Direct Observation of Lattice Aluminum Environments in Li-Ion Cathodes NCA and Al-Doped NMC Via 27al MAS NMR Spectroscopy

Abstract

Direct observations of local lattice aluminum environments have been a major challenge for aluminum bearing Li-ion battery materials such as LiNi1-y-zCoyAlzO2 (NCA) and aluminum doped LiNixMnyCozO2 (NMC). 27Al magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy is the only structural probe currently available that can qualitatively and quantitatively characterize lattice and non-lattice (i.e. surface, coatings, segregation, secondary phase etc.) aluminum coordination and provide information that helps discern its effect in the lattice. In the present study, we use NMR to gain new insights into transition metal (TM)-O-Al coordination and evolution of lattice aluminum sites upon cycling. With the aid of first principles DFT calculations, we show direct evidence of lattice Al sites, non-preferential Ni/Co-O-Al ordering in NCA, and the lack of bulk lattice aluminum in aluminum doped NMC. Aluminum coordination of paramagnetic (lattice) and diamagnetic (non-lattice) nature is investigated for Al-doped NMC and NCA. For the later, the evolution of the lattice site upon cycling is also studied. A clear re-ordering of lattice aluminum environments due to nickel migration is observed in NCA upon extended cycling.