Investigating the Synthesis Pathway of a Disordered Rocksalt Phase Via in Situ X-Ray Characterization

Abstract

Recently, Li-rich disordered rocksalt (DRX) materials have gathered a lot of attention as prospective Li-ion battery cathode materials owing to their high electrochemical storage capacity and high specific energy. In contrast to the current commercialized layered Li-ion battery cathode materials (e.g. LiCoO2, LiNixMnyCo1-x-yO2), which contain toxic and expensive metals, DRX can accommodate a large variety of transition metals, such as Mn and Ti, which are cheap and earth abundant. These characteristics make them a cheaper and a more sustainable alternative to the current commonly used layered cathodes.Unlike the commercial Li-ion battery cathodes, where the transition metals and the Li are arranged in an ordered manner, DRX exhibits a highly disordered cation framework. Fluorine incorporation into DRX was extensively studied, where the fluorine anions are distributed disorderly at the anion sites. Previous studies have shown that the electrochemical performance of DRX cathodes can be enhanced via fluorination, as fluorine can effectively suppress the oxygen redox activity observed in many Li-rich cathodes. However, a large degree of fluorine incorporation into DRX was found to be difficult. As a result, it is important to study the mechanism in which fluorine is incorporated into the structure, and the role of LiF during synthesis of DRX.In addition to fluorination, the degree and type of short-range cation ordering (SRO) presenting in the materials were also found to have a significant impact on the electrochemical performance of DRX materials. The SRO present can be modified with synthesis techniques, where samples annealed at the synthesis temperature for a longer period exhibit a higher degree of SRO. This observation has demonstrated that the synthesis conditions and SRO are highly correlated. It is hence possible to control the SRO in DRX materials via varying synthesis conditions, thereby enhancing their electrochemical performance. Keeping this in mind, it is hence important to understand the mechanism by which DRX is formed.In this work, we have performed in situ synchrotron X-ray diffraction monitoring the phase progression during the solid-state synthesis of Li1.1Mn0.8Ti0.1O1.9F0.1 DRX oxyfluoride. We have observed several crystalline intermediate phases emerged before the formation of the DRX phase. Coupled with X-ray absorption spectroscopy, we are able to gain additional insights into the intermediate phases formed during the synthesis. This poster will describe the investigation of the synthesis pathway of Li1.1Mn0.8Ti0.1O1.9F0.1 DRX oxyfluoride using X-ray characterization techniques. The formation mechanism of this DRX phase will also be discussed.