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

Abstract

Substituted Li-layered transition-metal oxide (LTMO) electrodes such as Li_<i>x</i>Ni_<i>y</i>Mn_<i>z</i>Co_{1-<i>y</i>-<i>z</i>}O₂ (NMC) and Li_<i>x</i>Ni_<i>y</i>Co_{1-<i>y</i>-<i>z</i>}Al_<i>z</i>O₂ (NCA) show reduced first cycle Coulombic efficiency (90-87% under standard cycling conditions) in comparison with the archetypal Li_<i>x</i>CoO₂ (LCO; ∼98% efficiency). Focusing on Li_<i>x</i>Ni_0.8Co_0.15Al_0.05O₂ as a 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 <i>x</i> > 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 cutoff voltage during delithiation and 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 <i>x</i> > 0.88. ⁷Li NMR measurements indicate that the kinetic limitation reflects limited Li transport at <i>x</i> > 0.86. Electrochemical measurements on a wider range of LTMOs including Li_<i>x</i>(Ni,Fe)_<i>y</i>Co_1-<i>y</i>O₂ 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 nonmetallic LTMOs in general.