Rémi Dedryvère

Li–S batteries: simple approaches for superior performance

Although promising improvements have been made in the field of Li–S rechargeable batteries, they are still far from reaching the market place due to several drawbacks. To combat the solubility of polysulphides, confinement approaches aiming to trap sulphur within the cathode side have been pursued, but success has been limited. Herein, we drastically deviate from this approach and use a liquid cathode obtained either by dissolving polysulphides within the electrolyte or by placing sulphur powders in contact with the Li negative electrode. Such approaches are shown to result in greater performance than confinement approaches. Such a strategy eliminates the detrimental Li2S formation inside a porous carbon matrix and moreover leads to the formation of a protective SEI layer at the Li electrode, as deduced by impedance spectroscopy and XPS, which seems beneficial to the cell cycling performance.

XPS Identification of the Organic and Inorganic Components of the Electrode/Electrolyte Interface Formed on a Metallic Cathode

X-ray photoelectron spectroscopy (XPS) was used to determine the nature and composition of electrode/electrolyte interfaces forming during the 55°C cycling of Li-based cells in ethylene carbonate:dimethyl carbonate LiPF 6 electrolyte using a heat-treated stainless steel substrate as the positive electrode. From a classical analysis of the XPS C 1s, O 1s, F 1s, P 2p, and Li Is core peak spectra complemented by an unusual detailed interpretation of XPS valence spectra, we could follow, as a function of the cell cycling history, the evolution and nature of the species constituting the organic/inorganic layer as well as determine its approximate composition. We have shown that this surface layer mainly consists of PEO oligomers (-CH 2 -CH 2 -O-) n , carbonates Li 2 CO 3 and/or CH 3 OCO 2 Li, LiPF 6 salt, and of degradation products of the salt such as LiF and phosphates. Moreover, we give evidence that this layer does not only grow but also becomes richer in CH 3 OCO 2 Li and LiF species upon cycling.

Improved Performances of Nanosilicon Electrodes Using the Salt LiFSI: A Photoelectron Spectroscopy Study

Silicon is a very good candidate for the next generation of negative electrodes for Li-ion batteries, due to its high rechargeable capacity. An important issue for the implementation of silicon is the control of the chemical reactivity at the electrode/electrolyte interface upon cycling, especially when using nanometric silicon particles. In this work we observed improved performances of Li//Si cells by using the new salt lithium bis(fluorosulfonyl)imide (LiFSI) with respect to LiPF6. The interfacial chemistry upon long-term cycling was investigated by photoelectron spectroscopy (XPS or PES). A nondestructive depth resolved analysis was carried out by using both soft X-rays (100-800 eV) and hard X-rays (2000-7000 eV) from two different synchrotron facilities and in-house XPS (1486.6 eV). We show that LiFSI allows avoiding the fluorination process of the silicon particles surface upon long-term cycling, which is observed with the common salt LiPF6. As a result the composition in surface silicon phases is modified, and the favorable interactions between the binder and the active material surface are preserved. Moreover a reduction mechanism of the salt LiFSI at the surface of the electrode could be evidenced, and the reactivity of the salt toward reduction was investigated using ab initio calculations. The reduction products deposited at the surface of the electrode act as a passivation layer which prevents further reduction of the salt and preserves the electrochemical performances of the battery.

XPS investigation of surface reactivity of electrode materials: effect of the transition metal.

The role of the transition metal nature and Al2O3 coating on the surface reactivity of LiCoO2 and LiNi(1/3)Mn(1/3)Co(1/3)O2 (NMC) materials were studied by coupling chemisorption of gaseous probes molecules and X-ray photoelectron (XPS) spectroscopy. The XPS analyses have put in evidence the low reactivity of the LiMO2 materials toward basic gaseous probe (NH3). The reactivity toward SO2 gaseous probe is much larger (roughly more than 10 times) and strongly influenced by the nature of metal. Only one adsorption mode (redox process producing adsorbed sulfate species) was observed at the LiCoO2 surface, while NMC materials exhibit sulfate and sulfite species at the surface. On the basis of XPS analysis of bare materials and previous theoretical work, we propose that the acid-base adsorption mode involving the Ni(2+) cation is responsible for the sulfite species on the NMC surface. After Al2O3 coating, the surface reactivity was clearly decreasing for both LiCoO2 and NMC materials. In addition, for LiCoO2, the coating modifies the surface reactivity with the identification of both sulfate and sulfite species. This result is in line with a change in the adsorption mode from redox toward acid-base after Al/Co substitution. In the case of NMC materials, the coating induced a decrease of the sulfite species content at the surface. This phenomenon can be related to the cation mixing effect in the NMC.

Role of propane sultone as an additive to improve the performance of a lithium-rich cathode material at a high potential

This study presents the use of 1,3-propane sultone (PS) in the [EC–DMC + 1 mol L−1 LiPF6] electrolyte as a protective additive for the Li-rich-NMC xLi2MnO3–(1 − x)LiMO2 (x ≫ 1; M = Ni, Co, Mn) cathode–electrolyte interface during cathode material activation and cycling at a high potential (5 V vs. Li). The results showed that the presence of 1% PS (w/w) ensured complete and better electrode activation during the first cycle than EC–DMC + 1 mol L−1 LiPF6. Thus, Li//Li-rich-NMC half-cell and Gr//Li-rich-NMC full-cell provided capacities as high as C = 330 mA h g−1 during charge and C = 275 mA h g−1 during discharge with a higher cut-off voltage of 5 V. Measurements by cyclic voltammetry demonstrated that activating at such a voltage enhanced the redox activity from Li2MnO3 activation. At same time, the contribution of nickel and cobalt electroactivity is decreased at their regular voltage. This feature was attributed to structural modifications occurring on the surface to the bulk of the material. Long-cycling tests of Li//Li-rich-NMC half-cells with PS provided a higher reversible capacity and superior capacity retention (245 mA h g−1 after 240 cycles) with good coulombic efficiency (99 ± 1%) and better high-discharge rate capability (above 180 mA h g−1 at 1 C regime) than those obtained using conventional electrolytes without additive.

Fundamental interplay between anionic/cationic redox governing the kinetics and thermodynamics of lithium-rich cathodes

Reversible anionic redox has rejuvenated the search for high-capacity lithium-ion battery cathodes. Real-world success necessitates the holistic mastering of this electrochemistry’s kinetics, thermodynamics, and stability. Here we prove oxygen redox reactivity in the archetypical lithium- and manganese-rich layered cathodes through bulk-sensitive synchrotron-based spectroscopies, and elucidate their complete anionic/cationic charge-compensation mechanism. Furthermore, via various electroanalytical methods, we answer how the anionic/cationic interplay governs application-wise important issues—namely sluggish kinetics, large hysteresis, and voltage fade—that afflict these promising cathodes despite widespread industrial and academic efforts. We find that cationic redox is kinetically fast and without hysteresis unlike sluggish anions, which furthermore show different oxidation vs. reduction potentials. Additionally, more time spent with fully oxidized oxygen promotes voltage fade. These fundamental insights about anionic redox are indispensable for improving lithium-rich cathodes. Moreover, our methodology provides guidelines for assessing the merits of existing and future anionic redox-based high-energy cathodes, which are being discovered rapidly.Anionic redox chemistry has enabled the design of high-capacity battery cathodes for energy storage. Here, the authors demonstrate reversible anionic redox in an archetypical lithium-rich oxide via bulk-sensitive spectroscopies, further revealing its crucial role in practically important properties.

Energy Storage - Batteries and Supercapacitors - ISTE- WILEY

The electrochemical energy storage is a means to conserve electrical energy in chemical form. This form of storage benefits from the fact that these two energies share the same vector, the electron. This advantage allows us to limit the losses related to the conversion of energy from one form to another. The RS2E focuses its research on rechargeable electrochemical devices (or electrochemical storage) batteries and supercapacitors. The materials used in the electrodes are key components of lithium-ion batteries. Their nature depend battery performance in terms of mass and volume capacity, energy density, power, durability, safety, etc. This book deals with current and future positive and negative electrode materials covering aspects related to research new and better materials for future applications (related to renewable energy storage and transportation in particular), bringing light on the mechanisms of operation, aging and failure.The electrochemical energy storage is a means to conserve electrical energy in chemical form. This form of storage benefits from the fact that these two energies share the same vector, the electron. This advantage allows us to limit the losses related to the conversion of energy from one form to another. The RS2E focuses its research on rechargeable electrochemical devices (or electrochemical storage) batteries and supercapacitors. The materials used in the electrodes are key components of lithium-ion batteries. Their nature depend battery performance in terms of mass and volume capacity, energy density, power, durability, safety, etc. This book deals with current and future positive and negative electrode materials covering aspects related to research new and better materials for future applications (related to renewable energy storage and transportation in particular), bringing light on the mechanisms of operation, aging and failure.

Fluorinated reduced graphene oxide as a protective layer on the metallic lithium for application in the high energy batteries

Metallic lithium is considered to be one of the most promising anode materials since it offers high volumetric and gravimetric energy densities when combined with high-voltage or high-capacity cathodes. However, the main impediment to the practical applications of metallic lithium is its unstable solid electrolyte interface (SEI), which results in constant lithium consumption for the formation of fresh SEI, together with lithium dendritic growth during electrochemical cycling. Here we present the electrochemical performance of a fluorinated reduced graphene oxide interlayer (FGI) on the metallic lithium surface, tested in lithium symmetrical cells and in combination with two different cathode materials. The FGI on the metallic lithium exhibit two roles, firstly it acts as a Li-ion conductive layer and electronic insulator and secondly, it effectively suppresses the formation of high surface area lithium (HSAL). An enhanced electrochemical performance of the full cell battery system with two different types of cathodes was shown in the carbonate or in the ether based electrolytes. The presented results indicate a potential application in future secondary Li-metal batteries.