Molecular dynamics simulations have become an indispensable tool for investigating chemo-mechanical degradation mechanisms at solid-state battery interfaces, particularly in systems like lithium metal and lithium lanthanum zirconium oxide (LLZO). These simulations provide atomic-scale insights into interfacial reactions, space charge layer formation, and void nucleation that are challenging to observe experimentally. By tracking the trajectories of individual atoms under various electrochemical conditions, MD reveals how mechanical stresses evolve during cycling and how they contribute to contact loss between electrode and electrolyte materials.
At the Li/LLZO interface, MD simulations have demonstrated that lithium ion transfer creates localized stress concentrations due to mismatches in ionic radii and lattice parameters. When lithium metal is deposited during charging, the simulations show compressive stresses can exceed 100 MPa near the interface, while during discharge, tensile stresses develop as lithium is extracted. This cyclic stress variation leads to plastic deformation in the lithium metal and microcrack formation in the LLZO ceramic. The simulations capture how these mechanical processes initiate at the atomic scale before propagating to form visible defects.
Space charge layers form due to the different chemical potentials of lithium in the metal and the ceramic electrolyte. MD studies quantify this effect by tracking lithium ion distributions across the interface. The simulations reveal an enrichment of lithium vacancies in the LLZO near the interface, extending approximately 2-5 nm into the electrolyte, consistent with experimental measurements. This space charge region creates an additional barrier for ion transport, increasing interfacial resistance. The MD models further show how this layer evolves during cycling, with the width fluctuating by up to 30% between charge and discharge states.
Void formation is another critical degradation mode revealed by MD. As lithium is stripped from the interface during discharge, the simulations show vacancies initially nucleate at grain boundaries and triple junctions in the lithium metal. These vacancies then coalesce into nanoscale voids that grow with continued cycling. The void growth rate depends strongly on current density, with simulations predicting a cubic relationship between void volume and cycle number at moderate current densities. This leads to loss of interfacial contact and increased local current density at remaining contact points, accelerating degradation.
The chemo-mechanical coupling at these interfaces is particularly evident in MD studies of lithium dendrite propagation. The simulations demonstrate that mechanical stress gradients can direct dendrite growth along specific crystallographic directions in LLZO. When local stresses exceed 1 GPa, the simulations predict spontaneous crack formation in the ceramic electrolyte, creating pathways for dendrite penetration. This stress-assisted dendrite growth mechanism explains experimental observations of dendrites following grain boundaries in polycrystalline LLZO.
Stress accumulation during cycling has been quantified through MD simulations tracking the evolution of stress tensors at the interface. The results show that after 50 cycles at practical current densities, the interfacial stress can reach 80% of the yield strength of LLZO. This stress accumulation leads to progressive contact loss, with simulations predicting a 15-20% reduction in effective contact area per 100 cycles under typical operating conditions. The contact loss occurs non-uniformly, creating localized hot spots for current concentration.
MD insights have guided several strategies for stabilizing these interfaces. Simulations of thin interlayer materials have identified aluminum oxide as particularly effective at reducing interfacial stress, decreasing peak stresses by up to 40% compared to bare interfaces. The simulations show this benefit arises from the interlayer's ability to accommodate strain through amorphous phase formation rather than crack propagation. Surface treatments that create lithium alloy layers have also shown promise in MD studies, with lithium-gold interlayers demonstrating improved adhesion and reduced void formation rates.
Another stabilization approach revealed by MD involves nanostructuring the LLZO surface. Simulations of nanopatterned interfaces show reduced stress concentrations compared to flat surfaces, with the optimal groove spacing being approximately 50 nm. This geometry allows lithium to deform into the grooves during plating, maintaining better contact while reducing overall stress. The simulations predict this approach could extend cycle life by a factor of 3-5 compared to conventional flat interfaces.
Experimental validation of these MD predictions has come from transmission electron microscopy studies showing excellent agreement with simulated void distributions and crack propagation paths. TEM observations confirm the predicted 2-5 nm space charge layer width and its cycling-induced fluctuations. Impedance spectroscopy data aligns with MD predictions of interfacial resistance increases, showing a 25-35% rise in resistance after 100 cycles that correlates well with simulated contact loss progression.
The combination of MD simulations with experimental characterization provides a comprehensive picture of chemo-mechanical degradation at solid-state battery interfaces. The atomic-scale insights from MD guide the development of mitigation strategies while experimental techniques validate the simulation predictions. This synergistic approach accelerates the optimization of interface designs for improved cycle life and safety in solid-state batteries. Future MD studies will likely focus on more complex multicomponent interfaces and the effects of manufacturing variations on long-term degradation behavior.