Phase diagram in NaxMO2 layered oxides – Dinamo

Sodium layered oxides NaxMO2 (where x is comprised between 0 and 1 and M is a 3d cation) were initially studied thirty years ago for use as positive electrode materials in secondary batteries. However, competing lithium based battery technology soon showed more promise and research on sodium compounds for battery applications was largely neglected. More recently, sodium batteries are once again generating interest, in part because sodium is much less expensive than lithium and it is widely available around the world. Numerous studies over the past few years have examined the structure and properties of sodium layered oxide systems, including their performance as electrode materials in sodium battery technologies for stationary applications. Moreover, some phases in these systems exhibited fascinating physical properties such as superconductivity, high thermoelectric power, and metal-insulator transitions. Therefore it appears very attractive to explore new systems in sodium layered oxides.
In this project, we aim to explore new phase diagrams in sodium layered oxide systems NaxMO2 with 4d cations (in a first step with M = Mo, then with M = Nb, Ru or Rh) and to study the structure and the transport and magnetic properties of the single phases existing in these systems. This project is based on an innovative synthetic approach; the controlled electrochemical deintercalation/ intercalation of sodium ions in a battery by fixing the Fermi level of the targeted NaxMO2 phase versus the Na+/Na redox couple. It will be organised in three main work packages. The first package will be the synthesis of the new materials by solid state chemistry, either as powder or as single crystals, followed by the controlled sodium electrochemical deintercalation/ intercalation at room temperature. The electronic and the magnetic properties of single phases obtained in the first package will be systematically examined in the second package as a function of temperature. Thirdly, the structure of new phases with the most promising physical properties (high electronic and ionic conductivity, superconductivity, high thermoelectric power...) will be studied in detail using multiple complementary probes including crystallographic diffraction techniques (X-rays, neutrons or electrons) and local-scale probes such as Pair Distribution Function (PDF) analysis or solid state Nuclear Magnetic Resonance (NMR) Spectroscopy, to elucidate the composition-structure-property relationships.
Whereas the NaxMO2 phases that I propose to study in this project may not find immediate wide spread use as active positive electrode materials in commercial sodium-ion batteries due to their cost and a high atomic weight / charge ratio for the incorporated 4d transition metals, the gained fundamental knowledge of composition-structure-properties relationships is of paramount importance to understand the mechanisms occurring in the positive electrode during the cycling process of all sodium layered oxide based battery technologies. Finally, this project might allow discovering new materials with exceptional electronic properties.