Claude Delmas

Electrochemical investigation of the P2–NaxCoO2 phase diagram

Sodium layered oxides NaxCoO2 form one of the most fascinating low-dimensional and strongly correlated systems; in particular P2–NaxCoO2 exhibits various single-phase domains with different Na+/vacancy patterns depending on the sodium concentration. Here we used sodium batteries to clearly depict the P2–NaxCoO2 phase diagram for x≥0.50. By coupling the electrochemical process with an in situ X-ray diffraction experiment, we identified the succession of single-phase or two-phase domains appearing on sodium intercalation with a rather good accuracy compared with previous studies. We reported new single-phase domains and we underlined the thermal instability of some ordered phases from an electrochemical study at various temperatures. As each phase is characterized by the position of its Fermi level versus the Na+/Na couple, we showed that the synthesis of each material, even in large amounts, can be carried out electrochemically. The physical properties of the as-prepared Na1/2CoO2 and Na2/3CoO2 ordered phases were characterized and compared. Electrochemical processes are confirmed to be an accurate route to precisely investigate in a continuous way such a complex system and provide a new way to synthesize materials with a very narrow existence range.

Structural classification and properties of the layered oxides

Layer oxides with formula AxMO2 where M stands for a transition element with two oxidation states or for a mixture of tetravalent and trivalent (or eventually divalent) elements are obtained for 0.5 ≤ x ≤ 1. The lattice is built up by sheets of edge sharing MO6 octahedra between which the alkali ions are inserted with trigonal prismatic or octahedral environment. n nSimilar structures can be found among A2MO3 oxides, the alkali ions lying between (A13M23)O2 sheets. The influence of the pressure on the stability of the various packings is discussed. n nLayer structures are also obtained for the compositions Li8MO6, Li7L□O6 and Li6In2□O6. Structures of these pseudo-2D materials are characterized by a packing of octahedral and tetrahedral sheets where the alkali ions and the vacancies are distributed. n nTransport properties of these materials have been studied.

Electrochemical intercalation of sodium in naxcoo2 bronzes

The chemical study of the NaxCoO2 system (0.5⩽x⩽1) has shown previously the existence of bronze-type phases with layer structure. In each material the lattice is built up by sheets of edge-sharing CoO6 octahedra allowing Na+ ions to be intercalated with trigonal prismatic surrounding (AABBCC or AABB oxygen packing) for small values of x and octahedral environment (ABCABC oxygen packing) for larger ones. As these materials are mixed conductors, they have been used as cathodes in Na batteries at room temperature. Discharge potentials (2.0<V<3.5V) have been measured as a function of the composition. The electrochemical intercalation of sodium in all these materials is reversible within their existence range. During the electrochemical intercalation a reversible charge is observed between phases having AABBCC and ABCABC oxygen packing. This can be explained by the relatively small displacement of the sheets. In contrast, for the material with AABB oxygen packing no transition occurs since a packing change requires breaking of cobalt-oxygen bonds, which is apparently impossible at room temperature.

Optimization of the Composition of the Li1 − z Ni1 + z O 2 Electrode Materials: Structural, Magnetic, and Electrochemical Studies

Lithium nickel oxide, used as the positive electrode in lithium batteries, crystallizes in the rhombohedral system (SG :R3m) with a layered structure. In fact, stoichiometric LiNiO 2 has never been reported. The true formula is Li 1-z Ni 1+z O 2 (0.00 < z < 0.20) ; z is dependent on the experimental conditions. This nonstoichiometry leads to a strong decrease of the battery performance. Therefore, several methods of preparation were investigated to synthesize stoichiometric LiNiO 2 . The Li 0.98 Ni 1.02 O 2 composition, which is very close to the ideal one, was obtained from a mixture of Li 2 O and NiO heated at 700°C. This quasi-2D LiNiO 2 was submitted to several thermal treatments, in order to determine the influence of the temperature on the composition. Purposely lithium deficient phases were also prepared. Correlations between the composition of each material (deduced from the Rietveld refinement of the x-ray diffraction pattern) and the magnetic and electrochemical behavior are discussed

Electrochemical intercalation and deintercalation of NaxMnO2 bronzes

Abstract Various NaxMnO2 bronzes have been electrochemically deintercalated. Na0.40MnO2 has a channel structure which is maintained for a large intercalation range (0.30 ≤ × ≤ 0.58). In order to explain the upper intercalation limit, an ordered sodium distribution between two types of Na+ sites is proposed. Na0.70MnO2 and α-NaMnO2 have lamellar structures of P2 and 0′3 types. During intercalation the original P2 type is maintained for 0.45 ≤ × ≤ 0.85 while two reversible structural transitions are observed from α-NaMnO2. A similar behavior occurs during the deintercalation of the high-temperature β-NaMnO2 variety. In each case of the structural transition the double octahedral layers remain unchanged. Electronic localization (increased by Mn3+ Jahn—Teller effect) tends to trap the Na+ ions and therefore increases the relaxation time of the investigated materials.

An overview of the Li(Ni,M)O2 systems: syntheses, structures and properties

Lithium nickel oxide derivatives are promising positive electrode materials for the next generation of lithium-ion batteries. Partial substitution of certain cations for nickel in this family of oxides significantly modifies their properties and is therefore an attractive route to develop an optimised oxide electrode which satisfies the demanding requirements for rechargeable battery applications. In this paper the interest is focused on the effect of cobalt, iron, aluminium and magnesium for a general discussion of the effect of cationic substitution on the properties based on a review of results mostly obtained in our laboratories. Although iron substitution does not seem interesting for the practical aspect, iron Mossbauer spectroscopy allows very precise characterisations, interesting to understand the general behaviour of this family of materials. We deal with the optimisation of the synthesis conditions in order to obtain the most electrochemically active materials. The relations between the nature of the substituting cation, the presence of foreign cations in the lithium site, the electrochemical behaviour and the redox processes upon electrochemical cycling are discussed in detail. A new view of the relation between this latter point and the cationic distribution formed during the material synthesis is proposed.

The nasicon-type titanium phosphates Ati2(PO4)3 (A=Li, Na) as electrode materials

Lithium and sodium have been intercalated in LiTi 2 (PO 4 ) 3 and NaTi 2 (PO 4 ) 3 respectively. Despite the low electronic conductivity of the NASICON framework the intercalation can be realized either chemically or electrochemically. The electrochemical study shows the reversibility of the process and the existence of large biphased domains in both systems. The observed phase separation reactions result from Li + (Na + ) and e − migration without skeleton bond breaking and recombination. The large hexagonal c -parameter of Li 3 Ti 2 (PO 4 ) 3 results from a peculiar lithium ion distribution (M(1) empty, M(2) fully occupied) as shown by neutron diffraction.

Electrochemical Na-Deintercalation from NaVO2

This paper describes the preparation of NaxVO2 phases by means of electrochemical deintercalation. The curves V = f(x) show that the process is reversible up to x = 0.5 and they indicate the presence of multiple single phase domains in the range 1 < x < 0.5. Two single phases have been successfully isolated for Na1/2VO2 and Na2/3VO2 compositions; we report here the first X-ray powder diffraction measurements of these phases. They present a O3-type monoclinic distortion of the O3 rhombohedral phase observed for Na1VO2. The evolution of cell parameters with sodium deintercalation is comparable with other AxMO2 layered oxides.

A nasicon-type phase as intercalation electrode: NaTi2(PO4)3

Abstract Reversible intercalation of sodium in NaTi 2 (PO 4 ) 3 at room temperature can be achieved either chemically or electrochemically. Na 3 Ti 2 (PO 4 ) 3 is obtained as final product via a two phase mechanism. The non-existence between both extreme compounds of a Na 1+x Ti 2 (PO 4 ) 3 solid solution seems to result from a topotactic demixtion reaction which requires only Na + and e − transfers without skeleton bond breaking and recombination.

Etude par desintercalation electrochimique des systemes NaxCrO2 et NaxNiO2

NaxCrO2 and NaxNiO2 phases have been obtained for the first time by electrochemical desintercalation from NaCrO2 and NaNiO2. Structures of non-stoechiometric phases can be related to the starting material by sheet shifting. Their strong oxidizing power (3.5V vs Na for x ⋍ 0.85) emphasizes the unstability of tetravalent chromium and nickel.