Detailed Investigation of Anionic Redox Chemistry in Li-Rich Li2Ir0.75Sn0.5O3 Electrode Materials for Li-Ion Batteries
Authors: Yun Soyeong , Muhammad Shoaib , Kim Yunok , Lee Eunkang , Yoon Won-Sub 
Published: February 2020

Li-rich layered oxides, xLi2MnO3∙(1-x)LiMO2 (M = Ni, Co, Mn) have attracted significant attention as cathode materials for lithium ion batteries in recent years. Although they exhibit promising capacities, up to 300mAh/g, due to transition metal redox reactions and unconventional oxygen redox reaction, they have several critical problems: (1) large irreversible capacity loss in the first cycle due to the release of oxygen and lithium from the lattice at the end of first charge, (2) insufficient cycling performance, (3) side reactions with electrolyte at high cut-off voltage and, (4) phase transformation that leads to voltage decay during cycling [1]. In order to improve the structural stability and investigate single redox cation, Li2M1-xSnxO3 (M = Ru, Ir) type Li-rich materials have been recently designed by taking advantage of 4d, 5d transition metals that can increase the M-O covalency. These materials are structurally related to the 3d transition metal-based traditional Li-rich cathode materials. Increased M-O bond covalency results in lower voltage plateau compared to 3d transition metal based Li-rich cathode materials and thus enables the investigation of intrinsic redox chemistry by avoiding the side reactions with electrolytes at higher voltage. In these cathode materials electrochemical process is described as ‘a cumulative and reversible cationic (Mn+ → M(n+1)+) and anionic (O2- → O2
2-) redox process, due to the d-sp hybridization associated with a reductive coupling mechanism’. This mechanism is derived by using the X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) techniques [2,3]. However, the structural changes in Li-rich layered materials during anionic redox chemistry are very complex and still remain a controversial issue that needs further investigation.

In order to understand the origin of high capacity of Li-rich cathode materials, systematic investigation is required to understand the structural changes in the bulk materials during electrochemical cycling. In this study, we synthesized Li2Ir0.75Sn0.25O3 electrode material that delivered first cycle charge and discharge capacity of 214mAh/g (corresponds to removal of ~2 moles of Li+) and 188mAh/g, respectively and after 100 cycles, it was still able to deliver 128mAh/g of discharge capacity with 71% capacity retention. We performed synchrotron radiation-based high-resolution X-ray powder diffraction (HRPD) and hard & soft X-ray absorption spectroscopy (XAS) along with several electrochemical characterization techniques, to investigate the redox reaction in Li2Ir0.75Sn0.25O3 electrode in terms of local and bulk crystal structural changes, variations in the oxidation states of transition metal and oxygen ions in Li2Ir0.75Sn0.25O3, respectively. Sn K-edge XANES spectra do not show any edge shift implying that Sn-ions are not participating in the electrochemical reaction. However, we found that there is minimal contribution from Ir-ions in the charge compensation, as inferred by small changes in the Ir L3-edge XANES edge position. So the charge is mostly compensated by anionic redox chemistry mainly involving oxygen. O K-edge soft XAS spectra were measured to quantify the charge compensation by oxygen and metals. However, anionic redox chemistry is very complex as it involves significant structural changes. Understanding these structural changes during lithium extraction and insertion is a key to investigate the charge compensation mechanism of 4d and 5d metals-based Li-rich cathode materials. Rietveld refinement and profile refinement by FAULTS [4] were performed on the measured HRPD pattern to understand the detailed crystal structure variations and formation of stacking faults during charge and discharge. Detailed findings will be presented at the time of the meeting.

[1] J. Wang et al. Advanced Energy Materials6 (2016) 1600906

[2] M. Sathiya et al. Nature Materials12 (2013) 827–835

[3] E. McCalla et al. Science350 (2015) 1516–1521

[4] M. Casas-Cabanas et al.

Journal of Applied Crystallography49 (2016) 2259-2269