Effect of Stress on Li Transport in High Energy Density Electrode Materials

Abstract

Solid-state diffusion of lithium through the active material is a crucial aspect of lithium-ion battery operation. Quantitative analysis of transport properties is important to understand diffusion mechanisms and designing effective electrode architectures for Li-ion battery. There is a large body of literature exists on Li diffusion coefficient measurement techniques in various electrodes, among them, galvanostatic intermittent titration technique (GITT) and the potentiostatic intermittent titration technique (PITT) are the most widely used methods. However, none of the existent studies considered electrode stresses and the effect of stresses on the measured chemical diffusion coefficient. In this study, sputter deposited Ge films (as a model high energy electrode material) were subjected to potentiostatic intermittent titration technique (PITT) and galvanostatic intermittent technique (GITT) conditions while simultaneously measuring the stress evolution in the electrodes. It was observed that the stresses varied significantly in a single titration step during GITT experiment, which violates the assumptions of Fickian transport model where the electrode stresses are usually neglected. Therefore, only PITT data was analyzed to obtain the chemical diffusion coefficient of Li in Ge. As expected, the diffusion coefficient value increased considerably with Li concentration; however, the values obtained during delithiation are at least two times higher than those obtained during lithiation at any given concentration, with the difference becoming significantly higher with Li concentration. This difference is attributed to the state of stress, i.e., tensile state of stress enhances diffusion and compressive state of stress impedes the diffusion process. The data and observations presented here will be helpful in developing and using electro-chemo-mechanical models in producing optimized electrode microstructures.