Calcium-Ion Batteries: Identifying Ideal Electrolytes for Next-Generation Energy Storage Using Computational Analysis

Calcium ion batteries show promise as a high-density, next generation replacement for current lithium ion batteries. The precise chemical structure of the carbonate electrolyte solvent has a large impact on calcium battery efficacy. In this computational study, we have investigated the solvation behavior of calcium tetrafluoroborate in both neat carbonates and carbonate mixtures using combined molecular dynamics simulations and quantum mechanical calculations. Our results indicate that both neat ethyl methyl carbonate and a mixture of ethylene carbonate and diethyl carbonate show the highest free-energy of solvation for the Ca2+ ion, making them likely candidates for further focus. The cation’s interaction with the carbonyls of the coordinating solvents, rather than those with the tetrafluoroborate counterions, play the primary role in delocalizing the charge on Ca2+. Detailed calculations indicate that the HOMO-LUMO energy gap (Eg), electronic chemical potential (μ) and chemical hardness (η) of the calcium-carbonate complexes are directly proportional to the free energy of solvation of the complex. Comparison of these observed trends with our previous results from Li+, Na+ and Mg2+ ions show that this correlation is also observed in solvated magnesium ions, but not in lithium or sodium salts. This observation should assist in the rational design of next generation battery materials in the rational selection of additives, counterions, or electrolyte solvent

The effect of ionic liquid adsorption on the electronic and optical properties of fluorographene nanosheets

In the present study, we investigate the adsorption characteristics of six different ionic liquids (ILs) on a fully-fluorinated graphene (fluorographene, FG) surface using electronic structure studies and associated analysis methods. A systematic comparison of differences in IL binding energies (ΔEb) with fluorographene, graphene and hexagonal boron nitride surfaces indicates that fluorination strongly decreases the binding energy compared to the other two surfaces, hence resulting in the binding energetics: ΔEb (Graphene…IL) \u3e ΔEb (Hexagonal boron-nitride…IL) \u3e ΔEb (Fluorographene…IL). To probe the reasons for this difference, quantum theory of atoms in molecules (QTAIM) analysis and non-covalent interactions (NCI) analyses were carried out. Results indicate that the stability of complexes of FG surface with ILs (FG…IL) arises only due to the presence of the expected weak non-covalent intermolecular interactions. The calculation of charge transfers by employing the ChelpG method shows that the interaction of ILs with FG surface generally induces a negative charge on the FG surface. Furthermore, these interactions lead to a decrease of the HOMO-LUMO energy gap (Eg) of the FG surface, enhancing its electrical conductivity. In addition, a detailed analysis of the global molecular descriptors including the Fermi energy level (EFL), work function (WF), electronic chemical potential (μ), chemical hardness (η), global softness (S) and electrophilicity index (ω) was carried out for both the FG surface alone and the adsorbed complexes showing that there are small, but meaningful, differences in the reactivity of the surface depending on the nature of the IL. Finally, time-dependent DFT (TD-DFT) calculations of the optical properties of FG surface and FG…IL complexes reveal that the absorption spectrum of the FG surface undergoes a red shift following IL adsorption. This study demonstrates that FG provides a useful complementary tool to graphene and boron nitride materials, allowing for the fine-tuning of the optoelectronic properties of these monolayer materials. These results will assist in the development of these types of ILs for applications in optoelectronics

A DFT study of the adsorption of deep eutectic solvents onto graphene and defective graphene nanoflakes

The interaction of four deep choline chloride-derived eutectic solvents (DESs) with both graphene nanoflakes (GNF) and its defective double-vacancy and Stone–Wales forms (DV-GNF and SW-GNF), was evaluated using density functional theory (DFT). The presence of defects increases the adsorption energy of DESs, following the order DES∩DV-GNF \u3e DES∩SW-GNF \u3e DES∩GNF. Non-covalent interaction and energy decomposition analyses show that the interactions are noncovalent and dominated by dispersive forces. Furthermore, we find that the presence of aromatic moieties in the DESs increases the van der Waals interactions with the surfaces. These interactions decrease the HOMO-LUMO (Eg) energy gap of the surfaces and thus increase reactivity. Reactivity parameter calculations indicate that the chemical potential (μ) and chemical hardness (η) of the complexes follow the order DES∩GNF \u3e DES∩SW-GNF \u3e DES∩DV-GNF. This order is reversed for the global softness (S) and electrophilicity index (ω). Time-dependent DFT (TD-DFT) calculations predict that the adsorption of DESs onto DV-GNF and SW-GNF should red shift absorption, while the absorption spectrum of GNF surface remains unchanged upon DES adsorption. The biggest changes in the absorption spectra are observed upon adsorption of DESs on the DV-GNF surface due to the stronger affinity of the DESs for this surface

Ionic Liquid Based on α-Amino Acid Anion and N7,N9-Dimethylguaninium Cation ([dMG][AA]): Theoretical Study on the Structure and Electronic Properties

The interactions between five amino acid based anions ([AA]⁻ (AA = Gly, Phe, His, Try, and Tyr)) and N7,N9-dimethylguaninium cation ([dMG]⁺) have been investigated by the hybrid density functional theory method B3LYP together with the basis set 6-311++G­(d,p). The calculated interaction energy was found to decrease in magnitude with increasing side-chain length in the amino acid anion. The interaction between the [dMG]⁺ cation and [AA]⁻ anion in the most stable configurations of ion pairs is a hydrogen bonding interaction. These hydrogen bonds (H bonds) were analyzed by the quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analysis. Finally, several correlations between electron densities in bond critical points of hydrogen bonds and interaction energy as well as vibrational frequencies in the most stable configurations of ion pairs have been checked

Trends in Na-Ion Solvation with Alkyl-Carbonate Electrolytes for Sodium-Ion Batteries: Insights from First-Principles Calculations

Classical molecular dynamics (MD) simulations and M06-2X hybrid density functional theory calculations have been performed to investigate the interaction of various nonaqueous organic electrolytes with Na+ ion in rechargeable Na-ion batteries. We evaluate trends in solvation behavior of seven common electrolytes namely pure carbonate solvents (ethylene carbonate (EC), vinylene carbonate (VC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC)) and four binary mixtures of carbonates (EC:PC, EC:DMC, EC:EMC, and EC:DEC). Thermochemistry calculations for the interaction of pure and binary mixtures of carbonate solvents with Na+ ion, Na+ ion coordinated with carbonate clusters obtained from molecular dynamics simulations, show that the formation of Na-carbonate complexes is exothermic and proceeds favorably. Based on the highest binding energy (ΔEb), enthalpy of solvation (ΔH(sol)), and Gibbs free energy of solvation (ΔG(sol)) values for the interaction of Na+ ion with carbonate solvents, our results conclusively show that pure EC and binary mixture of (EC:PC) are the best electrolytes for sodium-ion based batteries. Quantum chemical analyses are performed to understand the observed trends in ion solvation. Quantum theory of atoms in molecules (QTAIM) analysis shows that the interactions in Na-carbonate complexes are classified as a closed-shell (electrostatic) interaction. The localized molecular orbital energy decomposition analysis (LMO-EDA) also indicates that the electrostatic term (ΔEele) in the interaction energy between Na+ ion and carbonate solvents has the highest value and confirms the results of QTAIM about the electrostatic nature of Na+ ion interaction. The noncovalent interaction (NCI) plots indicate that the noncovalent interactions responsible for the formation of Na-carbonate complexes are strong to weak attractive interactions. Density of state (DOS) calculations show that the HOMO−LUMO energy gap in the EC, VC, PC, BC, DMC, EMC, and DEC increases as they interact with Na+ ion, although the HOMO−LUMO energy gap decreases with the addition of EC as an electrolyte additive to PC, DMC, and EMC. Calculated trends based on these quantum chemical calculations suggest that EC and binary mixture of EC:PC emerge as the best electrolytes in sodium-ion batteries, which is in excellent agreement with previously reported in silico experimental results

Effect of Mono-Vacant Defects on the Adsorption Properties of Deep Eutectic Solvents onto Hexagonal Boron-Nitride Nanoflakes

Hexagonal boron nitride is a promising material for a variety of electronic, optical, and material science applications. Both the synthesis of the material through exfoliation, and its various applications almost inevitably require its solvation. Deep eutectic solvents (DES) are extremely useful solvents for these types of applications due to their non-volatility, inflammability, biocompatibility, and reasonable cost. There are many different deep eutectic solvents available, and their suitability for any given application is particularly dependent on the specific of their structure. DES have been examined computationally for use with boron nitride, but these calculations use idealized, perfect boron nitride sheets instead of the more realistic, defect-containing systems. In this report, we investigated four DESs with two experimentally observed defective boron nitride, one with a single boron vacancy, the other with a single nitrogen vacancy. All DESs bound with higher affinity to the defective boron nitride than to the pristine surface. Charge transfer was minimal in all cases although the surfaces tended to donate electron density to the solvents. The interactions between the solvents and the surfaces are primarily non-covalent although in several cases natural bond order analysis indicates a partial covalent interaction that helps explain the higher-than-expected affinity for particular DES. The DESs have little effect on the predicted optical behaviour of the pristine boron nitride but do significantly change the adsorption spectrum of the defective boron nitride nanoflakes; the effect on bulk material might be limited. Together these results suggest that the choice of DES can either be made to limit any effect on the properties of the material (urea-choline chloride) or to affect the optical and electronic nature of the material (benzoic acid-choline chloride)

Meta-Hybrid Density Functional Theory Study of Adsorption of Imidazolium- and Ammonium-Based Ionic Liquids on Graphene Sheet

In this study, two types of ionic liquids (ILs) based on 1-butyl-3-methylimidazolium [Bmim]⁺ and butyltrimethylammonium [Btma]⁺ cations, paired to tetrafluoroborate [BF₄]⁻, hexafluorophosphate [PF₆]⁻, dicyanamide [DCA]⁻, and bis­(trifluoromethylsilfonyl)­imide [Tf₂N]⁻ anions, were chosen as adsorbates to investigate the influence of cation and anion type on the adsorption of ILs on the graphene surface. The adsorption process on the graphene surface (circumcoronene) was studied using M06-2X/cc-pVDZ level of theory. Empirical dispersion correction (D3) was also added to the M06-2X functional to investigate the effects of dispersion on the binding energy values. The graphene···IL configurations, binding energies, and thermochemistry of IL adsorption on the graphene surface were investigated. Orbital energies, charge transfer behavior, the influence of adsorption on the hydrogen bond strength between cation and anion of ILs, and the significance of noncovalent interactions on the adsorption of ILs on the graphene surface were also considered. ChelpG analysis indicated that upon adsorption of ILs on the graphene surface the overall charge on the cation, anion, and graphene surface changes, enabled by the charge transfer that occurs between ILs and graphene surface. Orbital energy and density of states calculations also show that the HOMO–LUMO energy gap of ILs decreases upon adsorption on the graphene surface. Quantum theory of atoms in molecules analysis indicates that the hydrogen-bond strength between cation and anion in ILs decreases upon adsorption on the graphene surface. Plotting the noncovalent interactions between ILs and graphene surface shows the role and significance of cooperative π···π, C–H···π, and X···π (X = N, O, F atoms from anions) interactions in the adsorption of ILs on the graphene surface. The thermochemical analysis also indicates that the free energy of adsorption (Δ<i>G</i>_ads) of ILs on the graphene surface is negative, and thus the adsorption occurs spontaneously

Effect of mono-vacant defects on the adsorption properties of deep eutectic solvents onto hexagonal boron-nitride nanoflakes

Hexagonal boron nitride is a promising material for a variety of electronic, optical, and material science applications. Both the synthesis of the material through exfoliation, and its various applications almost inevitably require its solvation. Deep eutectic solvents (DES) are extremely useful solvents for these types of applications due to their non-volatility, inflammability, biocompatibility, and reasonable cost. There are many different deep eutectic solvents available, and their suitability for any given application is particularly dependent on the specific of their structure. DES have been examined computationally for use with boron nitride, but these calculations use idealized, perfect boron nitride sheets instead of the more realistic, defect-containing systems. In this report, we investigated four DESs with two experimentally observed defective boron nitride, one with a single boron vacancy, the other with a single nitrogen vacancy. All DESs bound with higher affinity to the defective boron nitride than to the pristine surface. Charge transfer was minimal in all cases although the surfaces tended to donate electron density to the solvents. The interactions between the solvents and the surfaces are primarily non-covalent although in several cases natural bond order analysis indicates a partial covalent interaction that helps explain the higher-than-expected affinity for particular DES. The DESs have little effect on the predicted optical behaviour of the pristine boron nitride but do significantly change the adsorption spectrum of the defective boron nitride nanoflakes; the effect on bulk material might be limited. Together these results suggest that the choice of DES can either be made to limit any effect on the properties of the material (urea-choline chloride) or to affect the optical and electronic nature of the material (benzoic acid-choline chloride)

The interaction of deep eutectic solvents with pristine carbon nanotubes and their associated defects: A density functional theory study

In this study, the interaction of four deep eutectic solvents (DESs): [Choline chloride][Urea] ([ChCl][U]), [Choline chloride][Ethylene glycol] ([ChCl][EG]), [Choline chloride][Glycerol] ([ChCl][Gly]) and [Choline chloride][Benzoic acid] ([ChCl][BA]), with pristine carbon nanotube (CNT) and its defects: double-vacancy and Stone–Wales structures (CNT-DV and CNT-SW) is investigated using density functional theory (DFT) calculations. The geometry optimization, electronic property calculations, noncovalent interaction analysis and optical properties of the DES@nanotube complexes were carried out at the M06-2X/cc-pVDZ level of theory. The adsorption energy (Eads) calculations show that the presence of the DV and SW defects on the CNT increases the adsorption strength of the DESs, DES@CNT-SW \u3e DES@CNT-DV \u3e DES@CNT. On the other hand, the adsorption energy values increase with an increase in the volume of DESs due to the increase of noncovalent interactions, following the order [ChCl][BA] \u3e [ChCl][Gly] \u3e [ChCl][U] \u3e [ChCl][EG]. The calculation of the HOMO-LUMO energy gap (Eg) and chemical hardness (η) of the DES@nanotube complexes indicates that the DES@CNT-SW complexes have the largest Eg and η values and thus the lowest chemical reactivity. The analysis of the interactions between the nanotubes and the DESs using noncovalent interaction (NCI) plots and energy decomposition analysis (EDA) suggests that the DESs adsorb onto the nanotubes through van der Waals interactions and that dispersive interactions dominate (dispersion interaction energy (ΔEdisp) \u3e electrostatic interaction energy (ΔEelec) \u3e orbital interaction energy (ΔEorb)). Predicted ultraviolet–visible absorption spectra of the complexes show that the adsorption of DESs on the nanotubes has only a very marginal effect on the optical response of the nanotubes. Transition density matrix heat maps reveal that the electrons and holes localize to the CNT, CNT-DV and CNT-SW surfaces in the DES@nanotube complexes, indicating that the charge transfer occurs mostly on the surfaces