IntrinsicKinetic Limitations in Substituted Lithium-LayeredTransition-Metal Oxide Electrodes
2020
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% under standard cycling conditions) in comparison with
the archetypal LixCoO2 (LCO;
∼98% efficiency). Focusing on LixNi0.8Co0.15Al0.05O2 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 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 nonmetallic LTMOs in general.
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