Sodium Redox Intercalation at Layered Na2/3Mg1/3Mn2/3O2 Oxides
Rositsa Kukeva, Mariya Kalapsazova and Radostina Stoyanova
Abstract: Layered Mn-rich oxides stand out among the cathode materials for sodium-ion battery due to their diversity of structural modifications and unique intercalation properties towards Na+ ions [1]. The intercalation of Na+ into layered oxides proceeds thanks to the redox properties of transition metal ions [1]. Recently, it has been found that, in addition to transition metal ions, lattice oxygen participates in intercalation reactions too [2]. The involvement of the O-/O2- redox couple in the electrochemical reactions, as well as the resulting structural changes, is a subject of this investigation.
Herein, we examine in details the sodium intercalation properties of Mn-rich oxide, Na2/3Mg1/3Mn2/3O2, having a P3-type layered structure. The studied oxides were obtained by freeze-drying method, followed by heating up to 600⁰C. The low-temperature synthesis allows to stabilize the P3-type structure over P2 one, the lattice parameters being of a=2.8558(0.0003) Å, c=16.5705 (0.0040) Å. The sodium intercalation into layered oxides was examined in model sodium-ion cells using sodium electrolyte 1 M NaPF6 in PC. CV curves were recorded in a broad voltage range 1.5-5.5 V, thus the oxygen redox reaction is found to occur at about 4.5V. Taking into account these results, two voltage ranges are selected for electrochemical characterization: a narrow range between 1.5 and 4.0V, where Mn-ions are electrochemically active, and a broad range between 1.5 and 4.8V, where lattice oxygen is activated. As a result, the oxides deliver higher discharge capacity (more than two times) when the upper voltage limit increases from 4.0 to 4.8 V (Fig. 1).
In order to understand the underlying mechanism of reversible oxygen redox activity the electrode materials were studied by post-mortem XRD and EPR analysis. After the first charge up to 4.8 V (corresponding to complete extraction of Na+), the XRD pattern displays a phase transformation from P3- to O3 type structure concomitant with a strong interlayer contraction (around 13 %). The subsequent oxide discharging to 1.5 V (corresponding to 1 mol intercalated Na+ into oxide) leads to a formation of mixture between phases with P3 and O3-structures, which have intra- and inter-layer distances different from that of the pristine oxide. When the number of cycles is increased up to 50 cycles, there are no further structural changes: P3- and O3-phases in a ratio of 35-to-65 wt. % account for the cycled electrodes. This disclose that all structural changes occurring during cycling in a broad voltage window are reversible and they are a function of the participation of lattice oxygen in the electrochemical reaction. Supporting this findings, the pristine P3-structure remains when the oxide is cycled numerously in a narrow voltage range, i.e. where the manganese ions are electrochemically active.
The post-mortem EPR analysis is carried on the same type of electrodes that are studied byXRD. This method is undertaken for monitoring of oxidation state of manganese ions after the electrochemical reaction and it is also useful in distinguishing manganese ions that are positioned in different surroundings. In the pristine Na2/3Mg1/3Mn2/3O2 oxide, manganese ions are stabilized in oxidation state of 4+. The EPR data give evidence that, at potentials above 4.0 V, Mn4+ ions are electrochemically inactive, while, at potentials lower than 3.0 V, Mn4+ ions are partially reduced to Mn3+. Therefore, the activation of the oxygen redox activity can be related with Mg2+– induced variation in the electronic structure of oxides. Besides, the EPR spectroscopy demonstrates some structural changes occurring at the interface electrolyte-electrode where Mn2+ ions are included.
The present studies shed further light on the design of electrode materials with oxygen redox activity.
References
[1] E. Gonzalo, M. Zarrabeitia, N. E. Drewett, J. Miguel López del Amo, Teófilo Rojo, Energy Storage
Materials 34 (2021) 682.
[2] B. Song, E. Hu, J. Liu, Y. Zhang, X.-Q. Yang, J. Nanda, A. Huq, K. Page, J. Mater. Chem. A7 (2019) 149.
Acknowledgements:
– Project BG05M2OP001-1.001-0008, „National center of mechatronics and clean technologies“ funded by the Operational Programme Science and Education for Smart Growth, co-financed by the European Union through the European Regional Development Fund.
– CARiM (NSP Vihren, КП-06-ДB-6/16.12.2019) for the research program.
– The authors are also grateful to the project Д01-92/06.2022 – “European Network on Materials for Clean Technologies”, to present the obtained results in the SizeMat4.
The authors from CARiM’s Research Team are bolded.
VIHREN-CARiM 2019 