Surface impurity species, most notably Li 2 CO 3 , that develop on layered oxide positive electrode materials with atmospheric aging have been reported to be highly detrimental to the subsequent electrochemical performance. LiNi 0.8 Co 0.15 Al 0.05 O 2 (NCA) was used as a model layered oxide compound to evaluate the growth and subsequent electrochemical impact of H 2 O, LiHCO 3 , LiOH and Li 2 CO 3 . Methodical high temperature annealing enabled the systematic removal of each impurity specie, thus permitting the determination of each specie's individual effect on the host material's electrochemical performance. Extensive cycling of exposed and annealed materials emphasized the cycle life degradation and capacity loss induced by each impurity, while rate capability measurements correlated the electrode impedance to the impurity species present. Based on these characterization results, this work attempts to clarify decades of ambiguity over the growth mechanisms and the electrochemical impact of the specific surface impurity species formed during powder storage in various environments.

Enabling practical utilization of layered R3̅m positive electrodes near full delithiation requires an enhanced understanding of the complex electrode-electrolyte interactions that often induce failure. Using Li[NiCoAl]O (NCA) as a model layered compound, the chemical and structural stability in a strenuous thermal and electrochemical environment was explored. Operando microcalorimetry and electrochemical impedance spectroscopy identified a fingerprint for a structural decomposition and transition-metal dissolution reaction that occurs on the positive electrode at full delithiation. Surface-sensitive characterization techniques, including X-ray absorption spectroscopy and high-resolution transmission electron microscopy, measured a structural and morphological transformation of the surface and subsurface regions of NCA. Despite the bulk structural integrity being maintained, NCA surface degradation at a high state of charge induces excessive transition-metal dissolution and significant positive electrode impedance development, resulting in a rapid decrease in electrochemical performance. Additionally, the impact of electrolyte salt, positive electrode surface area, and surface LiCO content on the magnitude and character of the dissolution reaction was studied.

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.

The emergence of oxidized oxygen RIXS features at high voltages for Ni-rich layered oxide cathodes.