The Limited Incorporation and Role of Fluorine in Mn-rich Disordered Rocksalt Cathodes.

Disordered rocksalt oxide (DRX) cathodes are promising candidates for next-generation Co- and Ni-free Li-ion batteries. While fluorine substitution for oxygen has been explored as an avenue to enhance their performance, the amount of fluorine incorporated into the DRX structure is particularly challenging to quantify and impedes our ability to relate fluorination to electrochemical performance. Herein, an experimental-computational method combining Li and F solid-state nuclear magnetic resonance, and cluster expansion Monte Carlo simulations, is developed to determine the composition of DRX oxyfluorides.

An Experimental Approach to Assess Fluorine Incorporation into Disordered Rock Salt Oxide Cathodes.

Disordered rock salt oxides (DRX) have shown great promise as high-energy-density and sustainable Li-ion cathodes. While partial substitution of oxygen for fluorine in the rock salt framework has been related to increased capacity, lower charge-discharge hysteresis, and longer cycle life, fluorination is poorly characterized and controlled. This work presents a multistep method aimed at assessing fluorine incorporation into DRX cathodes, a challenging task due to the difficulty in distinguishing oxygen from fluorine using X-ray and neutron-based techniques and the presence of partially amorphous impurities in all DRX samples.

Giant Modulation of Refractive Index from Picoscale Atomic Displacements.

It is shown that structural disorder-in the form of anisotropic, picoscale atomic displacements-modulates the refractive index tensor and results in the giant optical anisotropy observed in BaTiS, a quasi-1D hexagonal chalcogenide. Single-crystal X-ray diffraction studies reveal the presence of antipolar displacements of Ti atoms within adjacent TiS chains along the c-axis, and threefold degenerate Ti displacements in the a-b plane. Ti solid-state NMR provides additional evidence for those Ti displacements in the form of a three-horned NMR lineshape resulting from a low symmetry local environment around Ti atoms.

Polymorphism in Weberite NaFeF and its Effects on Electrochemical Properties as a Na-Ion Cathode.

Weberite-type sodium transition metal fluorides (Na'F) have emerged as potential high-performance sodium intercalation cathodes, with predicted energy densities in the 600-800 W h/kg range and fast Na-ion transport. One of the few weberites that have been electrochemically tested is NaFeF, yet inconsistencies in its reported structure and electrochemical properties have hampered the establishment of clear structure-property relationships. In this study, we reconcile structural characteristics and electrochemical behavior using a combined experimental-computational approach.

Tuning Bulk Redox and Altering Interfacial Reactivity in Highly Fluorinated Cation-Disordered Rocksalt Cathodes.

Lithium-excess, cation-disordered rocksalt (DRX) materials have been subject to intense scrutiny and development in recent years as potential cathode materials for Li-ion batteries. Despite their compositional flexibility and high initial capacity, they suffer from poorly understood parasitic degradation reactions at the cathode-electrolyte interface. These interfacial degradation reactions deteriorate both the DRX material and electrolyte, ultimately leading to capacity fade and voltage hysteresis during cycling.