Controlling Covalency and Anion Redox Potentials through Anion Substitution in Li-Rich Chalcogenides

Development of next-generation battery technologies is imperative in the pursuit of a clean energy future. Toward that end, battery chemistries capable of multielectron redox processes are at the forefront of studies on Li-based systems to increase the gravimetric capacity of the cathode. Multielectron processes rely either on the iterative redox of transition metal cations or redox involving both the transition metal cations and the anionic framework. Targeting coupled cation and anion redox to achieve multielectron charge storage is difficult, however, because the structure–property relationships that govern reversibility are poorly understood. In an effort to develop fundamental understanding of anion redox, we have developed a materials family that displays tunable anion redox over a range of potentials that are dependent on a systematic modification of the stoichiometry. We report anion redox in the chalcogenide solid solution Li₂FeS_(2–y)Se_y, wherein the mixing of the sulfide and selenide anions yields a controllable shift in the high voltage oxidation plateau. Electrochemical measurements indicate that reversible multielectron redox occurs across the solid solution. X-ray absorption spectroscopy supports the oxidation of both iron and selenium at high states of charge, while Raman spectroscopy indicates the formation of Se–Se dimers in Li₂FeSe₂ upon Li deintercalation, providing insight into the charge mechanism of the Li-rich iron chalcogenides. Anion substitution presents direct control over the functional properties of multielectron redox materials for next generation battery technologies

Promoting Reversibility of Multielectron Redox in Alkali-Rich Sulfide Cathodes through Cryomilling

Conventional cathodes for Li-ion batteries (LIBs) are reaching their theoretical capacity limits. One way to meet the growing demands for high-capacity LIBs is by developing so-called Li-rich cathode materials that greatly benefit from additional capacities from anionic moieties in the structure. Li-rich materials are intrinsically subject to higher degrees of (de)­intercalation, leaving the particles more prone to fractures and thus rapid capacity fade. Alkali-rich LiNaFeS2 reversibly cycles with capacities exceeding 300 mAh g–1, but its capacity fades faster than an isostructural material Li2FeS2. Using synchrotron-based transmission X-ray microscopy (TXM), we demonstrate that the capacity fade of LiNaFeS2 stems from particle fractures in the first charge cycle. We improve the cycling performance of LiNaFeS2 by means of cryomilling, which enhances capacity retention at cycle 50 by 76%. Through crystallographic and morphological characterization techniques, we confirm that cryomilling not only decreases particle and crystallite size while increasing microstrain but also prevents particles from fracturing. Cryomilling is a powerful tool to engineer nanoscale battery materials, and TXM allows the direct observation of morphological changes of the particles, which can be leveraged to develop next-generation cathode materials for LIBs