Layered sodium transition metal oxides : structure - properties relationship – LaNaMox

In this project several layered NaxMO2 phases (M = Co, V, Ni or substituted phases with M = Co/Fe/ Mn) were synthetized by direct synthesis using several routes. The materials have been studied as positive electrode in sodium cells.
We aimed to understand deeply the mechanisms associated with the Na intercalation/deintercalation (structural changes and redox processes) by different experimental, in situ or ex situ, techniques (X-ray diffraction (XRD), neutron diffraction, Transmission Electronic Microscopy (TEM), Nuclear Magnetic Resonance (NMR), x-ray absorption spectroscopy (XAS), Mössbauer, Raman …). For some materials, we improved their cycling properties by optimizing the material and the electrolyte composition. For others, we used the electrochemical route to prepare new intermediate NaxMO2 phases, measured their physical properties and study in details their structure.

• Evidence of original redox mechanisms : simultaneous Co3+ and Mn4+ reduction during electrochemical sodium intercalation in P2-Na2/3Co2/3Mn1/3O2 and activity at high voltage of the Fe3+/Fe4+ redox couple in the Nax(Fe,Mn)O2 phases.
• Controlling the synthesis of Nax(Fe,Mn)O2 phases, P2 vs. O3 by changing the Na content and Fe/Mn ratio. New phases were discovered with interesting electrochemical properties in Na cells.
• Vanadium clustering in sodium layered vanadium oxide: structural and electronic transition for the P2-Na1/2VO2 phase.

In the perspective of the development at very large scale of renewable energy systems that require stationary batteries, the prevailing parameters are the lifetime, the price and the material availability. From these points of view, rechargeable sodium-ion batteries are investigated. For this application, we obtained within the joint ANR-NSC project, really interesting results for some of the P2-type layered NaxMO2 (M = 3d transition metal oxide) materials as positive electrode materials for Na-ion batteries. Especially, some of the phases like Nax(Co,Mn)O2 and Nax(Fe,Mn)O2 exhibit very interesting properties. The perspective of the work will be i) to optimize those materials in order to improve their cycling properties (Especially to understand the phenomena occurring at high voltage that seems to limit to lifetime of the materials); ii) To propose new P2-NaxMO2 materials with other 3d elements, dopants or surface coating to improve the electrochemical properties (capacity or stability upon cycling); iii) use modelling (Density Functional Theory (DFT) calculations) to better understand the mechanisms involved in the sodium intercalation/deintercalation (structural changes, redox processes and the sodium diffusion paths in the structures).

- 7 publications and multiple participations to international meeting.

In the NaxMO2 phases, the sodium content plays the main role in the structure and clearly drives the system properties. Regarding the large number of potential applications of these phases : batteries materials, thermoelectric devices, superconductors … the knowledge of their structure versus the Na+ ions content in relation with their physical properties is crucial. Depending on this content, one can observe ordered distribution if the Na+ / vacancy ratio correspond to peculiar values. A change in sodium composition of 1 or 2% is sometimes high enough to induce the formation of a new distribution. The sodium/vacancy distribution in the interslab space has a strong influence on the electronic distribution in the MO2 slab, that can lead to electron localization (with or without charge ordering), electron delocalization or coexistence of both localized and delocalized electrons. This effect can lead to very interesting physical properties of interest for solid state physicists.
From the our (ICMCB) experience on electrochemical intercalation (deintercalation) chemistry we recently investigated the P2-NaxCoO2 phase diagram by performing Na+ ions electrochemical intercalation (deintercalation) in the galvanostatic mode, which allows tuning continuously the sodium ions amount. From the variation of the cell voltage vs. the sodium amount it is also possible to determine the voltage (vs. Na+/Na) range for which a given composition is stable and then prepare by the electrochemical route single phases in a reproducible manner. The phase preparation can be done from sintered pellets (no additives) that will allow the measurements of the physical properties.

In this project, using a similar approach, we aim to:
i) Prepare several NaxMO2 phases (M = Co, V, substituted phases, M = Mo or other 4d transition metal ions) by direct solid state reactions followed by electrochemical Na+ intercalation/deintercalation in order to modify the Na content.
ii) Characterize their structures through long-range techniques (laboratory or synchrotron XRD (ex situ or in situ), neutron diffraction, TEM) and through local-scale techniques (NMR, EXAFS, XANES, Raman spectroscopy). The local-scale characterization will allow to evidence defects vs. ideal structure, partial ordering vs. statistical occupational disorder or large-scale ordering (superstructure), electron localization on some transition metal ions, etc.
iii) Study the physical properties of the phases (electrical and thermal conductivities, Seebeck coefficient, magnetic properties).
iv) To discuss the structure-properties relationship and design new materials with interesting physical properties mainly for thermoelectric applications (and eventually interesting electrochemical properties)


This project should lead in the future to very interesting collaboration with solid state physicists. As our project deals with energy systems: battery and thermoelectric materials, it is in good agreement with the priority topics of the international ANR call in collaboration with Taiwan.

The three laboratories involved in the ANR-NSC project, we apply here for, have complementary expertise and a solid internationally recognized experience in the field of interest:
- the “Institut de Chimie de la Matière Condensée de Bordeaux” (ICMCB) has considerable experience in the domain of layered alkali-transition metal oxides and their electrochemical properties;
- the “Institut des Sciences Moléculaires” (ISM) has a strong expertise in the investigation by Raman spectroscopy of the structure and phase transformations of solid-state materials and in particular of nano-structured materials;
- the group of the Nano-Electrochemistry Laboratory in National Taiwan University of Science and Technology (NTUST) has been devoted to the development of techniques for nano-scale characterization based on ex situ and in situ synchrotron- based x-ray absorption spectroscopy (XAS) and x-ray diffraction (XRD).