Y. Chabre

Self-discharge of LiMn2O4/C Li-ion cells in their discharged state: Understanding by means of three-electrode measurements

The potential distribution through plastic Li-ion cells during electrochemical testing was monitored by means of three- or four-electrode measurements in order to determine the origin of the poor electrochemical performance (namely, premature cell failure, poor storage performance in the discharged state) of LiMn{sub 2}O{sub 4}/C Li-ion cells encountered at 55 C. Several approaches to insert reliably one or two reference electrodes that can be either metallic lithium or an insertion compound such as Li{sub 4}Ti{sub 5}O{sub 12} into plastic Li-ion batteries are reported. Using a reference electrode, information regarding the evolution of (1) the state of charge of each electrode within a Li-ion cell, (2) their polarization, and (3) their rate capability can be obtained. From these three-electrode electrochemical measurements, coupled with chemical analyses, X-ray diffraction, and microscopy studies, one unambiguously concludes that the poor 55 C performance is mainly due to the instability of the LiMn{sub 2}O{sub 4} phase toward Mn dissolution in LiPF{sub 6}-type electrolytes. A mechanism, based on Mn dissolution, is proposed to account for the poor storage performance of LiMn{sub 2}O{sub 4}/C Li-ion cells.

Li / β ‐ VOPO 4: A New 4 V System for Lithium Batteries

Lithium intercalation into {beta}-VOPO{sub 4} has been performed, both chemically and electrochemically, with X-ray diffraction characterization of the resulting phases at various levels of intercalation-deintercalation. It is shown that intercalation occurs through a first-order transition at 3.98 V vs. Li{sup +}/Li{sup 0}, which leads to a {beta}-LiVOPO{sub 4}-like structure but with lithium ordering which doubles the a and b unit cell parameters. Starting from chemically intercalated {beta}-Li{sub 0.92}VOPO{sub 4}, the reversible process involves 0.55 electron per transition metal, i.e., 90 mAh/g, at a C/50 rate which owing to the 3.95 V potential plateau, gives an attractive specific energy of 355 Wh/kg. Fundamentally, it is shown herein that the redox energy of V{sup 5+}N{sup 4+} couple in an octahedral coordination involving (V = O){sup m+} unit is included between those observed in oxides and in M{sub 2}(PO{sub 4}){sub 3} compounds with (PO{sub 4}){sup 3{minus}} oxo-anions.

On the origin of the 3.3 and 4.5 V steps observed in LiMn{sub 2}O{sub 4}-based spinels

Different series of LiMn{sub 2}O{sub 4}-based spinels were studied, all presenting two reduction steps at 4.5 and 3.3 V in addition to the normal spinel plateaus around 4 V. A correlation was found between the capacity recovered on these additional steps, the manganese oxidation degree, and the cell parameter of a given spinel. By means of in situ synchrotron diffraction the authors were able to detect the appearance of a new set of diffraction peaks upon oxidation of the 3.3 V step at 3.95 V, that disappeared on subsequent reduction at 3.3 V. Electron diffraction and high resolution electron microscopy studies on partially delithiated samples revealed the formation of double hexagonal layers upon oxidation, consistent with the additional peaks observed in the in situ experiments. Finally, a model to explain the existence of the redox steps at 4.5 and 3.95/3.3 V based on the creation of these double hexagonal layers is proposed.

On the electrochemical reactivity mechanism of CoSb3 vs. lithium

The electrochemical reactivity of CoSb 3 vs. lithium has been studied. This phase reacts with more than 9.5 lithium in a two-step process, consisting of the uptake of 9 Li at a constant voltage close to 0.6 V, and of about one lithium over the final voltage decay to 0.01 V. Upon recharge, only 8 lithium can be extracted. From in situ X-ray diffraction, microscopy, and magnetic measurements, we provide evidence that the constant voltage process is rooted in the decomposition of CoSb 3 , leading to the formation of a composite made of Co and Li 3 Sb nanograins. We also illustrate that the mechanism by which the internal nanostructured (Co + Li 3 Sb) electrode, formed during reduction, converts back to CoSb 3 , is quite unusual. It involves, concomitant with the Li 3 Sb → Li 2 Sb → Sb dealloying reaction, a chemical reaction between Co and Sb nanograins. The extra capacity, measured at low potential, appears to be nested in a decomposition-type reaction catalyzed by the cobalt nanoparticles, in a manner similar to that previously reported for CoO. Although these materials can reversibly uptake about 8 lithium, they are of negligible value, since their capacity rapidly decays with cycling, independent of the electrode processing.

On the Origin of the Second Low‐Voltage Plateau in Secondary Alkaline Batteries with Nickel Hydroxide Positive Electrodes

The nickel oxyhydroxide electrode (NOE) that acts as the positive electrode in Ni-based rechargeable alkaline batteries was studied. A survey of the influence of crystal-chemistry factors (nature and ratio of the various nickel hydroxide phases), cycling parameters (charge/discharge rates, charge/discharge cutoff voltages, and percentage of overcharge), and technological parameters (nature of the current collector, active material morphology, and type of additives) on the appearance of the second voltage plateau was performed. Direct experimental evidence shows that the appearance of the second plateau is directly linked to the amount of the {gamma}-phase present in the nickel oxyhydroxide electrode prior to its discharge. The occasional appearance of this phase in the electrode results from a poor active material/current collector interface related to the electrode-forming technology, or to secondary reactions that can lead to a physical disconnection of the active material upon cycling. Based on these findings, the presence of this {gamma}-phase accounts for the ohmic drop that has led to previous speculation of a barrier layer as the origin of the second plateau. Finally, tentative recommendations to eliminate or mitigate the appearance of the second plateau phenomena are given.

Structural study of NiO2 and CoO2 as end members of the lithiated compounds by in situ high resolution X-ray powder diffraction

Abstract In-situ X-ray diffraction has been carried out on Li x NiO 2 and Li x CoO 2 batteries, using the high resolution powder diffractometer BM16 at ESRF (Grenoble, France). Diffraction was performed in transmission geometry through 1-mm thick Bellcore-type plastic batteries. The batteries were controlled in potentiostatic mode. During these experiments, end-members NiO 2 and CoO 2 have been isolated, and high resolution patterns have been recorded in order to solve their structures from powder data. NiO 2 structure is of CdI 2 type, with a monoclinic distortion. CoO 2 seems to be of CdCl 2 type also with a monoclinic distortion.

Electrochemical and structural study of the 3.3 V reduction step in defective LixMn2O4 and LiMn2O(4−y)Fy compounds

Abstract Non-stoichiometric or fluorine-substituted Li x Mn 2 O 4 are known to present a reversible redox step at 4.5 V as well as a reduction step at 3.3 V appearing to the expense of the usual 4.0–4.15 V. We present results of very slow stepwise potentiodynamic studies which allowed to localise the oxidation step associated to the 3.3 V reduction level close to 3.95 V. We propose the cubic spinel lattice parameter value as the key parameter for knowing a priori if these redox states will be present or not, with 8.215 A as threshold value. In situ XRD studies performed across the 3.3↔3.95 V redox step indicate that these additional states are probably associated to structural features.

NMR studies of lamellar intercalation compounds

Abstract We present a review of recent NMR investigations of lamellar intercalation compounds. We first give a general discussion of the various information that NMR can shed on these systems: the symmetry of the sites occupied by the intercalated species, their degree of ionization, the change in the electronic and magnetic properties of the host matrix, and the dynamics of the intercalated species. As far as this last point is concerned, we underline the importance of the two-dimensional character of the atomic diffusion in the interpretation of NMR data. Experimental data in various lamellar intercalated compounds - transition metal dichalcogenides, transition metal phosphorus trichalcogenides, and graphite - are first considered from the point of view of charge donation and change in the electronic properties of the host matrix. Finally, NMR studies of the dynamic of intercalated species are reviewed, in particular in the case of cathode materials.

Competition between the Two Reduction Reactions of the γ(III) Phase in the Ni-Based Batteries

Abstract We show that the γ(III)/β(II) phase transition is at the origin of the second plateau at 0.8V in the Ni-based batteries by means of transmission electron microscopy (TEM) and X-ray diffraction (XRD) studies and intermittent galvanostatic measurements (GITT). This study shows that the equilibrium voltage of this transition is 1.25V vs Cd-Cd(OH)2, thus implying an overvoltage of 400 mV. Direct evidence for a textural memory effect trough the γ(III)/β(II) phase transition is given by TEM studies. The reoxidation of the β(II) phase to γ(III) phase occurs without going trough the β(III) phase.