Basic Concepts
Keywords: Chemical Diffusion, Charge-Transfer Reactions, Non-equilibrium Thermodynamics
Our Research
The rate limiting process in battery particles
Identifying the rate-limiting step in a device or system is one of the first steps toward finding routes beyond simple empirical optimization. In this context, one of our earliest questions was whether battery active-material particles are truly diffusion-limited (that is, limited by bulk transport) as is commonly assumed in standard battery models. Our first body of evidence emerged from studies of kinetically induced apparent phase separation in Li layered oxides. When these particles are delithiated above a certain rate threshold, their compositions are known to segregate into two populations, as though the material were undergoing phase separation. We referred to this phenomenon as "fictitious phase separation."
Depending on whether delithiation is limited by bulk transport through solid-state chemical diffusion or by interfacial charge transfer at the surface, fictitious phase separation should occur at either the intraparticle or interparticle level. Using several complementary approaches including X-ray spectromicroscopy and quench-and-relaxation studies with X-ray diffraction, we showed that interparticle heterogeneity prevails across a range of Li layered oxide particles. This finding established that many commonly used and commercially optimized Li layered oxide particles are not bulk-transport-limited, at least at lithiation levels of approximately 70% and above.