Intrinsic Kinetic Limitations in Substituted Lithium Layered Transition-Metal Oxide Electrodes.

Substituted Li layered transition metal oxide (LTMO) electrodes such as LixNiyMnzCo1-y-zO2 (NMC) and LixNiyCo1-y-zAlzO2 (NCA) show reduced first cycle Coulombic efficiency (90-87 % in standard cycling conditions) compared with archetypal LixCoO2 (LCO - ~98 % efficiency). Focusing on LixNi0.8Co0.15Al0.05O2 as model compound, we use operando synchrotron x-ray diffraction (XRD) and nuclear magnetic resonance (NMR) spectroscopy to demonstrate that the apparent first cycle capacity loss is a kinetic effect linked to limited Li mobility at x > 0.88, with near full capacity recovered during a potentiostatic hold following the galvanostatic charge-discharge cycle. This kinetic capacity loss, unlike many capacity losses in LTMOs, is independent of the cut-off voltage during delithiation-it is a reversible process. The kinetic limitation manifests not only as the kinetic capacity loss during discharge, but as a subtle bimodal compositional distribution early in charge and, also, a dramatic increase of the charge-discharge voltage hysteresis at x > 0.88. 7Li NMR measurements indicate that the kinetic limitation reflects limited Li transport at x > 0.86. Electrochemical measurements on a wider range of LTMOs including Lix(Ni,Fe)yCo1-yO2 suggest that 5 % substitution is sufficient to induce the kinetic limitation, and that the effect is not limited to Ni substitution. We outline how, in addition to a reduction in the number of Li vacancies and shrinkage of the Li-layer size, the intrinsic charge storage mechanism (two-phase vs. solid-solution), and localization of charge give, rise to additional kinetic barriers in NCA and non-metallic LTMOs in general.