Open-Cage Fullerene as a Selective Molecular Trap for LiF/[BeF].

The insertion of ionic compounds into open-cage fullerenes is a challenging task due to the electropositive nature of the cavity. The present work reports the preparation of an open-cage C60 derivative with a hydroxy group pointing towards the centre of the cavity, which can coordinate to a metal cation, thus acting as a bait/hook to trap the metal cation such as the lithium cation in neutral LiF and the beryllium cation in the cationic [BeF]+ species. Other metal salts could not be inserted under similar conditions. The structure of MF in the cage was unambiguously determined by single-crystal X-ray diffraction. Owing to its tendency to undergo polycoordination, Li+ monomer salts have not been isolated before, despite extensive research on Li bonds. The present results provide a unique example of a Li bond.

An Unprecedented Fireproof, Anion-Immobilized Composite Electrolyte Obtained via Solidifying Carbonate Electrolyte for Safe and High-Power Solid-State Lithium-Ion Batteries.

The update of electrolytes from a liquid state to a solid state is considered effective in improving the safety and energy density of lithium-ion batteries (LIBs). Although numerous efforts have been made, solid-state electrolytes' (SSEs) insufficient charge transfer capability remains a significant obstruction to practical applications. Herein, a fireproof and anion-immobilized composite electrolyte is designed by solidifying carbonate electrolyte, exhibiting superior Li-ion conductivity (11.5 mS cm-1 at 30 °C) and Li-ion transference number (0.90), which endows LIBs excellent rate capability and cycling stability. Elaborate characteristics and theoretical calculations demonstrate the presence of robust anion-molecule coordination (composed of lithium bond and Coulomb force) enables a more efficient ion transport, where the mobility of Li+ ion is enhanced meanwhile the anions are immobilized. This work highlights how the strong interactions between electrolyte components can be used to simultaneously regulate the migration of Li+ ion and anion, and realize a one-step conversion of inflammable liquid-state electrolyte to nonflammable solid-state electrolyte.

Enhanced Cycling Performance of All-Solid-State Li-S Battery Enabled by PVP-Blended PEO-Based Double-Layer Electrolyte.

Despite the high specific capacity of Li-S battery, shuttle effect of lithium polysulfides (LiPSs) and safety issue pose a great challenge to realize its commercial application. Replacing liquid electrolyte with poly (ethylene oxide) (PEO) -based solid-state electrolyte is considered as a promising method to boost the safety, but the shuttle effect of LiPSs cannot be completely eliminated. In this work, a new kind of double-layer PEO-based polymer electrolyte is designed to restrict the LiPSs. The layer next to cathode consists of PEO and poly(vinylpyrrolidone) (PVP). The other layer consists of PEO. PVP with abundant of amide groups has been proved to have strong affinity to LiPSs. The strong interaction between LiPSs and carbonyl groups in amide is verified by Attenuated Total Reflection-Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy tests. As a result, the assembled Li-S battery exhibits a specific capacity of 1100 mAh g-1 and capacity retention of 347 mAh g-1 after 200 cycles at 60 °C and 0.05 C, while the capacity retention of the battery without PVP-blended PEO electrolyte remains only 27 % at the same conditions.

Proton and Lithium Cations Linked to π-Electron and σ-Electron Systems: Are Such Interactions beyond or within the Definition of Hydrogen/Lithium Bond?

MP2/aug-cc-pVTZ calculations were performed on systems containing a proton or a lithium cation located between two π-electron systems or between π-electron and σ-electron units. The proton or the lithium cation attached to the acetylene or its derivative may be treated as the Lewis acid unit while the remaining part of the complex, the π-electron species or the dihydrogen, act as the Lewis base through their π-electrons or σ-electrons, respectively. The complexes analysed here are linked by the π⋅⋅⋅H+ /Li+ ⋅⋅⋅π and π⋅⋅⋅H+ /Li+ ⋅⋅⋅σ interactions. It is discussed whether these interactions are classified as hydrogen and lithium bonds. Therefore, different definitions of the latter interactions are presented. The Electron Localization Function (ELF) and the Natural Bond Orbital (NBO) approaches were applied to analyse the above-mentioned complexes. The unique properties of interactions with the proton and with the lithium cation that occur in complexes analysed here are described.

Hydrogen and Lithium Bonds-Lewis Acid Units Possessing Multi-Center Covalent Bonds.

MP2/aug-cc-pVTZ calculations were carried out on complexes wherein the proton or the lithium cation is located between π-electron systems, or between π-electron and σ-electron units. The acetylene or its fluorine and lithium derivatives act as the Lewis base π-electron species similarly to molecular hydrogen, which acts as the electron donor via its σ-electrons. These complexes may be classified as linked by π-H∙∙∙π/σ hydrogen bonds and π-Li∙∙∙π/σ lithium bonds. The properties of these interactions are discussed, and particularly the Lewis acid units are analyzed, because multi-center π-H or π-Li covalent bonds may occur in these systems. Various theoretical approaches were applied here to analyze the above-mentioned interactions-the Quantum Theory of Atoms in Molecules (QTAIM), the Symmetry-Adapted Perturbation Theory (SAPT) and the Non-Covalent Interaction (NCI) method.

Lithium Bond Chemistry in Lithium-Sulfur Batteries.

The lithium-sulfur (Li-S) battery is a promising high-energy-density storage system. The strong anchoring of intermediates is widely accepted to retard the shuttle of polysulfides in a working battery. However, the understanding of the intrinsic chemistry is still deficient. Inspired by the concept of hydrogen bond, herein we focus on the Li bond chemistry in Li-S batteries through sophisticated quantum chemical calculations, in combination with 7 Li nuclear magnetic resonance (NMR) spectroscopy. Identified as Li bond, the strong dipole-dipole interaction between Li polysulfides and Li-S cathode materials originates from the electron-rich donors (e.g., pyridinic nitrogen (pN)), and is enhanced by the inductive and conjugative effect of scaffold materials with π-electrons (e.g., graphene). The chemical shift of Li polysulfides in 7 Li NMR spectroscopy, being both theoretically predicted and experimentally verified, is suggested to serve as a quantitative descriptor of Li bond strength. These theoretical insights were further proved by actual electrochemical tests. This work highlights the importance of Li bond chemistry in Li-S cell and provides a deep comprehension, which is helpful to the cathode materials rational design and practical applications of Li-S batteries.

Modulating the Coordination Environment of Lithium Bonds for High Performance Polymer Electrolyte Batteries.

The new-generation lithium metal batteries require polymer electrolytes with high ionic conductivity and mechanical properties. However, the performance of the polymer electrolytes is severely influenced by the lithium bond formation between the functional groups and lithium ions (Li+), which has barely been considered in the past. Herein, a lithium bond enriched polymer gel (PAEV) is elaborately designed by copolymerizing 4-acryloylmorpholine (ACMO) and 1-vinyl-3-ethyl imidazolium bis(trifluoromethylsulfonyl)imide ([VEIM][TFSI]) in 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) with the presence of LiFSI. The lithium bonds formed between LiFSI and carbonyl groups in PACMO can be regulated by the Li+ coordination number, and further weakened by the hydrogen bonds with [EMIM][TFSI] and poly[VEIM][TFSI], to effectively render the polymer electrolyte with adjustable ionic conductivity and tunable mechanical property. In addition, with the regulated coordination environment of Li+, the LiF and Li3N layer can be uniformly formed on the Li surface to facilitate Li+ nucleation and deposition. As a consequence, the PAEV electrolyte confers the Li/LiFePO4 (LFP) battery with high capacity of 124 mA h g-1 at 1 C under 25 °C, and 152 mA h g-1 under 50 °C. This work can promote the development of high performance polymer electrolyte via lithium bond manipulation.

Cationic P⋯N interaction in XH3P(+)⋯NCY complexes (X=H, F, CN, NH2, OH; Y=H, Li, F, Cl) and its cooperativity with hydrogen/lithium/halogen bond.

The geometries, interaction energies and bonding properties of cationic pnicogen bond (CPB) interactions are studied in binary XH3P(+)⋯NCY (X=H, F, CN, NH2, OH; Y=H, Li, F, Cl) complexes by means of MP2/aug-cc-pVTZ calculations. Interaction energies of these binary complexes span a large range, from -16.36kcal/mol in (NH2)H3P(+)⋯NCF to -71.36kcal/mol in FH3P(+)⋯NCLi complex. The spin-spin coupling constant across P⋯N interaction depends considerably on the nature of X and Y substituents. The characteristic of CPB interactions is analyzed in terms of parameters derived from quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses. The charge transfer from the nitrogen base to the cationic acid stabilizes these pnicogen-bonded complexes. For a given XH3P(+), the net charge transfer value increases as the interaction energy of the complex becomes more negative, i.e., NCLi>NCCl>NCH>NCF. Moreover, mutual influence between the CPB and hydrogen/halogen/lithium bond is studied in the ternary XH3P(+)⋯NCY⋯NCH complexes. The results indicate that the formation of a Y⋯N interaction tends to strengthen CPB in the ternary systems.

Comparison of the directionality of the halogen, hydrogen, and lithium bonds between HOOOH and XF (X = Cl, Br, H, Li).

Detailed electrostatic potential (ESP) analyses were performed to compare the directionality of halogen bonds with those of hydrogen bonds and lithium bonds. To do this, the interactions of HOOOH with the molecules XF (X = Cl, Br, H, Li) were investigated. For each molecule, the percentage of the van der Waals (vdW) molecular surface that intersected with the ESP surface was used to roughly quantify the directionality of the halogen/hydrogen/lithium bond associated with the molecule. The size of the region of intersection was found to increase in the following order: ClF < BrF < HF < LiF. The maximum ESP in the region of intersection, V S, max, was observed to become more positive according to the sequence ClF < BrF < HF < LiF. For ClF and BrF, the positive electrostatic potential was concentrated in a very small region of the vdW molecular surface. On the other hand, for HF and LiF, the positive electrostatic potential was more diffusely scattered across the vdW surface than for ClF and BrF. Also, the optimized geometries of the dipolymers HOOOH··· XF (X = Cl, Br, H, Li) indicated that halogen bonds are more directional than hydrogen bonds and lithium bonds, consistent with the results of ESP analyses.

Cooperative effects in FH/Li⋯HCCX⋯OH2 complexes (X=F, Cl, Br, H).

The dimers with general formula FH/FLi⋯HCCX and HCCX⋯OH2, and the trimers FH⋯HCCX⋯OH2 (X=F, Cl, Br, H), were optimized computationally to stable structures. These model systems derive their strength from a combination of H⋯π (or Li⋯π) electrostatic interactions in the T-shaped FH/FLi⋯HCCX dimers and halogen bonding between the X and the O atom of H2O (or CH⋯O hydrogen-bonding in HCCH complexes). These cooperative interactions in the trimer clusters yield a non-additive energy which enhances the stability by between 7% and 10%. The variation in the interaction energies, as well as other selected properties, for different X is rationalized and discussed.

Synergistic and diminutive effects between halogen bond and lithium bond in complexes involving aromatic compounds.

Quantum chemical calculations have been performed to study the interplay between halogen bond and lithium bond in the ternary systems FX-C6H5CN-LiF, FLi-C6H5CN-XF, and FLi-C6H5X-NH3 (X = Cl, Br, and I) involving aromatic compounds. This effect was studied in terms of interaction energy, electron density, charge transfer, and orbital interaction. The results showed that both FX-C6H5CN-LiF and FLi-C6H5CN-XF exhibit diminutive effects with the weakening of halogen bond and lithium bond, while FLi-C6H5X-NH3 displays synergistic effects with the strengthening of halogen bond and lithium bond. The nature of halogen bond and lithium bond in the corresponding binary complexes has been unveiled by the quantum theory of atoms in molecules methodology and energy decomposition analysis.

Mutual influence between conventional and unconventional lithium bonds.

The interplay between conventional and unconventional lithium bonds interactions in NCLi⋯NCLi⋯XCCX and CNLi⋯CNLi⋯XCCX (X=H, F, Cl, Br, OH, CH3, and OCH3) complexes is studied by ab initio calculations. Cooperative effects are observed when Li⋯N(C) and Li⋯π bonds coexist in the same complex. These effects are analyzed in terms of geometric, energetic and electron charge density properties of the complexes. The cooperative effects are larger in those complexes with shorter intermolecular distances than in those with the longest ones. The electron density at the lithium bond critical points can be regarded as a good descriptor of the degree of cooperative effects. An excellent linear correlation can be obtained between the cooperative energies and the calculated spin-spin coupling constants across the lithium bonds.