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

Substituted Li-layered transition-metal oxide (LTMO) electrodes such as Li_xNi_yMn_zCo_1-y-zO₂ (NMC) and Li_xNi_yCo_1-y-zAl_zO₂ (NCA) show reduced first cycle Coulombic efficiency (90-87% under standard cycling conditions) in comparison with the archetypal Li_xCoO₂ (LCO; ∼98% efficiency). Focusing on Li_xNi_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 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 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 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 Li_x(Ni,Fe)_yCo_1-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 nonmetallic LTMOs in general.