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

• Published in: Journal of physical chemistry. C 2016-09, Vol.120 (34), p.19049-19063
• Main Authors: Buzlukov, Anton, Mouesca, Jean-Marie, Buannic, Lucienne, Hediger, Sabine, Simonin, Loïc, Canevet, Emmanuel, Colin, Jean-Francois, Gutel, Thibaut, Bardet, Michel
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.6b07532

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–x MO2-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.