A Coupled Diffusion-Mechanical Lattice Model for the Degradation of Graphite Active Particles of Li-Ion Battery Anodes

Abstract

The performance and durability of lithium-ion batteries (LIBs) are constrained by the degradation mechanisms that take place during charge and discharge cycles. Degradation of active particles (APs) of LIBs is a complex problem involving several physical phenomena (e.g., diffusion, mechanical deformation, heat transfer, to cite a few). During lithium insertion and extraction cycles, volume changes in the AP result in high mechanical stresses and, consequently, mechanical damage that promotes capacity fade. In this work, we present a microscale 3D finite element model that takes into account the coupled effects between lithium diffusion and mechanical stress within the AP. Using the surface of an ellipsoid as the base for the geometrical construction, we are able to generate different shapes of APs, with both concave and convex surfaces. Porosity and other types of defects that may be present inside the AP are explicitly modeled, and different volume fractions, shapes, and orientations are also accounted for. In our approach, the material is discretized into a lattice of one-dimensional elements: we consider beam elements for the mechanical problem, while in the diffusive approach, the material is treated as an assembly of “nanopipes” through which the flow of Li-ions takes place. The same lattice network is used for both simulations. We follow a classical lattice model approach to characterize the fracture behavior of a single AP of a LIB anode when subjected to charge/discharge cycles. The material of the APs analyzed in this work is graphite, which presents a brittle, disordered material structure, making it suitable for lattice modeling. The mechanical problem is solved, obtaining the crack patterns associated with specific charge and discharge strategies and potential initial defects. The simulation results correctly reproduce the experimental observations on mechanical stresses and the evolution of damage. This lattice model framework analyzing the degradation in the APs of LIBs (durability) can be used to provide more information regarding the microstructural evolution, morphological changes, and mechanical degradation in APs and identify improvement strategies