O3–Na_xMn_1/3Fe_2/3O₂as a positive electrode material for Na-ion batteries: structural evolutions and redox mechanisms upon Na⁺(de)intercalation

Abstract

International audienceThe electrochemical properties of the O3-type NaxMn1/3Fe2/3O2 (x = 0.77) phase used as positive electrode material in Na batteries were investigated in the 1.5–3.8 V, 1.5–4.0 V and 1.5–4.3 V ranges. We show that cycling the Na cells in a wider voltage range do not induce a significant gain on long term cycling as the discharge capacities reached for the three experiments are identical after the 14th cycle. The structural changes the material undergoes from 1.5 V (fully intercalated state) to 4.3 V were investigated by operando in situ X-ray powder diffraction (XRPD) and were further characterized by ex situ synchrotron XRPD. We show that the low amount of Mn3+ ions (≈33% of total Mn+ ions) is enough to induce a cooperative Jahn–Teller effect for all MO6 octahedra in the fully intercalated state. Upon deintercalation the material exhibits several structural transitions: O′3 → O3 → P3. Furthermore, several residual phases are observed during the experiment. In particular, a small part of the O3 type is not transformed to P3 but is always involved in the electrochemical process. To explain this behaviour the hypothesis of an inhomogeneity, which is not detected by XRD, is suggested. All phases converge into a poorly crystallized phase for x ≈ 0.15. The short interslab distance of the resulting phase strongly suggests an octahedral environment for the Na+ ions. X-ray absorption spectroscopy and 57Fe Mössbauer spectroscopy were used to confirm the activity of the Mn4+/Mn3+ and Fe4+/Fe3+ redox couples in the low and high voltage regions, respectively. 57Fe Mössbauer spectroscopy also showed an increase of the disorder into the material upon deintercalation

Intermediate honeycomb ordering to trigger oxygen redox chemistry in layered battery electrode

“…2), D-Na x RuO 3 undergoes a structural change from O3 to P3 (with Na in prismatic sites owing to the ABBCCA oxide-ion stacking) on charging23. This O3→P3 transition is commonly observed in O3-NaMO 2 materials through gliding of the [Na 1/3 Ru 2/3 ]O 2 slabs from ABCABC (O3) to ABBCCA (P3) stacking925. The synchrotron XRD pattern of the charged state (D-Na 1 RuO 3 ; Supplementary Fig.…”

The NaxMoO2 Phase Diagram (1/2 ≤ x < 1): An Electrochemical Devil’s Staircase

“…Transitions from one stacking to another are possible through slab gliding requiring only small atomic displacement and no breaking of M-O bonds. They can therefore occur at room temperature during the cycling of the battery 29,30,39,40 , as it was more recently shown in the O3-NaxMn1/3Fe2/3O2 system 41 . This difference in stacking arrangements leads to a significant change in intensity ratio between the reflections 104h and 105h in the Xray diffraction patterns indexed with an undistorted hexagonal cell.…”

Layered Oxide Cathodes for Sodium‐Ion Batteries: Phase Transition, Air Stability, and Performance

“…Specially, a small amount of the O3 type could not completely transform to P3 but always involve in the electrochemical process. [171,172] Sharma et al reported that O3-Na 2/3 [Fe 2/3 Mn 1/3 ]O 2 undergoes a series of twophase and solid-solution reactions leading to a disordered phase at the charged state before undergoing solid solution and two behavior on discharge, but the structural motif and the space group are preserved. [173,174] The physical origin of bistability and differences in Na ion diffusion between P2 and O3 phases may result from the electrostatic interactions within oxide layers which control the bistable nature of these two phases.…”