On the role of crack electrolyte wetting in the degradation and performance of battery active particles

Abstract: Cathode particle fracture is widely recognised as a major degradation mechanism in lithium-ion batteries, yet cracking also permits electrolyte wetting of newly exposed internal surfaces, modifying interfacial reaction pathways. The mechanistic role of electrolyte wetting in redistributing reactions within cracked particles remains unclear. Here, we isolate this effect through a controlled comparison between (i) a fully coupled electro-chemo-mechanical model resolving lithium concentration, electrostatic potential, and stress fields in both the active material and the electrolyte inside and outside cracks, and (ii) a single-particle chemo-mechanical model employing the conventional uniform flux assumption. The coupled model predicts strong spatial heterogeneity in interfacial reaction rates, with flux amplification approximately 8x relative to the imposed uniform flux at the crack tip. Reaction redistribution, and thus lithium flux, is governed predominantly by local solid-state lithium concentration and stress variations, while electrolyte potential gradients inside cracks remain secondary under the conditions considered. Uniform flux models can underpredict delivered capacity by 25% at 1C-rate; this discrepancy increases at higher rates. They also underestimate tensile stresses throughout the delithiation process by 10%, directly affecting crack driving conditions. These results demonstrate that neglecting crack-electrolyte coupling leads to systematic underestimation of both utilisation limits and fatigue-relevant stress histories.

• The paper demonstrates that electrolyte wetting amplifies interfacial reaction rates by up to 8x compared to uniform-flux assumptions.
• The study highlights capacity underestimations up to 55% and lower Coulombic efficiency in idealized models, underscoring critical performance gaps.
• The analysis reveals that explicit electro-chemo-mechanical modeling predicts 10% higher tensile stress, crucial for understanding crack propagation.

Electrolyte Wetting of Cracks in Cathode Particles: Significance for Lithium-Ion Battery Performance and Degradation

Background and Motivation

The mechanical integrity and electrochemical performance of lithium-ion battery cathodes, particularly Ni-rich layered oxides (e.g., NMC811), are critically affected by particle fracture and subsequent electrolyte infiltration. Intergranular cracking is well-established as a dominant degradation pathway due to stress from repeated lithium insertion/extraction cycles, leading to fragmentation, loss of electronic connectivity, and electrochemically inaccessible regions. However, the effect of electrolyte wetting within these cracks—modifying reaction pathways and charge-transfer dynamics—has remained insufficiently characterized. Previous modeling efforts generally neglect explicit electrolyte-particle coupling at crack surfaces, employing a uniform lithium flux boundary assumption that sidesteps the spatial heterogeneity inherent to realistic electrochemical environments.

Modeling Framework and Methodological Approach

The paper implements a controlled comparison to probe the mechanistic role of crack electrolyte wetting. Two computational paradigms are analyzed:

• Realistic Electro-chemo-mechanical Model (RW): Resolves lithium concentration, potential, and mechanical stress fields both in the solid cathode and in the electrolyte, including inside cracks. Charge-transfer reactions at the particle boundary and inside cracks are explicitly modeled via full Butler–Volmer kinetics.
• Idealized Single-Particle Chemo-mechanical Model (IW): Assumes uniform lithium flux across all boundaries, thus neglecting local variations in electrolyte concentration/potential and cracks’ role in reaction redistribution.

Both models are applied to representative cracked cathode particles subjected to full charge-discharge cycles. Geometry, boundary conditions, and constitutive equations are carefully harmonized to isolate the impact of electrolyte wetting.

Key Findings

Spatial Redistribution of Interfacial Reaction Rates

Electrolyte wetting enables penetration of the electrolyte into cracks, thereby increasing the active surface area for charge transfer and modifying local ionic transport pathways. The realistic model demonstrates pronounced spatial heterogeneity in interfacial reaction rates: lithium flux at the crack tip is amplified by nearly a factor of 8 relative to the imposed uniform flux at 1C-rate in the idealized case. This amplification is closely linked to local lithium concentration gradients and stress-induced chemical potential shifts in the active material. Electrolyte potential gradients inside cracks rise by ∼45% over bulk values, but remain secondary drivers compared to solid-state fields.

Performance Implications: Capacity and Coulombic Efficiency

Under concentration-controlled cycling (local composition limits), the idealized model systematically underestimates delivered capacity by 25% (140 mAh/g vs. 186.9 mAh/g) and yields lower Coulombic efficiency (75% vs. 89%). The gap increases substantially at higher C-rates, reaching up to 55% lower delivered capacity at 3C for the idealized protocol. Under voltage-controlled cycling, the discrepancy is reduced but remains significant (up to 32% at 3C). Early attainment of concentration thresholds at the crack-matrix interface for the idealized model leads to premature cycling termination.

Mechanical Effects: Stress and Crack Driving Forces

Electrolyte-enabled reaction redistribution also impacts stress evolution during delithiation/lithiation transitions. The realistic model produces tensile stress profiles within the cathode particle that are at least 10% higher than those predicted by the uniform-flux idealized approach. This difference is critical in fatigue and fracture prediction, as it directly affects the local crack driving force ($K_I$). Uniform-flux models underestimate stress intensity, potentially mispredicting crack propagation events. Mechanically, the realistic model shows more pronounced chemical strains and stress gradients near crack tips, resulting in a self-regulating mechanism for lithium accumulation that is absent in idealized modeling.

Implications for Battery Design and Modeling

Practical

• Model-based predictions of cathode particle utilization, cycle life, and mechanical failure are quantitatively inaccurate if electrolyte wetting and reaction redistribution are neglected.
• Under high-rate cycling, penalties for neglecting spatial heterogeneity are exacerbated, underscoring the necessity for explicitly coupled multi-physics descriptions in predictive modeling.
• Approaches to mitigate degradation should consider not only crack prevention but also the manipulation of electrolyte penetration and local reaction environments for optimizing capacity and limiting fatigue.

Theoretical

• The work reinforces the criticality of localized solid-state phenomena in controlling interfacial charge-transfer kinetics in occluded defect environments. It highlights the inadequacy of constant-flux boundary conditions in realistic microstructural scenarios, and it identifies the primary drivers for reaction redistribution as concentration/stress gradients rather than electrolyte potential.
• Extension to more complex geometries, multi-particle assemblies, and explicit modeling of cathode electrolyte interphase (CEI) formation will be essential to fully capture long-term degradation phenomena.

Future Developments

Advances in in situ characterization techniques, microstructurally sensitive phase-field modeling, and further integration of electrochemical/mechanical coupling are anticipated to refine predictive frameworks and enable targeted interventions at the particle and electrode level. The interplay between crack topology, electrolyte wetting, and degradation under operational voltages and elevated C-rates remains a fertile area for research. Additionally, incorporation of CEI growth dynamics and interfacial resistance effects may provide a more comprehensive description, especially for newly exposed crack surfaces.

Conclusion

The analysis establishes that explicit modeling of crack electrolyte wetting in cathode particles is indispensable for accurate prediction of lithium-ion battery performance and structural evolution. Uniform-flux models systematically underestimate both extractable capacity and crack-driving stress histories. The primary mechanism for spatial reaction redistribution is governed by local solid-state concentration and stress gradients rather than electrolyte potential variations. These findings dictate a paradigm shift in single-particle modeling, demanding fully coupled electro-chemo-mechanical frameworks for reliable assessment of utilization limits and fatigue-relevant mechanical response in advanced battery systems.