Mesoscopic Phase Transition Kinetics in Secondary Particles of Electrode-Active Materials in Lithium-Ion Batteries

Many compounds used as battery storage electrodes undergo large composition changes during use that are accompanied by a first-order phase transition. Most studies of these phase transitions have focused on the unit cell to single-crystallite scale, whereas real battery electrodes are typically composed of mesoscopic assemblies of nanocrystallites, for which phase transformation mechanisms are poorly understood. In this work, a systematic study is conducted of the potentiostatic (constant driving force) kinetics of phase transition in secondary particles of representative intercalation compounds: LiFePO₄, LiMn_1–<i>x</i>Fe_<i>x</i>PO₄, and Li₄Ti₅O₇. Storage kinetics are studied as a function of overpotential, material composition, primary particle size, and temperature. We find that in regimes where phase transformation occurs, the results can be self-consistently explained as nucleation and growth kinetics within the framework of the Johnson–Mehl–Avrami–Kolmogorov model. This implies that despite the common secondary particle topology, the electrochemically driven phase transformations occur by nucleation and growth with little apparent resistance to phase propagation across the grain boundaries. Growth appears to be one-dimensional in nature, consistent with a hybrid growth model in which rapid surface propagation is followed by slower growth into particles

Mesoscopic Phase Transition Kinetics in Secondary Particles of Electrode-Active Materials in Lithium-Ion Batteries

Many compounds used as battery storage electrodes undergo large composition changes during use that are accompanied by a first-order phase transition. Most studies of these phase transitions have focused on the unit cell to single-crystallite scale, whereas real battery electrodes are typically composed of mesoscopic assemblies of nanocrystallites, for which phase transformation mechanisms are poorly understood. In this work, a systematic study is conducted of the potentiostatic (constant driving force) kinetics of phase transition in secondary particles of representative intercalation compounds: LiFePO₄, LiMn_1–<i>x</i>Fe_<i>x</i>PO₄, and Li₄Ti₅O₇. Storage kinetics are studied as a function of overpotential, material composition, primary particle size, and temperature. We find that in regimes where phase transformation occurs, the results can be self-consistently explained as nucleation and growth kinetics within the framework of the Johnson–Mehl–Avrami–Kolmogorov model. This implies that despite the common secondary particle topology, the electrochemically driven phase transformations occur by nucleation and growth with little apparent resistance to phase propagation across the grain boundaries. Growth appears to be one-dimensional in nature, consistent with a hybrid growth model in which rapid surface propagation is followed by slower growth into particles

Metalation Triggers Single Crystalline Order in a Porous Solid

We report the dramatic triggering of structural order in a Zr­(IV)-based metal–organic framework (MOF) through docking of HgCl₂ guests. Although as-made crystals were unsuitable for single crystal X-ray diffraction (SCXRD), with diffraction limited to low angles well below atomic resolution due to intrinsic structural disorder, permeation of HgCl₂ not only leaves the crystals intact but also resulted in fully resolved backbone as well as thioether side groups. The crystal structure revealed elaborate HgCl₂-thioether aggregates nested within the host octahedra to form a hierarchical, multifunctional net. The chelating thioether groups also promote Hg­(II) removal from water, while the trapped Hg­(II) can be easily extricated by 2-mercaptoethanol to reactivate the MOF sorbent