(Invited) P2-Nax(Mn,Fe,Co,Ni) Layered Oxides in Na-Batteries

In the perspective of the development of electrical vehicles and also of very large scale renewable energy systems the prevailing parameters are the lifetime, the price and the material availability. From these points of view, sodium based batteries have to be investigated. Our research group studied layered oxides as positive electrode for 30 years. Recently, a general investigation was undertaken with a focus on P2-type layered phases. One of the main interests of this structure is the existence of an ion conduction plane made of face sharing trigonal prims which exhibits a high ionic diffusivity thanks to the existence of a large bottleneck (oxygen rectangle) for sodium diffusion. This structure is able to accommodate a lot of transition metal cations, allowing the optimization of the properties by cationic substitution. The study will be focused on the P2-Nax(Mn,Fe)O2, P2-Nax(Mn,Co)O2 Nax(Mn,Fe,Co)O2 and Nax(Mn,Fe,Ni)O2 systems with various amounts of transition metal ions. All materials crystallize in the hexagonal system (P63/mmc space group). The structure is made of MO2 slabs built of corner sharing MO6 octahedra. The sodium ions are in trigonal prismatic sites. Half of them share edges (Nae) with the MO6 octahedra while the other one share faces (Naf). As the two types of prisms share a common faces, they cannot be occupied simultaneously. The sodium distribution depends of the cationic charge distribution and of the sodium amount is order to minimize the Na+-Na+ and Na+-Mn+ electrostatic repulsions. In all systems, the P2 type packing is preserved in the 0.3 < x < 1 range. The fully intercalated phase is difficult to be obtained due to the very low ionic conductivity when the amount of vacancies is very small. For these compositions, only the Naf prisms are occupied. The specific capacity is in the 130-170 mAh.g-1 depending on the voltage range. In the case of materials with a single transition metal ion like NaxCoO2 and NaxVO2 the shape of the electrochemical curves present a lot of plateau resulting from ordering Na+/Vacancies in the sodium layer. For multi-cations systems, the transition metal disordering is the MO2 slabs prevents from most of the ordering except for the Na.050MO2 composition. A general overview of the electrochemical behavior of these materials will be presented with a special focus on the relation between the oxidation process and the shape of the electrochemical curve.