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

• Published in: Journal of the American Chemical Society 2020-04, Vol.142 (15), p.7001-7011
• Main Authors: Grenier, Antonin, Reeves, Philip J, Liu, Hao, Seymour, Ieuan D, Märker, Katharina, Wiaderek, Kamila M, Chupas, Peter J, Grey, Clare P, Chapman, Karena W
• Format: Article
• Language: English
• Subjects: Battery; ENERGY STORAGE; Layered Oxide
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.9b13551

Substituted Li-layered transition-metal oxide (LTMO) electrodes such as Li x Ni y Mn z Co1–y–z O2 (NMC) and Li x Ni y Co1–y–z Al z O2 (NCA) show reduced first cycle Coulombic efficiency (90–87% under standard cycling conditions) in comparison with the archetypal Li x CoO2 (LCO; ∼98% efficiency). Focusing on Li x Ni0.8Co0.15Al0.05O2 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) y Co1–y 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.