Meaning
Transport mechanisms describe the movement of ions within a solid host driven by gradients other than concentration alone. Stress driven diffusion occurs when mechanical pressure inside a particle forces lithium ions to move from regions of high compression to regions of tension. This phenomenon alters the distribution of lithium during rapid charging and discharging.
Chemical Potential
Internal energy in a lattice is a combination of chemical and mechanical states. In the presence of stress driven diffusion, the net flux of ions is determined by the total gradient of the chemical potential. High stress can actually push ions against a concentration gradient if the mechanical energy savings are large enough.
Mechanical Coupling
Volumetric expansion creates internal forces that feed back into the ion transport process. As the center of a particle lithiates and swells, it exerts pressure on the outer shells. This interaction makes stress driven diffusion a critical factor in modeling the rate capability of silicon and other high expansion materials.
Particle Transport
Diffusion pathways are modified by the local stress field around defects and grain boundaries. When stress driven diffusion is active, the time required to reach a uniform state of lithiation is different than predicted by simple laws of mass transfer. This can lead to localized over-lithiation at the surface of a particle even if the core remains empty.
Such uneven distribution increases the risk of fracture and chemical degradation. Understanding this coupling allows engineers to design particle geometries that minimize internal gradients. By shaping the material or controlling the surface coating, the impact of mechanical forces on ion mobility can be managed to improve the charging speed.
