In situ measurement of the effect of stress on the chemical diffusion coefficient of Li in high-energy-density electrodes

Sputter deposited germanium thin films were assembled in a half-cell configuration with lithium foil as counter/reference electrode and 1M LiPF₆ in EC, DEC, DMC solution (1:1:1, wt%) as electrolyte. The Ge films 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 electrode stresses varied significantly in a single titration step during a GITT experiment, which violates the assumptions of simple Fickian transport model where the electrode stresses are usually neglected. Therefore, only the PITT data was analyzed to obtain the chemical diffusion coefficient D∼ of Li in Ge. As expected, the diffusion coefficient value increased considerably with Li concentration; however, the D∼ values obtained during delithiation are at least two times greater than those obtained during lithiation at any given Li concentration, with the difference becoming significantly higher at higher Li concentration. This difference is attributed to the stress state, i.e., tensile stress during delithiation leads to higher D∼ values compared to the compressive stresses during lithiation. The data and observations presented here will be helpful in developing and using electrochemomechanical models in producing optimized electrode microstructures.