First-principles study of MXene properties with varying hydrofluoric acid concentration.

With varying hydrofluoric acid (HF) concentrations under three etching conditions, we presented a comparative study of the effects of both the ordered and randomly ternary mixed terminated Ti3C2Tx surfaces with a wide variation of O/OH/F stoichiometry on the thermodynamic stability and electronic properties. Regardless of the HF concentration, an OH-rich surface is found to be thermodynamically stable and the electrical conductivity of Ti3C2Tx is substantially affected by the OH concentration. The charge density difference and electron localization function demonstrated a significant electron localization at the hydroxyl group on the O/OH/F mixed terminated surface, which could yield a locally induced dipole on the surface that renders favorable reaction sites on the functionalized surface. In addition, a large tunability in the work function (ΔΦ âˆ¼ 3.5 eV) is predicted for Ti3C2Tx. These findings provide a pathway for strategically tuning the electronic and structural properties of Ti3C2 MXenes etched with HF.

Template assisted lithium superoxide growth for lithium-oxygen batteries.

Developing batteries with energy densities comparable to internal combustion technology is essential for a worldwide transition to electrified transportation. Li-O2 batteries are seen as the 'holy grail' of battery technologies since they have the highest theoretical energy density of all battery technologies. Current lithium-oxygen (Li-O2) batteries suffer from large charge overpotentials related to the electronic resistivity of the insulating lithium peroxide (Li2O2) discharge product. One potential solution is the formation and stabilization of a lithium superoxide (LiO2) discharge intermediate that exhibits good electronic conductivity. However, LiO2 is reported to be unstable at ambient temperature despite its favorable formation energy at -1.0 eV per atom. In this paper - based on our recent work on the development of cathode materials for aprotic lithium oxygen batteries including two intermetallic compounds, LiIr3 and LiIr, that are found to form good template interfaces with LiO2 - a simple goodness of fit R factor to gauge how well a template surface structure can support LiO2 growth, is developed. The R factor is a quantitative measurement to calculate the geometric difference in the unit cells of specific Miller Index 2D planes of the template surface and LiO2. Using this as a guide, the R factors for LiIr3, LiIr, and La2NiO4+δ, are found to be good. This guide is attested by simple extension to other noble metal intermetallics with electrochemical cycling data including LiRh3, LiRh, and Li2Pd. Finally, the template concept is extended to main group elements and the R factors for LiO2 (111) and Li2Ca suggest that Li2Ca is a possible candidate for the template assisted LiO2 growth strategy.

Unusual Melting Trend in an Alkali Asymmetric Sulfonamide Salt Series: Single-Crystal Analysis and Modeling.

Developing low-melting alkali salts is of interest for both battery electrolytes and inorganic ionic liquids. In this study, we report a series of asymmetric alkali-metal sulfonamide salts based upon the (3-methoxypropyl)((trifluoromethyl)sulfonyl)amide (MPSA) anion. This family of salts features an unusual melting point trend, where the melting point of the salts decreases as the cation increases in size from Li to K but then the melting point increases as the cation further increases in size from K to Cs. Analyses of single crystals reveal that the unusual higher melting points of RbMPSA and CsMPSA in comparison to KMPSA can be attributed to the greater cation-cation distances as well as the increased rigidity of anion-cation coordination due to an increase in cyclic structures in comparison to KMPSA. Exceptionally, KMPSA features a very low melting point of only 50.79 ± 0.31 °C. This low melting point can be attributed to a relatively high degree of disorder, an unusual uncoordinated ether moiety, and a very short K-K distance of only 3.4348(7) Å among other factors, which is supported by the low cohesive energy and small elastic moduli among the rest according to density functional theory (DFT) calculations. The low melting point of KMPSA makes it interesting for low-temperature ionic liquids.

In Situ Formed Ir3Li Nanoparticles as Active Cathode Material in Li-Oxygen Batteries.

Lithium-oxygen (Li-O2) batteries are a promising class of rechargeable Li batteries with a potentially very high achievable energy density. One of the major challenges for Li-O2 batteries is the high charge overpotential, which results in a low energy efficiency. In this work size-selected subnanometer Ir clusters are used to investigate cathode materials that can help control lithium superoxide formation during discharge, which has good electronic conductivity needed for low charge potentials. It is found that Ir particles can lead to lithium superoxide formation as the discharge product with Ir particle sizes of ∼1.5 nm giving the lowest charge potentials. During discharge these 1.5 nm Ir nanoparticles surprisingly evolve to larger ones while incorporating Li to form core-shell structures with Ir3Li shells, which probably act as templates for growth of lithium superoxide during discharge. Various characterization techniques including DEMS, Raman, titration, and HRTEM are used to characterize the LiO2 discharge product and the evolution of the Ir nanoparticles. Density functional calculations are used to provide insight into the mechanism for formation of the core-shell Ir3Li particles. The in situ formed Ir3Li core-shell nanoparticles discovered here provide a new direction for active cathode materials that can reduce charge overpotentials in Li-O2 batteries.

Structure-Property of Lithium-Sulfur Nanoparticles via Molecular Dynamics Simulation.

Lithium-sulfur (Li-S) batteries offer higher energy densities than most reported lithium-ion batteries. However, our understanding of Li-S battery is still largely unknown at the level of the nanoscale. The structural properties of Li-S materials were investigated via molecular dynamics (MD) simulations using the ReaxFF force field. Several Li-S nanoparticles with different Li/S composition ratios (2:1 and 2:8) and various structures are studied. Our MD simulations show that among the four structures we constructed for Li2S8 nanoparticles, the core-shell structure is the most thermodynamically stable one during the charging (delithiation) process. In contrast to bulk crystal Li2S, we find the presence of mixed lithium sulfide and polysulfide species are common features for these Li-S (Li2S, Li2S8) nanoparticles. The complex distribution of these sulfide and polysulfide speciation are dictated by both stoichiometry and local atomic structures in the nanoparticle. These findings will provide insight into further development of functionalized lithium-sulfur cathodes.

A lithium-oxygen battery with a long cycle life in an air-like atmosphere.

Lithium-air batteries are considered to be a potential alternative to lithium-ion batteries for transportation applications, owing to their high theoretical specific energy. So far, however, such systems have been largely restricted to pure oxygen environments (lithium-oxygen batteries) and have a limited cycle life owing to side reactions involving the cathode, anode and electrolyte. In the presence of nitrogen, carbon dioxide and water vapour, these side reactions can become even more complex. Moreover, because of the need to store oxygen, the volumetric energy densities of lithium-oxygen systems may be too small for practical applications. Here we report a system comprising a lithium carbonate-based protected anode, a molybdenum disulfide cathode and an ionic liquid/dimethyl sulfoxide electrolyte that operates as a lithium-air battery in a simulated air atmosphere with a long cycle life of up to 700 cycles. We perform computational studies to provide insight into the operation of the system in this environment. This demonstration of a lithium-oxygen battery with a long cycle life in an air-like atmosphere is an important step towards the development of this field beyond lithium-ion technology, with a possibility to obtain much higher specific energy densities than for conventional lithium-ion batteries.

Dendrite-Free Potassium-Oxygen Battery Based on a Liquid Alloy Anode.

The safety issue caused by the dendrite growth is not only a key research problem in lithium-ion batteries but also a critical concern in alkali metal (i.e., Li, Na, and K)-oxygen batteries where a solid metal is usually used as the anode. Herein, we demonstrate the first dendrite-free K-O2 battery at ambient temperature based on a liquid Na-K alloy anode. The unique liquid-liquid connection between the liquid alloy and the electrolyte in our alloy anode-based battery provides a homogeneous and robust anode-electrolyte interface. Meanwhile, we manage to show that the Na-K alloy is only compatible in K-O2 batteries but not in Na-O2 batteries, which is mainly attributed to the stronger reducibility of potassium and relatively more favorable thermodynamic formation of KO2 over NaO2 during the discharge process. It is observed that our K-O2 battery based on a liquid alloy anode shows a long cycle life (over 620 h) and a low discharge-charge overpotential (about 0.05 V at initial cycles). Moreover, the mechanism investigation into the K-O2 cell degradation shows that the O2 crossover effect and the ether-electrolyte instability are the critical problems for K-O2 batteries. In a word, this study provides a new route to solve the problems caused by the dendrite growth in alkali metal-oxygen batteries.

Effect of Hydrofluoroether Cosolvent Addition on Li Solvation in Acetonitrile-Based Solvate Electrolytes and Its Influence on S Reduction in a Li-S Battery.

Li-S batteries are a promising next-generation battery technology. Due to the formation of soluble polysulfides during cell operation, the electrolyte composition of the cell plays an active role in directing the formation and speciation of the soluble lithium polysulfides. Recently, new classes of electrolytes termed "solvates" that contain stoichiometric quantities of salt and solvent and form a liquid at room temperature have been explored due to their sparingly solvating properties with respect to polysulfides. The viscosity of the solvate electrolytes is understandably high limiting their viability; however, hydrofluoroether cosolvents, thought to be inert to the solvate structure itself, can be introduced to reduce viscosity and enhance diffusion. Nazar and co-workers previously reported that addition of 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) to the LiTFSI in acetonitrile solvate, (MeCN)2-LiTFSI, results in enhanced capacity retention compared to the neat solvate. Here, we evaluate the effect of TTE addition on both the electrochemical behavior of the Li-S cell and the solvation structure of the (MeCN)2-LiTFSI electrolyte. Contrary to previous suggestions, Raman and NMR spectroscopy coupled with ab initio molecular dynamics simulations show that TTE coordinates to Li+ at the expense of MeCN coordination, thereby producing a higher content of free MeCN, a good polysulfide solvent, in the electrolyte. The electrolytes containing a higher free MeCN content facilitate faster polysulfide formation kinetics during the electrochemical reduction of S in a Li-S cell likely as a result of the solvation power of the free MeCN.

Concentrated Electrolyte for the Sodium-Oxygen Battery: Solvation Structure and Improved Cycle Life.

Alkali metal-oxygen batteries are of great interests for energy storage because of their unparalleled theoretical energy densities. Particularly attractive is the emerging Na-O2 battery because of the formation of superoxide as the discharge product. Dimethyl sulfoxide (DMSO) is a promising solvent for this battery but its instability towards Na makes it impractical in the Na-O2 battery. Herein we report the enhanced stability of Na in DMSO solutions containing concentrated sodium trifluoromethanesulfonimide (NaTFSI) salts (>3 mol kg-1 ). Raman spectra of NaTFSI/DMSO electrolytes and ab initio molecular dynamics simulation reveal the Na+ solvation number in DMSO and the formation of Na(DMSO)3 (TFSI)-like solvation structure. The majority of DMSO molecules solvating Na+ in concentrated solutions reduces the available free DMSO molecules that can react with Na and renders the TFSI anion decomposition, which protects Na from reacting with the electrolyte. Using these concentrated electrolytes, Na-O2 batteries can be cycled forming sodium superoxide (NaO2 ) as the sole discharge product with improved long cycle life, highlighting the beneficial role of concentrated electrolytes for Na-based batteries.

A lithium-oxygen battery based on lithium superoxide.

Batteries based on sodium superoxide and on potassium superoxide have recently been reported. However, there have been no reports of a battery based on lithium superoxide (LiO2), despite much research into the lithium-oxygen (Li-O2) battery because of its potential high energy density. Several studies of Li-O2 batteries have found evidence of LiO2 being formed as one component of the discharge product along with lithium peroxide (Li2O2). In addition, theoretical calculations have indicated that some forms of LiO2 may have a long lifetime. These studies also suggest that it might be possible to form LiO2 alone for use in a battery. However, solid LiO2 has been difficult to synthesize in pure form because it is thermodynamically unstable with respect to disproportionation, giving Li2O2 (refs 19, 20). Here we show that crystalline LiO2 can be stabilized in a Li-O2 battery by using a suitable graphene-based cathode. Various characterization techniques reveal no evidence for the presence of Li2O2. A novel templating growth mechanism involving the use of iridium nanoparticles on the cathode surface may be responsible for the growth of crystalline LiO2. Our results demonstrate that the LiO2 formed in the Li-O2 battery is stable enough for the battery to be repeatedly charged and discharged with a very low charge potential (about 3.2 volts). We anticipate that this discovery will lead to methods of synthesizing and stabilizing LiO2, which could open the way to high-energy-density batteries based on LiO2 as well as to other possible uses of this compound, such as oxygen storage.

Interstitial and Interlayer Ion Diffusion Geometry Extraction in Graphitic Nanosphere Battery Materials.

Large-scale molecular dynamics (MD) simulations are commonly used for simulating the synthesis and ion diffusion of battery materials. A good battery anode material is determined by its capacity to store ion or other diffusers. However, modeling of ion diffusion dynamics and transport properties at large length and long time scales would be impossible with current MD codes. To analyze the fundamental properties of these materials, therefore, we turn to geometric and topological analysis of their structure. In this paper, we apply a novel technique inspired by discrete Morse theory to the Delaunay triangulation of the simulated geometry of a thermally annealed carbon nanosphere. We utilize our computed structures to drive further geometric analysis to extract the interstitial diffusion structure as a single mesh. Our results provide a new approach to analyze the geometry of the simulated carbon nanosphere, and new insights into the role of carbon defect size and distribution in determining the charge capacity and charge dynamics of these carbon based battery materials.

An atomistically informed mesoscale model for growth and coarsening during discharge in lithium-oxygen batteries.

An atomistically informed mesoscale model is developed for the deposition of a discharge product in a Li-O2 battery. This mescocale model includes particle growth and coarsening as well as a simplified nucleation model. The model involves LiO2 formation through reaction of O2(-) and Li(+) in the electrolyte, which deposits on the cathode surface when the LiO2 concentration reaches supersaturation in the electrolyte. A reaction-diffusion (rate-equation) model is used to describe the processes occurring in the electrolyte and a phase-field model is used to capture microstructural evolution. This model predicts that coarsening, in which large particles grow and small ones disappear, has a substantial effect on the size distribution of the LiO2 particles during the discharge process. The size evolution during discharge is the result of the interplay between this coarsening process and particle growth. The growth through continued deposition of LiO2 has the effect of causing large particles to grow ever faster while delaying the dissolution of small particles. The predicted size evolution is consistent with experimental results for a previously reported cathode material based on activated carbon during discharge and when it is at rest, although kinetic factors need to be included. The approach described in this paper synergistically combines models on different length scales with experimental observations and should have applications in studying other related discharge processes, such as Li2O2 deposition, in Li-O2 batteries and nucleation and growth in Li-S batteries.

The Effect of Potassium Impurities Deliberately Introduced into Activated Carbon Cathodes on the Performance of Lithium-Oxygen Batteries.

Rechargeable lithium-air (Li-O2) batteries have drawn much interest owing to their high energy density. We report on the effect of deliberately introducing potassium impurities into the cathode material on the electrochemical performance of a Li-O2 battery. Small amounts of potassium introduced into the activated carbon (AC) cathode material in the synthesis process are found to have a dramatic effect on the performance of the Li-O2 cell. An increased amount of potassium significantly increases capacity, cycle life, and round-trip efficiency. This improved performance is probably due to a larger amount of LiO2 in the discharge product, which is a mixture of LiO2 and Li2O2, resulting from the increase in the amount of potassium present. No substantial correlation with porosity or surface area in an AC cathode is found. Experimental and computational studies indicate that potassium can act as an oxygen reduction catalyst, which can account for the dependence of performance on the amount of potassium.

A Mo2C/Carbon Nanotube Composite Cathode for Lithium-Oxygen Batteries with High Energy Efficiency and Long Cycle Life.

Although lithium-oxygen batteries are attracting considerable attention because of the potential for an extremely high energy density, their practical use has been restricted owing to a low energy efficiency and poor cycle life compared to lithium-ion batteries. Here we present a nanostructured cathode based on molybdenum carbide nanoparticles (Mo2C) dispersed on carbon nanotubes, which dramatically increase the electrical efficiency up to 88% with a cycle life of more than 100 cycles. We found that the Mo2C nanoparticle catalysts contribute to the formation of well-dispersed lithium peroxide nanolayers (Li2O2) on the Mo2C/carbon nanotubes with a large contact area during the oxygen reduction reaction (ORR). This Li2O2 structure can be decomposed at low potential upon the oxygen evolution reaction (OER) by avoiding the energy loss associated with the decomposition of the typical Li2O2 discharge products.

Interfacial effects on lithium superoxide disproportionation in Li-O2 batteries.

During the cycling of Li-O2 batteries the discharge process gives rise to dynamically evolving agglomerates composed of lithium-oxygen nanostructures; however, little is known about their composition. In this paper, we present results for a Li-O2 battery based on an activated carbon cathode that indicate interfacial effects can suppress disproportionation of a LiO2 component in the discharge product. High-intensity X-ray diffraction and transmission electron microscopy measurements are first used to show that there is a LiO2 component along with Li2O2 in the discharge product. The stability of the discharge product was then probed by investigating the dependence of the charge potential and Raman intensity of the superoxide peak with time. The results indicate that the LiO2 component can be stable for possibly up to days when an electrolyte is left on the surface of the discharged cathode. Density functional calculations on amorphous LiO2 reveal that the disproportionation process will be slower at an electrolyte/LiO2 interface compared to a vacuum/LiO2 interface. The combined experimental and theoretical results provide new insight into how interfacial effects can stabilize LiO2 and suggest that these interfacial effects may play an important role in the charge and discharge chemistries of a Li-O2 battery.

Understanding side reactions in K-O2 batteries for improved cycle life.

Superoxide based metal-air (or metal-oxygen) batteries, including potassium and sodium-oxygen batteries, have emerged as promising alternative chemistries in the metal-air battery family because of much improved round-trip efficiencies (>90%). In order to improve the cycle life of these batteries, it is crucial to understand and control the side reactions between the electrodes and the electrolyte. For potassium-oxygen batteries using ether-based electrolytes, the side reactions on the potassium anode have been identified as the main cause of battery failure. The composition of the side products formed on the anode, including some reaction intermediates, have been identified and quantified. Combined experimental studies and density functional theory (DFT) calculations show the side reactions are likely driven by the interaction of potassium with ether molecules and the crossover of oxygen from the cathode. To inhibit these side reactions, the incorporation of a polymeric potassium ion selective membrane (Nafion-K(+)) as a battery separator is demonstrated that significantly improves the battery cycle life. The K-O2 battery with the Nafion-K(+) separator can be discharged and charged for more than 40 cycles without increases in charging overpotential.

Effect of the size-selective silver clusters on lithium peroxide morphology in lithium-oxygen batteries.

Lithium-oxygen batteries have the potential needed for long-range electric vehicles, but the charge and discharge chemistries are complex and not well understood. The active sites on cathode surfaces and their role in electrochemical reactions in aprotic lithium-oxygen cells are difficult to ascertain because the exact nature of the sites is unknown. Here we report the deposition of subnanometre silver clusters of exact size and number of atoms on passivated carbon to study the discharge process in lithium-oxygen cells. The results reveal dramatically different morphologies of the electrochemically grown lithium peroxide dependent on the size of the clusters. This dependence is found to be due to the influence of the cluster size on the formation mechanism, which also affects the charge process. The results of this study suggest that precise control of subnanometre surface structure on cathodes can be used as a means to improve the performance of lithium-oxygen cells.

Structure and Stability of Lithium Superoxide Clusters and Relevance to Li-O2 Batteries.

The discharge mechanism of a Li-O2 battery involves lithium superoxide (LiO2) radicals. In this Letter, we have performed high-level quantum chemical calculations (G4MP2) to investigate the structure and stability of LiO2 clusters. The clusters have planar ring-shaped structures, high spins, and are thermodynamically more stable than LiO2 dimer. The computed energy barrier for disproportionation of the larger clusters is also significantly higher than the corresponding barrier in the LiO2 dimer (1.0 eV vs 0.5 eV). This means that disproportionation rate should be much slower if the reaction involves LiO2 clusters other than the dimer. As a result, the clusters may survive long enough to be incorporated into the growing discharge product. These results are discussed in terms of recent experimental studies of the electronic structure and morphology of the discharge products in Li-air batteries.

Raman Evidence for Late Stage Disproportionation in a Li-O2 Battery.

Raman spectroscopy is used to characterize the composition of toroids formed in an aprotic Li-O2 cell based on an activated carbon cathode. The trends in the Raman data as a function of discharge current density and charging cutoff voltage provide evidence that the toroids are made up of outer LiO2-like and inner Li2O2 regions, consistent with a disproportionation reaction occurring in the solid phase. The LiO2-like component is found to be associated with a new Raman peak identified in the carbon stretching region at ∼1505 cm(-1), which appears only when the LiO2 peak at 1123 cm(-1) is present. The new peak is assigned to distortion of the graphitic ring stretching due to coupling with the LiO2-like component based on density functional calculations. These new results on the LiO2-like component from Raman spectroscopy provide evidence that a late stage disproportionation mechanism can occur during discharge and add new understanding to the complexities of possible processes occurring in Li-O2 batteries.

Disproportionation in Li-O2 batteries based on a large surface area carbon cathode.

In this paper we report on a kinetics study of the discharge process and its relationship to the charge overpotential in a Li-O2 cell for large surface area cathode material. The kinetics study reveals evidence for a first-order disproportionation reaction during discharge from an oxygen-rich Li2O2 component with superoxide-like character to a Li2O2 component. The oxygen-rich superoxide-like component has a much smaller potential during charge (3.2-3.5 V) than the Li2O2 component (∼4.2 V). The formation of the superoxide-like component is likely due to the porosity of the activated carbon used in the Li-O2 cell cathode that provides a good environment for growth during discharge. The discharge product containing these two components is characterized by toroids, which are assemblies of nanoparticles. The morphologic growth and decomposition process of the toroids during the reversible discharge/charge process was observed by scanning electron microscopy and is consistent with the presence of the two components in the discharge product. The results of this study provide new insight into how growth conditions control the nature of discharge product, which can be used to achieve improved performance in Li-O2 cell.