Microscale Engineering for the Structural Integrity and Transport Properties of Electrode Material in Li-ion Batteries.

Abstract

Aggregation between additive particles and active particles in the electrodes of Li-ion batteries is an important process for lithium-ion batteries in that it strongly affects mechanical stability, transport properties, utilization, and gravimetric and volumetric power/energy density. Transport properties, which encompass electrical conductivity, diffusion of lithium ions and reaction rate at the solid/electrolyte interface, determine the power performance of batteries. However, fracture has been experimentally observed in the electrode aggregates. Fracture is a putative degradation mechanism of lithium-ion batteries, one that may cause the rupture of electrode particles and disruption of conductive path, and thus adversely affect the structural integrity as well as transport properties of electrodes. This thesis proposes two three-dimensional simulation models, in which Brownian dynamics and the Monte Carlo method are employed, respectively, to simulate the aggregation process of conductive additives and active particles. The aggregated structures generated from these models are characterized to analyze the effects of a few manufacturing parameters such as temperature, particle size, particle aspect ratio, and additive to active material mass ratio on the final morphology and transport properties of the electrode. Further, a stress and fracture analysis was carried out on these aggregated structures, as well as on single particles with unconstrained surfaces. The effects of particle geometry and electrochemical loading conditions on fracture propagation in single electrode particles are investigated, as is the location of initial defects. It is also demonstrated through stress analysis that particles in a cluster have a significantly higher propensity to fracture compared to single particles with unconstrained surfaces under the same electrochemical cycling conditions. Finally, based on the findings of these studies, recommendations are provided for the optimization of Li-ion batteries in terms of conductivity, specific energy, and fracture resistance.