Chemo-Mechanics Interactions in Electrochemically Reactive Architectures

Due to the global rise in energy storage demands, there is a growing need for powerful, long-lasting, and safe batteries. The rechargeability and high energy density of lithium-ion batteries has kept them at the forefront of key applications such as electric vehicles. However, the instability of lithium anodes leads to early cell failure and safety concerns, which must be addressed to increase their viability for commercial use. Over repeated cycling under suboptimal conditions, lithium dendrites push through the cell layers, leading to capacity fade, short-circuits, and fire. Potential mitigation methods include the application of external pressure and selection of materials with optimal properties. To inform cell manufacturing and material choices, the interactions between electrochemistry and mechanical deformation at the microscale must be understood.

The nature of this problem requires the coupling of electrochemical processes with realistic porous material deformation. The multiphase nature of porous materials requires special modeling approaches to track conservation between the solid matrix and fluid. Multiphysical behaviors which lead to surface mass loss/gain are typically approached with phase field modeling. However, these approaches cannot accurately resolve mechanical stresses that result from the surface changes. While typical finite-element models can easily model solid-solid contact, the use of meshes that respond only to applied forces presents difficulties while modeling mass loss/gain within the domain, as well as resulting changes in contact between the solid materials.

In this work, a novel modeling approach is developed which couples solid and fluid mechanics in a 3D, finite-element framework. The pressure condition is first applied to solid finite element blocks to initialize contact and the state of stress across the system. The deformed system is then remeshed to resolve fluid-filled gaps between the solid blocks. The electrochemical response to an applied current density is evaluated across both the solid and fluid regions. Local reaction rate is used to calculate the amount of mass stripping/plating that a surface experiences, and the solid block nodes are manipulated to align with the calculated deformed profile. By repeating this process, the long-term evolution of a surface due to both electrochemical stripping/plating and mechanical deformation may be evaluated.

The approach is first used to model the electrochemical response across a lithium anode at the onset of cell operation. The application of pressure is found to induce a nonconformal anode-separator contact surface that greatly impacts the distribution of reaction rate across the anode surface. The peaks of the rough lithium anode experience extremely high stress compared to the pressure condition. In response, the separator experiences local pore closure around these protrusion peaks, which leads to suppression of ion transport across the contact surface, as well as a significant increase in ion transport at the boundary of the contact surface. Parametric studies are conducted to further explore the impact of material properties on both mechanical and electrochemical transport limitations.

The full transient approach is then used to model the evolution of a lithium anode impacted by stack pressure over the course of one full stripping/plating cycle. During the stripping cycle, mass loss at the contact surface induces lithium yielding to continue supporting the pressure condition. Over time, this simultaneous stripping and yielding process leads to the hardening of lithium protrusions, and a long-term evolution towards a sharp peak profile. At this stage, anode-separator contact is unstable and may result in extreme local separator deformation. The application of stack pressure increases contact stability and delays the onset of this profile. The lithium yield strength assumed at the microscale greatly impacts the predicted early interfacial evolution, but over long-term cell operation similar end state profiles are achieved.