Structural and Chemical Evolution of Li-Excess Li2IrO3 during Electrochemical Cycling

Abstract

The family of Li-excess layered Li2TMO3 (TM=Mn, Ru, Ir, etc.) compounds has recently drawn considerable attention in the pursuit of improved capacity of Li-ion batteries. Many studies have unambiguously demonstrated the extra capacity of Li2TMO3 associated with oxygen redox behavior.1,2 Unlike Li2MnO3, which suffers from severe structural degradation and capacity fading upon cycling, Li2IrO3 shows negligible structural change and much improved reversibility,3 and thus serves as a model system to understand the structural response of Li-excess cathodes to Li extraction, as well the exact role of oxygen in compensating Li+ loss. In this study, we employed various experimental techniques as well as first-principles density functional theory (DFT) to investigate the structural and electronic evolution of Li2IrO3 upon electrochemical cycling. It is found that Li2IrO3 undergoes phase transition and slight capacity fading at each cycle. All the thermodynamically stable phases of Li2-xIrO3 (0≤x<2) are identified through a complete structural screening based on electrostatic energy and DFT calculations, and compared with Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) experiments. Core Level Spectroscopy is adopted to probe the changes in the oxygen electronic structure at various (de)lithiation stages, and corresponding oxygen K-edge spectra are simulated based on first-principles calculations. The cationic/anionic charge compensation mechanism of Li2IrO3 upon delithiation, as well as its resistance to O2 loss are discussed based on the spectroscopic observations. Reference: 1. D. Seo, J. Lee, A. Urban, and R. Malik, Nat. Chem., 8, 692–697 (2016) 2. M. Saubanere, E. McCalla, J.M. Tarascon, and M.L. Doublet, Energy Environ. Sci., 984–991 (2016) 3. E. McCalla et al., Science, 350, 1516–1521 (2015)