Li-Rich Mn/Ni Layered Oxide as Electrode Material for Lithium Batteries: A 7Li MAS NMR Study Revealing Segregation into (Nanoscale) Domains with Highly Different Electrochemical Behaviors

Abstract

We present a 7Li MAS NMR study carried out before (pristine material) and during the first cycle of charge/discharge of Li[Li0.2Mn0.61Ni0.18Mg0.01]O2 layered oxide, a promising active material for positive electrode in Li-ion batteries. For the pristine material, at least five NMR signals were observed. To analyze these results, we developed an 18 cation local model (first and second spheres) aiming at identifying very precise cationic (Li+, Mn4+/Ni2+) configurations compatible with all our NMR data while satisfying local electroneutrality constraints (the key ingredient of our approach). Our results strongly suggest that the material presents two types of coexisting nanoscale domains. The first type is highly ordered and consists of pure Li2MnO3 cores (volume ∼58%), while the second more disordered type concentrates most of the Ni and is labeled LiMO2-like (volume ∼20%) where M = Mn1/2Ni1/2. Finally, at the interphase of these two Ni-free and Ni-rich domains, there are slightly Ni-contaminated Li2MnO3-li...