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Origins of Bistability and Na Ion Mobility Difference in P2‐ and O3‐Na 2/3 Fe 2/3 Mn 1/3 O 2 Cathode Polymorphs
Author(s) -
Katcho Nebil A.,
Carrasco Javier,
Saurel Damien,
Gonzalo Elena,
Han Man,
Aguesse Frederic,
Rojo Teofilo
Publication year - 2017
Publication title -
advanced energy materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 10.08
H-Index - 220
eISSN - 1614-6840
pISSN - 1614-6832
DOI - 10.1002/aenm.201601477
Subject(s) - materials science , oxide , bistability , electrochemistry , ion , cathode , chemical physics , stacking , transition metal , diffusion , phase transition , stoichiometry , electrode , thermodynamics , chemistry , physics , optoelectronics , nuclear magnetic resonance , biochemistry , metallurgy , catalysis , organic chemistry
Transition metal layered oxides are promising cathode materials for sodium‐ion batteries. Phase transitions involving different stacking sequences of the oxide layers often plague the electrochemistry of these materials during cycling, which strongly impacts in their electrochemical performance. However, the underlying mechanisms of these processes remain elusive. Interestingly, P2‐ and O3‐Na 2/3 Fe 2/3 Mn 1/3 O 2 phases are the first transition metal layered oxide polymorphs that have been synthesized with exactly the same composition. This offers unprecedented access to the study of bistability in these systems as well as isolates the effect of local structure on Na ion mobility. Here, first‐principles calculations and experiments are combined to unveil the physical origin of such bistability and identify important differences in Na ion diffusion between these two phases. It has been found that electrostatic interactions between oxide layers control the bistable nature of P2 and O3 phases. It is also put forward that the interlayer distance between oxide layers may be a useful descriptor to rationalize the relative stability of other P and O phases in general. Furthermore, this study tracks down to the molecular level the differences regarding Na ion mobility in P2‐ and O3‐Na 2/3 Fe 2/3 Mn 1/3 O 2 by computing activation energies and estimating diffusion coefficients.