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Reversible anionic redox reactions represent a transformational change for creating advanced high-energy-density positive-electrode materials for lithium-ion batteries. The activation mechanism of these reactions is frequently linked to ligand-to-metal charge transfer (LMCT) processes, which have not been fully validated experimentally due to the lack of suitable model materials. Here we show that the activation of anionic redox in cation-disordered rock-salt Li1.17Ti0.58Ni0.25O2involves a long-lived intermediate Ni3+/4+species, which can fully evolve to Ni2+during relaxation. Combining electrochemical analysis and spectroscopic techniques, we quantitatively identified that the reduction of this Ni3+/4+species goes through a dynamic LMCT process (Ni3+/4+–O2−→ Ni2+–On−). Our findings provide experimental validation of previous theoretical hypotheses and help to rationalize several peculiarities associated with anionic redox, such as cationic–anionic redox inversion and voltage hysteresis. This work also provides additional guidance for designing high-capacity electrodes by screening appropriate cationic species for mediating LMCT.

Anionic redox is a double-edged sword for Li-ion cathodes because it offers a transformational increase in energy density that is also negated by several detrimental drawbacks to its practical implementation. Among them, voltage hysteresis is the most troublesome because its origin is still unclear and under debate. Herein, we tackle this issue by designing a prototypical Li-rich cation-disordered rock-salt compound Li1.17Ti0.33Fe0.5O2that shows anionic redox activity and exceptionally large voltage hysteresis while exhibiting a partially reversible Fe migration between octahedral and tetrahedral sites. Through combined in situ and ex situ spectroscopic techniques, we demonstrate the existence of a non-equilibrium (adiabatic) redox pathway enlisting Fe3+/Fe4+and O redox as opposed to the equilibrium (non-adiabatic) redox pathway involving sole O redox. We further show that the charge transfer from O(2p) lone pair states to Fe(3d) states involving sluggish structural distortion is responsible for voltage hysteresis. This study provides a general understanding of various voltage hysteresis signatures in the large family of Li-rich rock-salt compounds.

Sodium ion batteries, because of their sustainability attributes, could be an attractive alternative to Li-ion technology for specific applications. However, it remains challenging to design high energy density and moisture stable Na-based positive electrodes. Here, we report an O3-type NaLi1/3Mn2/3O2phase showing anionic redox activity, obtained through a ceramic process by carefully adjusting synthesis conditions and stoichiometry. This phase shows a sustained reversible capacity of 190 mAh g−1that is rooted in cumulative oxygen and manganese redox processes as deduced by combined spectroscopy techniques. Unlike many other anionic redox layered oxides so far reported, O3-NaLi1/3Mn2/3O2electrodes do not show discernible voltage fade on cycling. This finding, rationalized by density functional theory, sheds light on the role of inter- versus intralayer 3dcationic migration in ruling voltage fade in anionic redox electrodes. Another practical asset of this material stems from its moisture stability, hence facilitating its handling and electrode processing. Overall, this work offers future directions towards designing highly performing sodium electrodes for advanced Na-ion batteries.

Calcium batteries represent a promising alternative to lithium metal systems. The combination of the low redox potential and low cost and the energy-dense calcium anode (2073 mAh/cm3, similar to 2044 mAh/cm3for Li) with appropriate low-cost cathode materials such as sulfur could produce a game-changing technology in several fields of applications. In this work, we present the reversible activity of a proof-of-concept Ca/S battery at room temperature, characterized by a surprising medium-term cycling stability with low polarization, promoted by the use of a simple positive electrode made of sulfur supported on an activated carbon cloth scaffold, and a state-of-the-art fluorinated alkoxyborate-based electrolyte. Insights into the electrochemical mechanism governing the chemistry of the Ca/S system were obtained for the first time by combining X-ray photoelectron spectroscopy and X-ray absorption spectroscopy. The mechanism implies the formation of different types of soluble polysulfide species during both charge and discharge at room temperature, and the formation of solid CaS at the end of discharge. The reversible electrochemical activity is proven by the reformation of elemental sulfur at the end of the following charge. These promising results open the way to the comprehension of emerging Ca/S systems, which may represent a valid alternative to Mg/S and Li/S batteries.

The recent discovery of anionic redox as a means to increase the energy density of transition-metal oxide positive electrodes is now a well-established approach in the Li-ion battery field. However, the science behind this new phenomenon pertaining to various Li-rich materials is still debated. Thus, it is of paramount importance to develop a robust set of analytical techniques to address this issue. Herein, we use a suite of synchrotron-based X-ray spectroscopies as well as diffraction techniques to thoroughly characterize the different redox processes taking place in a model Li-rich compound, the tridimensional hyperhoneycomb β-Li2IrO3. We clearly establish that the reversible removal of Li+from this compound is associated with a previously described reductive coupling mechanism and the formation of the M-(O–O) and M-(O–O)* states. We further show that the respective contributions to these states determine the spectroscopic response for both Ir L3-edge X-ray absorption spectroscopy and X-ray photoemission spectroscopy. Although the high covalency and the robust tridimensional structure of this compound enable a high degree of reversible delithiation, we found that pushing the limits of this charge compensation mechanism has significant effects on the local as well as average structure, leading to electrochemical instability over cycling and voltage decay. Overall, this work highlights the practical limits to which anionic redox can be exploited and sheds some light on the nature of the oxidized species formed in certain lithium-rich compounds.

The development of efficient water splitting heterogeneous catalysts in acidic media is largely hampered by both the slow kinetics of the oxygen evolution reaction (OER) and the limited stability of the electrocatalyst under these harsh working conditions. At the origin of both the activity and the instability of iridium oxides used as OER catalysts in acid conditions is the formation of high-valence IrOxintermediates such as IrO3. Nevertheless, owing to its reactivity with water, this intermediate was never stabilized, and its electrochemical/chemical behavior was never studied in depth. Here, we report a strategy to stabilize this elusive IrO3intermediate. Furthermore, we demonstrate that upon reactivity with water, gaseous oxygen is generated, while the intermediate is protonated following the reaction IrO3+ xH2O → H2xIrO3+ x/2O2. The resulting hydrated iridium oxide H2IrO3, which possesses three-dimensional proton intercalation/deintercalation channels, can be considered as an enlightening OER model electrocatalyst in acidic media. Owing to its unique proton insertion ability, we could study a yet ill-described charge compensation mechanism upon OER relying on the bulk reversible exchange of protons, which shows remarkable performances and stability. Indeed, enhanced OER is measured, with a dissolution rate lower than the one measured for the “stellar” IrOx/SrIrO3catalyst. Such a finding opens the route toward the design of new catalysts enlisting proton insertion that could be competitive for water splitting in acidic media.

A better control of the metal–electrolyte interface is mandatory to help the development of rechargeable magnesium batteries. Protecting the magnesium surface by a coating layer to avoid its passivation with conventional liquid electrolytes is a promising strategy. Herein, in contact with a mercury droplet, a crystalline amalgam layer is created on the surface of magnesium. After the creation of this surface coating, a remarkable improvement of the plating/stripping process is observed with magnesium bis(trifluoromethanesulfonyl)imide/dimethoxyethane electrolyte, whereas strong surface passivation hindering electrochemical cycling occurs when using bare magnesium. The detailed investigation of the evolution of the amalgam layer during cycling, however, shows substantial chemical and morphological electrochemical changes, eventually showing that such a layer does not act as a protecting coating, but rather as an independent electrode material deposited on an inert Mg substrate. The amalgam process enables the formation of the surface layer mainly containing crystalline MgHg2on the magnesium electrode. Enhanced magnesium plating/stripping is observed with magnesium bis(trifluoromethanesulfonyl)imide/dimethoxyethane electrolyte. The amalgam surface layer undergoes chemical and morphological changes upon cycling.

Density Functional Theory is applied to understand the large difference in surface reactivity of LiCoO2due to Al/Co substitution which is experimentally observed. In this way, we explore the SO2and CO2adsorption modes on the (110) surface of LiCoO2and α-LiAlO2. For SO2adsorption, chemisorption produces sulfite species (for LiCoO2and α-LiAlO2) and sulfate species (in the case of LiCoO2). We demonstrate that the modification of the surface reactivity when Co3+ions are substituted by Al3+ions is due to a change from an adsorption mode controlled by redox properties for LiCoO2to a less energetically favorable adsorption mode controlled by acid–base properties for α-LiAlO2. For CO2adsorption, the formation of carbonate species is observed for both compounds, illustrating the fundamental difference in the factors controlling SO2adsorption compared to CO2adsorption.

Li-ion batteries are growing in demand and such growth calls for the quest for high-energy-density electrode materials. Li-rich layered oxides that show both cationic and anionic redox are expected to meet the high energy requirement. However, the oxygen anion activity triggers numerous structural and electronic rearrangements that need to be understood prior to envisioning applications. Here, we chemically design two new LixIrS2polymorphs to further interrogate the mechanisms of the ligand redox process. By combined structural and spectroscopic characterizations, we show that electrochemical lithiation/delithiation of the polymorphs involve different sulfur redox couples that stand as unusual behavior. These structure-dependent kinetic pathways lead to an ∼1 V difference between the two polymorphs, hence providing the missing link between the structure and hysteresis in anionic redox systems. These insights into the origin of hysteresis can guide proper parameters to cure it, hence laying the groundwork for the design of new practical electrode materials.

In this work, abundant and environmentally friendly nano-fibrillated (NFC) cellulose is used for fabrication of porous separator membranes according to the procedure adopted from papermaking industry. As-prepared NFC separators were characterized in terms of thickness, porosity, wettability, electrochemical stability and electrochemical performance in lithium-sulfur and Li-symmetrical pouch cells and compared to a commercial Celgard 2320 separator membrane. Results demonstrated that morphology and electrochemical performance of NFC separator outperforms the conventional polyolefin separator. Due to exceptional interplay between lithium metal and cellulose, this research provides a self-standing NFC separator that can be used besides the lithium-sulfur also in other lithium metal battery configurations.

In this study, a gel polymer electrolyte (GPE) based on polymer ionic liquid (PIL) is used in a solvent-free and in a hybrid electrolyte configuration for Li-S batteries. Results obtained in the solvent-free configuration show a high discharge capacity in the first cycle and excellent coulombic efficiency during cycling. Capacity fading and polarization increase during cycling are explained based on the XPS and EIS measurements. The results of the present study are indicating that the increase of various internal resistance contributions and capacity fading are related with an accumulation of polysulfides in the GPE-PIL layer or/and on the surface of the lithium anode. Within a hybrid battery configuration, the thickness of the GPE-PIL layer is thinner, and the volume where polysulfides can be trapped is smaller. Such a configuration shows better cycling stability. The hybrid configuration outperforms cycling stability of the conventional configuration with a liquid electrolyte. This is explained by increased internal resistance in the convential configuration while the polarization in the first 100 cycles is constant in the hybrid configuration. Additionally, the hybrid configuration exhibits excellent C-rate performance.

Li(Ni,Mn,Co)O2/graphite lithium-ion batteries designed to work at high temperature were tested upon cycling at 85°C in various voltage ranges. Aging processes in relation with changes in the electrode/electrolyte interfaces at both electrodes were investigated by X-ray Photoelectron Spectroscopy (XPS). Changes were observed in the composition of the solid electrolyte interphase (SEI) at the surface of graphite electrode due to the high cycling temperature: disappearance of carbonates and increase of inorganic compounds coming from degradation of the salt. A slight dissolution process of the PVdF positive electrode binder was evidenced. It was also observed that the formation and thickness of the passivation layer at the positive electrode surface at 85°C depends on the cycling voltage range.