Surface degradation of Li1–xNi0.80Co0.15Al0.05O2 cathodes: Correlating charge transfer impedance with surface phase transformations

Abstract

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 Li1–xNi0.8Co0.15Al0.05O2 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.

Synchrotron‐Based X‐ray Absorption Fine Structures, X‐ray Diffraction, and X‐ray Microscopy Techniques Applied in the Study of Lithium Secondary Batteries

“…The reduction of Co at the surface results in an initial degradation to the capacity. Moreover, Co and Ni L 3,2 ‐edge XANES spectra of LiNi 0.8 Co 0.15 Al 0.05 O 2 also confirm the reduction of elements in electrodes materials following electrolyte contact and results in a cationic disordered phase near the surface and seriously impedes charge transfer . As the side reaction at the interface can be probed by XAFS, related challenges can be overcome by coating with a passivation layer prior to contact with the electrolyte.…”

“…During electrochemical cycling, valence states as well as the local geometric environment undergo changes due to the diffusion of lithium and other elements in the material. Based on this theoretical prediction, lithium secondary batteries have been intensively studied with the use of XAFS to find out what happens during the process, including electrochemical reaction, formation of interfaces between electrodes and electrolytes . In this section, the study of electrochemical reactions as well as degradation mechanisms of lithium secondary batteries will be introduced and summarized with a focus on the application of XAFS.…”

A Comprehensive Analysis of the Interphasial and Structural Evolution over Long‐Term Cycling of Ultrahigh‐Nickel Cathodes in Lithium‐Ion Batteries

“…[61][62][63][64][65][66] AEI Thickness, Active-Li Trapping, and Transition-Metal Dissolution: The quantification of AEI thickness, which reflects the degree of anode degradation, has been a daunting challenge due to its air sensitivity, vulnerability, and the limitation of techniques available. Initially, a "healthy" graphite AEI is featured with a thin mono-layer architecture.…”

Editors' Choice—Growth of Ambient Induced Surface Impurity Species on Layered Positive Electrode Materials and Impact on Electrochemical Performance

“…6,[19][20][21][22] It has been proposed that the transition metal reduction from a valence state of +3 to +2 produces active oxygen species which concurrently catalyzes the formation of * Electrochemical Society Member.…”