Aluminium-carbon batteries are promising next-generation secondary batteries with high power density that can potentially be manufactured much cheaper than current Li-ion batteries. The Al-C battery cell is made up of an Al anode and a graphite cathode. The first reported electrolyte is still the state-of-the art; AlCl 3 -[EMIm]Cl [1], which contains EMIm + , Al 2 Cl 7 - and AlCl 4 - ionic species, of which the two latter are active in the anode and cathode half reactions, respectively. Importantly, the electrolyte is electrochemically stable against the Al anode. However, the interfacial properties of the ionic liquid electrolyte against the Al anode and graphite cathode are still poorly understood [2]. In this work, we present a modelling study of the interfaces composed of Al electrodes and the ionic liquid electrolyte, including the electric double layer (EDL) formed at the interface. We utilize constant potential method molecular dynamics simulations to model the electrified interface [3-5] for different potential differences across the cell and different electrolyte compositions. We analyze the density profile of electrolyte species in the interface and find that the presence of only charged species causes charge overscreening and ionic layering extending far (>3 nm) into the electrolyte. The ionic species are highly oriented in the interface region next to the electrode, increasing the packing density of the species. The residence time of the species is about an order of magnitude higher in the interfaces compared to the bulk, indicating that the interface constitute a considerable barrier against ion transport, however there are large variations in the species’ mobility. We examine the influence of adding neutral solvent species to the electrolyte, e.g. tetraglyme and sulfones. The influence on the transport properties of the bulk electrolyte and the interfaces next to electrodes are studied. Reference s : 1. Lin, M.C., Gong, M., Lu, B. et al. An ultrafast rechargeable aluminium-ion battery. Nature. 520 (2015) 324–328. 2. Das, S.K., Mahapatra, S., Lahan, H. Aluminium-ion batteries: developments and challenges.J. Mater. Chem. A. 5 (2017) 6347-636. 3. Siepmann, J. I.; Sprik, M. Influence of surface topology and electrostatic potential on water/electrode systems. The Journal of Chemical Physics. 102 (1995) 511–524. 4. Reed, S. K.; Lanning, O. J.; Madden, P. A. Electrochemical interface between an ionic liquid and a model metallic electrode. The Journal of Chemical Physics. 126 (2007) 084704. 5. Ahrens-Iwers, L. J. V.; Janssen, M.; Tee, S. R.; Meißner, R. H. ELECTRODE: An electrochemistry package for atomistic simulations. The Journal of Chemical Physics. 157 (2022) 084801.

Small single-stranded non-coding ribonucleic acids (RNAs) perform several critical functions within living cells. However, the thermodynamics of nanoscale RNA strains remain poorly understood. In this work, we studied the non-equilibrium and size-dependent thermodynamics of stretching and relaxation for various RNA strain lengths (N = 5, 10, and 50), compositions (poly-adenosine, guanosine, cytidine, and uridine), and secondary structure complexities (hairpin loops: 1ZIG and 2KOC) using molecular dynamics simulations. We calculate the Gibbs and Helmholtz energies during stretching, as well as the degree of base stacking for each molecule as a function of elongation. Trends in thermo-mechanical properties dependent on RNA size, type, and structural complexity are highlighted. We observe fluctuations in force during length-controlled (isometric) simulations and fluctuations in length during force-controlled (isotensional) simulations. Our findings indicate that the role of base stacking on chain stability during stretching is influenced by RNA size, nucleobase type, and chain complexity (single-stranded vs. hairpin). Lastly, we investigate the energetic cost of repeated stretching and relaxation in both water and vacuum environments.

In order to increase the adoption of batteries for sustainable transport and energy storage, improved charging and discharging capabilities of lithium-ion batteries are necessary. To achieve this, accurate data that describe the internal state of the cells are essential. Several models have been derived, and transport coefficients have been reported for use in these models. We report for the first time a complete set of transport coefficients to model the concentration and temperature polarization in a lithium-ion battery ternary electrolyte, allowing us to test common assumptions. We include effects due to gradients in chemical potentials and in temperature. We find that the voltage contributions due to salt and solvent polarization are of the same order of magnitude as the ohmic loss and must be taken into account for more accurate modeling and understanding of battery performance. We report new Soret and Seebeck coefficients and find thermal polarization to be significant in cases relevant to battery research. The analysis is suitable for electrochemical systems, in general.

In thermosetting epoxies, thermomechanical properties can be enhanced by conscious selection of curing agents. Full cross-linking leads to a maximum in the glass- transition temperature. However, the relation between the glass transition temperature and the epoxy matrix depends on several factors beyond the cross-linking degree, such as the molecular weight of the polymers, network organization, amount of branching, and the presence of hydrogen bonds. In this study, we investigated adding non- stoichiometric ratios of the epoxy resin Epikote 828 and the curing agent Jeffamine D230. The investigations were done through a combination of molecular dynamics simulations and experiments, primarily differential scanning calorimetry and nanoindentation. Reorganization of the network to fewer clusters with a higher degree of linearity overcomes the effect of cross-linking and leads to a reduction of glass-transition temperature with increasing concentrations of curing agent to epoxy. The elastic, shear, and compressive moduli remained constant. Hence, moderating the curing agent con- tent has the potential to improve thermal properties while maintaining mechanical properties for this epoxy system.

Improvements in the thermomechanical properties of epoxy upon inclusion of well-dispersed SiO2 nanoparticles have been demonstrated both experimentally and through molecular dynamics simulations. The SiO2 was represented by two different dispersion models: dispersed individual molecules and as spherical nanoparticles. The calculated thermodynamic and thermomechanical properties were consistent with experimental results. Radial distribution functions highlight the interactions of different parts of the polymer chains with the SiO2 between 3 and 5 nm into the epoxy, depending on the particle size. The findings from both models were verified against experimental results, such as the glass transition temperature and tensile elastic mechanical properties, and proved suitable for predicting thermomechanical and physicochemical properties of epoxy-SiO2 nanocomposites.

Ionic correlations are important in concentrated PEO–LiTFSI polymer electrolytes. Onsager coefficients are necessary to characterize the ionic motion. Li and TFSI are anticorrelated, increasing the ionic conductivity and Li-ion transport number.

Transport properties are essential for the understanding and modeling of electrochemical cells, in particular complex systems like lithium-ion batteries. In this study, we demonstrate how a certain degree of freedom in the choice of variables allows us to efficiently determine a complete set of transport properties. We apply the entropy production invariance condition to different sets of electrolyte variables and obtain a general set of formulas. We demonstrate the application of these formulas to an electrolyte typical for lithium-ion batteries, 1M lithium hexafluoro-phosphate in a 1:1 wt. % mixture of ethylene and diethyl carbonates. While simplifications can be introduced, they provide inadequate predictions of conductivity and transport numbers, and we argue that a full matrix of Onsager coefficients is needed for adequate property predictions. Our findings highlight the importance of a complete set of transport coefficients for accurate modeling of complex electrochemical systems and the need for careful consideration of the choice of variables used to determine these properties.

A systematic description of microscopic mechanisms is necessary to understand mass transport in solid and liquid electrolytes. From Molecular Dynamics (MD) simulations, transport properties can be computed and provide a detailed view of the molecular and ionic motions. In this work, ionic conductivity and transport numbers in electrolyte systems are computed from equilibrium and nonequilibrium MD simulations. Results from the two methods are compared with experimental results, and we discuss the significance of the frame of reference when determining and comparing transport numbers. Two ways of computing ionic conductivity from equilibrium simulations are presented: the Nernst-Einstein approximation or the Onsager coefficients. The Onsager coefficients take ionic correlations into account and are found to be more suitable for concentrated electrolytes. Main features and differences between equilibrium and nonequilibrium simulations are discussed, and some potential anomalies and critical pitfalls of using nonequilibrium molecular dynamics to determine transport properties are highlighted.

Data for publication: Coupled Ion Transport in Concentrated PEO-LiTFSI Polymer Electrolytes in New Journal of Chemistry.

The Kirkwood-Buff theory is a cornerstone of the statistical mechanics of liquids and solutions. It relates volume integrals over the radial distribution function, so-called Kirkwood-Buff integrals (KBIs), to particle number fluctuations and thereby to various macroscopic thermodynamic quantities such as the isothermal compressibility and partial molar volumes. Recently, the field has seen a strong revival with breakthroughs in the numerical computation of KBIs and applications to complex systems such as bio-molecules. One of the main emergent results is the possibility to use the finite volume KBIs as a tool to access finite volume thermodynamic quantities. The purpose of this Perspective is to shed new light on the latest developments and discuss future avenues.

The 2D layered Ti3C2T2 MXene is known for its diverse chemistry and has been investigated as potential anode and cathode in Li, Na, and Mg batteries. Ti3C2T2 layers can stack in at least two different ways depending on the termination group chemistry. In addition, stacking and termination groups influence the diffusivity and energy of ions intercalated in the MXene. How stacking influences the diffusivity, and how intercalated ions influence the stacking stability are not fully understood. In this study, density functional theory simulations explore Li, Na, and Mg ions intercalated in Ti3C2T2 stacked in four different ways; two are experimentally verified, and previously discussed in literature, and two with limited experimental evidence. It is shown that the stacking can reduce diffusion by 8–20 orders of magnitude. It is also explained how the termination group chemistry and the intercalated Li/Na/Mg ions change the relative stability of the stackings. The stacking's influence on diffusion properties is explained by examining the coordination of the ions at different points along the migration path. It is suggested that Ti3C2T2 with significant fluorine termination can be well‐suited for especially Na anode use, and regardless of termination is unsuited as Mg cathode.

Fluids confined in small volumes behave differently than fluids in bulk systems. For bulk systems, a compact summary of the system's thermodynamic properties is provided by equations of state. However, there is currently a lack of successful methods to predict the thermodynamic properties of confined fluids by use of equations of state, since their thermodynamic state depends on additional parameters introduced by the enclosing surface. In this work, we present a consistent thermodynamic framework that represents an equation of state for pure, confined fluids. The total system is decomposed into a bulk phase in equilibrium with a surface phase. The equation of state is based on an existing, accurate description of the bulk fluid and uses Gibbs' framework for surface excess properties to consistently incorporate contributions from the surface. We apply the equation of state to a Lennard-Jones spline fluid confined by a spherical surface with a Weeks-Chandler-Andersen wall-potential. The pressure and internal energy predicted from the equation of state are in good agreement with the properties obtained directly from molecular dynamics simulations. We find that when the location of the dividing surface is chosen appropriately, the properties of highly curved surfaces can be predicted from those of a planar surface. The choice of the dividing surface affects the magnitude of the surface excess properties and its curvature dependence, but the properties of the total system remain unchanged. The framework can predict the properties of confined systems with a wide range of geometries, sizes, interparticle interactions, and wall-particle interactions, and it is independent of ensemble. A targeted area of use is the prediction of thermodynamic properties in porous media, for which a possible application of the framework is elaborated.

Dendrite formation occurs on Li, Na, and Mg metal anodes in rechargeable batteries, and is a safety challenge, as well as a limiting factor for increasing energy- and power density. However, the behaviour of the dendrites differs depending on the anode material. In this study, we investigate the local bulk and surface crystal structure of Li, Na, and Mg surfaces to shed light on how differences in the morphology and structure of the anode surface and its metal deposits can explain differences in dendrite formation on Li, Na, and Mg anodes. The local bulk- and surface structure are found using molecular dynamics simulations in combination with the surface adaptive common neighbour analysis, and indicate that Li and Na surfaces are more prone to surface instabilities and formation of protrusions than Mg surfaces, which remain flat and hexagonal close-packed even near room temperature. Additionally, the equilibrium shapes of the Mg deposits obtained from density functional theory assume more flat and hexagonal shapes than the Li and Na deposits. Together, these results shed light on atomic mechanisms that may contribute to the different propensities of Li, Na, and Mg metal anodes to form dendrites.

We present a new method for computing chemical potential differences of macroscopic systems by sampling fluctuations in small systems. The small system method, presented by Schnell et al. [Schnell et al., J. Phys. Chem. B, 2011, 115, 10911], is used to create small embedded systems from molecular dynamics simulations, in which fluctuations of the number of particles are sampled. The sampled fluctuations represent the Boltzmann distributed probability of the number of particles. The overlapping region of two such distributions, sampled from two different systems, is used to compute their chemical potential difference. Since the thermodynamics of small systems is known to deviate from the classical thermodynamic description, the particle distributions will deviate from the macroscopic behavior as well. We show how this can be utilized to calculate the size dependence of chemical potential differences and eventually extract the chemical potential difference in the thermodynamic limit. The macroscopic chemical potential difference is determined with a relative error of 3% in systems containing particles that interact through the truncated and shifted Lennard-Jones potential. In addition to computing chemical potential differences in the macroscopic limit directly from molecular dynamics simulation, the new method provides insights into the size dependency that is introduced to intensive properties in small systems.

A versatile group of 2D carbide materials from the past decade, MXenes, have attracted attention for their rich chemistry and wide range of properties. The perhaps best known MXene, namely, Ti3C2Tx, has been observed to stack in two distinct ways, and simulations show that this influences interlayer bonding energy and Li diffusion. In this DFT study, six types of Ti3C2T2 interlayer bonds resulting from O, F, and OH termination groups are assessed with respect to stability. It is shown that OH termination groups are highly stable up to 50% coverage, but unstable for higher coverage. A model to predict stacking type based on termination group chemistry shows that the degree of hydrogen bonding is the deciding factor. The model is also tested on V2CT2 and Zr3C2T2, giving similar results to those of Ti3C2T2. By calculating migration barriers for Ti3C2O2, we show that Li, Na, and Mg have orders of magnitude faster diffusion in the stacking favored by hydrogen bonds. XRD patterns calculated for both stackings show they are close to indistinguishable, highlighting the need for caution when classifying stacking.

Li adatom SDBs are affected by the local crystal structure of the surroundings. Cu substrates introduce vacancies and disorder in the Li metal anode, which increases the adatom SDB and therefore the tendency to form Li dendrites on the surface.

The inevitable nexus between energy use and CO2 emission necessitates the development of sustainable energy systems. The conversion of CO2 to CH4 using green H-2 in power-to-gas applications in such energy systems has attracted much interest. In this context, the present study provides a thermodynamic insight into the effect of water removal on CO2 conversion and irreversibility within a CO2 methanation reactor. A fixed-bed reactor with one intermediate water removal point, representing two reactors in series, was modeled by a one-dimensional pseudo-homogeneous model. Pure CO2 or a mixture of CO2 and methane, representing a typical biogas mixture, were used as feed. For short reactors, both the maximum conversion and the largest irreversibilities were observed when the water removal point was located in the middle of the reactor. However, as the length of the reactor increased, the water removal point with the highest conversion was shifted towards the end of the reactor, accompanied by a smaller thermodynamic penalty. The largest irreversibilities in long reactors were obtained when water removal took place closer to the inlet of the reactor. The study discusses the potential benefit of partial water removal and reactant feeding for energy-efficient reactor design.

Mass transfer across a liquid-repelling gas permeable membrane is influenced by the state(s) of the liquid-vapor interface(s) on the surface of the membrane, the pore geometry, and the solid-fluid interactions inside the membrane. By tuning the different local contributions, it is possible to enhance the temperature difference-driven mass flux across the membrane for a constant driving force. Non-equilibrium molecular dynamics simulations were used to simulate a temperature difference-driven mass flux through a gas permeable membrane with the evaporating liquid on one side and the condensing liquid on the other. Both sides were simulated for Wenzel- and Cassie-Baxter-like states. The interaction between the fluid and the solid inside the gas permeable membrane varied between the wetting angles of θ = 125° and θ = 103°. For a constant driving force, the Cassie-Baxter state led to an increased mass flux of almost 40% in comparison to the Wenzel state (given a small pore resistance). This difference was caused by an insufficient supply of vapor particles at the pore entrance in the Wenzel state. The difference between the Wenzel and Cassie-Baxter states decreased with increasing resistance of the pore. The condensing liquid-vapor interface area contributed in the same manner to the overall transport resistance as the evaporating liquid-vapor interface area. A higher repulsion between the fluid and the solid inside the membrane decreased the overall resistance.

Small systems have higher surface area-to-volume ratios than macroscopic systems. The thermodynamics of small systems therefore deviates from the description of classical thermodynamics. One consequence of this is that properties of small systems can be dependent on the system's ensemble. By comparing the properties in grand canonical (open) and canonical (closed) systems, we investigate how a small number of particles can induce an ensemble dependence. Emphasis is placed on the insight that can be gained by investigating ideal gases. The ensemble equivalence of small ideal gas systems is investigated by deriving the properties analytically, while the ensemble equivalence of small systems with particles interacting via the Lennard-Jones or the Weeks-Chandler-Andersen potential is investigated through Monte Carlo simulations. For all the investigated small systems, we find clear differences between the properties in open and closed systems. For systems with interacting particles, the difference between the pressure contribution to the internal energy, and the difference between the chemical potential contribution to the internal energy, are both increasing with the number density. The difference in chemical potential is, with the exception of the density dependence, qualitatively described by the analytic formula derived for an ideal gas system. The difference in pressure, however, is not captured by the ideal gas model. For the difference between the properties in the open and closed systems, the response of increasing the particles' excluded volume is similar to the response of increasing the repulsive forces on the system walls. This indicates that the magnitude of the difference between the properties in open and closed systems is related to the restricted movement of the particles in the system. The work presented in this paper gives insight into the mechanisms behind ensemble in-equivalence in small systems, and illustrates how a simple statistical mechanical model, such as the ideal gas, can be a useful tool in these investigations.

The ideal gas model is an important and useful model in classical thermodynamics. This remains so for small systems. Molecules in a gas can be adsorbed on the surface of a sphere. Both the free gas molecules and the adsorbed molecules may be modeled as ideal for low densities. The adsorption energy, Us, plays an important role in the analysis. For small adsorbents this energy depends on the curvature of the adsorbent. We model the adsorbent as a sphere with surface area Ω=4πR2, where R is the radius of the sphere. We calculate the partition function for a grand canonical ensemble of two-dimensional adsorbed phases. When connected with the nanothermodynamic framework this gives us the relevant thermodynamic variables for the adsorbed phase controlled by the temperature T, surface area Ω, and chemical potential μ. The dependence of intensive variables on size may then be systematically investigated starting from the simplest model, namely the ideal adsorbed phase. This dependence is a characteristic feature of small systems which is naturally expressed by the subdivision potential of nanothermodynamics. For surface problems, the nanothermodynamic approach is different, but equivalent to Gibbs' surface thermodynamics. It is however a general approach to the thermodynamics of small systems, and may therefore be applied to systems that do not have well defined surfaces. It is therefore desirable and useful to improve our basic understanding of nanothermodynamics.