Insights into the Nature and Evolution upon Electrochemical Cycling of Planar Defects in the β-NaMnO2 Na-Ion Battery Cathode: An NMR and First-Principles Density Functional Theory Approach

beta-NaMnO2, is a high-capacity Na-ion battery cathode, delivering ca. 190 mAh/g of reversible capacity when cycled at a rate of C/20. Yet, only 70% of the initial reversible capacity is retained after 100 cycles. We carry out a combined solid-state Na-23 NMR and first-principles DFT study of the evolution of the structure of beta-NaMnO2 upon electrochemical cycling. The as synthesized structure contains planar defects identified as twin planes between nanodomains of the alpha and beta forms of NaMnO2. GGA+U calculations reveal that the formation energies of the two polymorphs are within 5 meV per formula unit, and a phase mixture is likely in any NaMnO2 sample at room temperature. Na-23 NMR indicates that 65.5% of Na is in beta-NaMnO2 domains, 2.5% is in alpha-NaMnO2 domains, and 32% is close to a twin boundary in the as-synthesized material. A two-phase reaction at the beginning of charge and at the end of discharge is observed by NMR, consistent with the constant voltage plateau at 2.6-2.7 V in the electrochemical profile. GGA+U computations of Na deintercalation potentials reveal that Na extraction occurs first in alpha-like domains, then in beta-like domains, and finally close to twin boundaries. Na-23 NMR indicates that the proportion of Na in alpha-NaMnO2-type sites increases to 11% after five cycles, suggesting that structural rearrangements occur, leading to twin boundaries separating larger alpha-NaMnO2 domains from the major beta-NaMnO2 phase.