Kinetic Limitations in Single‐Crystal High‐Nickel Cathodes

Abstract

High-nickel cathodes attract immense interest for use in lithium-ion batteries to boost Li-storage capacity while reducing cost. For overcoming the intergranular-cracking issue in polycrystals, single-crystals are considered an appealing alternative, but aggravating concerns on compromising the ionic transport and kinetic properties. We report here a quantitative assessment of redox reaction in single-crystal LiNi 0.8 Mn 0.1 Co 0.1 O 2 using operando hard X-ray microscopy/ spectroscopy, revealing a strong dependence of redox kinetics on the state of charge (SOC). Specifically, the redox is sluggish at low SOC but increases rapidly as SOC increases, both in bulk electrodes and individual particles. The observation is corroborated by transport measurements and finite-element simulation, indicating that the sluggish kinetics in singlecrystals is governed by ionic transport at low SOC and may be alleviated through synergistic interaction with polycrystals integrated into a same electrode.

Origin of Rapid Delithiation In Secondary Particles Of LiNi0.8Co0.15Al0.05O2 and LiNiyMnzCo(1-y-z)O2 Cathodes

“…This behavior was observed experimentally by two independent mapping techniques (µ-XRD and TXM), with very different energies of the X-ray beam (≈8 keV for TXM vs 25 keV for µ-XRD). The persistence of the phenomena suggests that the behavior is inherent to layered cathode materials rather than being an artifact of the techniques, and is consistent with previous reports using other techniques on related LiNi y Mn z Co 1 -y -z O 2 and LiCoO 2 layered materials [13,15,16].…”

“…The evolution of Li content in the particles is unique to experiments by µ-XRD and adds new important insight to literature observations by powder X-ray diffraction (i.e. averaging at the ensemble) and spectromicroscopy [3,4,5,6,7,8,9,10,11,12,13,14,15,16]. For both P1 and P2, the rate of delithiation, estimated from the slope in Figure 1e, was initially low, proceeding at ≈0.025 Li/h, or C/40.…”

“…Therefore, experimental observations of rapid delithiation in such electrodes suggest that the behavior is governed by material properties rather than electrode morphology. Additionally, several studies instead attributed the rapid delithiation to limited diffusion of Li in the cathode active material [15,16]. Their models and those described above (our model with the step form of D s and the model presented by Park et al [13]) predict spatial heterogeneity within secondary particles in the diffusion limited case [16].…”

“…While impurities forming blocking surface layers help explain a seemingly two-phase transformation [11], limitations arising from diffusion kinetics within the material have also been proposed [12]. Recently, this apparent two-phase behavior in layered oxides has been connected to individual particles delithiating rapidly and at seemingly random times [13,14,15,16], however conflicting explanations for this behavior are presented. Park et al [13] used in-situ PXRD to indirectly study the transformation kinetics of LiNi 1/3 Mn 1/3 Co 1/3 O 2 and showed that interfacial exchange current is the underlying reason for the observed pseudo-two-phase behavior rather than the solid-state diffusion limitation.…”

“…In separate work with LiNi 0.5 Mn 0.3 Co 0.2 O 2 particles, [14] direct observations of asynchronous delithiation were attributed to spatial inhomogeneity in the electrostatic potential caused by limitations in electrical conductivity due to the incomplete coverage of the particles by carbon additives. Lastly, Merryweather et al [15] and Ge et al [16] ascribed the sudden changes in rates of delithiation in different layered oxides to sluggish solid-state transport in the Lirich phase. Clearly, there is no clear consensus on the origin of this rapid delithiation behavior in layered cathode particles.…”