At the end of the 70’s, Huggins, Whittingham, Murphy, Armand and then Rouxel research groups open the way to reversible Lithium batteries with aprotic liquid electrolytes. Most of the researches was devoted to chalcogenides at the positive electrode. Concerning Na battery, only intercalation in TiS₂ and WO₃ was considered.

Working on layered potassium and sodium layered oxides, only from the solid state chemistry point of view, I decided to study intercalation processes in the Na_xCoO₂ system because various structural types could be obtained by high temperature chemistry depending on the Na amount. This led to the first paper on the Na//Na_xCoO₂ battery. Simultaneously, J. Goodenough do the same in the homologous Li system (he made the good choice). Then my work was extended to the Na_xMO₂ (M = Ti, Mn, Ni, Nb) materials and to mix Co,Ni phases. Various systems were explored with Li and Na intercalation. This led, for example to the first intercalation in Nasicon phases.

At this time only alkali metals were used as negative electrode; therefore, the applications were limited. When Sony proposed in 1989 the first Li-ion Battery a fantastic market opened and most of the researches were then focused on materials with promising applications. In 1991, we proposed the LiNi_1-xCo_xO₂ system which is used till now in most of the batteries for spatial applications. The Ni-rich phases, which are now very promising for EV, where intensively studied in my group till 2005, then followed by the lithium-rich system.

Recently, we revisited the Na layered phases with V and Mo. These ions with d² and d³ configurations like to form M-M bonds via their t_2g orbitals, therefore the study of the structural changes upon Na intercalation (deintercalation) was interesting.

In the perspective of the development of very large scale renewable energy systems the prevailing parameters are the lifetime, the price and the material availability. A general investigation was undertaken on Na layered oxides with several transition metal ions. The studies were focused 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.

In all our studies we try to use a solid state chemist approach: structural and physical properties characterization in addition to the electrochemical characterization. The electrochemistry is a probe inside the material which is able to detect all changes (structural or electronic). The solid state chemistry is able to understand what is going on.

Several examples will be given in this general overview.