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

β-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 ²³Na NMR and first-principles DFT study of the evolution of the structure of β-NaMnO₂ upon electrochemical cycling. The as-synthesized structure contains planar defects identified as twin planes between nanodomains of the α and β 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. ²³Na NMR indicates that 65.5% of Na is in β-NaMnO₂ domains, 2.5% is in α-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 α-like domains, then in β-like domains, and finally close to twin boundaries. ²³Na NMR indicates that the proportion of Na in α-NaMnO₂-type sites increases to 11% after five cycles, suggesting that structural rearrangements occur, leading to twin boundaries separating larger α-NaMnO₂ domains from the major β-NaMnO₂ phase.