Laurence Croguennec

Polyanionic (Phosphates, Silicates, Sulfates) Frameworks as Electrode Materials for Rechargeable Li (or Na) Batteries

For more than 20 years, most of the technological achievements for the realization of positive electrodes for practical rechargeable Li battery systems have been devoted to transition metal oxides such as LixMO2 (M = Co, Ni, Mn), LixMn2O4, LixV2O5, or LixV3O8. The first two classes of materials built on close-packed oxygen stacking adopt bidimensional and tridimensional crystal structures, respectively (Figure 1), from which lithium ions may be easily intercalated or extracted in a reversible manner. These oxides are reasonably good ionic and electronic conductors, and lithium insertion/extraction proceeds while operating on the M4+/M3+ redox couple, located between 4 and 5 V versus Li+/Li...

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.

Synthesis and characterization of new LiNi1-yMgyO2 positive electrode materials for lithium-ion batteries

New LiNi 1-y Mg y O 2 (0 ≤ y ≤ 0.20) layered oxides were synthesized by a coprecipitation method followed by a high-temperature thermal treatment. Rietveld refinements of their X-ray diffraction patterns showed that they exhibit a quasi-two-dimensional structure, isostructural to LiNiO 2 . for small substitution amounts (y ≤ 0.10). For larger amounts (y = 0.15, 0.20), the Li/(Ni + Mg) ratio is significantly lower than unity. In all cases, the extra ions located in the inter-slab space for lithium deficiency compensation are preferentially Mg 2+ ions. A magnetic study confirmed the cationic distributions which result from the size difference between Ni 3+ and Mg 2+ ions. An electrochemical study showed reversible behavior for all materials. A high capacity (≥ 150 Ah kg -1 ) was found for LiNi 1-y Mg y O 2 phases (y ≤ 0.02), which decreased when y increased. The presence of Mg 2+ cations in the inter-slab space, which cannot be oxidized and have a size close to Li + , prevents the local collapses of the structure which occurs for the Li 1-zNi1+zO2 system; therefore good cycling stability is observed.

On the structure of Li3Ti2(PO4)3

The structure of the Nasicon-type phase Li3Ti2(PO4)3, obtained by chemical lithiation of LiTi2(PO4)3, has been characterised using neutron diffraction for the long range structure and 7Li NMR for more local information. The lithium atoms were precisely located from the neutron diffraction data, using nuclear difference Fourier maps. The lithium ions, which were known to be in the large M2 cavity, occupy two M3 and M′3 subsites (distorted tetrahedra) with occupation factors of 2/3 and 1/3, respectively. From these two intermediate sites, it was shown that the diffusion pathway between two M1 sites in these Nasicon-type structures consists of a set of seven face-sharing tetrahedra. A variable temperature 7Li MAS NMR study showed a set of signals due to a distribution of environments for a given Li+ ion, in terms of occupied or vacant M3/M′3 sites in its vicinity. Elevation of the temperature to 353 K leads to a reversible exchange of these signals, due to fast hopping of Li between the two sites within a given M2 cavity.

Structural and electrochemical properties of LiNi0.70Co0.15Al0.15O2

LiNi0.70Co0.15Al0.15O2 has been synthesized by a coprecipitation method. The structural characterization by X-ray and neutron diffraction, associated with a Rietveld analysis, has confirmed a segregation tendency for cobalt and aluminum ions. However, a quasi-ideal lamellar structure was obtained for this phase, with less than 1% extra-nickel ions in the interslab space. Cycling tests have shown a very good cycling stability with a high reversible capacity of about 150 mAh/g in the 3–4.15 V range at the C/20 rate.

Electrochemical cyclability of orthorhombic LiMnO{sub 2}: Characterization of cycled materials

The electrochemical removal of lithium from orthorhombic LiMnO 2 (o-LiMnO 2 ) leads to a phase transition with a first plateau at about 3.7 V. This corresponded to the formation of a spinel-like material; a possible transition to a rhombohedral Li 2 MnO 2 phase was ruled out through structural and crystal-site energy considerations. Several electrochemical cycles were necessary to achieve a complete phase transformation; the smaller the crystallites/crystals the fewer the number of cycles needed. The capacity difference between large and small crystallite/crystal compounds was ascribed to kinetic reasons as shown by ex situ x-ray diffraction analyses and quasi-equilibrium electrochemical studies. Capacities as high as 200 Ah/kg were found for 0.3 μm crystal size materials. Contrary to the spinel prepared at high temperature, the electrochemically obtained spinel-like phase cycled very well in the 2.5 to 4.3 V range, suggesting structural differences between the two materials. An extended x-ray absorption fine structure study at the manganese K edge confirmed this observation through a marked difference between the manganese second neighbors for two compounds. This can be related to the orthorhombic-to-cubic phase transition itself and/or to the memory effect of the stacking faults originally present in o-LiMnO 2 .

Layered Li1 + x ( Ni0.425Mn0.425Co0.15 ) 1 − x O2 Positive Electrode Materials for Lithium-Ion Batteries

Layered Li 1 + x (Ni 0 . 4 2 5 Mn 0 . 4 2 5 Co 0 . 1 5 ) 1 - x O 2 materials (0 ≤ x ≤ 0.12) were prepared at 1000°C for 12 h in air by a coprecipitation method. As x increased in Li 1 + x (Ni 0 . 4 2 5 Mn 0 . 4 2 5 Co 0 . 1 5 ) 1 - x O 2 , the substitution of x Li + ions for x transition metal ions induced for charge compensation an increase in the average transition metal oxidation state. X-ray photoelectron spectroscopy analyses showed that cobalt and manganese were present in these materials in the trivalent and tetravalent states, respectively, and that increasing overlithiation led to the oxidation of Ni 2 + ions into Ni 3 + ions. The refinement of the crystal structure of these materials in the R3m space group and magnetic measurements showed a decrease in the Ni occupancy in the Li layers with increasing overlithiation. From an electrochemical point of view, the reversible capacity in the 2-4.3 V range decreased with overlithiation.

Raman and FTIR Spectroscopy Investigations of Carbon-Coated Li x FePO4 Materials

Raman and Fourier transform infrared (FTIR) spectroscopy investigations were performed on carbon-coated LiFePO 4 materials differing by the temperature of their thermal treatments (575 and 800°C) and by their electrochemical performance, with that obtained at a higher temperature showing larger reversible capacity and better capacity retention at high rates. Raman spectra gave information on the carbon located at the surface of the LiFePO 4 particles, which was shown for the two samples to be highly disordered with small in-plane correlation lengths (<3 nm). A UV Raman study has shown that these carbon coatings contain almost no sp 3 -type carbon hybridization. This study has also highlighted again that the sp 3 -type C/sp 2 -type C ratio cannot be determined straightforwardly from Raman spectra recorded with visible excitation (such as 632.8 nm), and thus that no direct correlation can be done between the Raman band intensity ratio I D /I G and the sp 3 -type C/sp 2 -type C ratio; a UV Raman study is necessary to get the true information on the sp 3 -type C contribution. The baseline and absolute intensity of the FTIR spectra were shown to be sensitive to changes in the electronic conductivity of the C-LiFePO 4 samples. Furthermore, good crystallinity was maintained for Li x FePO 4 materials upon cycling, showing good reversibility of the lithium deintercalation/intercalation reaction.

Electrochemical behavior of orthorhombic LiMnO2: influence of the grain size and cationic disorder

Abstract Stoichiometric orthorhombic O-LiMnO 2 , prepared at high temperature from a mixture of Mn 2 O 3 and LiOH.H 2 O, is electrochemically active in a lithium deintercalation/intercalation process. The samples with the smallest grains ( O μ m), and corresponding to the phases prepared with some lithium hydroxide deficiency developed the higher weight capacities. These could reach up to 160 mAh/g under a C/15 regime following a forming that spanned over about thirty cycles. The cells capacity remained constant after forming, showing the very good cyclability of the system. It has not been clearly demonstrated whether the Li/Mn substitution in the O-LiMnO 2 samples played an important role in the electrochemical characteristics of the Li/Li x MnO 2 cells, although the group with small grains also showed, on average, a higher cationic disorder. A phase transition took place upon the first oxidation of O-LiMnO 2 . The new phase presented an electrochemical behavior resembling that of spinel LiMn 2 O 4 . A modeling of the discharge curves showed a progressive forming of the materials during a few cycles.

7Li and 1 H MAS NMR Observation of Interphase Layers on Lithium Nickel Oxide Based Positive Electrodes of Lithium-Ion Batteries

7Li and 1 H magic-angle spinning (MAS) NMR spectra were recorded for Li(Ni.Co,Al)O 2 samples. The 7 Li MAS NMR signal of the layer on the pristine material is close to that of Li 2 CO 3 in close contact with it. Its lineshape, due to dipolar interaction with electron spins of the material, is drastically different from that of pure Li 2 CO 3 . Upon hydration, this layer grows, while 1 H NMR suggests that some lithium in the material is replaced by protons. Upon electrochemical cycling, the original layer is replaced by a different one, the solid electrolyte interphase, with distinct 7 Li and 1 H NMR signatures, showing its largely organic nature.