Mechanical stress at the electrode-electrolyte interface plays a critical role in the performance and longevity of lithium-ion batteries. The formation and evolution of the solid-electrolyte interphase (SEI) layer, along with its mechanical stability, directly influence cycle life, capacity retention, and safety. Understanding these stresses requires a combination of computational modeling and experimental validation to capture the complex interactions between materials and their degradation mechanisms.

The SEI layer forms as a result of electrochemical reactions between the electrode and electrolyte, typically during the initial charging cycles. This layer, while essential for passivation and preventing further electrolyte decomposition, is mechanically fragile. Repeated lithiation and delithiation cycles induce volumetric changes in the electrode material, leading to stress accumulation at the interface. When the SEI layer cannot accommodate these stresses, cracking and delamination occur, exposing fresh electrode surfaces to further reactions and accelerating capacity fade.

Computational modeling provides a powerful tool for analyzing stress evolution at these interfaces. Cohesive zone models (CZMs) are particularly effective for simulating interfacial delamination and crack propagation. These models represent the interface as a series of cohesive elements that follow a traction-separation law, defining the relationship between stress and displacement until failure. By incorporating material properties such as elastic modulus, fracture energy, and interfacial strength, CZMs can predict the conditions under which the SEI layer will fail.

For example, simulations using CZMs have shown that higher fracture energy in the SEI layer correlates with improved mechanical stability. A study on graphite anodes demonstrated that an SEI with a fracture energy of 0.5 J/m² could withstand greater strain before cracking compared to weaker layers. However, excessive stiffness in the SEI can also be detrimental, as it may transfer more stress to the underlying electrode, leading to particle fracture. Optimizing the balance between flexibility and strength is therefore crucial for durable interfaces.

In-situ scanning electron microscopy (SEM) has emerged as a key experimental technique for validating these computational predictions. By observing electrode-electrolyte interfaces under operational conditions, researchers can directly measure crack formation and propagation in real time. In one study, in-situ SEM revealed that SEI cracks initiate at localized defects and propagate along regions of high stress concentration, consistent with CZM simulations. The ability to correlate mechanical failure with electrochemical performance provides valuable feedback for refining models.

Another critical aspect of stress modeling is accounting for the heterogeneous nature of the SEI. The layer is not uniform but consists of inorganic components near the electrode and organic compounds closer to the electrolyte. This compositional gradient results in varying mechanical properties across the thickness. Multiscale modeling approaches integrate atomistic simulations of SEI components with continuum-level stress analysis to capture this complexity. For instance, molecular dynamics simulations have shown that inorganic components like LiF exhibit higher stiffness but lower toughness compared to organic species such as lithium alkyl carbonates.

Experimental techniques such as nanoindentation and atomic force microscopy (AFM) further complement these studies by measuring local mechanical properties. AFM force-distance curves have quantified the Young’s modulus of SEI layers, with values ranging from 0.1 to 10 GPa depending on composition and formation conditions. These measurements align with model inputs, ensuring accurate predictions of interfacial behavior.

Despite these advances, challenges remain in fully capturing the dynamic evolution of the SEI. The layer continuously reforms and thickens over cycles, altering its mechanical response. Coupling electrochemical degradation models with mechanical stress analysis is necessary to predict long-term performance. Recent work has incorporated SEI growth kinetics into CZMs, showing that slower, more uniform growth leads to a more stable interface compared to rapid, uneven deposition.

Practical implications of this research extend to battery design and material selection. Electrodes with engineered porosity or graded stiffness can mitigate interfacial stresses by accommodating volume changes more effectively. Additives that promote flexible SEI formation, such as fluoroethylene carbonate, have demonstrated improved cycling stability in experimental cells. These strategies highlight the importance of interdisciplinary approaches combining mechanics, electrochemistry, and materials science.

In summary, stress modeling at electrode-electrolyte interfaces provides critical insights into battery degradation mechanisms. Cohesive zone models, supported by in-situ SEM and nanomechanical testing, offer a robust framework for understanding SEI stability. Future work should focus on integrating dynamic SEI evolution into these models and exploring novel electrode architectures to enhance mechanical resilience. By addressing these challenges, the development of longer-lasting, safer batteries becomes increasingly achievable.