Investigating the Synthesis Pathway of Disordered Rocksalt Cathode Materials Using Synchrotron X-Ray Characterization

The flourishing of the electric vehicle (EV) industry has led to a surge in demand for cheap and sustainable Li-ion batteries (LiBs). Among all the components in a LiB, the cost and energy density of a battery is mainly dependent on the cathode material. Current commercially LiBs mainly utilize layered cathodes such as LiCoO₂, containing rare and expensive metals, which raise the cost of LIBs. As a result, a cathode material based on cheap and sustainable metals is needed to satisfy the increasing market demand.
Li-rich cation disordered rocksalt (DRX) oxides/oxyfluorides have recently emerged as a new class of cathode material owing to its ability to deliver high electrochemical capacity and high energy density. This class of cathode materials adopt the cubic rocksalt structure (S.G. Fm-3m), with Li and transition metal cations distributed in a disordered manner. Due to its disordered nature, this structural framework can accommodate a large variety of elements, such as Mn and Ti, both of which are earth abundant and low cost. DRX cathodes based on Mn and Ti deliver high specific energy (~ 1000 Wh kg^-1) and large specific capacity (beyond 300 mAhg^-1), making them prospective cathode materials.
The electrochemical performance of DRX oxyfluorides has also been extensively studied. Fluorine incorporation into DRX system was found to enhance the electrochemical performance of the cathode materials, as fluorine can help suppress oxygen redox, which reduces the hysteresis in the voltage profile. DRX oxyfluorides can be prepared via various synthetic routes, among all the synthesis techniques, solid-state synthesis is the most appealing technique as it is most amenable to scale-up. The fluorination agent used for solid state synthesis is mostly LiF. However, the solubility of fluorine into the DRX frameworks is observed to be small, especially in Mn-rich DRX cathodes. This opens the question of whether the low fluorine solubility is intrinsic to DRX systems, or it is due to the high temperature during solid-state synthesis. In order to gain insights into this problem and to optimize the synthesis conditions of DRX materials, it is hence important to study the synthesis pathways of DRX materials and understand the mechanism at which fluorine is incorporated into the DRX framework.
Powder X-ray diffraction has been widely utilized to study the structural evolution of a variety of battery materials during their synthesis, such as LiFePO₄ and Ni-rich layered cathode materials. However, X-ray sources in most laboratory diffractometers have low flux and contain both Kα₁ and Kα₂ wavelengths, which leads to a longer data collection time and lower resolution of the data. In contrast, synchrotron facilities provide highly collimated X-ray beams with high flux, which allows fast collection of high-resolution X-ray data monitoring the synthesis process at real time. As a result, this allows a further quantitative analysis of the in situ X-ray diffraction data, resolving the intermediate crystalline phases and detecting impurities present in the final product.
In this work, we have conducted in situ synchrotron X-ray diffraction at Beamline 2-1 at the Stanford Synchrotron Radiation Lightsource (SSRL). This experiment allows the probing of phase evolution during the solid-state synthesis of a series of Mn and Ti-containing Li-rich DRX materials. Coupled with X-ray absorption spectroscopy, we are able to gain a deeper understanding into the formation pathway and the effect of fluorine during the synthesis of the DRX materials, which may potentially help optimize their synthesis conditions. This presentation will describe and compare the synthesis pathways of fluorinated and non-fluorinated Mn and Ti-containing Li-rich DRX using synchrotron X-ray characterization techniques.