Understanding Na+ Diffusion, Physicochemical Behavior, and Electrochemical Performance of a Gel Polymer Electrolyte.

Gel polymer electrolytes (GPEs) represent a credible alternative to organic liquid electrolytes (LEs) for safer sodium metal batteries. As a compromise between solid polymer electrolytes and LEs, GPEs ensure a good ionic conductivity, improve the electrolyte/electrode interface, and prevent solvent leaks. Herein, a GPE based on acrylate-bifunctionalized polyethylene glycol chains mixed with an ether solvent (TEGDME) and a polyethylene glycol diacrylate (PEG600DA) in a 50/50 wt % ratio was prepared by ultraviolet photopolymerization. Sodium bis(fluorosulfonyl)imide salt (NaFSI) was added at different concentrations to study its interactions with the solvent and/or the cross-linked polymer. Infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and swelling ratio characterizations were combined to determine the physicochemical properties of the GPE. Complementary characterizations including electrochemical impedance spectroscopy, chronopotentiometry, and cyclic voltammetry allowed correlating the physicochemical properties of the GPE to its electrochemical performance. Then, improvements were obtained by careful combination of its components. The cross-linking agent allowed us to obtain a polymer matrix that traps the organic solvent and prevents leakage. Such a solvent inclusion reduces the rigidity of the membrane and lowers its viscosity, offering a room temperature ionic conductivity of 4.8 × 10-4 Ω-1 cm-1. The control of polymer's tortuosity leads to a stable cycling vs sodium metal over several hundred hours without increase of the polarization. Finally, optimization of the salt loading plays a major role in electrostatic cross-linking, leading to an improvement of the mechanical properties of the GPE without reducing its conductivity.

Lithium and cobalt extraction from LiCoO2 assisted by p(VBPDA-co-FDA) copolymers in supercritical CO2.

Supercritical CO2 (scCO2) extraction assisted by complexing copolymers is a promising process to recover valuable metals from lithium-ion batteries (LIBs). CO2, in addition to being non-toxic, abundant and non-flammable, allows an easy separation of metal-complexes from the extraction medium by depressurization, limiting the wastewater production. In this study, CO2-philic gradient copolymers bearing phosphonic diacid complexing groups (poly(vinylbenzylphosphonic diacid-co-1,1,2,2-tetrahydroperfluorodecylacrylate), p(VBPDA-co-FDA)) were synthesized for the extraction of lithium and cobalt from LiCoO2 cathode material. Notably, the copolymer was able to play the triple role of leaching agent, complexing agent and surfactant. The proof of concept for leaching, complexation and extraction was achieved, using two different extraction systems. A first extraction system used aqueous hydrogen peroxide as reducing agent while it was replaced by ethanol in the second extraction system. The scCO2 extraction conditions such as extraction time, temperature, functional copolymer concentration, and the presence of additives were optimized to improve the metals extraction from LiCoO2 cathode material, leading to an extraction efficiency of Li and Co up to ca. 75 % at 60 °C and 250 bar.

Single-Atomic Dispersion of Fe and Co Supported on Reduced Graphene Oxide for High-Performance Lithium-Sulfur Batteries.

High theoretical energy density and low cost make lithium-sulfur (LSB) batteries a promising system for next-generation energy storage. LSB performance largely depends on efficient reversible conversion of elemental sulfur to Li2S. Here, well-designed sulfur host materials including Fe or Co single atoms embedded on N-doped reduced graphene oxide (MNC/G with M = Fe or Co) are proposed to tackle the LSB challenges and enhance the electrochemical performance. Using a combination of Mössbauer spectroscopy and high-resolution scanning electron microscopy, the atomic dispersion of Co and Fe was revealed up to relatively high mass loadings. After optimization of the electrolyte/sulfur (E/S) ratio, FeNC/G shows the most promising cycle performance combining a constant high discharge capacity at low E/S values with the lowest polarization. In particular, the material FeNC/G@S with a high sulfur loading (9.4 mg cm-2) delivers a high area capacity of 7.7 mAh cm-2 under lean electrolyte conditions (6 mL g-1).

Unveiling redox mechanism at the iron centers in the mechanochemically activated conversion of CO2 in the presence of olivine.

The transformation of olivine during the conversion of CO2 to light hydrocarbons activated by mechanochemical treatments at different impact frequencies was studied by a combination of several complementary characterization methods including X-ray diffraction, Raman and 57Fe Mössbauer spectroscopy. Several olivine samples were studied as a function of the milling time, indicating the gradual transformation of FeII-containing olivine into new FeIII-containing weathering products including iron oxides, magnesium iron carbonates and silicates. The results presented here complement those of a previous study on the weathering process of olivine promoted by mechanochemical activation, by demonstrating the role of the redox activity of the iron species during the activation process. These additional spectroscopic results allow us to thoroughly understand the complex weathering mechanism and to correlate it with the efficiency of the CO2 conversion and storage properties of mechanochemically activated olivine. Supplementary Information: The online version contains supplementary material available at 10.1007/s10853-022-06962-x.

Importance of Halide Ions in the Stabilization of Hybrid Sn-Based Coatings for Lithium Electrodes.

The properties of hybrid Sn-based artificial solid electrolyte interphase (SEI) layers in protecting Li-metal electrodes toward surface instabilities were investigated via a combined experimental and theoretical approach. The performance of coating layers can be coherently explained based on the nature of the coating species. Notably, when starting from a chloride precursor, the hybrid coating layer is formed by an intimate mixture of Li7Sn2 and LiCl: the first ensures a high bulk ionic conductivity, while the second forms an external layer allowing a fast surface diffusion of Li+ to avoid dendrite growth, a low surface tension to guarantee the thermodynamic stability of the protective layer, and a negative underneath plating energy (UPE) to promote lithium plating at the interface between the Li metal and the coating layer. The synergy between the two components and, in particular, the crucial role of LiCl in the promotion of such an underneath plating mechanism are shown to be the key properties to improve the performance of artificial SEI layers.

The rise of X-ray spectroscopies for unveiling the functional mechanisms in batteries.

Synchrotron-based techniques have been key tools in the discovery, understanding, and development of battery materials. In this review, some of the most suitable X-ray spectroscopy related techniques employed for addressing diverse scientific cases connected to battery science are highlighted. Furthermore, current shortcomings, intrinsic limitations, and ongoing challenges of individual techniques are pointed out, providing an outlook of future trends that are relevant to the battery research community. In particular, the ongoing development of next generation synchrotrons, machine learning algorithms for data analysis and combined theoretical/experimental approaches will enhance the already powerful assets of these advanced spectroscopic methods.

Impact of Solution Chemistry on Growth and Structural Features of Mo-Substituted Spinel Iron Oxides.

The effect of crystallizing solution chemistry on the chemistry of subsequently as-grown materials was investigated for Mo-substituted iron oxides prepared by thermally activated co-precipitation. In the presence of Mo ions, we find that varying the oxidation state of the iron precursor from Fe(II) to Fe(III) causes a progressive loss of atomic long-range order with the stabilization of 2-4 nm particles for the sample prepared with Fe(III). The oxidation state of the Fe precursor also affects the distribution of Fe and Mo cations within the spinel structure. Increasing the Fe precursor oxidation state gives decreased Fe-ion occupation and increased Mo-ion occupation of tetrahedral sites, as revealed by the extended X-ray absorption fine structure. The stabilization of Mo within tetrahedral sites appears to be unexpected, considering the octahedral preferred coordination number of Mo(VI). The analysis of the atomic structure of the sample prepared with Fe(III) indicates a local ordering of vacancies and that the occupation of tetrahedral sites by Mo induces a contraction of the interatomic distances within the polyhedra as compared to Fe atoms. Moreover, the occupancy of Mo into the thermodynamic site preference of a Mo dopant in Fe2O3 assessed by density functional theory calculations points to a stronger preference for Mo substitution at octahedral sites. Hence, we suggest that the synthetized compound is thermodynamically metastable, that is, kinetically trapped. Such a state is suggested to be a consequence of the tetrahedral site occupation by Mo ions. The population of these sites, known to be reactive sites enabling particle growth, is concomitant with the stabilization of very small particles. We confirmed our hypothesis by using a blank experiment without Mo ions, further supporting the impact of tetrahedral Mo ions on the growth of iron oxide nanoparticles. Our findings provide new insights into the relationships between the Fe-chemistry of the crystallizing solution and the structural features of the as-grown Mo-substituted Fe-oxide materials.

Ganyong Starch Derived Hard Carbon Anodes for Sodium Ion Batteries.

Hard carbons are one of the most promising carbon anode materials for sodium ion batteries (SIBs) due to the high specific capacity and excellent cycle properties. Among the precursors used to synthesize hard carbon, natural starches are of great interest due to their unique morphologies. In this paper, ganyong starch based hard carbons (GSHC) were prepared by direct carbonization at various temperatures (700-1100) °C. The obtained hard carbons exhibit high reversible capacities of sodium-ion batteries of about 239 mAh g-1 at current density of 0.1 C. after 100 cycles. The excellent cycle profiles are attributed to the unique morphology and defect carbon structures.

Effect of the electrolyte on K-metal batteries.

The comparison of different electrolytes showed that both salt concentration and anion are key parameters for controlling the performance of K-metal batteries. Among the different tested electrolytes, 5 M KTFSI in DME exhibits the best stability at high potential and good performance in K|Prussian blue cells.

Quantum-Chemical Study of the FeNCN Conversion-Reaction Mechanism in Lithium- and Sodium-Ion Batteries.

We report a computational study on 3d transition-metal (Cr, Mn, Fe, and Co) carbodiimides in Li- and Na-ion batteries. The obtained cell voltages semi-quantitatively fit the experiments, highlighting the practicality of PBE+U as an approach for modeling the conversion-reaction mechanism of the FeNCN archetype with lithium and sodium. Also, the calculated voltage profiles agree satisfactorily with experiment both for full (Li-ion battery) and partial (Na-ion battery) discharge, even though experimental atomistic knowledge is missing up to now. Moreover, we rationalize the structural preference of intermediate ternaries and their characteristic lowering in the voltage profile using chemical-bonding and Mulliken-charge analysis. The formation of such ternary intermediates for the lithiation of FeNCN and the contribution of at least one ternary intermediate is also confirmed experimentally. This theoretical approach, aided by experimental findings, supports the atomistic exploration of electrode materials governed by conversion reactions.

Elaboration and Characterization of Active Films Containing Iron-Montmorillonite Nanocomposites for O2 Scavenging.

Iron particles of sizes between 6 and 20 nm forming aggregates of 57 ± 17 nm were synthesized by chemical reduction of iron precursors on the surface of montmorillonite (MMT). This active MMT-Fe powder was then uniformly distributed in a linear low-density polyethylene (LLDPE) matrix by extrusion at atmospheric conditions, as confirmed by wide-angle X-ray scattering (WAXS), which also detected a partial exfoliation of the nanoclays. Thermogravimetric analysis (TGA) did not detect any significant modification of the degradation temperature between nanocomposites and active nanocomposites. 57Fe Mössbauer spectroscopy evidenced the formation of a majority of iron boride in MMT-Fe as well as in the active film containing it. The LLDPE.Fu15.MMT-Fe3.75 and LLDPE.Fu15.MMT-Fe6.25 films had oxygen-scavenging capacities of 0.031 ± 0.002 and 0.055 ± 0.009 g(O2)/g(Fe), respectively, while the neat powder had an adsorption capacity of 0.122 g(O2)/g(Fe). This result confirms that the fresh film samples were partially oxidized shortly after thermomechanical processing (60% of oxidized species according to Mössbauer spectroscopy). No significant difference in oxygen permeability was observed when MMT-Fe was added. This was related to the relatively small film surface used for measuring the permeability. The reaction-diffusion model proposed here was able to reproduce the observed data of O2 adsorption in an active nanocomposite, which validated the O2 adsorption model previously developed for dried MMT-Fe powder.

Dehydration of Alginic Acid Cryogel by TiCl4 vapor: Direct Access to Mesoporous TiO2 @C Nanocomposites and Their Performance in Lithium-Ion Batteries.

A new strategy for the synthesis of mesoporous TiO2 @C nanocomposites through the direct mineralization of seaweed-derived alginic acid cryogel by TiCl4 through a solid/vapor reaction pathway is presented. In this synthesis, alginic acid cryogel can have multiple roles; i) mesoporous template, ii) carbon source, and iii) oxygen source for the TiO2 precursor, TiCl4 . The resulting TiO2 @alginic acid composite was transformed either into pure mesoporous TiO2 by calcination or into mesoporous TiO2 @C nanocomposites by pyrolysis. By comparing with a nonporous TiO2 @C composite, the importance of the mesopores on the performance of electrodes for lithium-ion batteries based on mesoporous TiO2 @C composite was clearly evidenced. In addition, the carbon matrix in the mesoporous TiO2 @C nanocomposite also showed electrochemical activity versus lithium ions, providing twice the capacity of pure mesoporous TiO2 or alginic acid-derived mesoporous carbon (A600). Given the simplicity and environmental friendliness of the process, the mesoporous TiO2 @C nanocomposite could satisfy the main prerequisites of green and sustainable chemistry while showing improved electrochemical performance as a negative electrode for lithium-ion batteries.

Paving the Way for K-Ion Batteries: Role of Electrolyte Reactivity through the Example of Sb-Based Electrodes.

Developing potassium-ion batteries remains a challenge so far due to the lack of efficient electrolytes. Moreover, the high reactivity of K metal and the use of half-cells may greatly alter both the electrochemical performance and the solid electrolyte interphase formation. Here, it is shown that in K metal/Sb half-cells, Coulombic efficiency improvement is achieved by the addition of fluoroethylene carbonate + vinylene carbonate to propylene carbonate (PC), the replacement of PC by ethylene carbonate/diethyl carbonate, and the replacement of KPF6 by potassium bis(fluorosulfonyl)imide. Surprisingly, however, storage of cells containing K metal leads to the coloration of K metal, separators, and Sb electrodes, whereas no change occurs for cells prepared without K metal. These results demonstrate that for all electrolytes, the high electrolyte reactivity with K metal also influences the Sb/electrolyte interface via a cross-talk mechanism. This observation is supported by gas chromatography/mass spectrometry analysis of electrolytes and X-ray photoelectron spectroscopy analysis of Sb electrodes. In summary, these results indicate that the search for efficient electrolytes for potassium-ion batteries must be carried out in full cells if one wants to obtain meaningful correlations between electrochemical performance and electrode/electrolyte interfacial properties. Overall, the results presented here are also likely to benefit the development of other emerging Na- and Mg-ion cell chemistries.

Carbodiimides as energy materials: which directions for a reasonable future?

A little less than a decade after their quantum-chemical prediction and eventual synthesis, solid-state transition-metal carbodiimides and closely related compounds have somewhat unexpectedly emerged as energy materials. In these carbodiimides, the O2- oxide dianion has been replaced by the complex NCN2- dianion, and the outstanding properties of such materials are likely related to their metastability and their higher amount of covalency compared to related oxides. When used as anode materials in rechargeable Li- and Na-ion batteries, one finds a conversion reaction, and further improving their performance will likely involve studying the redox behavior of NCN2-, the synthesis of novel ternary carbodiimides, in particular those with redox-active transition metals, and controlling their morphology. At present, such materials serve as catalysts in photochemical water oxidation, where they outperform their oxide cousins.

Electrochemical Alloying of Lead in Potassium-Ion Batteries.

The electrochemical alloying of lead-based electrodes with potassium was investigated by galvanostatic measurements as well as by ex situ and operando X-ray diffraction. The electrochemical reduction must be activated by an initial high current pulse which prevents the passivation of the lead electrode. The alloying process leads to the formation of crystalline KPb. During the discharge, two intermediate phases are observed, K10Pb48 and K4Pb9, whereas only K4Pb9 seems to form during the charge. High capacity retention is observed, with, however, a limited specific capacity value because of high weight of lead.

Alginic acid aquagel as a template and carbon source in the synthesis of Li4Ti5O12/C nanocomposites for application as anodes in Li-ion batteries.

We report here a simple process for the synthesis of Li4Ti5O12(LTO)/carbon nanocomposites by a one-pot method using an alginic acid aquagel as a template and carbon source, and lithium acetate and TiO2 nanoparticles as precursors to the LTO phase. The carbon content can be tuned by adjusting the relative amount of alginic acid. The obtained materials consist of nanosized primary particles of LTO (30 nm) forming micron-sized aggregates covered by well-dispersed carbon (from 3 to 19 wt%). The homogeneous dispersion of carbon over the particles improves the electrochemical performance of LTO electrodes such as rate capability (>95 mA h g-1 at 40C) and cycling performance (>98% of retention after 500 cycles at 5C), even with only 3% of carbon black additive in the electrode formulation. With a simple and easily up-scalable synthesis, the LTO/carbon nanocomposites of this study are promising candidates as anode materials for practical application in lithium-ion batteries.

Transition-Metal Carbodiimides as Molecular Negative Electrode Materials for Lithium- and Sodium-Ion Batteries with Excellent Cycling Properties.

We report evidence for the electrochemical activity of transition-metal carbodiimides versus lithium and sodium. In particular, iron carbodiimide, FeNCN, can be efficiently used as negative electrode material for alkali-metal-ion batteries, similar to its oxide analogue FeO. Based on (57)Fe Mössbauer and infrared spectroscopy (IR) data, the electrochemical reaction mechanism can be explained by the reversible transformation of the Fe-NCN into Li/Na-NCN bonds during discharge and charge. These new electrode materials exhibit higher capacity compared to well-established negative electrode references such as graphite or hard carbon. Contrary to its oxide analogue, iron carbodiimide does not require heavy treatments (such as nanoscale tailoring, sophisticated textures, or coating) to obtain long cycle life with current density as high as 9 A g(-1) for hundreds of charge-discharge cycles. Similar to the iron compound, several other transition-metal carbodiimides M(x)(NCN)y with M=Mn, Cr, Zn can cycle successfully versus lithium and sodium. Their electrochemical activity and performance open the way to the design of a novel family of anode materials.

Synthesis of Titania@Carbon Nanocomposite from Urea-Impregnated Cellulose for Efficient Lithium and Sodium Batteries.

Nanostructured TiO2 and TiO2@C nanocomposites were prepared directly from urea-impregnated cellulose by a simple reaction/diffusion process and evaluated as negative electrode materials for Li and Na batteries. By direct treatment with TiCl4 under anhydrous conditions, the urea impregnation of cellulose impacts both the TiO2 morphology and the carbon left by cellulose after pyrolysis. Hierarchical TiO2 structures with a flower-like morphology grown from-and-at the surface of the cellulose fibers are obtained without any directing agent. The resulting TiO2/cellulose composite is then transformed either into pure TiO2 flowers by calcination in air at 600 °C, or into TiO2@C nanocomposites by pyrolysis under Ar at 600 °C. Electrochemical studies demonstrate that both samples can (de)insert lithium and sodium ions and are promising electrode materials.

Characterization of Phosphate Species on Hydrated Anatase TiO2 Surfaces.

The adsorption/interaction of KH2PO4 with solvated (100) and (101) TiO2 anatase surfaces is investigated using periodic DFT calculations in combination with GIPAW NMR calculations and experimental IR and solid state (17)O, and (31)P NMR spectroscopies. A complete and realistic model has been used to simulate the solvent by individual water molecules. The most stable adsorption configurations are characterized theoretically at the atomic scale, and experimentally supported by NMR and IR spectroscopies. It is shown that H2PO4(-) chemisorbs on the (100) and (101) anatase surfaces, preferentially via a bidentate geometry. Dimer (H3P2O7(-)) and trimer (H4P3O10(-)) adsorption models are confronted with monomer adsorption models, in order to rationalize their occurrence.

The Quest for Polysulfides in Lithium-Sulfur Battery Electrolytes: An Operando Confocal Raman Spectroscopy Study.

Confocal Raman spectra of a lithium-sulfur battery electrolyte are recorded operando in a depth-of-discharge resolved manner for an electrochemical cell with a realistic electrolyte/sulfur loading ratio. The evolution of various possible polysulfides is unambiguously identified by combining Raman spectroscopy data with DFT simulations.