In this paper, we have investigated the effect of nanoconfinement with the slit-pore width of multilayer Tin+1CnTx (n = 1, 2) MXenes as a tunable parameter on the hydrogen storage capacity using a revised thermodynamic model developed for slit-pore materials across a wide range of pressure and temperature. We highlight that the surface termination plays an important role in dictating the surface adsorption energy of hydrogen molecules, which effectively determines the optimum slit-pore width to store hydrogen and, subsequently, the optimum hydrogen storage capacity. Our calculations showed that an upper bound of slit-pore width ranging from 8 to 40 & Aring;, for uniform and ternary O-/OH-/F-mixed terminated Tin+1CnTx (n = 1, 2), is suitable for enhancing the hydrogen storage capacity while maintaining a substantial nanoconfinement effect and thermodynamic stability. Our study suggests that a 40% increase in the surface adsorption energy could result in a 4-fold increase in the predicted hydrogen storage capacity. The calculated upper bound of the hydrogen gravimetric storage capacity of the multilayer Ti3C2Tx can achieve up to similar to 6.5 wt % H-2 at 77 K and 5 MPa and an even higher capacity of similar to 9.5 wt % H-2 at 77 K and 5 MPa for multilayer Ti2CTx, demonstrating their suitability for hydrogen storage applications. By determining the range of optimum slit-pore width that balances the gravimetric and volumetric storage capacity for hydrogen, the optimum slit-pore width of multilayer Tin+1CnTx (n = 1, 2) is about 13-18 & Aring;. Based on this optimum slit-pore width, the predicted optimum hydrogen gravimetric and volumetric storage capacity of the multilayer Ti3C2Tx slit pore is found to be 3.10 wt % and 47.96 g/L at 77 K and 5 MPa and is found to be comparable to metal organic frameworks (MOFs). Meanwhile, compared with the monolayer, the multilayer with slit pores demonstrated a significant improvement in terms of gravimetric storage capacity and desorption of hydrogen due to the nanoconfinement effect. This slit-pore nanoconfinement helps in stabilizing the hydrogen molecules' desorption with a steady and slower rate of desorption and is beneficial for hydrogen storage applications.

While we are pursuing a fully electrified society, high-energy rechargeable batteries are undergoing intensive investigation. In this respect, atomic and molecular layer deposition (ALD and MLD) have been drawing increasing interest, due to their unmatched capabilities to precisely modify electrodes’ surfaces for better electrochemical performance. In this work, we reviewed the recent studies using ALD/MLD for interface engineering of several important electrode materials, including nickel (Ni)-rich metal oxide cathodes, silicon (Si), and lithium (Li) anodes in lithium-ion and lithium metal batteries. We particularly discussed the most promising coatings from these studies and explored the underlying mechanisms based on experiments and modeling. We anticipate that this work will inspire more studies using ALD/MLD as an important technique for securing new solutions for batteries.

In this work, we systematically investigated the hydrogen storage properties of multilayer Ti3C2Tx MXene using density functional theory (DFT) coupled with the quantum-thermodynamic model to include the thermodynamic effect on hydrogen adsorption and storage behavior over a wide range of applied pressure and temperature. In addition to the surface-adsorbed hydrogen, we show that the interlayer spacing is a plausible storage route that could contribute to an additional hydrogen storage capacity. Due to hydrogen-bond bounded multilayer Ti3C2Tx, the insertion, diffusion, and adsorption of hydrogen molecules into the interlayer spacing of the multilayer structure require sufficient external pressure to overcome the energy penalty to separate the multilayer structure. Using DFT calculations, we presented a novel model in attempt to unveil the mechanism of the nanopump effect, and providing new insights into its underlying process from a theoretical perspective. The calculated upper bound of theoretical gravimetric storage capacity using DFT calculation for multilayer Ti3C2Tx is similar to 3.8 wt% H-2. While at 77 and 300 K with external pressure of 25 MPa, the predicted gravimetric capacity employing the DFT and quantum-thermodynamic model are found to be similar to 2.1 and 0.67 wt% H-2,H- respectively, and experimentally, these H-2-stored multilayer Ti3C2Tx structures can be verified based on our simulated XRD analysis. Based on this work, we believe that our current simulation model can provide a reasonable and realistic prediction of hydrogen storage capacity and a systematic study of hydrogen storage mechanisms in other two-dimensional (2D) layered materials, besides multilayer Ti3C2Tx.

Hexagonal diamond (HD), with anticipated physical properties superior than the known cubic diamond, has been pursued relentlessly since its inception 60 years ago1. However, natural and synthetic HD has only been preserved as a highly disordered component in fragile, heterogeneous mixtures of other nanocarbon structures that precludes determination of bulk properties and identification of HD as a bona fide crystalline phase2-4. Here we report the synthesis, recovery and extensive characterization of bulk HD by compressing and heating high-quality graphite single crystals under controlled quasi-hydrostatic conditions. We demonstrate the successful synthesis of 100-µm-sized to mm-sized, highly ordered, bulk HD. We observed direct transformation of graphite ( 10 1 ¯ 0 ) orientation to HD (0002) and graphite (0002) to HD ( 10 1 ¯ 0 ). The bulk sample consists of threefold intergrowth of tightly knitted 100-nm-sized crystals, predominantly HD with trace imperfections of cubic diamond. The interlayer bonds in HD are shortened with respect to intralayer bonds to optimize the HD structure. Notably, the hardness of HD is only slightly higher than cubic diamond. We anticipate that purifying the precursor graphite carbon and fine-tuning the high pressure-temperature (P-T) synthesis conditions may lead to higher-quality HDs.

In this work, we evaluated two closo-borate salts (Li2B12H12 and Li2B12F12) in propylene carbonate from theoretical and experimental perspectives to understand how the coordination environment influences their spectroscopic and electrochemical properties. The coordination environments of the closo-borate salts were modeled via density functional theory (DFT) and molecular dynamics (MD). Vibrational spectra calculated from the predicted coordination environments are in agreement with experimentally measured steady-state FTIR data. This theoretical investigation also suggested that Li2B12F12 would possess a higher ionic conductivity than Li2B12H12, which was corroborated experimentally. Additionally, an electrochemical cell was designed and fabricated that enabled operando optical and FTIR spectroelectrochemical (OP-IR-SEC) measurements. This allowed for the simultaneous measurement of the relative changes of species at a lithium electrode-liquid electrolyte interface and the visualization of lithium plating at the electrode surface. This technique could provide new chemical insights and potentially link optical changes at the electrode-electrolyte interface to specific chemical species in similar electrochemical systems. The Li2B12F12 electrolyte was found to have a higher thermal stability, which may find utility in applications for batteries that are subject to high-temperature conditions.

MXene, with its high aspect ratio and adjustable surface properties, has garnered significant attention in the realm of hydrogen storage research. For the first time, considering a ternary/quaternary mixed terminated MXene surface, the authors have investigated comprehensively the hydrogen storage potential of twodimensional (2D) titanium carbide Ti3C2Tx monolayer MXene using density functional theory (DFT). By considering mixed terminated surfaces, this study indicated the locally induced dipole due to the mixed termination is beneficial in facilitating hydrogen adsorption with stronger average adsorption energies than that of the uniform F-/O-/OH-/H-terminated surfaces. The authors estimated a compelling average H2 surface adsorption energy on the ternary mixed termination and total surface storage capacity to be -0.14 eV/H2 and -2 wt% H2, which is comparable to that of the metal-organic frameworks (MOFs). This study also reveals the importance of the local surface chemistry effects on hydrogen adsorption.

The family of two-dimensional (2D) carbides and/or nitrides, also known as MXenes, has generated great excitement within the scientific community and has been proposed for a wide variety of applications since its discovery. Despite this attention, there have been only limited studies of the atomically resolved local electronic and physical structure of MXene surfaces strongly affecting the physicochemical properties of these materials. Here, we report local structural, spectroscopic, and chemical investigations of the surfaces of Ti3C2Tx flakes using scanning tunneling microscopy and spectroscopy, closely coupled to theoretical studies. Terminal groups are visualized and characterized, along with surface TiO2 clusters formed upon exposure to air. Fundamental insight into the local electronic and chemical properties associated with different terminal groups on MXenes and their oxidation products is presented.

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 (DF ⠁ 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.

We proposed a more realistic albeit slightly complicated multilayer Ti3C2Tx model and performed a comprehensive theoretical study of its structural and electronic properties. In this work, we constructed various multilayer Ti3C2Tx structures considering different concentrations of hydrofluoric acid (HF; 5, 10, and 48 wt%) as the etchant. The validity of our ternary mixed O/OH/F-terminated Ti3C2Tx multilayer models is confirmed by the consistency of the calculated d-spacing (9.60 ± 0.07 Å), simulated X-ray diffraction (XRD) spectra and the predicted adhesion energy (0.77 ± 0.15 J m-2) with the reported experimental measurements. The uniform terminated and mixed terminated multilayer Ti3C2Tx exhibit metallic characteristics, similar to those of monolayer Ti3C2Tx. We found a stronger interaction between the interlayers with OH-rich ternary mixed terminated Ti3C2Tx surfaces, due to the formation of hydrogen bonds between the hydroxyl groups and adjacent layers of F/O terminal groups as supported by the crystal orbital Hamilton population (COHP) calculation. From this finding, we propose that multilayer Ti3C2Tx etched with a strong HF acid could be easily exfoliated into monolayer sheets due to smaller adhesion energy. Based on this work, we believe that the current findings will offer a fundamental understanding and a useful baseline multilayer model for the future investigation of the hydrogen and ion storage and diffusion properties in the MXene multilayer application.

Based on the unique ubiquity of similar solvate structures found in solvate crystals and superconcentrated electrolytes, we performed a systematic study of four reported solvate crystals which consist of different lithium salts (i.e., LiMPSA, LiTFSI, LiDFOB, and LiBOB) solvated by acetonitrile (MeCN) based on first principles calculations. Based on the calculations, these solvate crystals are predicted to be electronic insulators and are expected to be similar to their insulating liquid counterpart (e.g., 4 M superconcentrated LiTFSI-MeCN electrolyte), which has been confirmed to be a promising electrolyte in lithium batteries. Although the MeCN molecule is highly unstable during the reduction process, it is found that the salt-MeCN solvate molecules (e.g., LiTFSI-(MeCN)2, LiDFOB-(MeCN)2) and their charged counterparts (anions and cations) are both thermodynamically and electrochemically stable, which can be confirmed by Raman vibrational modes through the unique characteristic variation in C≡N bond stretching of MeCN molecules. Therefore, in addition to the development of new solvents or lithium salts, we suggest it is possible to utilize the formation of superconcentrated electrolytes with improved electrochemical stability based on existing known compounds to facilitate the development of novel electrolyte design in advanced lithium 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+delta, 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. A simple goodness of fit R factor to gauge how well a template surface structure can support LiO2 growth is developed. The R factor may be extended to other transition and main group element LiMx catalysts, as potential LiO2 growth supports.

Ionic liquids (ILs) are a new group of novel solvents with great potential in design-synthesis. They are promising electrolyte candidates in energy storage applications, especially in rechargeable batteries. However, in practice, their usage remains limited due to the unfavorable high-viscosity (η) property at ambient conditions. To optimize the design synthesis of ILs, a systematic fundamental study of their structure-property relationship is deemed necessary. In this study, we employed a deep-learning (DL) model to predict the room-temperature viscosity of a wide range of ILs that consist of various cationic and anionic families. Based on this DL model, accurate prediction of IL viscosity can be realized, reaching an R2 score of 0.99 with a root mean square error of ~45 mPa·s. To further help identify low- and high-η ILs, a low/high-η binary classification model with an overall accuracy of 93% for test prediction is obtained based on the DL model. From the important structure-property relationship analysis governed by the top-rank molecular descriptors of this model, a list of very low-η ILs (i.e., η < 30 mPa·s) that could be potentially useful in battery electrolytes is identified. Based on the finding of the DL model, it suggests that in order to achieve low-η, grafting IL cations into smaller sizes (e.g., smaller head rings) and short alkyl chains and reducing ionization potentials/energies will help. Meanwhile, for the same cations, further reducing anions in sizes, chain lengths, and hydrogen bonds might be useful to further reduce the viscosity. Thus, with a fine selection and molecular grafting of anionic and cationic species in ILs, we believe fine-tuning IL viscosities can be achieved through the proper design synthesis of functional groups in ILs.

The Li-S battery is exceptionally appealing as an alternative candidate beyond Li-ion battery technology due to its promising high specific energy capacity. However, several obstacles (e.g., polysulfides’ dissolution, shuttle effect, high volume expansion of cathode, etc.) remain and thus hinder the commercialization of the Li-S battery. To overcome these challenges, a fundamental study based on atomistic simulation could be very useful. In this work, a comprehensive investigation of the adsorption of electrolyte (solvent and salt) molecules, lithium sulfide, and polysulfide (Li2Sx with 2 ≤x≤ 8) molecules on the amorphous Al2O3 atomic layer deposition (ALD) surface was performed using first-principles density functional theory (DFT) calculations. The DFT results indicate that the amorphous Al2O3 ALD surface is selective in chemical adsorption towards lithium sulfide and polysulfide molecules compared to electrolytes. Based on this work, it suggests that the Al2O3 ALD is a promising coating material for Li-S battery electrodes to mitigate the shuttling problem of soluble polysulfides.

Li-O-2 batteries suffer from large charge overpotentials due to the high charge transfer resistance of Li2O2 discharge products. A potential solution to this problem is the development of LiO2-based batteries that possess low charge overpotentials due to the lower charge transfer resistance of LiO2. In this report, IrLi nanoparticles were synthesized and implemented for the first time as a LiO2 battery cathode material. The IrLi nanoparticle synthesis was achieved by a temperature-and time-optimized thermal reaction between a precise ratio of iridium nanoparticles and lithium metal. Li-O-2 batteries employing the IrLi-rGO cathodes were cycled up to 100 cycles at moderate current densities with sustained low cell charge potentials (< 3.5 V). Various characterization techniques, including SEM, DEMS, TEM, Raman, and titration, were used to demonstrate the LiO2 discharge product and the absence of Li2O2. On the basis of first-principles calculations, it was concluded that the formation of crystalline LiO2 can be stabilized by epitaxial growth on the (111) facets of IrLi nanoparticles present on the cathode surface. These findings demonstrate that, in addition to the previously studied Ir3Li intermetallic, the IrLi intermetallic also provides a means by which LiO2 discharge products can be stabilized and confirms the importance of templating for the formation process.

A K+ single cation ionic liquid (K-SCIL) contains only K+ cations. When used as an electrolyte in potassium batteries, it not only possesses the properties of conventional ionic liquids such as nonflammability, negligible vapor pressure, and good electrochemical stability but also increases the K+ transport number, which reduces concentration polarization of the electrolyte and improves the battery power performance. In this study, we report K-SCILs based on the low melting (T-m similar to 50 degrees C) potassium salt of (3-methoxypropyl)((trifluoromethyl)sulfonyl) amide (MPSA(-)). Combined with potassium bis(fluorosulfonyl)imide (KFSI), the mixture of the two salts exists as a liquid at as low as -13 degrees C, generating for the first time a room temperature K-SCIL. The phase diagram is established, and the ionic conductivities of various compositions are measured. Although the room temperature ionic conductivity is low, mild heating to 55 degrees C enhances the conductivity to > 1 mS/cm. Evidence for a strong resistance to K+ concentration gradients as well as a near unity K transport number were also found in this K-SCIL. This report hopes to serve as inspiration for the discovery of even lower melting K salts to enable low temperature K-SCIL electrolytes and to explore the benefits of SCILs in practical cells.

Poor oxidation stability of ether solvents at the cathode restricts the use of dilute ether electrolytes with conventional concentrations around 1 M in high-voltage lithium metal batteries. Here we develop an anion-adsorption approach to altering the ether solvent environment within the electrical double layer (EDL) at the cathode, by adding a small amount of nitrate, so that the oxidation tolerance of nitrate-containing dilute ether electrolytes is enhanced up to 4.4 V (versus Li/Li+), leading to complete compatibility with high-voltage cathodes and exhibiting superior cycling stability. Constant-potential molecular dynamics simulations reveal that ether molecules are mostly excluded from the cathode because of nitrate occupation in the inner layer of the EDL, thus suppressing ether oxidative decomposition. This work highlights that regulating the interfacial structure by adding surface adsorbates, rather than passivating cathode-electrolyte interphase or changing ion solvation, can help to enhance the oxidation stability of ether solvents. It also provides design criteria for adsorption-type additives to achieve high-voltage dilute ether electrolytes.

Ionic liquids (ILs) have great potential for application in energy storage and conversion devices. They have been identified as promising electrolytes candidates in various battery systems. However, the practical application of many ionic liquids remains limited due to the unfavorable melting points (Tm) which constrain the operating temperatures of the batteries and exhibit unfavorable transport property. To fine tune the Tm of ILs, a systematic study and accurate prediction of Tm of ILs is highly desirable. However, the Tm of an IL can change considerably depending on the molecular structures of the anion and cation and their combination. Thus, a fine control in Tm of ILs can be challenging. In this study, we employed a deep-learning model to predict the Tm of various ILs that consist of different cation and anion classes. Based on this model, a prediction of the melting point of ILs can be made with a reasonably high accuracy, achieving an R2 score of 0.90 with RMSE of ~32 K, and the Tm of ILs are mostly dictated by some important molecular descriptors, which can be used as a set of useful design rules to fine tune the Tm of ILs.

Corrosion protection is vital to ensure the reliability and long-term durability of metal components. Coating surfaces with two-dimensional (2D) materials is an attractive approach due to the stren...

Lithium–sulfur (Li–S) batteries are one of the most promising substitutes for the current Li-ion battery system as a next-generation storage system because of their high theoretical energy density ...