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

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-like regions, most probably surrounding the Li₂MnO₃ domains and thus labeled “Ni-poor boundaries” (volume ∼21%). This partition is confirmed by the behavior of the NMR signals during the first electrochemical cycle. At the initial state of charge (≤4.3 V), Li-ion extraction occurs mainly from the (Ni-rich) Li₁–ₓMO₂-like domains via Ni²⁺ oxidation. At higher states of charge (≥4.5 V), the Li₂MnO₃-like domains become highly involved via oxygen-based (ir)reversible oxidation processes, leading to significant structural transformations. During discharge, only ∼60% of the initial lithium is reinserted into the structure. The (Ni-rich) LiMO₂-like domains are fully refilled (via reversible Ni⁴⁺ reduction into Ni²⁺), while the ordered Li₂MnO₃-like domains experience a significant size decrease after the first cycle of charge/discharge. The originality of the present approach consists of analyzing NMR data with a new model that includes at its heart local electroneutrality constraints. This model allowed us to shed light on the processes occurring in the Li-rich Mn/Ni layered oxide compound during the first electrochemical cycle on the microscopic level.