Impact of Annealing Time on Lithium Local Environment of LiMn0.5Ni0.5O2: DFT-Supported 6 Li Solid-State NMR Study

Abstract

To quench the thirst of ever-increasing demand for high energy and low-Co lithium-ion batteries (LIB), LiMn0.5Ni0.5O2 has received considerable attention as a promising alternative cathode. However, it has been plagued by the structural failure driven by the irreversible phase change upon subsequent cycling and the structural degradation caused by the unavoidable Li/Ni exchange.[i] This brings the importance of understanding the relation with lithium local environments and domains on overall electrochemical properties of materials.In this study, we have focused on two different samples, prepared by different annealing times (150 vs. 15 hours) to elucidate the change of local lithium surroundings and its sensitivity to the annealing time. We use a combination of 6Li solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR)[ii] and density functional theory (DFT)[iii] techniques to illuminate the impact of annealing time to the local lithium environment of LiMn0.5Ni0.5O2. Examination of NMR spectra reveals peak shift and intensity change, which is scrutinized by the assistance of DFT that is capable of deconvoluting the NMR spectra and obtain the corresponding local lithium environment. We compare the NMR spectra of the previously reported transition metal arrangements such as a zigzag, row, and honeycomb structures and describe the influence of Li/Ni exchange on the shape of the spectra. Overall, we combine the various computed-NMR spectra that show in close proximity to the experimental spectra, which provides new insights to understand the correlation between annealing time and local lithium environment for a promising LIB cathode design. [i] Ceder, G. et al., Phase Transitions in the LiNi0.5Mn0.5O2 System, Chem. Mater. 2007, 19, 7, 1790–1800 [ii] Grey, C. P. et al., NMR Studies of Cathode Materials for Lithium-Ion Rechargeable Batteries, Chem. Rev. 2004, 104, 4493−4512[iii] Casas-Cabanas, M. et al., DFT-Assisted Solid-State NMR Characterization of Defects in Li2MnO3, Inorg. Chem. 2019, 58, 8347−8356