O3–NaxMn1/3Fe2/3O2 as a positive electrode material for Na-ion batteries: structural evolutions and redox mechanisms upon Na+ (de)intercalation

Abstract

The electrochemical properties of the O3-type Na_xMn_1/3Fe_2/3O₂ (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 14^th 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.