Surface degradation of Li_1–xNi_0.80Co_0.15Al_0.05O₂ cathodes: Correlating charge transfer impedance with surface phase transformations

The pronounced capacity fade in Ni-rich layered oxide lithium ion battery cathodes observed when cycling above 4.1 V (versus Li/Li⁺) is associated with a rise in impedance, which is thought to be due to either bulk structural fatigue or surface reactions with the electrolyte (or combination of both). Here, we examine the surface reactions at electrochemically stressed Li_1–xNi_0.8Co_0.15Al_0.05O₂ binder-free powder electrodes with a combination of electrochemical impedance spectroscopy, spatially resolving electron microscopy, and spatially averaging X-ray spectroscopy techniques. We circumvent issues associated with cycling by holding our electrodes at high states of charge (4.1 V, 4.5 V, and 4.75 V) for extended periods and correlate charge-transfer impedance rises observed at high voltages with surface modifications retained in the discharged state (2.7 V). The surface modifications involve significant cation migration (and disorder) along with Ni and Co reduction, and can occur even in the absence of significant Li₂CO₃ and LiF. These data provide evidence that surface oxygen loss at the highest levels of Li⁺ extraction is driving the rise in impedance.