NaxCoO2 System Research Articles

(Plenary) Layered Oxides in Li- and Na- Batteries: Structural and Electronic Effects

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 NaxCoO2 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//NaxCoO2 battery. Simultaneously, J. Goodenough do the same in the homologous Li system (he made the good choice). Then my work was extended to the NaxMO2 (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 LiNi1-xCoxO2 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 t2g 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.

Restoration of long range order of Na ions in NaxCoO2 at high temperatures by sodium site doping

We have systematically investigated the NaxCoO2 system doped with Cu, Y, Sn, W, Au and Bi for x=0.5, 0.75 and 1.00 using density functional theory. Sn, W and Bi always substitute a Co while Au always substitute a Na regardless of Na concentration. However, for, Cu and Y, the substitution site depends on Na concentration. When compared to the available experimental data, we find that thermoelectric performance is enhanced when the dopants substitute a Na site. In this case, surprisingly, resistivity decreases despite the reduced hole concentration caused by carrier recombination. We propose improved carrier mobility to be the cause of observed reduced resistivity.

Standard enthalpy of formation of NaxCoO2 system

Layered sodium cobalt oxide (NaxCoO2) materials have attracted lot of attention in recent times because of their large thermoelectric power. Information on thermodynamic properties is necessary to predict the stability of these compounds. The present study deals with determination of enthalpy of formation of sodium-cobalt oxides [NaxCoO2 (x=0.1, 0.2…0.9)] using a solution calorimeter. The standard molar heat of dissolutions of NaxCoO2 (x=0.1, 0.2…0.9), NaCl(s), CoCl3(s) and H2O (l) in 0.150dm3 (1:1 ratio) aqueous HCl solution of concentration 5.96moldm−3 were measured by an isoperibol solution calorimeter. Using the values of the measured enthalpy of dissolution and other auxiliary data, the standard molar enthalpies of formation (ΔfHm°298) of the compounds were determined. The values of ΔfHm°298 for the Na0.1CoO2(s), Na0.2CoO2(s), Na0.3CoO2(s), Na0.4CoO2(s), Na0.5CoO2(s), Na0.6CoO2(s), Na0.7CoO2(s), Na0.8CoO2(s) and Na0.9CoO2(s) compounds are found to be −(598.61±6.03), −(637.40±6.04), −(676.26±6.04), −(716.53±6.05), −(758.03±6.06), −(800.04±6.07), −(841.58±6.08), −(883.38±6.09), −(924.93±6.10)kJmol−1, respectively.

Electronic phase diagram of LixCoO2revisited with potentiostatically deintercalated single crystals

Electronic phase diagram of LixCoO2 has been re-examined using potentiostatically de-intercalated single crystal samples. Stable phases of x ~ 0.87, 0.72, 0.53, 0.50, 0.43, and 0.33 were found and isolated for physical property studies. A-type and chain-type antiferromagnetic orderings have been suggested from magnetic susceptibility measurement results in x ~ 0.87 and 0.50 below ~ 10 K and 200 K, respectively, similar to those found in NaxCoO2 system. There is no Li vacancy superlattice ordering observed at room temperature for the electronically stable phase Li0.72CoO2 as revealed by synchrotron X-ray Laue diffraction. The peculiar magnetic anomaly near ~ 175 K as often found in powder samples of x ~ 0.46-0.78 cannot be isolated through this single crystal potentiostatic method, which supports the previously proposed explanation to be surface stabilized phase of significant thermal hysteresis and aging character.

Low temperature electric pulse induced resistance switching in Na0.5−δCoO2

We have been able to control the resistance state of the NaxCoO2 system for the x=0.5−δ sample at low temperatures by the application of current pulses. We can reversibly commute between two resistance states, controlling the amplitude and polarity of the applied pulses. We also studied the resistance temperature dependence of the different state and we interpret the switching effect in term of the displacement of Na+ ions induced by current.

Crystal structure and elementary properties ofNaxCoO2(x=0.32, 0.51, 0.6, 0.75, and 0.92) in the three-layerNaCoO2family

The crystal structures of the NaxCoO2 phases based on three-layer NaCoO2, with x=0.32, x=0.51, x=0.60, x=0.75 and x=0.92, determined by powder neutron diffraction, are reported. The structures have triangular CoO2 layers interleaved by sodium ions, and evolve with variation in Na content in a more complex way than has been observed in the two-layer NaxCoO2 system. The highest and lowest Na containing phases studied (x=0.92 and x=0.32) are trigonal, with three CoO2 layers per cell and octahedral Na ion coordination. The intermediate compositions have monoclinic structures. The x=0.75 compound has one CoO2 layer per cell, with Na in octahedral coordination and an incommensurate superlattice. The x=0.6 and x=0.5 phases are also single-layer, but the Na is found in trigonal prismatic coordination. The magnetic behavior of the phases is similar to that observed in the two-layer system. Both the susceptibility and the electronic contribution to the specific heat are largest for x=0.6.