Effect of boron on the stability of monoclinic NaMnO2: Theoretical and experimental studies

The structural stability of NaBxMn1−xO2, where x = 0.0–0.375 has been studied using the Vienna ab initio simulation package (VASP), employing density functional theory (DFT) within the generalized gradient approximation (GGA). In addition, the structural stability of the oxides with 75% vacancy of Na ions for each concentration of B has been investigated. In order to validate the results of the ab initio study, a series of experimental investigations have been conducted for NaBxMn1−xO2 and LiBxMn1−xO2 synthesized via conventional solid-state and the chimie-douce ion-exchange reactions. Results of the simulation show that the monoclinic phase of these oxides is thermodynamically stable for all the B concentrations studied. Moreover, substitution of a larger amount of boron results in a higher energy difference between the monoclinic phase and its nearest concurrent orthorhombic phase. Also, deintercalation of Na ions from the parent oxide has shown that the monoclinic phase is stable and the energy difference between the monoclinic and spinel crystal structures increases with increasing amounts of boron. A rationale for this electrochemical response of boron substituted sodium manganese oxide has been provided. Electrochemical cycling results of the experimentally synthesized materials are also discussed in light of these theoretical predictions.