Layered oxides are widely used as cathode materials in lithium-ion batteries due to their high energy density and structural stability. Conventionally, these materials are considered single-phase systems that undergo reversible lithiation and delithiation without undergoing phase transitions, especially when cycled under conditions near equilibrium. This understanding is supported by monotonic Nernst potential profiles and X-ray diffraction (XRD) data from equilibrated samples. However, recent operando XRD studies on porous, multi-particle electrodes have revealed apparent phase separation during delithiation, particularly at high rates. Notably, this phenomenon is not observed during lithiation or subsequent cycles, leading to the prevailing interpretation that it is a first-cycle effect, possibly due to irreversible surface passivation or slow lithium diffusion near full lithiation.
Despite these explanations, inconsistencies remain. The observed behavior—such as rate and path dependence, and particle-by-particle variations in lithium content—cannot be fully explained by diffusion limitations alone. Moreover, the fact that the phenomenon persists beyond the first cycle in various compositions challenges the notion of a one-time transient effect. To resolve this paradox, we investigate the origin of this so-called “fictitious” phase separation using a combination of operando XRD, nanoscale X-ray microscopy, and a population-dynamics model grounded in electro-autocatalysis.
Our experiments focus on Lix(Ni1/3Mn1/3Co1/3)O2 (NMC111), a representative layered oxide, as well as Ni-rich (LiNi0.HAS1 Antibody Epigenetics 5Mn0.3Co0.2O2, NMC532) and Li/Mn-rich (Li1.17Ni0.21Mn0.54Co0.08O2, LMR-NMC) variants. We prepare composite electrodes with both agglomerate and platelet particles to decouple effects related to morphology.HRP-conjugated Goat Anti-Human IgG Fc Purity By using low active material loading and thin electrodes (~20 µm), we minimize transport limitations.PMID:35059944 Operando XRD measurements during second and tenth cycles reveal a clear dichotomy: fast delithiation induces a bifurcation of the (003) Bragg peak, while slow cycling or lithiation results in continuous, linear shifts consistent with single-phase behavior. This asymmetry persists across multiple cycles and compositions, indicating a fundamental kinetic mechanism rather than a transient artifact.
To probe the underlying cause, we perform scanning transmission X-ray microscopy (STXM) on quenched electrodes. After fast delithiation, we observe a non-unimodal distribution of lithium composition across particles—many remain nearly lithiated (red), while others are fully delithiated (green)—despite uniform average lithium fractions. In contrast, slow cycling and lithiation yield unimodal distributions. This inter-particle heterogeneity, confirmed by diffraction on quenched samples, rules out intra-particle diffusion gradients as the source of the apparent phase separation.
We propose that the observed behavior arises from an autocatalytic reaction mechanism: the interfacial exchange current increases with the extent of delithiation. During fast delithiation, reacted particles experience a higher driving force for further reaction, accelerating their progress and creating a feedback loop that amplifies compositional differences between particles. This leads to a bimodal ensemble distribution—a “fictitious” two-phase state—without any actual thermodynamic phase transition. Conversely, during lithiation, the reverse process acts autoinhibitory: unreacted particles catch up, suppressing inhomogeneity.
This hypothesis is validated through a reaction-limited population-dynamics model. Using Bayesian model selection and inverse solving of a Fokker-Planck equation based on multi-stream data (XRD, STXM, electrochemistry), we extract a composition-dependent exchange current j0 that increases exponentially near full lithiation. This functional form, consistent across multiple compositions and measurement techniques, confirms the autocatalytic nature of the kinetics.
The findings challenge long-standing assumptions in battery science. First, they demonstrate that inter-particle inhomogeneity in single-phase materials is primarily governed by reaction kinetics—not diffusion. Second, they show that even non-phase-separating materials can exhibit metastable, pseudo-phase-separated states under fast driving conditions. Third, they establish that ensemble stability is not determined solely by equilibrium thermodynamics but depends critically on kinetic pathways and autocatalytic feedback.
In practical terms, this work suggests that controlling reaction rates—particularly avoiding high C-rates near full lithiation—can suppress fictitious phase separation and improve cycle life. It also highlights the need to reevaluate standard models of electrode behavior, which often assume equilibrium or diffusion-limited dynamics. Ultimately, our study reveals that electro-autocatalysis plays a central role in shaping the dynamic response of battery electrodes, offering new design principles for next-generation materials.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com
