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We have used Atomic Force Microscopy (AFM) as a powerful tool to study the structure of the separator in Li-ion batteries. The measurements were performed in LiTFSI-based electrolytes with different concentrations in two different solvents. We chose propylene carbonate (PC) from the family of carbonate solvents because it has a low vapor pressure at room temperature. The results are compared to measurements in water-based electrolytes. We find that the surface of the separator is always electrically charged, regardless of water or PC solvent. In water-based electrolytes, there is an additional hydrophobic attractive interaction at surface distances below 20 to 60 nm, which is due to the formation of gas microbubbles on hydrophobic surfaces. Adding LiTFSI salt to water reduces the hydrophobic interaction and shields the electric field of the charged polymer surface. For PC-based electrolytes, there is no evidence of hydrophobic attractive interactions, but we observe an electrically charged separator-polymer surface. The addition of LiTFSI salt to PC not only shields the electric field emanating from the charged surface of the polymer but it also has an interesting effect on the morphology of the separator surface. At average LiTFSI salt concentrations of around 0.1 M, we see a swelling of the separator surface. This is confirmed by contact AFM images of the separator surface in the presence of 0.1 M salt, which shows long and curled polymer filaments of 10 nm diameter, forming a random and soft filament mesh on the surface of the separator. Our results show that in situ AFM imaging and force spectroscopy of Li-ion separator–electrolyte interfaces provide valuable information about the structure and electrical properties of the interface.

MXenes, i.e., two-dimensional transition metal carbides and nitrides, have been reported as promising materials for various applications, including energy storage, biomedicine, and electronics. The family of MXenes has proliferated, and the chemical space of synthesized MXenes has expanded to 13 transition metals and a dozen elements in surface terminations. The diverse chemistry of MXenes enables systematical tuning of MXene properties to meet the needs of target applications. However, synthesizing new MXene compositions largely relies on a trial-and-error approach. To overcome it, computational predictions of MXene compositions that are thermodynamically stable are crucial to rationalize experimental efforts. Here, we report a comprehensive computational screening for thermodynamically stable MXenes across 29 transition metals and 11 surface terminations. Density functional theory calculations are employed to compute the energy above the convex energy hull as a descriptor of thermodynamic stability. The results are analyzed to explore factors crucial for determining the thermodynamic stability of MXenes, by which the chemistry of surface terminations is found to play a crucial role. The insights on the chemistry of 998 MXene compositions predicted to be (meta)stable are given to systematically guide further research on MXene synthesis and application.

Despite the rapid expansion of the organic cathode materials field, we still face a shortage of materials obtained through simple synthesis that have stable cycling and high energy density. Herein, we report a two-step synthesis of a small organic molecule from commercially available precursors that can be used as a cathode material. Oxidized tetraquinoxalinecatechol (OTQC) was derived from tetraquinoxalinecatechol (TQC) by the introduction of additional quinone redox-active centers into the structure. The modification increased the voltage and capacity of the material. The OTQC delivers a high specific capacity of 327 mAh g–1with an average voltage of 2.63 V vs Li/Li+in the Li-ion battery. That corresponds to an energy density of 860 Wh kg–1on the OTQC material level. Furthermore, the material demonstrated excellent cycling stability, having a capacity retention of 82% after 400 cycles. Similarly, the OTQC demonstrates increased average voltage and specific capacity in comparison with TQC in aqueous Zn–organic battery, reaching the specific capacity of 326 mAh g–1with an average voltage of 0.86 V vs Zn/Zn2+. Apart from good electrochemical performance, this work provides an additional in-depth analysis of the redox mechanism and degradation mechanism related to capacity fading.

Cost-effective, simple, and easily reproducible synthesis methods of polymers are of profound significance when it comes to extracting high battery performance metrics from polymeric redox-active materials. This work reports a procedure for the solvothermal synthesis of a poly(hydroquinonyl-benzoquinonyl sulfide) (PHBQS) polymer and the development of its nanostructured composites with multiwalled carbon nanotubes (MWCNTs). Polymers are tested as high-performance cathode materials for Li+and Mg2+batteries. In configurations, compared to neat PHBQS, the PHBQS@5%MWCNT cathode exhibits superior electrochemical performance with high active material utilization owing to improved ion/electron transport pathways. Galvanostatic characterization of the PHBQS@5%MWCNT cathode in lithium batteries exhibited peak capacity up to 358 mAh g–1at a current density of 50 mA g–1(C/8) and excellent rate performance with a discharge capacity of 236 mAh g–1maintained even at high current density of 10C. The galvanostatic characterization in Mg batteries reveals more sluggish kinetics with a stable capacity of 200 mAh g–1at 50 mA g–1.

High-performance electrolytes are at the heart of magnesium battery development. Long-term stability along with the low potential difference between plating and stripping processes are needed to consider them for next-generation battery devices. Within this work, we perform an in-depth characterization of the novel Mg[Al(hfip)4]2salt in different glyme-based electrolytes. Specific importance is given to the influence of water content and the role of additives in the electrolyte. Mg[Al(hfip)4]2-based electrolytes exemplify high tolerance to water presence and the beneficial effect of additives under aggravated cycling conditions. Finally, electrolyte compatibility is tested with three different types of Mg cathodes, spanning different types of electrochemical mechanisms (Chevrel phase, organic cathode, sulfur). Benchmarking with an electrolyte containing a state-of-the-art Mg[B(hfip)4]2salt exemplifies an improved performance of electrolytes comprising the Mg[Al(hfip)4]2salt and establishes Mg[Al(hfip)4]2as a new standard salt for the future Mg battery research.

Insights into the electrochemical processes occurring at the electrode–electrolyte interface are a crucial step in most electrochemistry domains and in particular in the optimization of the battery technology. However, studying potential-dependent processes at the interface is one of the biggest challenges, both for theoreticians and experimentalists. The challenge is pushed further when stable species also depend on the concentration of specific ligands in the electrolyte, such as chlorides. Herein, we present a general theoretical ab initiomethodology to compute a Pourbaix-like diagram of complex electrolytes as a function of electrode potential and anion’s chemical potential, that is, concentration. This approach is developed not only for the bulk properties of the electrolytes but also for electrode–electrolyte interfaces. In the case of chlorinated magnesium complexes in dimethoxyethane, we show that the stability domains of the different species are strongly shifted at the interface compared to the bulk of the electrolyte because of the strong local electric fields and charges occurring in the double layer. Thus, as the interfacial stability domains are strongly modified, this approach is necessary to investigate all interface properties that often govern the reaction kinetics, such as solvent degradation at the electrode. Interface Pourbaix diagram is used to give some insights into the improved stability at the Mg anode induced by the addition of chloride. Because of its far-reaching insights, transferability, and wide applicability, the methodology presented herein should serve as a valuable tool not only for the battery community but also for the wider electrochemical one.

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 electrochemical degradation of two solvent-based electrolytes for Mg-metal batteries is investigated through a grand canonical density functional theory (DFT) approach. Both electrolytes are highly reactive in the double layer region where the solvated species have no direct contact with the Mg-surface, hence emphasizing that surface reactions are not the only phenomena responsible for electrolyte degradation. Applied to dimethoxyethane (DME) and ethylene carbonate (EC), the present methodology shows that both solvents should thermodynamically decompose in the double layer prior to the Mg2+/Mg0reduction, leading to electrochemically inactive reaction products. Based on thermodynamic considerations, Mg0deposition should not be possible, which contrasts with experiments, at least for DME-based electrolytes. This apparent contradiction is here addressed through the rationalization of the electrochemical mechanism underlying solvent electroactivation. An extended operation potential window(OPW) is extracted, in which the Mg2+/Mg0reduction can compete with electrolyte decomposition, thus enabling battery operation beyond the solvated species thermodynamic stability. The chemical study of the degradation products is in excellent agreement with experiments and offers rationale for the Mg-battery failure in EC electrolyte and capacity fade in DME electrolyte. The potential-dependent approach proposed herein is thus able to successfully tackle the challenging problem of interface electrochemistry. Being fully transferable to any other electrochemical systems, this methodology should provide rational guidelines for the development of viable electrolytes for multivalent batteries and, more generally, energy conversion and storage devices.

We report the synthesis and detailed structural and chemical characterization including electrochemical properties of a lithium vanadium oxy/fluoro-phosphate material. To the best of our knowledge, we have for the first time synthesized a LiVPO4O-type phase with a mixed O/F ligand. In the synthesis procedure, the LiVPO4O precursor compound was fluorinated via LiF incorporation, with preservation of the LiVPO4O framework structure. The operating potential of the synthesized material is increased compared to that of the LiVPO4O precursor (4.12 V vs 3.95 V versus metallic lithium, respectively). The related increase in operating potential was assigned to the effect of the intermixing O/F ligand, which is attained via the successful fluorine incorporation into the LiVPO4O structure. A characterization of the investigated materials was performed using microscale-covering XRD, XANES, and NMR techniques as well as nanoscale spatially resolved imaging and analytical STEM techniques. The obtained oxy/fluoro-phosphate phase is isostructural to LiVPO4O; however, the presence of the mixed O/F ligand promoted a higher symmetry of vanadium octahedra. These variations of the vanadium local environment along with the observed inhomogeneous distribution of the incorporated fluorine gave rise to the minor local deviations in vanadium valence. Our results clearly emphasize the connection among the fluorine ligand incorporation, its local distribution, and the electrochemical properties of the material.

Organic active materials are seen as next-generation battery materials that could circumvent the sustainability and cost limitations connected with the current Li-ion battery technology while at the same time enabling novel battery functionalities like a bioderived feedstock, biodegradability, and mechanical flexibility. Many promising research results have recently been published. However, the reproducibility and comparison of the literature results are somehow limited due to highly variable electrode formulations and electrochemical testing conditions. In this Perspective, we provide a critical view of the organic cathode active materials and suggest future guidelines for electrochemical characterization, capacity evaluation, and mechanistic investigation to facilitate reproducibility and benchmarking of literature results, leading to the accelerated development of organic electrode active materials for practical applications.

The impact of the solid film deposit (mainly Li2S) on the complex electrochemistry of a Li–S cell is studied in detail. Already a simple, straightforward experiment strongly indicates that this impact might be much smaller than usually assumed. Notably, a similar phenomenon is demonstrated for another battery operated on the same basic principle: the magnesium–sulfur battery. In order to better detect the surface-specific phenomena associated with formation and properties of the solid surface deposit, we construct special electrochemical cells with a flat glassy carbon disc or other well-defined materials. Different model systems are prepared in which crucial variables such as the electrode configuration, separator type, and state of charge are varied in a systematic and controlled way. Electrochemical results are supplemented with data from microstructural analysis, in particular focused ion beam–scanning electron microscopy (FIB-SEM) imaging and X-ray diffraction analysis. We show that the growth of the surface film is more complex than generally assumed and that its defect-rich morphology hardly represents any obstacle for electrochemical reaction(s) to take place. Rather, the cell operation is limited by diffusional processes and depletion of polysulfide concentration in electrolyte. The new insight into the occurrence, properties, and especially the impact of solid film deposits on operation of the Li–S system is expected to have important implications for future design of Li–S practical cells.

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.

The impedance response of a Li electrode enclosed in a pouch cell casing is compared with the response of the same electrode in coin cell and Swagelok cell casing. A significant difference is observed in the high-to-medium-frequency part of the response. The difference is explained using a simple equivalent circuit. The explanation is further confirmed by a series of experiments in which the electrodes are either transferred from one cell type to the other, or the surface area of lithium metal electrode and contactingmetal from the cell casing is varied. Six different electrolytes are used to demonstrate the generality of the phenomenon due to (inadvertent) wetting of the stainless steel case in coin cells and Swagelok cells; such wetting results in distortion of the main impedance arc, which may even split into two separate arcs. A similar situation can occur in pouch cells when a significant surface area of the metal used for contacting the lithium metal electrode is in the direct electrochemical field of the cell. Solutions to this problem are briefly presented.

The search for high Li-ion conducting ceramics has regained tremendous interest triggered by the renaissance of the all-solid-state battery. Within this context we herein reveal the impact of structural polymorphism of lithium copper pyroborate Li6CuB4O10on its ionic conductivity. Using combined in situsynchrotron X-ray and neutron powder diffraction, a structural and synthetic relationship between α- and β-Li6CuB4O10could be established and its impact on ionic conductivity evolution was followed using electrochemical impedance spectroscopy. We show that the high temperature form of Li6CuB4O10exhibits a high Li-ion conductivity (2.7 mS cm–1at 350 °C) and solve its crystal structure for the first time. Our results emphasize the significant impact of structural phase transitions on ionic conductivity and show possible high Li-ion mobility within borate based compounds.

A collection of different polymeric ionic liquids (PILs) were explored as cathode binders in lithium–sulfur batteries. The PIL molecular structure, polymer backbone, and polymer architecture were found to influence the cell capacity, the cyclability, and the morphology of the cathode itself. PILs with styrene backbones performed better than the vinyl-based polymer, while cross-linked PILs imparted further improved capacities, cyclability, and reduced overpotentials. Unlike polyvinylidene fluoride, PIL binders mixed with the sulfide species, resulting in more uniformly distributed sulfides in the cathode and better sulfide transport. These features helped to mitigate volume change-induced degradation that typically plagues Li–S batteries. The uptake of polysulfides by PILs also constrains the polysulfide shuttle during battery cycling, leading to better cycling stability. While traditionally binders are viewed only as a “glue” to hold the active material together, PIL binders have additional functions and play an active role during Li–S battery working operation.

Li-S batteries are promising energy storage technology for the future, however there two major problems remained which need to be solved before successful commercialization. Capacity fading due to polysulfide shuttle and corrosion of lithium metal are directly connected with the type and quantity of electrolyte used in the cells. Several recent works show dependence of the electrochemical behavior of Li-S batteries on type of the electrolyte. In this work we compare and discuss a discharge mechanism of sulfur conversion in three different electrolytes based on measurements with sulfur K-edge XAS. The sulfur conversion mechanism in the ether based electrolytes, the most studied type of solvents in the Li-S batteries, which are enabling high solubility of polysulfides are compared with the fluorinated ether based electrolytes with a reduced polysulfide solubility and in carbonate based electrolytes with the sulfur confined into a ultramicroporous carbon. In all three cases the sulfur reduction proceeds through polysulfide intermediate phases with a difference on the type polysulfides detected at different steps of discharge.

Polysulfides are central compounds in lithium–sulfur battery cells. However, the fundamental redox and diffusion properties of polysulfides are still poorly understood. We try to fill this gap by performing an accurate impedance spectroscopy investigation using symmetrical cells consisting of two planar glassy carbon electrodes separated with catholyte-soaked separator. The catholyte contains a mixture of selected polysulfides with predetermined nominal concentrations. Impedance measurements reveal textbook shapes of spectra for most polysulfide compounds or their mixtures. This allows reliable and accurate determination of the rate constant (exchange current density) for a given redox reaction as well as the diffusion coefficient and diffusion length for the rate-determining polysulfide species. Further, it is confirmed that polysulfides tend to disproportionate with time, which significantly changes the chemistry and electrochemistry of the system. Two approaches are proposed for identification of the prevailing redox mechanism in the resulting mixtures. The values of kinetic and transport parameters obtained for different cases of interest are commented on in significant detail. The study provides a solid basis for better understanding of the complex processes in polysulfide mixtures.

In the search for new cathode materials for sodium ion batteries, the exploration of polyanionic compounds has led to attractive candidates in terms of high redox potential and cycling stability. Herein we report the synthesis of the two new sodium transition-metal pentaborates Na3MB5O10(M = Fe, Co), where Na3FeB5O10represents the first sodium iron borate reported at present. By means of synchrotron X-ray diffraction, we reveal a layered structure consisting of pentaborate B5O10groups connected through M2+in tetrahedral coordination, providing possible three-dimensional Na-ion migration pathways. Inspired by these structural features, we examined the electrochemical performances versus sodium and showed that Na3FeB5O10is active at an average potential of 2.5 V vs Na+/Na0, correlated to the Fe3+/Fe2+redox couple as deduced from ex situMössbauer measurements. This contrasts with the case for Na3CoB5O10, which is electrochemically inactive. Moreover, we show that their electrochemical performances are kinetically limited, as deduced by complementary ac/dc conductivity measurements, hence confirming once again the complexity in designing high-performance borate-based electrodes.

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.

A mechanism for Li–S battery operation with a composite electrode and an adsorption additive obtained by using operando ultraviolet/visible (UV/vis) spectroscopy and X-ray absorption spectroscopy confirms the role of the adsorption additive and reflects the conversion mechanism of sulfur into Li2S. Operando UV/vis spectroscopy shows a reversible appearance of the long-chain polysulfides in the separator in the fifth cycle, whereas the appearance of mid- and short-chain polysulfides suggests a polysulfide shuttle mechanism. By using a nonsulfur-containing electrolyte, a high-precision analysis of sulfur K-edge X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) spectra is possible. The XANES analysis shows that polysulfides reach the maximum concentration at the end of the high-voltage plateau, and the low-voltage plateau is characteristic of the polysulfides/Li2S equilibrium. The relative amount of Li2S increases linearly until the end of discharge and reaches a relative amount of 75%. This is confirmed by sulfur K-edge EXAFS analysis. Additionally, a quantitative analysis of EXAFS spectra measured during discharge evidences a decrease of the average S–S coordination number. This can be interpreted as a decrease of the chain length of polysulfides. EXAFS analysis showed that there are no specific interactions of the polysulfide species with the matrix or with other species in the electrolyte.

The effect of mild oxidation in air of LiFe1[?]yMnyPO4 (0 [?] y [?] 1) powders is studied via in situ and ex situ X-ray diffraction, thermogravimetric analysis and Mossbauer spectroscopy. Structural modifications involve an anisotropic change in the lattice parameters upon heat-treatment with conservation of the olivine-type structure up to [?]500degC. The extrusion of part of iron in the form of a-Fe2O3 is observed, which results in the oxidation of iron in the structure and the creation of vacancies, while Mn remains stable at a +2 oxidation state. Mass gain and unit-cell volume variations upon heat-treatment under air are found to be proportional to y in the pristine composition. In addition, the electrochemical signature of powders annealed at [?]300degC significantly differs from the stoichiometric ones. Two solid solution contributions arise from the Fe3+/Fe2+ redox couple, and the Mn3+/Mn2+ reaction upon Li+ electrochemical insertion/extraction is reported for the first time as a solid solution reaction.

Co2+-doped sodium titanate nanoribbons were grown under hydrothermal conditions from Co2+-doped TiO2and NaOH(aq) at 175 °C. The obtained nanoribbons, with diameters of 30–150 nm and length up to several micrometers, were determined to have a trititanate structure ((Na,H)2Ti3O7). Transmission electron microscopy revealed nanoparticles, not exceeding 15–20 nm in length, as well as hexagonal nanoflakes located on the surface of the nanoribbons. These nanoflakes are most likely originating from a β-Co(OH)2side-product, according to the X-ray diffraction investigation. High angle annular dark filed scanning transmission electron microscopy combined with electron energy loss spectroscopy showed that the amount of cobalt in the surface nanoparticles is much higher than that in the nanoribbons. X-ray photoelectron spectroscopy (XPS) revealed that atomic concentration of cobalt in the sample is 1.5 wt %, of which a small amount is in the oxidation state 3+. A detailed electron paramagnetic resonance characterization of this sample proved that Co2+ions occupy octahedral sites with rhombic distortion in a high-spin S= 3/2 state. Temperature-dependent susceptibility measurement revealed the prevailing paramagnetic behavior from which a mass ratio 1.3 wt % of Co2+was obtained and which is in agreement with the elemental analysis results and the value extracted from the XPS measurements. A weak antiferromagnetic transition at 12 K is associated with β-Co(OH)2nanoflakes.