Hui-Chia Yu

Designing the next generation high capacity battery electrodes

Much of current research in electrochemical energy storage is devoted to new electrode chemistries and reaction mechanisms that promise substantial increases in energy density. Unfortunately, most high capacity electrodes exhibit an unacceptably large hysteresis in their voltage profile. Using a first-principles multi-scale approach to examine particle level dynamics, we identify intrinsic thermodynamic and kinetic properties that are responsible for the large hysteresis exhibited by many high capacity electrodes. Our analysis shows that the hysteresis in the voltage profile of high capacity electrodes that rely on displacement reactions arises from a difference in reaction paths between charge and discharge. We demonstrate that different reaction paths are followed (i) when there is a large mismatch in ionic mobilities between the electrochemically active species (e.g. Li) and displaced ionic species and (ii) when there is a lack of a thermodynamic driving force to redistribute displaced ions upon charging of the electrode. These insights motivate the formulation of design metrics for displacement reactions in terms of fundamental properties determined by the chemistry and crystallography of the electrode material, properties that are now readily accessible with first-principles computation.

Localized concentration reversal of lithium during intercalation into nanoparticles

Inhomogeneous Li intercalation and localized concentration reversal in nanoparticles are investigated on a nanometer scale. Nanoparticulate electrodes, such as LixFePO4, have unique advantages over their microparticulate counterparts for the applications in Li-ion batteries because of the shortened diffusion path and access to nonequilibrium routes for fast Li incorporation, thus radically boosting power density of the electrodes. However, how Li intercalation occurs locally in a single nanoparticle of such materials remains unresolved because real-time observation at such a fine scale is still lacking. We report visualization of local Li intercalation via solid-solution transformation in individual LixFePO4 nanoparticles, enabled by probing sub-angstrom changes in the lattice spacing in situ. The real-time observation reveals inhomogeneous intercalation, accompanied with an unexpected reversal of Li concentration at the nanometer scale. The origin of the reversal phenomenon is elucidated through phase-field simulations, and it is attributed to the presence of structurally different regions that have distinct chemical potential functions. The findings from this study provide a new perspective on the local intercalation dynamics in battery electrodes.