at 100% DOD as compared with that cycled to 70% DOD.